Voltage Regulator Characteristics

Introduction to Voltage Regulator

A power supply is an important element of any type of electronic circuit. It provides the supply for the proper operation of the circuit. The successful operation of the circuit depends on the proper functioning of the power supply. Most of the electronic circuits require a smooth d.c. voltage as that of batteries. The power supply in a circuit tries to provide such a constant voltage. A block diagram containing the parts of a typical power supply and nature of the voltages at various points is shown in the Fig. 5.2.1.

The a.c. voltage is connected to a transformer. The transformer steps down the a.c. voltage down to the level required for the desired d.c. output. The rectifier converts a.c. voltage to a d.c. voltage. The filter circuit is used after the rectifier to reduce the ripple content in the d.c., to make it smoother. Still then the d.c. voltage usually has some ripple or a.c. voltage variation. This voltage is called unregulated d.c. voltage. A regulator circuit is a circuit used after the filter, which not only makes the d.c. voltage smooth and almost ripple free but also keeps the d.c. output voltage constant though input d.c. voltage varies under certain conditions. It keeps the output d.c. voltage constant under the variable load conditions, as well. Thus input to a regulator is an unregulated d.c. voltage while the output of a regulator is a regulated d.c. voltage, to which the load is connected. Such a regulator block is shown dotted in the Fig. 5.2.1. Now a days, complete regulator circuits are available in integrated circuit form.

Review Question

1. Draw and explain the block diagram of a typical power supply.

Voltage Regulator Characteristics

Let us study some important regulator characteristics and the factors affecting the load voltage.

1. Load Regulation

The load regulation is the change in the regulated output voltage when the load current is changed from minimum (no load) to maximum (full load).

Consider the block diagram of regulator circuit shown in the Fig. 5.3.1.

The load regulation is denoted as LR and mathematically expressed as,

LR = V_NL – V_FL

where V_NL = Load voltage with no load current.

V_FL = Load voltage with no load current.

The load regulation is often expressed as percentage by dividing the LR by full load voltage and multiplying result by 100.

The graph of load current against load voltage is called regulation characteristics of a power supply. The ideal value of load regulation is zero. Less the regulation, better is the performance of regulator. The regulation characteristics is shown in the Fig. 5.3.2.

2. Source Regulation/Line Regulation

The input to the unregulated power supply i.e. rectifier circuit is 230 V a.c. supply. This line voltage may change, under the different load conditions. This affects the output voltage of rectifier which is V_in for a regulator circuit. Hence the characteristics which gives source effect on regulator performance is defined.

The source regulation is also called line regulation or source effect and denoted as SR. 

The SR is defined as the change in the regulated load voltage for a specified range of line voltage, typically 230 V ± 10 %.

Mathematically it is expressed as,

SR = V_HL - V_LL

where V_HL = Load voltage with no load current.

V_LL = Load voltage with no load current.

The percentage source regulation is defined as,

% SR = SR / V_nom × 100

where V_nom - Nominal load voltage

3. Output Impedance

The output impedance of regulated power supply is very small. It can supply different loads keeping load voltage constant. In a series regulator, the pass transistor Q₂ is an emitter follower which has very low output impedance Z_out. The use of voltage feedback reduces it to,

Z_out (CL) = Z_out / 1 + AB, A = Forward gain, B = Feedback factor

Hence regulated power supply has output impedance in milliohms so it is very stiff voltage source.

4. Ripple Rejection

The output of rectifier and filter circuit consists of ripples. The ripple is equivalent to periodic changes in input voltage. Due to the negative feedback, the ripple voltage gets attenuated by large amount. The factor by which it gets reduced is 1+AB. Mathematically the output ripple of a voltage regulator is given by,

V_R(out) = V_R(in) / 1 + AB

The performance parameter ripple rejection denoted as RR is defined as,

RR = V_R(out) / V_R(in)

In datasheet, it is expressed in decibels (dB)

RR' = 20 log RR = 20 log V_R(out) / V_R(in)  dB

Key Point As V_R(out) is always less than V_R(in) RR’ i.e. RR in dB is always negative when difined as V_R(out) / V_R(in)

5. The Load Current (I_L)

As discussed, the load current affects the load voltage. The variation in load is indicated by the variation in the load current. Ideally the output voltage should remain constant for the variation of load from no load to the full load condition.

6. Temperature

The rectifier using the components like diodes is temperature sensitive. Hence the temperature is an important factor responsible for the changes in the load voltage. The semiconductor devices used in power supplies, have their characteristics which is temperature dependent.

The voltage regulator circuit mainly, has to consider the above discussed factors and has to provide constant d.c. load voltage irrespective of changes in load, line voltage and the temperature.

Review Question

1. What are the various performance factors of a regulator ? Define them.