Switching Regulators

Switching Regulators

Dec.-03,04,06,10,12,14,15,17, May-06,07,08,09,10,11,15,16,17,18

The operating principle of switching regulators is completely different than that of linear regulators. The switching regulators are also called as switched mode regulators. Such a switching regulator requires an external transistor and a choke. The series pass transistor in such a regulator is used as a controlled switch and is operated in cut-off region or saturation region. Hence the power transmitted across such a transistor is in the form of discrete pulses rather than a steady flow of current.

When the transistor is operated in the cut-off region, there is no current and dissipates no power. While when it is operated in the saturation region, a negligible voltage drop appears across it and hence dissipates very small power, providing maximum current to load. In any case, the power dissipated in the transistor is very small. Almost the entire power gets transmitted to the load. Hence the efficiency of the switching regulators is always very high.

Key Point The pulse width modulation is the basic principle of the switching regulators. The average value of repetitive pulse waveform is proportional to the area under the waveform.

So switching regulators use the fact that if duty cycle of the pulse waveform is varied, the average value of the voltage also changes proportionally.

Key Point The duty cycle of the pulse waveform is the ratio of the on time ton to the period T of the pulse waveform.

Mathematically it can be expressed as,

This basic pulse width is shown in the Fig. 5.13.1.

The basic switching regulator consists of four major components :

a) Voltage source V_in

b) Switching transistor

c) Pulse generator, V_pulse

d) Filter F₁ 

These blocks are connected together as shown in the Fig. 5.12.2, to obtain the switching regulator.

A voltage source V_in is a d.c. supply which is a battery, unregulated or regulated voltage.

It has to satisfy the requirements as :

i) It has to supply required power and the losses associated with the regulator.

ii) It must be high to satisfy the minimum requirements of the regulator.

iii) It must be large to supply sufficient dynamic range of line and load changes.

The switch is generally a transistor. The pulse generator output makes it on and off. The pulse generator produces a required pulse waveform. The most effective range of pulse waveform frequency is 20 kHz. The typical operating frequency range is 10 to 50 kHz. The filter F₁ may be RC, RL or RLC. Most commonly used filter is RLC. It converts the pulse waveforms obtained from the switch into a d.c. output voltage.

1. Block Diagram of SMPS

The Fig. 5.13.3 shows the functional block diagram of basic switching voltage regulator, which uses transistor Q₁ as a switch.

The part R₂/R₁ + R₂ of the output is fedback to the inverting input of error amplifier. It is compared with the reference voltage. The difference is amplified and given to the comparator inverting terminal.

The oscillator generates a triangular waveform at a fixed frequency. It is applied to the non-inverting terminal of the comparator. The output of the comparator is high when the triangular voltage waveform is above the level of the error amplifier output. Due to this the transistor Q₁ remains in cut-off state. Thus the output of the comparator is nothing but a required pulse waveform.

The period of this pulse waveform is same as that of oscillator output say T. The duty cycle is denoted as 5 = t_on/T or ton f as mentioned earlier. This duty cycle is controlled by the difference between the feedback voltage and the reference voltage.

When Q₁ is on in saturation state, V_CE(sat) for Q₁ is zero. Hence entire input voltage appears at point A. Thus the current flows through inductor L₁.

When Q₁ is off, L₁ still continue to supply current through itself to the load. The diode D₁ provides the return path for the current. 

The capacitor C₁ acts to smooth out the voltage and the voltage at the output is almost d.c. in nature. The output voltage V_o of the switching regulator is a function of duty cycle and the input voltage V^. Mathematically it is expressed as,

V_o = (t_on / T) V_in = δ V_in … (5.13.3)

Thus when T is constant, output is proportional to ton. This method is called Pulse Width Modulation (PWM). When ton is constant, the output is inversely proportional to the period T i.e. proportional to frequency of the pulse waveform. This method is called frequency modulation.

Key Point The PWM technique is commonly used though it is more complex as it is most suitable for the high current applications.

A high switching frequency allows small values of L₁ and C₁ and thus reduces size, cost and weight. It also reduces the ripple at the output. But the efficiency decreases and electrical noise increases. On the other hand, low switching frequency improve efficiency and reduce noise but require large filtering components. As a result of this, the range of operating frequency to get maximum efficiency, is 10 to 50 kHz.

2. Types of Switching Regulators

The term switched mode regulator is used to describe a circuit which takes d.c. input (unregulated) and provides a single d.c. output of the same or opposite polarity, and of a lower or higher voltage. The switched mode regulators use an inductor and there is no input to output isolation.

