Figure 1: Circuit Diagram of Push Pull Converter.

Circuit diagram & Working of Push Pull Converter

Fig. 1 shows the circuit diagram of a push pull converter. As shown it consists of two transistors, a center tapped transformer and two diodes. V_s is the dc input voltage while V_o is the variable dc output voltage.

1. When Q₁ is on :

Figure 2: Equivalent Circuit Diagram for mode I (Push Pull Converter).

When transistor Q₁ is turned on, the dc input voltage V_s appears across the lower half of the primary winding of the transformer. The equivalent circuit is as shown in Fig. 2. The current through the primary induces the secondary voltage with the polarities shown in Fig. 2. This will forward bias diode D₁ and the average output voltage V_o will be positive equal to V₂.

\[{{V}_{o}}={{V}_{2}}=({{N}_{s}}/{{N}_{p}}){{V}_{1}}=a{{V}_{1}}=a{{V}_{s}}\]

After half cycle period i.e. t = T/2, transistor Q₁ is turned off and Q is turned on. Transistors Q₁ and Q₂ operate at a 50% duty cycle.

2. When Q₂ is on :

Figure 3: Equivalent Circuit Diagram for mode II (Push Pull Converter).

When transistor Q₂ is turned on, the dc input voltage V_s appears across the upper half of the primary winding of the transformer. The equivalent circuit is as shown in Fig. 3. The current through the primary induces the secondary voltage with the polarities shown in Fig. 3. This will forward bias diode D₂ and the average output voltage V_o will be positive equal to V₂.

\[{{V}_{o}}={{V}_{2}}=({{N}_{s}}/{{N}_{p}}){{V}_{1}}=a{{V}_{1}}=a{{V}_{s}}\]

After the full cycle period i.e. t = T, transistor Q₂ is turned off and Q₁ is turned on and the cycle of operation repeats itself. Transistors Q₁ and Q₂ operate at a 50 % duty cycle.

Waveforms of Push Pull Converter

Various voltage and current waveforms for a push-pull converter at 50% duty cycle are as shown in Fig. 4.

Figure 4: Waveforms of Pushk Boost Converter.

Analysis of Push Pull Converter :

1.    The average current through each transistor, due to 50% duty cycle is given by :

\[{{I}_{A}}={{I}_{s}}/2\]

2.    The peak current through each transistor is equal to the source current is.

\[{{I}_{p}}={{I}_{s}}\]

3.    The voltage across a non-conducting transistor is twice the supply voltage. Therefore the push pull configuration is suitable only for low-voltage applications.

Advantages of Push Pull Converter :

The major advantages of a push-pull converter are as follows :

1. Low noise operation
2. High efficiency
3. Multiple outputs
4. It provides electrical isolation between input and output.
5. Output voltage can be varied above and below the input voltage.

Disadvantages of Push Pull Converter :

The major disadvantages of a push-pull converter are as follows :

1. It needs a pair of perfectly matched transistors
2. There is a possibility of transformer saturation
3. The duty cycle should be kept constant at 50 % to avoid saturation of core.

Applications of Push Pull Converter :

Some of the applications of push-pull converter are as given below :

1. Power Supplies
2. DC to AC inverter
3. DC to DC converter
4. Photovoltaic application
5. Automotive applications.

The post What is Push Pull Converter? Working Principle, Waveforms, Circuit Diagram & Formula appeared first on Electrical and Electronics Blog.

Circuit diagram & Working of Cuk Converter

Initially when the input voltage V_s is applied and the transistor T is in the off state, the capacitor C₁ charges through L₁ and D_m. to a voltage equal to V_s. The equivalent circuit of this mode is shown in Fig. 2. The circuit operation can be divided into two modes.

Figure 2.

Mode 1 (0 to t₁ ):

Figure 3: Equivalent Circuit Diagram for mode I (Cuk Converter).

At t = 0, transistor T is turned ON and it starts acting as a closed switch. So current through L₁ starts increasing. The voltage across C₁ gets applied across D_m to reverse bias it, and turns it off. The equivalent circuit of this mode is shown in Fig. 3. The inductor L₁ continues to store energy. Capacitor C₁ will discharge its energy through the circuit formed by C₁, C₂, load and L₂, as shown in Fig. 3. Mode 1 comes to an end at t = t₁ and the circuit enters into mode 2.

Mode 2 (t₁ to t₂):

Figure 4: Equivalent Circuit Diagram for mode II (Cuk Converter).

At t = t₁ the transistor T is turned off. Capacitor C₁ is charged from the input supply and the energy stored in L₂ is transferred to the load. The diode D_m and transistor T provide a synchronous switching action. C₁ acts as a medium to transfer energy from source to load. Fig. 4 shows the equivalent circuit for mode 2.

Waveforms of Cuk Converter

Figure 5: Waveforms of Cuk Converter.

Various voltage and current waveforms when the circuit reaches its steady state are shown in Fig. 5 assuming the load current to be continuous.

Advantages of Cuk Converter

Some of the major advantages of the Cuk converter are as follows :

1. The cuk converter operation is based on the transfer of capacitor energy. Hence the input current is continuous.
2. This circuit has low switching losses.
3. It has a high efficiency.

Disadvantages of Cuk Converter

Some of the major disadvantages of the Cuk converter are as follows :

1. A high value peak current flows through the transistor.
2. Ripple current of the capacitor C₁ is high.
3. This circuit requires an additional capacitor and an inductor.

Features of Cuk Converter

Some of the important features of the Cuk converter are as follows :

1. It can provide an output voltage which less that or greater than the input voltage.
2. Its input current is constant and it has low switching looses.
3. It has high efficiency.

The post What is Cuk Converter? Working Principle, Waveforms, Circuit Diagram, Formula & Derivation appeared first on Electrical and Electronics Blog.

