Chapter 6 Voltage Regulators - Part 1-

VOLTAGE REGULATION Two basic categories of voltage regulation are:  line regulation;  load regulation. line regulation input voltage The purpose of line regulation is to maintain a nearly constant output voltage when the input voltage varies. load regulation load The purpose of load regulation is to maintain a nearly constant output voltage when the load varies.

Figure 6–2 Line regulation. A change in input (line) voltage does not significantly affect the output voltage of a regulator (within certain limits). Line Regulation

Line regulation can be defined as the percentage change in the output voltage for a given change in the input voltage. Line Regulation Line regulation in %/V can be calculated using the following formula: Δ means “a change in”. (6-1) (6-2)

Figure 6–3 Load regulation. A change in load current has practically no effect on the output voltage of a regulator (within certain limits). Load Regulation

Load regulation can be defined as the percentage change in the output voltage from no-load (NL) to full-load (FL). where V NL = the no-load output voltage V FL = the full-load output voltage Load Regulation (6-3)

Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification. Power Supply In this case, V OUT can be found by applying the voltage divider rule: In terms of resistances, load regulation can be expressed as: Load Regulation

TYPES OF REGULATOR The fundamental classes of voltage regulators are linear regulators and switching regulators. Two basic types of linear regulator are the series regulator and the shunt regulator. The series regulator is connected in series with the load and the shunt regulator is connected in parallel with the load. Figure 6.4 Series and shunt regulators.

Series Regulator Circuit Figure 6.5 Block diagram of the basic connection of a series regulator circuit. The control element is a pass transistor in series with the load between the input and output. The error detector compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.

Basic Op-amp Series Regulator Figure 6.6 Basic op-amp series regulator. The resistor R 2 and R 3 sense a change in the output voltage and provide a feedback voltage. The error detector compares the feedback voltage with a Zener diode reference voltage. The resulting difference voltage causes the transistor Q 1 controls the conduction to compensate the variation of the output voltage. The output voltage will be maintained at a constant value.

(6-5) The closed-loop voltage gain is:

Determine the output voltage for the regulator in Figure below. EXAMPLE Answer: V OUT = 10.2 V

Current limiting prevents excessive load current. Q 2 will conduct when the current through R 4 develops 0.7 V across Q 2 ’s V BE. This reduces base current to Q 1, limiting the load current. The current limit is: For example, a 1.4  resistor, limits current to about 0.5 A. Series Regulator with constant-current limiting

An overload causes V R5 to drop because V OUT drops. This means that less current is needed to maintain conduction in Q 2 and the load current drops. Fold-back current limiting drops the load current well below the peak during overload conditions. Q 2 conducts when V R5 +V BE = V R4 and begins current limiting. V R5 is found by applying the voltage-divider rule: Regulator with Fold-back current limiting