The term switched mode converter is used to describe a circuit which takes d.c. input (unregulated) and provides single or multiple d.c. outputs, again of same or opposite polarity and of a lower or higher voltage. Converters use transformer and may provide input to output isolation.

There are three basic configurations of the switching regulators.

1. Step down or Buck switching regulator

2. Step up or Boost switching regulator.

3. Inverting type switching regulator.

Let us discuss the operation of three types of switching regulators.

3. Step Down Switching Regulator

The Fig. 5.13.4 shows the basic circuit of step down switching regulator, which is also called buck type switching regulator. 

 It uses an inductor L and series transistor Q₁ which acts as a switch. The reference for error amplifier is provided by zener voltage V_z. The output is fed back to error amplifier through potential divider. The pulse width oscillator controls the operation of Q₁ as on or off, depending on the load requirements.

Consider an equivalent circuit of the regulator as shown in the Fig. 5.13.4 (a). In this circuit, Q₁ is shown as a switch as it does the function of a switch.

The transistor Q₁ is used for switching the input voltage for the required period of time, which is dependent on load current requirement. The L-C filter averages the switched voltage.

Operation : When Q₁ is ON, the capacitor charges through it and when Q₁ is OFF, the capacitor discharges through the load resistance, as shown in the Fig. 5.13.5 (a) and (b).

The variable pulse width oscillator controls ON/OFF periods of Q₁ When ON time is more compared to OFF time, the capacitor charges more, increasing the output voltage. On the other hand, when OFF time is more than ON time for Q₁ the capacitor discharges more, reducing output voltage. Thus adjusting the duty cycle δ = (t_on / t_on + t_off) of Q₁ output voltage can be regulated. 

If the output voltage increases, the voltage across R₃ increases. The reference V_z is fixed. Thus error at the input of error amplifier decreases. The output of error amplifier controls the output of variable pulse width oscillator. It produces pulse of smaller width which reduces ton for Q₁ This makes the capacitor C to discharge more, to offset any attempt of increase in output voltage.

Thus output voltage is maintained constant by controlling duty cycle of Q₁.

The output voltage is given by,

The Fig. 5.13.6 shows the waveform of capacitor voltages for t_on > t_off and t_on < t_off.

The waveforms for step down switching regulator are shown in the Fig. 5.13.7.

a. Advantages

The advantages of buck regulator are,

1. Higher efficiency.

2. Simple to design.

3. Low ripple content.

4. Small output filter.

5. Low switch stress.    

6. Large tolerance of line voltage regulation.

7. Low cost, size and weight. 

b. Disadvantages

The disadvantages of buck regulator are,

1. Single output.

2. No isolation between input and output

3. High input ripple current.

4. The input voltage must be always slightly greater than output (by 1 or 2 V).

5. Slow transient response compared to linear regulator.

6. Due to finite reverse recovery time of commutating diode, the instantaneous short circuit occurs across the source, due to which active switches may fail.

4. Step Up Switching Regulator

The Fig. 5.13.8 shows the basic circuit of step up switching regulator which is also called boost type switching regulator.

The basic elements used in this type are identical to those used in step down type but their arrangement is different.

The transistor Q₁ works as an on/off switch. When Q₁ is driven into saturation, V_CE is very very small and it acts as short circuit. When Q₁ is driven into cut-off, it is off and it acts as an open circuit. Let us study these two cases in detail.

Case 1 : Let Q₁ is ON i.e. driven to saturation.

When Q₁ is ON, V_CE is denoted as VCE(sat) and the voltage across L suddenly becomes [V_in - V_CE(sat)] as shown in the Fig. 5.13.9 (a). This expands the magnetic field around the inductor very quickly. This voltage across L can be obtained by applying KVL to V_in, L, Q₁ and V_in closed path.

During the ON time (ton) of Q1, the voltage across the inductor starts decreasing exponentially from its initial maximum value [Vin – V_CE(sat)]

Key Point The longer the on time of Qlf the smaller will be the voltage across L.

Case 2 : Let Q₁ is switched OFF i.e. cut-off region. When Q₁ is OFF, the magnetic field of the inductor L collapses and its polarity gets reversed. This is because an inductor current can not change instantly. Thus value of V_L attained after exponential decrease when Q₁ is ON, now gets reversed as shown in the Fig. 5.13.9 (b). Due to reversal of polairty, it gets added to V_in.

Trace the path V_in, L, D₁, C and V_in. The diode D₁ is forward biased due to reversed V_L and capacitor C now charges to V_in + V_L. The output voltage is voltage across capacitor C which is V_in + V_L, which is more than Vin. Thus it acts as step up type regulator.