Circuit diagram & Working of Buck Boost Converter

The circuit diagram for the buck boost converter is as shown in Fig. 1. The buck boost switching converter is a non-isolated type converter and it is also known as inverting converter. The buck-boost converter is a type of flyback converter whose operation is very similar to a boost converter. A power BJT is used as a switching device in Fig. 1. But it is possible to use either a MOSFET or an IGBT in place of the power BJT. The operation can be divided into two modes.

1. Mode I (Q₁ ON):

When Q₁ is turned on at t = 0, the supply voltage V_S gets connected across the inductance L and Diode D₁ is reverse biased. The inductance current starts increasing linearly from I₁ to I₂. The inductance will store energy during this mode of operation. Fig. 2 shows the equivalent circuit for this mode.

2. Mode II (Q₁ off D₁ on):

As soon as the transistor Q₁ is turned off at t = t₁, the current through L is interrupted abruptly. A negative voltage is induced into L which will forward bias diode D₁. The load current starts flowing through D₁, C and L as shown in Fig. 3. Note that this current is negative. The capacitor charges with its lower plate positive with respect to its upper plate. During this mode the energy stored in L is delivered to the load and the inductor current decreases from I₂ to I₁ linearly until transistor Q₁ is turned on again. This mode comes to an end when Q is turned on again in the next cycle of operation. Fig. 3 shows the equivalent circuit for this mode.

Waveforms of Buck Converter

The waveforms for a buck boost converter in the steady state for the continuous conduction mode are as shown in Fig. 4.

Figure 4: Waveforms of Buck Boost Converter.

Analysis of Buck Boost Converter :

Refer Fig. 4. The inductor current increases from I₁ to I₂ in time t.

\[{{e}_{L}}=\frac{Ld{{i}_{L}}}{dt}….(1)\]

But in mode I, e_L = V_S, d i_L = I₂ – I₁ = ΔI and dt = t₁.

\[{{V}_{S}}=\frac{L({{I}_{2}}-{{I}_{1}})}{{{t}_{1}}}=\frac{L\Delta I}{{{t}_{1}}}\]

\[\text{OR  }{{t}_{1}}=\frac{L\Delta I}{{{V}_{S}}}\]

In mode II, the inductor current falls linearly from I₂ to I₁ in time t₂ and e_L = -V_o. Substituting all these values into Equation (1) we get,

\[{{V}_{o}}=\frac{-L\Delta I}{{{t}_{2}}}\]

Therefore,

\[{{t}_{2}}=\frac{-L\Delta I}{{{V}_{o}}}\]

Note that ΔI = (I₂ – I₁) is the peak to peak ripple in the inductor current. From above Equations we get,

\[\Delta I=\frac{{{V}_{S}}{{t}_{1}}}{L}=\frac{-{{V}_{o}}{{t}_{2}}}{L}\]

But t₁ = On time = DT and t₂ = Off time = (1 – D)T.

\[{{V}_{S}}(DT)=-{{V}_{o}}(1-D)T\]

Hence the average output voltage of the buck boost converter is,

\[{{V}_{o}}=\frac{-D{{V}_{S}}}{(1-D)}….(2)\]

\[(1-D)=\frac{-D{{V}_{S}}}{{{V}_{S}}}….(3)\]

Substitute t₁ = DT and t₂ = (1 – D) T in Equation (3) we get,

\[(1-D)=\frac{-{{V}_{S}}}{{{V}_{o}}-{{V}_{S}}}….(4)\]

Now substitute t₂ = (1 – D) T and the value of (1 – D) from Equation (4) into Equation (2) to get,

\[{{t}_{1}}=\frac{{{V}_{o}}}{({{V}_{o}}-{{V}_{S}})f}\]

Assume that the buck-boost converter is a lossless circuit.

\[{{V}_{S}}{{I}_{s}}=-{{V}_{o}}{{I}_{o}}\]

\[\text{But     }{{V}_{o}}=\frac{-D{{V}_{S}}}{(1-D)}\]

\[{{V}_{S}}{{I}_{s}}=\frac{D{{V}_{S}}}{(1-D)}.{{I}_{o}}\]

Hence the average input current I_s is given by,

\[{{I}_{s}}=\frac{D{{I}_{o}}}{1-D}\]

The switching time T is given by,

\[T={{t}_{1}}+{{t}_{2}}\]

Substituting t₁ and t₂ from above Equations we get,

\[T=\frac{L\Delta I}{{{V}_{S}}}-\frac{L\Delta I}{{{V}_{o}}}\]

\[T=\frac{1}{f}=\frac{L\Delta I({{V}_{o}}-{{V}_{S}})}{{{V}_{o}}{{V}_{S}}}\]

Hence the peak to peak ripple current is given by,

\[\Delta I=\frac{{{V}_{o}}{{V}_{S}}}{fL({{V}_{o}}-{{V}_{S}})}\]

Substitute \({{V}_{o}}=\frac{-D{{V}_{S}}}{(1-D)}\) to get,

\[\Delta I=\frac{-DV_{s}^{2}}{(1-D)}\times \frac{1}{fL\left[ \frac{-D{{V}_{S}}}{1-D}-{{V}_{S}} \right]}\]

\[=\frac{-DV_{s}^{2}}{(1-D)fL}.\frac{(1-D)}{-D{{V}_{S}}-(1-D){{V}_{S}}}\]

\[=\frac{-DV_{s}^{2}}{(1-D)fL}\times \frac{(1-D)}{-{{V}_{S}}(D-1-D)}\]

\[\Delta I=\frac{D{{V}_{S}}}{fL}\]

Expression for the voltage ripple :

When Q₁ is conducting during mode I, the filter capacitor C supplies the load current for t₁.

