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Switching Voltage Regulators

2015-09-14 22:12  
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Switching Voltage Regulators

All of the voltage regulators we have discussed so far are series regulators, also sometimes called the linear regulators because the pass transistor used in such a regulator operates in the active region. Series regulators are very popular and meet many of our needs. The main drawback of these regulators is the power dissipation in the pass transistor. As already mentioned above, the transistor used in these regulators is operated in its linear mode (active region). In essence, it is being used as a class A power amplifier. Conse-quently, it is not extremely efficient, and a great-deal of power is lost as heat. In fact, as the load current increases, the power levels handled by the series-pass resistor also in-crease dramatically. Obviously, the cost of the series-pass transistor increases, and the size of the required heat sink increases. Because of this, series regulators tend to get bulky at low voltages and large currents. In some cases, a fan may be required to remove the heat produced by the pass transistor.

Other drawbacks of linear regulators are that regulated power supplies using these regulators require a step-down transformer (most expensive and bulky component of the power supply) and large sized filter capacitors to reduce the ripple.

Solution to the above problems is the use of switching regulators. As its name implies, the mode of operation requires the pass transistor to perform as a switch i.e. the transistor is operated either in cut-off region or in saturation region. This results in much less power dissipation in the pass transistor. Switching regulators can provide large load currents at low voltages, precisely what is required in PCs (personal computers).

Switching regulators are available in three basic configurations viz. step-down, step-up and polarity inverting configurations.

Switching Voltage Regulators

Switching Voltage Regulators

Step-down versionis shown in figure. The rectangular pulses on the base saturate and cut-off the pass transistor during each cycle. This generates a rectangular voltage at the input to LC filter. This filter blocks the ac component and allows the dc component to pass to the out-put. Because of the on-off switching, the aver-age value is always less than the input voltage. This is why the circuit is called the step-down version.

Step-up versionof the switching regulator is shown in figure. Again, the transistor is alternately saturated and cut-off. When the transistor is saturated current flows through the inductor. When the transistor suddenly cuts off, the magnetic field around the coil collapses and induces a large voltage across the coil of opposite polarity. This keeps the current flowing in the same direction. Furthermore, the inductive kickback voltage is larger than the input voltage. This is why the circuit is called step-up configuration.

Polarity Inverting regulatoris shown in figure. When the transistor is saturated, current flows through the inductor. When the transistor cuts off, the magnetic field col-lapses, and the inductive kickback keeps current flowing in the same direction. Since the transistor is cut-off, the only path is through the capacitor. If the direction of charging current through the capacitor is checked, output voltage is found to be negative.

Switching Regulator

Switching Regulator

A low-power design, using circuits that we are already familiar with, is illustrated in figure. The relaxation oscillator generates a square wave whose frequency is deter-mined by R5and C3. The square-wave is integrated to provide a triangular wave, which is used to drive the non-inverting ( ) input of a triangular-to-pulse converter. The pulses of this circuit then drives the pass transistor. The duty cycle of these pulses will determine the output voltage.

The duty cycle D is the ratio of the on time W to the time period T. By controlling the duty cycle of the pulse generator, the duty cycle of the input voltage to the LC filter is controlled. The output of the LC filter is a dc voltage with only a small ripple. This output

Vout= DVin

Since D can vary from 0 to 1, Voutcan vary from 0 to Vin.

The output of the LC filter is sampled by a voltage divider, which returns a feedback voltage to the comparator. The feedback voltage is compared with a reference voltage VREFfrom a Zener diode or other source. The output of the comparator then drives the inverting input of the triangular-to-pulse generator.

Operation of the Circuit.Incase the regulated output voltage tends to increase, the comparator provides a higher output voltage, which raises the inverting input voltage to the triangular-to-pulse converter. This narrows the pulses at the base input of the pass transistor. Since the duty cycle is lower, the filtered output voltage is less, which tends to cancel almost all the original increase in the output voltage. It means that any attempted increase in output voltage generates a negative feedback voltage that almost eliminates the original increase. Reverse happens should the output voltage fall.

There is enough open-loop gain in the system to ensure a well-regulated output volt-age. Since the error voltage to the comparator is near zero, the voltage across R2is approximately equal to VREF. So the current through resistor R2is

I = VREF/ R2

This current flows through R1, which means the output voltage is

Vout= VREF(R1R2)


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