It can be seen that how much V_L should be added to i.e. by how much output should be stepped up can be controlled by ON period of Q₁. The shorter the ON period of Q₁ the greater is V_L as it will not decrease much and the greater voltage will get added to V^, increasing the output voltage. The longer the ON time of Q₁ the smaller is the inductor voltage V_L and less voltage will get added to V^, decreasing the output voltage.

When output voltage tries to decrease due to increase in load current or decrease in itself then ON time of Q₁ gets reduced, thus increases compensating for the decrease in it.

When output voltage tries to increase, then ON time of Q₁ gets increased. This reduces the flyback voltage i.e. voltage across the inductor. Thus the less voltage gets added to V_in, reducing it s value. This compensates for the attempted increase in the output voltage.

Expression for the output voltage

The output voltage is given by,

V_out = V_in / 1 – δ

where δ = t_on / T

The waveforms for step up switching regulator are shown in the Fig. 5.13.10.

a. Advantages

The advantages of boost regulator are :

1. The output voltage is higher than input voltage.

2. The efficiency is high, greater than 90 %.        

3. Low input ripple current.

4. Simple to design.

b. Disadvantages

The disadvantages of boost regulator are :

1. It provides single output. 

2. The duty cycle is limited to 50 % to avoid the continuous current mode. If regulator enters the continuous current mode, it stops regulating the output. Thus for a minimum input voltage range, maximum duty cycle is limited.

3. Due to restricted duty cycle, the peak collector current is very high. This limits its output power rating.

4. No isolation between input and output. Any surge or transient in input can reach to output directly.

5. Voltage Inverter Type Switching Regulator (Buck-Boost)

This type of switching regulator produces an output voltage having polarity opposite to that of the input voltage. This is also called buck boost type switching regulator. The Fig. 5.13.11 shows voltage inverter type switching regulator.

The elements are again identical to buck and boost type regulators but their connections are different. The basic action remains same. Any change in output produces error which gets amplified by op-amp error amplifier. This controls the on/off periods of Q₁ to regulate the output, through variable pulse width oscillator.

Let us analyse two cases.

Case 1 : Let Q₁ is switched ON. 

The Q₁ goes into saturation and the voltage across it drops to V_CE(sat) which is about 0.3 V. Due to this voltage across inductor suddenly rises to [V_in - V_CE(sat)] and magnetic field around it suddenly expands. Due to connection of diode D₁ in this situation, it is reverse biased. This is shown in the Fig. 5.13.12 (a).

The inductor voltage starts exponentially decresing from the intial value [V_in - V_CE(sat)]

Case 2 : Let Q₁ is OFF.

Now if Q₁ is turned OFF, the magnetic field across L gets collapsed. But inductor current cannot change instantaneously. Thus voltage across inductor V_L reverses its polarity as shown in the Fig. 5.13.12 (b). 

Due to reversed V_L, the diode D₁ is now forward biased. The capacitor charges through D₁ producing output voltage of opposite polarity to that of V_in. Hence the regulator is called voltage inverter type.

The repetitive on-off action of Q₁ produces a repetitive charging and discharging of the capacitor C which is smoothed by the LC filter action. The less period Q₁ is ON, higher is the output voltage, the greater time Q₁ is ON, smaller is the output voltage.

The advantages and disadvantages of this type are same as that of step up type regulator.

6. Application of SMPS

Depending upon the requirements such as low cost, isolation, multiple outputs, step up output, inverted output, step down output, output current, variable load current etc, various types of SMPS are used in the following variety of applications,

1. Adjustable high voltage constant current sources.

2. Battery powered systems.

4. Personal computers.

5. Printers

6. Video games.

7. Motor and industrial control systems.

8. Automotive applications.

a. Comparison of SMPS Linear Regulators

Review Questions

1. State differences between linear mode and switched mode power supply. Draw the complete block diagram of switching regulator and explain its operation.

10, Dec.-14, Marks 8

2. Explain the operation of switching regulator. Give its advantages.

Dec.-10, 17, May-16, 17, Marks 16

3. Write a note on switching regulator.

Dec.-12, May-18, Marks 8

4. What is the principle of switch-mode power supplies? Discuss its advantages and disadvantages.

May-15, Marks 8

5. Compare linear regulator with switched mode regulator.

6. With neat diagram explain the working of step down switching regulator.

May-06, 11, Dec.-12, 14, Marks 16

7. Briefly explain the working principle of step up switch mode power supply with necessary circuit diagram and waveforms.

8. Explain the working of voltage inverter type switching regulator.