The average discharging current of capacitor C is,

\[{{I}_{c}}={{I}_{o}}\]

Hence the peak to peak ripple voltage of the capacitor is given by,

\[\Delta {{V}_{c}}=\frac{1}{C}\int\limits_{0}^{{{t}_{1}}}{{{I}_{c}}dt}\]

\[=\frac{1}{C}\int\limits_{0}^{{{t}_{1}}}{{{I}_{o}}dt}\]

\[{{V}_{c}}=\frac{{{I}_{o}}{{t}_{1}}}{C}\]

Substituting t₁ we get,

\[\Delta {{V}_{c}}=\frac{{{I}_{o}}{{V}_{o}}}{C({{V}_{o}}-{{V}_{S}})f}\]

Substitute \(\Delta {{V}_{c}}=\frac{D{{V}_{d}}}{(1-D)}\) and simplify to get,

\[\Delta {{V}_{c}}=\frac{D{{I}_{o}}}{fC}\]

Advantages of Buck Boost Converter :

Some of the important advantages of buck-boost converter are as follows :

1. This circuit produces a negative output voltage without transformer.
2. Its efficiency is high.
3. The rate of change of fault current (di/dt) is limited to a safe value by the inductor L.
4. It is easy to implement the short circuit protection.

Disadvantages of Buck Boost Converter :

Some of the important disadvantages of buck boost converter are as follows :

1. Input current is discontinuous.
2. A high peak current flows through the transistor.

The post What is Buck Boost Converter? Working Principle, Waveforms, Circuit Diagram, Formula & Derivation appeared first on Electrical and Electronics Blog.

Figure 1: Voltage Source Converter.

Symbolic representation of a voltage sourced convener is as shown in figure 1. The symbol has a box with a gate tum-off device paralleled by a reverse diode, and a D.C capacitor as its voltage source.

In a voltage source converter, devices undergo sequential switching to present unidirectional D.C voltage of a D.C capacitor, as A.C voltage to the A.C side. The A.C output voltage can be varied in magnitude and also in any phase relationship to the A.C system voltage, using suitable converter topology. The power reversal here involves only the reversals of current but not the voltage. In case the storage capacity of the D.C capacitor reduces and if the D.C capacitor has no other source of power, the converter can no more impart or consume the real power for more than a cycle. A converter imparts or absorbs the reactive power alone, as long as the A.C voltage and A.C current (leading or lagging) are at 90º phase angle, where the A.C current is taken as reference.

Basic Operating Principle of a Voltage Source Converter

Figure 2: Voltage Source Converter working principle.

The basic operating principle of a voltage source converter generating reactive power is comparable to that of a conventional rotating synchronous machine (see Figure 2). Single line diagram for a basic voltage source convener scheme for reactive power generation is as shown in figure. The convener gives a set of controllable three phase output voltage with system frequency by the charged capacitor C_S when a D.C input voltage is given. Each output voltage is in phase with and coupled to the respective A.C system voltage through a small tie reactance. The tie reactance is of the order 0.1 to 0.15 p.u and it is provided by the per phase leakage inductance of the coupling transformer. The reactive power exchange between the converter and the A.C system is controlled by changing the amplitude of output voltage produced. This means that when the output voltage is increased than that of the A.C system voltage, current flows via the tie reactance from the converter to the A.C system. Thus reactive (capacitive) power for the A.C system is generated by the convener. Now if the output voltage is decreased then the reactive current flows to the converter from the A.C system. Thus reactive (inductive) power is absorbed by the converter. In case if the output voltage and the A.C system voltage are equal in amplitude, the reactive power exchange is zero.

Circuit Diagram of a Voltage Source Converter

Figure 3: Circuit Diagram of a Voltage Source Converter

The circuit diagram of single-phase full wave bridge converter is shown in figure (3).

Working of a Voltage Source Converter

Voltage source converter generates A.C voltage from D.C voltage. A single phase full wave bridge converter consists of 4 valves, namely valve-1, valve-2, valve-3 and valve-4 and each valve consists of a turn-off device T, and diode D, connected in series with each other. On the D.C side, as the voltage is unipolar, it is supported by a capacitor. The capacitor is used to handle the current that accompanies the switching sequence of the converter valve and shifts in phase angle of the switching valves without change in D.C voltage and two ac connection points A and B.

Conversion of AC voltage to D.C voltage is possible by changing the turn-ON and turn-OFF sequence of valves. In one cycle the single phase full wave bridge operates in four different operating modes as given below,

1. T₁ and T₄ ON, T₂ and T₃ OFF (Inverter)
2. T₁ and T₄ ON, T₂ and T₃ OFF (Rectifier)
3. T₁ and T₄ OFF, T₂ and T₃ ON (Inverter)
4. T₁ and T₄ OFF, T₂ and T₃ ON (Rectifier)

When the turn-off devices T₁ and T₄ are turned ON, voltage becomes positive i.e., +V_d for one half cycle and with T₂ and T₃ turned ON, V_AB becomes negative. The interaction of the converter generated A.C voltage with the A.C system voltage and impedance results in A.C current, which is generally a sinusoidal wave form.

From the 1^st operating mode time t_a to t_b with the turn-off devices T₁ and T₄ ON and T₂ and T₃ off, V_AB is positive and current i_AB is negative. Here power flow is from D.C to A.C. Hence inversion action takes place. In 2^nd operating mode time t_b to t_c, the current i_ab is positive and flows through diodes D₁ and with power flow A.C to D.C. Hence, it acts as a rectifier. Similarly 3^rd and 4^th operating modes are conducted. The operations of all the modes are shown below in comparison table.

The output waveforms of single phase converter is shown in figure (4).

Figure 4.

Figure 5.

Figure 6.

Figure 4 gives the output waveforms of voltage and current as per the given table. Figure 5 is the voltage across the valve 1 i.e., V_T1-D1. It is also known as lost waveform. Figure 6 shows the power flow from A.C to D.C with a power factor lagging. It gives the relationship between A.C voltage and current phasors.

Disadvantages of a Voltage Source Converter

The following are the disadvantages of voltage-source converters,

1. The output current limitation is low and it cannot control the capability of the semi-conduction device system faults.
2. In voltage-source converter, it is very difficult to protect converter against internal faults.
3. They have high-short circuit current and the transformer connections are complex compared with CSC.
4. Rapid increase in rise of capacitor discharge current results in the damage of valves.

