SWITCH MODE POWER SUPPLY – SMPS
The use of ICs and modular construction is very common is modern television receivers, Transmitter, Computer and many Electronics Equipment’s. This has led to the introduction of switching mode power supplies to meet the dc requirements of such receivers. These are smaller, lighter and dissipate less power than equivalent series regulated supplies.
In a switched mode supply the regulating elements consist of series connected transistors that act as rapidly opening and closing switches. The input ac is first converted to unregulated dc, which, in turn is chopped by the switching elements operating at a rapid rate, typically 20 KHz. The resultant 20 KHz pulse train is transformer coupled to an output network which provides final rectification and smoothing of the dc output. Regulation is accomplished by control circuits which vary the duty cycle (on-off periods) of the switching elements if the output voltage tends to vary.
Operating Advantages and Disadvantages of SMPS
The advantages of a SMPS over a conventional regulated supply are:
( i ) The switching transistors are basically on-off devices and hence dissipate very little power when either on (saturated) or off (non-conducting). Efficiencies ranging from 65 to 85 percent are typical of such supplies as compared to 30 to 45 percent efficiencies for linear supplies.
( ii ) On account of the higher switching rate (20 KHz) the power transformer, inductor and filter capacitors are much smaller and lighter than those required for operation at power line frequencies. Typically a switching power supply is less than one third in size and weight of a comparable series regulated supply.
( iii) A switched-mode supply can operate under low ac input voltage. It has a relatively long hold-up period if input power is lost momentarily. This is so because more energy can be stored in its input filter capacitors.
Disadvantages of SMPS. Although the advantages are impressive a SMPS has the following inherent disadvantages:
( i ) Electromagnetic interference (EMI) is a natural by-product of the on-off switching within these supplies. This interference can get coupled to various sections of the receiver and hinder their normal operation. For this reason, switching supplies have built-in shields and filter networks which substantially reduce EMI and also control output ripple and noise. In addition, special shields are provided around those sections of the receiver circuitry which are highly susceptible to electromagnetic interference.
( ii ) The control circuitry is expensive, quite complex and somewhat less reliable.
Typical Circuit of a SMPS
Figure shows simplified circuit and associated waveforms of a typical switching mode power supply. Regulation is achieved by a pair of push-pull switching transistors (Q1 and Q2) operating under the control of a feedback network consisting of a pulse-width modulator and a voltage comparison amplifier. The waveforms illustrate the manner in which the duty-cycle is controlled to deliver a constant dc output voltage. The voltage comparison amplifier continuously compares a fraction of the output voltage with a stable reference source Vr 1 and develops a
RECEIVER POWER SUPPLIES
Control voltage ( V control ) for the turn-on comparator. The comparator compares V control with a triangular ramp waveform occurring at a frequency of 40 KHz. When the ramp voltage is more positive than the control level, a turn-on signal is generated. As shown in the waveforms, any increase or decrease in the control voltage (V control) will very width of the turn-on voltage and this in turn will alter the width of drive pulses to both Q 1 and Q 2. The drive pulses pass through steering logic which ensures alternate switching of Q1 and Q 2. Thus each switch operates at 20 KHz, i.e., one-half of the ramp frequency. When Q1 is on, current flows in the upper half of the primary winding of transformer T1 and completes its path through its Centre tap. Similarly when Q 2 conducts current flows in opposite direction through the lower half of the same winding to complete its return path thus providing transformer action.
Since the dc output voltage is proportional to the duty-cycle of current through the transformer, increasing the ‘on’ periods of the switching transistor will increase the output voltage and vice-versa. Thus the control voltage automatically monitors the duty-cycle to maintain a constant dc output voltage despite any input voltage or load current variations. In some such supplies only one transistor is used as the chopping and control element. In such designs a 20 KHz clock pulse is used to time the on-off periods. The comparator, ramp generator and steering or control logic usually form part of a dedicated IC. Though, not shown, in the modular chip it contains additional circuits for over voltage protection, over-current protection and prevention of any inrush of ac current.