Chapter 6 Voltage Regulators - Part 2-. Basic Linear Shunt Regulators. Shunt Regulator. Fig. 6.9 Block diagram of a basic components of a shunt regulator. Basic op-amp Shunt Regulator with load resistor. Fig. 6.10 Transistor shunt voltage regulator.
- Part 2-
Fig. 6.9 Block diagram of a basic components of a shunt regulator.
Fig. 6.10 Transistor shunt voltage regulator.
The operation of the circuit is similar to the series regulator, except that regulation is achieved by controlling the current through the parallel transistor Q1 When the output voltage tries to decrease due to a change in input voltage or load current caused by a change in load resistance, the decrease is sensed by R3 and R4.
A feedback voltage obtained from voltage divider R3 and R4 is applied to the op-amp’s non-inverting input and compared to the Zener voltage to control the drive current to the transistor.
The difference voltage reduces the op-amp’s output, but increasing the collector voltage. This keeps the load output nearly constant.
Fig. 6.11 Block representation of three-terminal voltage regulator.
All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy.
A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement.
Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter.
In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage.
Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage.
In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source.
Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input.
The fixed voltage regulator has an unregulated dc input voltage Vi applied to one input terminal, a regulated output dc voltage Vo from a second terminal, and the third terminal connected to ground.
Fixed-Positive Voltage Regulator
The series 78XX regulators are the three-terminal devices that provide a fixed positive output voltage.
Fig. 6.12 (a) Standard configuration of the series 78XX regulator and
(b) typical packages
The figure above shows:
An unregulated input voltage Vi is filtered by a capacitor C1 and connected to the IC’s IN terminal.
The IC’s OUT terminal provides a regulated +12 V, which is filtered by capacitor C2.
The third IC terminal is connected to ground (GND).
The series 79XX regulators are the three-terminal IC regulators that provide a fixed negative output voltage.
This series has the same features and characteristics as the series 78XX regulators except the pin numbers are different.
Fig. 6.14 Standard configuration.
Voltage regulators are also available in circuit configurations that allow to set the output voltage to a desired regulated value.
The LM317 is an example of an adjustable-voltage regulator, can be operated over the range of voltage from 1.2 to 37 V.
Fig. 6.15 Connection of LM317 adjustable-voltage regulator.