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Figure 1: Circuit Diagram of Buck Converter.

Circuit diagram of Buck Converter

Capacitor C₁ is the input filter capacitor that may be connected to reduce the ripple in the DC input voltage V_S. L and C form an LC filter that connected to reduce the ripple contents in the output of the circuit. D_FW is the freewheeling diode. In place of transistor Q₁, we can connect any other power switching device like MOSFET or IGBT.

Working of Buck Converter

In the Fig. 1, Q₁ is a power transistor which is turned ON and OFF by the rectangular pulses applied at its base. We may connect any other power semiconductor switch in its place.  D_FW is a freewheeling diode, while L and C form a low pass filter. V_S is the unregulated dc power supply.

As we can vary the average output voltage by changing either the duty cycle or frequency. In most application the variation of duty cycle is preferred to variation in frequency. The expression for average output voltage in terms of duty cycle is given by :

\[{{V}_{o}}=D\times {{V}_{S}}\]

The duty cycle “D” can be varied between 0 and 1. Therefore average output voltage V_o will vary between 0 and V_S. As average output voltage V_o is less than or equal to V_S, this circuit is called as the “buck converter”, or a step down switching regulator. The buck converter is a step down type switching regulator. The operation is divided into two modes.

1. Mode I (When Q₁ is on) :

Figure 2: Equivalent Circuit Diagram for mode I (Buck Converter).

When Q₁ is turned on the input dc voltage V_S gets connected at the input of the LC filter. The output voltage is held constant by the large value capacitor. The current through L increases linearly from I₁ to I₂. The input current flows through Q₁, inductor L, capacitor C, and the load resistance R as shown in Fig. 2. Energy is given to the LC filter and the load during this mode of operation. The diode D_FW is reverse biased and remains off.

2. Mode II (When Q₁ is OFF) :

This mode begins at t = t₁ when Q₁ is turned off. Due to the interruption in current, there is a self induced voltage which appears across the inductance L. This voltage forward biases diode D_FW (which is also called as catch diode). The load current starts flowing through the D_FW. The inductor current continues to flow through L, C load and the freewheeling diode as shown in Fig. 3. The inductor current reduces linearly from I₂ to I₁ during this mode of operation. The output voltage can be varied by varying the duty cycle of the power transistor. The output voltage is approximately equal to V_out = DV_S

Where,

\[D=\frac{{{T}_{on}}}{{{T}_{on}}+{{T}_{off}}}\]

Waveforms of Buck Converter

The waveforms for a buck converter in the continuous conduction mode are as shown in Fig. 3.

Figure 4: Waveforms of Buck Converter.

Analysis of Buck Converter :

For the analysis of a buck converter we assume the current through filter inductance (i_L) is continuous and varies linearly. Also Q₁ and D_FW are assumed to be ideal devices. Refer the waveforms of Fig. 4, to write the expression for the voltage across L as,

\[{{e}_{L}}=L\frac{d{{i}_{L}}}{dt}\]

The inductor current i_L rises linearly from I₁ to I₂ during time t₁.

\[d{{i}_{L}}={{I}_{2}}-{{I}_{1}},dt={{t}_{1}}\]

And the voltage e_L during time t₁ is V_S – V_o as shown in Fig. 2 i.e. equivalent circuit of mode I.

\[{{e}_{L}}={{V}_{S}}-{{V}_{o}}\]

Substituting these value we get, \[{{V}_{S}}-{{V}_{o}}=\frac{L({{I}_{2}}-{{I}_{1}})}{{{t}_{1}}}\]

Let (I₂ – I₁) = ΔI i.e. the peak to peak ripple current.

\[{{V}_{S}}-{{V}_{o}}=\frac{L\Delta I}{{{t}_{1}}}\]

\[{{t}_{1}}=\frac{L\Delta I}{{{V}_{S}}-{{V}_{o}}}….(1)\]

Now consider mode II of operation. Here i_L changes linearly from I₂ to I₁ in time t₂. As shown in Fig. 3 the voltage e_L during this mode is equal to (-V_o).

\[{{e}_{L}}=\frac{Ld{{i}_{L}}}{dt}\]

\[\text{But, }{{i}_{L}}={{I}_{2}}-{{I}_{1}}\text{ and dt = }{{\text{t}}_{2}}\]

\[-{{V}_{o}}=\frac{-L({{I}_{2}}-{{I}_{1}})}{{{t}_{2}}}=\frac{-L\Delta I}{{{t}_{2}}}\]

\[{{t}_{2}}=\frac{L\Delta I}{{{V}_{o}}}….(2)\]

Equating the values of ΔI in Equations (1) and (2) we get,

\[\Delta I=\frac{{{t}_{1}}({{V}_{S}}-{{V}_{o}})}{L}=\frac{{{V}_{o}}{{t}_{2}}}{L}….(3)\]

But t₁ = DT and t₂ = (1 – D) T. Substituting these values into Equation (3) we get the average output voltage of a buck converter as,

\[DT({{V}_{S}}-{{V}_{o}})={{V}_{o}}(1-D)T\]

\[{{V}_{S}}DT-{{V}_{o}}DT={{V}_{o}}T-{{V}_{o}}DT\]

\[{{V}_{S}}DT={{V}_{o}}T\]

\[{{V}_{o}}=D{{V}_{S}}\]

Now assume that the buck converter circuit is lossless.

\[\text{Input power = Output power}\]

\[{{V}_{S}}{{I}_{S}}={{V}_{o}}{{I}_{o}}=D{{V}_{S}}{{I}_{o}}….(4)\]

From equation (4) we get the average source current as,

\[{{I}_{S}}=D{{I}_{o}}\]

Switching period T :

The switching period T is given by,

\[T={{t}_{1}}+{{t}_{2}}….(5)\]

