Switching Regulators

Switching Regulators

Introduction

The switching regulator offers the advantages

·           higher power conversion efficiency

·           Increased design flexibility (multiple output voltages of different polarities can be generated from a single input voltage).

·           a lot less heat and

·           Smaller size.

The primary filter capacitor is placed on the input to the regulator to help filter out the 60 cycle ripple. If the output voltage is 12 volts and the input voltage is 24 volts then we must drop 12 volts across the regulator. At output currents of 10 amps this translates into 120 watts (12 volts times 10 amps) of heat energy that the regulator must dissipate into heat.

The switching regulator is much more efficient than the linear regulator achieving efficiencies as high as 80% to 95% in some circuits. The obvious result is smaller heat sinks, less heat and smaller overall size of the power supply.

The switching regulator is really nothing more than just a simple switch. This switch goes on and off at a fixed rate usually between 50 Khz to 100Khz as set by the circuit.

Operation:

Diode D1 has to be a Schottky or other very fast switching diode. Inductor L1must be a type of core that does not saturate under high currents. Capacitor C1 is normally a low ESR (Equivalent Series Resistance) type.

To understand the action of D1 and L1, let’s look at what happens when S1 is closed as indicated below:

L1, which tends to oppose the rising current, begins to generate an electromagnetic field in its core. Diode D1 is reversed biased and is essentially an open circuit at this point.

When S1 opens, the electromagnetic field that was built up in L1 is now discharging and generating a current in the reverse polarity. As a result, D1 is now conducting and will continue until the field in L1 is diminished. This action is similar to the charging and discharging of capacitor C1. The use of this inductor/diode combination gives us even more efficiency and augments the filtering of C1.

Because the switching system operates in the 50 to 100 kHz region and has an almost square waveform, it is rich in harmonics way up into the HF and even the VHF/UHF region Four most commonly used switching converter types:

Buck: used the reduce a DC voltage to a lower DC voltage.

Boost: provides an output voltage that is higher than the input.

Buck-Boost (invert): an output voltage is generated opposite in polarity to the input.

Fly back: an output voltage that is less than or greater than the input can be generated, as well as multiple outputs.

Converters:

Push-Pull: A two-transistor converter that is especially efficient at low input voltages.

Half-Bridge: A two-transistor converter used in many off-line applications. Full-Bridge: A four-transistor converter (usually used in off-line designs) that can generate the highest output power of all the types listed.

Switching Regulator:

An example of general purpose regulator is Motorola’s MC1723. It can be used in many different ways, for example, as a fixed positive or negative output voltage regulator, variable regulator or switching regulator because of its flexibility.

To minimize the power dissipation during switching, the external transistor used must be a switching power transistor.

To improve the efficiency of a regulator, the series pass transistor is used as a switch rather than as a variable resistor as in the linear mode.

·           A regulator constructed to operate in this manner is called a series switching regulator. In such regulators the series pass transistor is switched between cut off & saturation at a high frequency which produces a pulse width modulated (PWM) square wave output.

·           This output is filtered through a low pass LC filter to produce an average dc output voltage.

·           Thus the output voltage is proportional to the pulse width and frequency.

·           The efficiency of a series switching regulator is independent of the input & output differential & can approach 95%

A basic switching regulator consists of 4 major components,

1.        Voltage source Vin

2.        Switch S1

3.        Pulse generator Vpulse

4.        Filter F1

1.        Voltage Source Vin:

It may be any dc supply – a battery or an unregulated or a regulated voltage. The voltage source must satisfy the following requirements.

·           It must supply the required output power & the losses associated with the switching regulator.

·           It must be large enough to supply sufficient dynamic range for line & load regulations.

·           It must be sufficiently high to meet the minimum requirement of the regulator system to be designed.

·           It may be required to store energy for a specified amount of time during power failures. 

2. Switch S1:

It is typically a transistor or thyristor connected as a power switch & is operated in the saturated mode. The pulse generator output alternately turns the switch ON & OFF

3.  Pulse generator Vpulse:

It provides an asymmetrical square wave varying in either frequency or pulse width called frequency modulation or pulse width modulation respectively. The most effective frequency range for the pulse generator for optimum efficiency 20 KHz. This frequency is inaudible to the human ear & also well within the switching speeds of most inexpensive transistors & diodes.

·           The duty cycle of the pulse wave form determines the relationship between the input & output voltages. The duty cycle is the ratio of the on time ton, to the period T of the pulse waveform.

Duty cycle = t_on/(t_on+t_off) = t_on/T =t_on.f

Where

t_on = On-time of the pulse waveform toff=off-time of the pulse wave form

T = time period = t_on + t_off

=1/frequency or T = 1/f

·           Typical operating frequencies of switching regulator range from 10 to 50 kHz.

·           Lower operating frequency improve efficiency & reduce electrical noise, but require large filter components (inductors & capacitors).

3.        Filter F1:

It converts the pulse waveform from the output of the switch into a dc voltage. Since this switching mechanism allows a conversion similar to transformers, the switching regulator is often referred to as a dc transformer.

The output voltage Vo of the switching regulator is a function of duty cycle & the input voltage Vin.

Vo is expressed as follows,

V_o= t_on V_in/T

·           This equation indicates that, if time period T is constant, Vo is directly proportional to the ON-time, ton for a given value of Vin. This method of changing the output voltage by varying ton is referred to as a pulse width modulation.

·           Similarly, if ton is held constant, the output voltage Vo is inversely proportional to the period T or directly proportional to the frequency of the pulse waveform. This method of varying the output voltage is referred to as frequency modulation (FM).

·           Switching regulator can operate in any of 3 modes

i)          Step – Down

ii)       Step – Up

iii)     Polarity inverting