Question

4. The circuit shown in P-4 is a switching regulator. Please describe the functions of the OP amps of a, c. f, and d with the output waveform at A, C. F, and D Find the output voltage Vout and describe the overall function of the circuit, 3 k2 P-4>

Answer 1

Solution :

• The given circuit is a simple Buck Converter Circuit with the output voltage control. The output voltage is controlled using simple analog circuitry performing discrete pulse width modulation. By discrete here, I mean that if the output voltage is less than desired voltage then the switch in Buck Converter is fully turned ON (100% Duty ratio) and if the output voltage is greater than desired voltage then the switch in Buck Converter is fully turned OFF (0% Duty ratio).

Functionalities of various op-amps :

• Op-amp 'a' is simply an astable multivibrator. A square waveform with 50 % duty ratio is generated by this op-amp.
• Op-amp 'c' is simply an integrator. It's function is to integrate the square waveform and generate a triangular waveform at it's output. This triangular waveform will serve as the carrier waveform for the PWM (Pulse Width Modulation) process used to control the output voltage of the circuit.
• Op-amp 'f' serves as an error amplifier. Strictly speaking, this component should perform the work of subtractor (V_out/4 - V_ref) and PI Controller. However, if 'f' is a simple op-amp, then it will simply act as a comparator and gives zero Volts or negative saturation voltage (depending on op-amp supply) if V_out/4 < Vref and gives positive saturation voltage if  V_out/4 > Vref.
• Op-amp 'd' simply acts as a comparator and is actually the Pulse Width Modulator block. If the modulating signal labelled 'F' in the circuit is logic high (implying that V_out needs to be reduced by controller) , then the Pulse width at the output of opamp 'd' will become zero leading to reduction in o/p voltage. Similarly,  if the modulating signal labelled 'F' in the circuit is logic low (implying that V_out needs to be increased by controller) , then the Pulse width at the output of opamp 'd' will become 100 % leading to increasein o/p voltage.

Expression for output voltage :   

V_out /4 (voltage at positive terminal of op-amp 'f') will always be equal to V_ref (voltage at negative terminal of op-amp 'f') due to control action discussed above. So,

V_out = 4*V_ref ........................(Ans)

The overall functioning of the circuit has already been explained while discussing the functionlities of various op-amp circuits.