Substituting Equations (1) and (2) into Equation (5) we get,

\[T=\frac{L\Delta I}{{{V}_{S}}-{{V}_{o}}}+\frac{L\Delta I}{{{V}_{o}}}\]

\[T=\frac{L\Delta I({{V}_{o}}+{{V}_{S}}-{{V}_{o}})}{{{V}_{o}}({{V}_{S}}-{{V}_{o}})}\]

\[T=\frac{L\Delta I\text{ }{{V}_{S}}}{{{V}_{o}}({{V}_{S}}-{{V}_{o}})}….(6)\]

Peak to peak ripple current ΔI :

From Equation (6) we get,

\[\Delta I=\frac{T{{V}_{o}}({{V}_{S}}-{{V}_{o}})}{L{{V}_{S}}}\]

But T = 1/f and V_o = DV_S.

\[\Delta I=\frac{D{{V}_{S}}({{V}_{S}}-D{{V}_{S}})}{fL{{V}_{S}}}\]

\[\Delta I=\frac{D{{V}_{S}}(1-D)}{fL}\]

Advantages of Buck Converter

The advantages of buck converter are as follows .

1. It needs only one transistor.
2. It is a simple circuit.
3. The circuit efficiency is high (higher than 90%).
4. The di/dt of the load current is limited by inductor L.

Disadvantages of Buck Converter

The disadvantages of buck converter are as follows :

1. Input current is discontinuous and a smoothing input filter is generally required.
2. It can produce an output voltage of only one polarity. Polarity reversal is not possible.
3. The output current is unidirectional.
4. It needs a separate protection circuit against a possible short circuit, across the diode path.

The post What is Buck Converter? Working Principle, Waveforms, Circuit Diagram, Formula & Derivation appeared first on Electrical and Electronics Blog.

Figure 1: Basic of Switch Mode Power supply (SMPS).

A switch mode power supply (SMPS) is a dc-to-dc series regulated power supply, in which the series pass transistor is operated as a switch. The output voltage of a SMPS is regulated by varying its duty cycle.

In SMPS, the series pass transistor (electronic switch in Fig. 1) does not operate in its active region. Instead it operates as a switch. This is how it is different from the conventional or linear power supply.

Block diagram of Switch Mode Power supply (SMPS)

The block diagram of a basic switching regulator is shown in Fig. 1. The block diagram shows that the SMPS is also a series regulator. The basic switch mode power supply consists of four components namely the unregulated dc voltage source V_in, an electronic switch S (a transistor or MOSFET), a pulse generator and a filter, as shown in Fig. 1.

Working of Switch Mode Power supply (SMPS)

The pulse generator generates rectangular pulses which are applied to the control terminal of an electronic switch. This switch is turned on and off with the help of these rectangular pulses. The switch is an electronic switch which is typically a transistor or MOSFET. It is used in its saturation and cut off regions and not in the active region. When the switch is on, it connects the unregulated dc input V_in as it is to the input of the filter and the filter input is disconnected from the dc input voltage V_in when the switch is open circuited.

Filter input voltage =  V_in        …..when switch is on

And, filter input voltage = 0       …..when switch is off

Figure 2: Waveforms of basic Switch Mode Power supply (SMPS).

This is shown in the waveforms of Fig. 2. Therefore at the input of the filter we get a rectangular waveform. The average value of this waveform can be adjusted by changing either the duty cycle or frequency of the rectangular pulses produced by the pulse generator. The duty cycle is defined as,

\[\text{Duty cycle (D) = }\frac{{{t}_{on}}}{{{t}_{on}}+{{t}_{off}}}=\frac{{{t}_{on}}}{T}\]

\[T={{t}_{on}}+{{t}_{off}}=\frac{1}{\text{Frequency}}\]

\[or\text{ T = }\frac{\text{1}}{f}\]

Typically, the operating frequency of the switching regulator will be in the range of 10 to 50 kHz. That means the total time T is of the order of 100 µs to 20 µs. The filter then converts the rectangular waveform at its input into a smooth dc voltage by removing the ripple contents. The expression for dc output voltage of a switching regulator is given by:

\[{{V}_{o}}=\frac{{{t}_{on}}}{T}\times {{V}_{in}}\]

\[or\text{   }{{V}_{o}}=D\times {{V}_{in}}\]

Thus the average output voltage is dependent on the duty cycle D. The average output voltage will increase with increase in the value of duty cycle as shown in Fig. 2.

Advantages of Switch Mode Power supply (SMPS)

The advantages of SMPS are as follows :

1. Low power dissipation in the series pass transistor as it operates as a switch and not in the active region.
2. High efficiency (upto 95%) due to reduced power dissipation in the transistor.
3. Small size : This is due to the smaller size of L and C at high operating frequencies and need of smaller heat sink for the series pass transistor.
4. Higher power handling capacity.

Disadvantages of Switch Mode Power supply (SMPS)

The disadvantages of SMPS are as follows :

1. Increased switching loss in the series pass transistor due to high frequency switching.
2. Radio Frequency Interference (RFD to the neighboring electronic circuits.
3. There is no isolation between input and output.
4. The load requires separate protection circuitry.
5. The transient response is slow as compared to the linear power supplies.
6. Ripple content in the output is higher than that for a linear power supply.
7. Load regulation is poor as compared to the linear regulators.

Difference between Linear regulator and Switch Mode Power Supply (SMPS) 

Types of Switch Mode Power supply (SMPS)

Figure 3: Classification of Switch Mode Power supply (SMPS).

The classification of SMPS is shown in Fig. 3. The SMPS are classified broadly into two categories namely :

1. Non-isolated type and
2. Isolated type.

No electrical isolation is provided between the load and source in the non-isolated type SMPS. Whereas, a transformer is included for providing the electrical isolation in case of the isolated type SMPS.

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Figure 1: RC Triggering of SCR.

The limited range of firing angle only upto 90° in the resistor trigger method of an SCR can be overcome by using RC trigger circuit of SCR. The limited range of firing angle can be increased from 90° to 180°, if the gate circuit of an SCR is supplied by a voltage that is shifted in its phase relationship to the anode voltage in such a manner that the positive gate current sufficient to trigger the SCR can be delayed beyond the peak of the anode voltage.

Circuit Diagram of RC Triggering of SCR

Fig. 1 shows the circuit diagram of an RC trigger for an SCR. The a.c. voltage is applied between the anode and cathode of an SCR. The variable resistor R is used to limit the gate current. It also controls the firing angle of an SCR. The diode D₁ is a blocking diode which is used as a preventive safeguard to the gate-cathode junction of an SCR from getting damaged in the negative half cycle of the applied a.c. voltage. The capacitor C is used to apply the reverse voltage and improve the firing angle from 90° to 180°. The diode D₂ is used to reset the capacitor C to the peak of the negative half cycle of the supply voltage.

Principle of Operation of RC Triggering of SCR

The a.c. voltage is applied between the anode and the cathode of an SCR T. Consider that the SCR is in the forward blocking (OFF) state. In the positive half cycle of the applied a.c. voltage, the capacitor C is initially charged through the variable resistor R upto the peak value of the applied voltage, as the diode D₂ is now reverse biased. The charging rate of the capacitor C can be controlled by the variable resistor R. The voltage across the capacitor C will be the time integral of current I_c admitted to it by a variable resistor R. When the voltage V_C across the capacitor C is sufficient, the diode D₁ allows the gate current. Depending on the voltage across the capacitor C and if the gate current at is sufficient, the SCR turns ON. The resistor R can control the firing angle. In the negative half cycle of the applied a.c. voltage, the capacitor C is charged upto the negative peak value through the diode D₂, thus resetting it for the next charging cycle. Now the diode D₁ is reverse biased and it is known as a blocking diode. It is used as a preventive safeguard against the reverse breakdown of the gate-cathode junction of an SCR during the negative half cycle. The diode D₁ must be rated to support at least the peak of the supply voltage V_SC. The diode D₂ resets the capacitor C to the peak of the negative cycle. The diode D₁ must be rated to support at least the peak of the supply voltage V_SC. The diode D₂ resets the capacitor C to the peak of the negative supply voltage V_S and hence it must be rated to withstand the voltage at least 2V_S, if there is a possibility of opening of the resistor R. The main function of the capacitor C is to shift the phase of the anode voltage so that a positive gate current can be supplied after the peak of the anode voltage. By varying the resistor R, the firing angle can be controlled from 0° to 180°.

Waveforms of RC Triggering of SCR

Figure 2: Waveforms for RC half-wave trigger circuit for two different values of R.

Fig. 2 shows voltage waveforms for RC trigger circuit for different values of R. When SCR turns ON, its ON-state voltage drop is approximately 1 V. This low voltage across SCR during turn-ON period keeps capacitor C discharged in positive half cycle until negative voltage cycle across C appears. When value of resistance R is high, the time taken by capacitor C to charge to required gate voltage is more because charging current is low. Therefore, firing angle α is more and output voltage V_o is less. Thus the output voltage V_o is inversely proportional to the firing angle α.

Advantages of RC Triggering of SCR

The RC trigger circuit has the following advantages :

1. It is the most simple and economical circuit.
2. It has a firing angle ranging from 0° to 180°.
3. It is less sensitive to temperature variations.

Disadvantages of RC Triggering of SCR

The disadvantages of RC trigger circuit are as given below :

1. It is not suitable for feedback control system because the control signal is a.c. and the feedback is through mechanical components.
2. It suffers from limited response time.
3. It does not have repeatability over a temperature range.

The post What is RC Triggering of SCR? Circuit Diagram, Working & Waveforms appeared first on Electrical and Electronics Blog.

Figure 1: Power MOSFET.

A power MOSFET is a voltage controlled device and it requires a very small amount of input voltage. When we call it as a voltage controlled device, then input should be voltage. Also we always assume gate current negligible because gate is insulated. It is a high-power version of the low-power MOSFET. Relatively recent developments in technology have resulted in the production of higher- power devices with large voltage and current capabilities.

Presently, it is available with typical current rating of tens of amperes and voltage rating of hundreds of volts. It is very fast in switching speed. The switching times are of the order of nano-seconds. It does not have the problem of secondary breakdown. However, it is sensitive to electrostatic discharge and it is relatively more difficult to protect it under short-circuit condition.

Both N-channel and P-channel MOSFETs are being made, but the N-channel devices are available in higher ratings. Two types of MOSFETs are available, one called as P-channel and the other as N-channel. The P-channel devices are fabricated on N-type substrate and the N-channel devices are fabricated on P-type substrate. Two distinct types of MOSFETs are available depending on the operating modes, depletion type and enhancement type. The P-channel MOSFET works in exactly the same way as an N-channel MOSFET, only with the voltage polarities and current directions reversed.

Construction of Power MOSFET

Power MOSFET is fabricated in the form of arrays (see Figure 1(a)). This means that a single power MOSFET is in reality a parallel combination of thousands of individual cells, each being a MOSFET in itself. The number of cells on a silicon pellet may be as high as 1000 on an area as small as 1mm². The MOSFET has three external terminals called Drain D, Source S and Gate G. The drain and source are the power terminals of the switch. The gate is the control terminal. The control voltage to implement turn-ON is applied between the gate and the source terminals. The direction of the forward current flow in an N-channel MOSFET is from the drain to the source. This results in the flow of electrons from the source to the drain. Fig. 1 (a) shows the junction structure of a depletion type N-channel power MOSFETs. The N-layer on the top constitutes the drain. This layer is actually made up of an outer N⁺ layer of low resistivity, i.e. higher concentration of carriers and an inner N⁺ layer of high resistivity, i.e. low impurity concentration. The inner high sensitivity region serves to give a high voltage capability, while the outer low resistivity region serves to make a strong low resistance electrical contact with the drain surface metal deposition. Adjacent to the N⁺ region, there is a relatively large P isolated as shown in Fig. 2.40 (a). Inside the P islands and also the middle part of the P island between the N⁺ islands. The gate terminal does not make any electrical contact with the silicon pellet because of the presence of layer of silicon dioxide (SiO₂) which is an insulator between the surface and the gate. The gate zone is over the P island between the N⁺ drain region and the N⁺ source region. A conditioning polycrystalline silicon layer, deposited over the gate zone, on the silicon dioxide layer serves as the gate layer. The polycrystalline silicon layer gives better performance than the deposited metal layer. Fig. 1 (b) shows the symbol of an N-channel depletion type power MOSFET.

Working Principle of Power MOSFET

Figure 2: Operation of N-channel power MOSFET

Consider the circuit of N-channel power MOSFET as shown in Fig. 2. The drain terminal D is made positive with respect to the emitter terminal E so that it is reverse biased. Now the gate is kept open i.e. V_GE = 0. Under this situation, the junction N^– P^– is reverse biased and hence no drain current I_D flows from drain-to-source. Only very low reverse leakage current can flow through it. Thus the power MOSFET does not conduct. This is a forward blocking (OFF) state of a power MOSET. When a low positive gate voltage V_GS is applied at the gate G, it creates the electric field which pulls the electrons from the N⁺ region into the P region immediately near the gate G. This forms an N-channel. Thus the source N⁺ region gets linked with the drain N⁺ region and the N-channel provides the path for flow of current. So, the drain current I_D flows from drain to source depending upon the voltage level V_DD. As then the drain voltage V_DS is increased further, the drain current I_D increases and then remains constant. This is the forward conduction (ON) state of a power MOSFET. This conduction is due to the majority charge carriers alone. Hence the time delay caused by removed or recombination with minority carriers is eliminated and so power MOSFET can switch ON only at frequencies in MHz range.

Characteristics of Power MOSFETs

Figure 3: Drain and transfer characteristics of N-channel power MOSFET

1. V-I characteristics :

These characteristics are plotted as the variation in drain current (I_D) as a function of drain source voltage (V_DS) for a given value of (V_GS). For low values of V_DS, the characteristics are linear and the device exhibits a constant ON-state resistance. If V_DS is increased further, then the drain current saturates. This is shown in Fig. 3 (a). Three regions are shown above in the output characteristics as :

1. Ohmic or linear : The output current I_D varies linearly with V_DS.
2. Saturation : The current I_D saturates if V_DS is increased further.
3. Cut-off : The current I_D is zero because V_GS is zero and no N channel is formed.

2. Transfer characteristics :

The characteristics of drain current (I_D) against gate-source voltage (V_GS) are called transfer characteristics. The threshold voltage (V_T) is that value of V_GS at which N-channel is formed and the drain current starts flowing. The transfer characteristics of N channel power MOSFET are shown in Fig. 3 (b).

Advantages of Power MOSFETs

The important advantages of power MOSFETs are as given below :

1. It has fast switching speed.
2. It has extremely low drain resistance r_DS(on).
3. It has very high input impedance of about 10⁹ Ω.
4. It is very simple for construction.
5. It is free from secondary breakdown.
6. It has ease of paralleling.
7. It has excellent temperature stability.
8. It has very high operating frequency.
9. The switching times are essentially independent of temperature.
10. It has very high current gain of the order of 10⁹.

The switching times (turn-ON and turn-OFF) are very low around 30 to 40 ns for a 40 V, 50 A power MOSFET and around 190 ns for a 500 V, 25 A power MOSET. Due to this low turn-OFF time, the power MOSFET can be operated at a frequency of I MHz to 10 MHz.

Applications of Power MOSFET ;

The important applications of power MOSFET are as given below :

1. It is widely used in analog and digital signal processing circuits both in discrete and integrated circuit (I_C) forms.
2. It can be used as a static switch or for analog operation.
3. It can be used in SMPS, solid state d.c. relay, brushless d.c. motor drives and automobile applications.

Features of Power MOSFET

MOSFETs are preferred over power BJTs in applications requiring high switching speeds. MOSFETs may be of P-channel type or N-channel type depending on whether the channel material is of P-type or N-type. Further MOSFET be either enhancement type or depletion type.

Difference between BJT and MOSFET

The post What is Power MOSFET? Working Principle, Symbol, Construction & V-I Characteristics appeared first on Electrical and Electronics Blog.

Figure 1: Single Phase Half Wave Controlled Rectifier.

Circuit diagram of Single Phase Half Wave Controlled Rectifier with resistive load

To understand the principle of phase controlled rectifier, refer Fig. 1. It is a half-wave controlled rectifier (HWCR) with a resistive load.

Working of Single Phase Half Wave Controlled Rectifier with resistive load

Operation in the positive half cycle :

During the positive half cycle of the ac supply, the thyristor SCR₁ is forward-biased. When it is turned on at ωt = α the thyristor acts like a closed switch and the input ac voltage appears as it is across the load, as shown in Fig. 2.

Due to the resistive nature of the load, the load current is in phase with the load voltage. And it has the same shape as that of the load voltage waveform. The instantaneous value of load current is equal to the ratio of instantaneous supply voltage and load resistance R. As the load voltage decreases, the load current also decreases and as this current reduces below the holding current of SCR₁, it is commutated due to natural commutation (at ωt = π).

Operation in the negative half cycle :

Figure 3: Equivalent circuit in the negative half cycle.

In the negative half cycle, the thyristor is reverse-biased, and acts like an open switch as shown in Fig. 3. The load is disconnected from the input and hence the load voltage is zero. The entire input voltage then appears across the turned-off SCR as shown in Fig. 4. The voltage across the SCR is almost equal to zero when it is in the on state. (α ≤ ωt ≤ π).

Waveforms of Single Phase Half Wave Controlled Rectifier with resistive load

Figure 4: Waveforms of Single Phase Half Wave Controlled Rectifier.

The voltage and current waveforms for the HWCR with resistive load are as shown in Fig. 4.

Average output voltage (V_Ldc):

From the load voltage waveform in Fig. 2 the average output voltage V_Ldc can be found as follows :

\[{{V}_{LDC}}=\frac{1}{2\pi }\int\limits_{\alpha }^{\pi }{{{V}_{m}}\sin \omega t\text{ }d\omega \text{t}}\]

\[=\frac{-{{V}_{m}}}{2\pi }\left[ \cos \omega t \right]_{\alpha }^{\pi }\]

\[=\frac{{{V}_{m}}}{2\pi }(1+\cos \alpha )….(1)\]

Equation (1) shows that the average load voltage V_Ldc can be varied from 0 to V_m/π by varying α between π to 0 radians respectively. The average voltage will be maximum when u and it is given by,

\[{{V}_{\text{LDC(max)}}}=\frac{{{V}_{m}}}{2\pi }(1+\cos 0)\]

\[=\frac{{{V}_{m}}}{\pi }\]

Concept of phase control:

Equation (1) shows that the average load voltage V_Ldc can be varied from 0 to (V_m/π) by varying α between π to 0 radians respectively. The average voltage will be maximum when α = 0 and it is given by,

\[{{V}_{\text{LDC(max)}}}=\frac{{{V}_{m}}}{2\pi }(1+\cos 0)=\frac{{{V}_{m}}}{\pi }\]

Thus it is possible to control the average load voltage and hence average load power by controlling the firing angle and phase angle α of the controlled rectifier. This is the basic concept of phase control.

Drawbacks of Single Phase Half Wave Controlled Rectifier with resistive load

1. The output voltage contains large ripple and the ripple frequency is low (50 Hz). This will make the filter design difficult and the filter becomes bulky.
2. The average output voltage \({{V}_{\text{LDC}}}=\frac{{{V}_{m}}}{2\pi }(1+\cos \alpha )\) is low due to half wave rectification and will not be useful in most of the applications.
3. The supply current is distorted and contains harmonic currents.
4. In addition to that the supply current contains dc component.
5. The input power factor is very poor.

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Figure 1: Structure of Two-transistor model of SCR.

Figure 2: Equivalent Circuit of Two-transistor model of SCR.

Two-transistor model is obtained by separating the two middle layer of SCR as shown in figure (1). The equivalent circuit of two-transistor analogy is shown in figure (2).

From figure (2), we have,

\[{{I}_{b1}}={{I}_{c2}}\]

\[{{I}_{c1}}={{I}_{b2}}\]

Cathode current = Anode current + Gate current

\[{{I}_{k}}={{I}_{a}}+{{I}_{g}}….(1)\]

The basic relation between collector current, I_c and emitter current I_E is given as,

\[{{I}_{c}}=\alpha {{I}_{E}}+{{I}_{CBO}}\]

α – Common-base current gain ≅ I_c / I_E

I_CBO – Leakage current of collector-base.

Hence, in the given equivalent circuit,

\[{{I}_{c1}}={{\alpha }_{1}}{{I}_{E1}}+{{I}_{CBO1}}\]

\[{{I}_{c2}}={{\alpha }_{2}}{{I}_{E2}}+{{I}_{CBO2}}\]

Here,

\[{{I}_{E1}}\text{ of }{{Q}_{1}}={{I}_{a}}\text{ and }{{I}_{E2}}\text{ of }{{Q}_{2}}={{I}_{c}}\]

Substituting the values of I_E1, I_E2 in equation I_C1 and I_C2, we get,

\[{{I}_{c1}}={{\alpha }_{1}}{{I}_{a}}+{{I}_{CBO1}}….(2)\]

\[={{\alpha }_{2}}{{I}_{c}}+{{I}_{CBO2}}….(3)\]

The sum of two collector currents is equal to an anode current, i.e.,

\[{{I}_{a}}={{I}_{c1}}+{{I}_{c2}}….(4)\]

Substituting equation (2) and (3) in equation (4), we get,

\[{{I}_{a}}={{\alpha }_{1}}{{I}_{a}}+{{I}_{CBO1}}+{{\alpha }_{2}}{{I}_{c}}+{{I}_{CBO2}}\]

From transistor analysis, we have,

\[{{I}_{e1}}={{I}_{b1}}+{{I}_{c1}}\]

\[{{I}_{e1}}={{I}_{b1}}+{{I}_{c1}}….(5)\]

Substituting equation (1) in equation (4), we get,

\[{{I}_{a}}={{\alpha }_{1}}{{I}_{a}}+{{I}_{CBO1}}+{{\alpha }_{2}}({{I}_{a}}+{{I}_{g}})+{{I}_{CBO2}}\]

\[{{I}_{a}}={{\alpha }_{1}}{{I}_{a}}+{{I}_{CBO1}}+{{\alpha }_{2}}{{I}_{a}}+{{\alpha }_{2}}{{I}_{a}}+{{\alpha }_{2}}{{I}_{g}}+{{I}_{CBO2}}\]

\[{{I}_{a}}={{I}_{a}}({{\alpha }_{1}}+{{\alpha }_{2}})+{{\alpha }_{2}}{{I}_{g}}+{{I}_{CBO1}}+{{I}_{CBO2}}\]

\[{{I}_{a}}(1-({{\alpha }_{1}}+{{\alpha }_{2}}))={{\alpha }_{2}}{{I}_{g}}+{{I}_{CBO1}}+{{I}_{CBO2}}\]

\[{{I}_{a}}=\frac{{{\alpha }_{2}}{{I}_{g}}+{{I}_{CBO1}}+{{I}_{CBO2}}}{1-({{\alpha }_{1}}+{{\alpha }_{2}})}\]

Since, I_CBO1 and I_CBO2 are very small, ‘I_a‘ can be written as,

\[{{I}_{a}}=\frac{{{\alpha }_{1}}{{I}_{g}}}{1-({{\alpha }_{1}}+{{\alpha }_{2}})}\]

If α₁ + α₂ = 1 then anode current I_a becomes infinity and hence thyristor enters into conduction state from OFF state.

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