2 POWER SUPPLIES (VOLTAGE REGULATORS) Fig Block diagram showing parts of a power supply.Power supply: a group of circuits that convert the standard ac voltage (120 V, 60 Hz) provided by the wall outlet to constant dc voltage.Transformer : a device that step up or step down the ac voltage provided by the wall outlet to a desired amplitude through the action of a magnetic field.
3 Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC.Step-up transformers increase voltage.Step-down transformers reduce voltage.
4 Rectifier: a diode circuits that converts the ac input voltage to a pulsating dc voltage.The pulsating dc voltage is only suitable to be used as abattery charger, but not good enough to be used as a dcpower supply in a radio, stereo system, computer and so on.
5 There are two basic types of rectifier circuits: 1. Half-wave rectifier2. Full-wave rectifieri) Center-tapped full-wave rectifierii) Bridge rectifierIn summary, a full-wave rectified signal has less ripple than ahalf-wave rectified signal and is thus better to apply to a filter.
6 Filter: a circuit used to reduce the fluctuation in the rectified output voltage or ripple. This provides a steadier dc voltage.Regulator: a circuit used to produces a constant dc output voltage by reducing the ripple to negligible amount.
7 VOLTAGE REGULATION Two basic categories of voltage regulation are: line regulation;load regulation.The purpose of line regulation is to maintain a nearly constant output voltage when the input voltage varies.The purpose of load regulation is to maintain a nearly constant output voltage when the load varies.
8 Figure 6–2 Line regulation Figure 6–2 Line regulation. A change in input (line) voltage does not significantly affect the output voltage of a regulator (within certain limits).
9 Figure 6–3 Load regulation Figure 6–3 Load regulation. A change in load current has practically no effect on the output voltage of a regulator (within certain limits).
10 Line RegulationLine regulation can be defined as the percentage change in the output voltage for a given change in the input voltage.(6-1)Δ means “a change in”.Line regulation in %/V can be calculated using the following formula:(6-2)
11 Load RegulationLoad regulation can be defined as the percentage change in the output voltage from no-load (NL) to full-load (FL).(6-3)whereVNL = the no-load output voltageVFL = the full-load output voltage
12 Load RegulationSometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification.In this case, VOUT can be found by applying the voltage divider rule:Power SupplyIn terms of resistances, load regulation can be expressed as:
13 Exercise ;A power supply has an output resistance of 25 mW and a full load current of 0.50 A to a 10.0 W load.(a)What is the load regulation?(b)What is the no load output voltage?The input of certain regulator increase by 3.5V. As a result, the output voltage increase by 0.042V. The nominal output 20V. Determine the line regulation in both % and in %/V.3) If a 5.0V power supply has an output resistance of 80mΩ and specific maximum output current of 1.0A, what is the load regulation in both % and in %/mA.
14 Figure 6.4 Series and shunt regulators. TYPES OF REGULATORTwo basic types of voltage 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 Series and shunt regulators.
15 Series Regulator Circuit Figure Block diagram of the basic connection of a series regulator circuit.The series element controls the amount of the input voltage that gets to the output.The output voltage is sampled by a circuit that provides a feedback voltage to be compared to a reference voltage.
16 Transistor Series Regulator Figure Pass-transistor regulator.The transistor series regulator is also called the pass-transistor regulator because the load current passes through the series transistor.
17 (6-4) Since Q1 is an npn transistor, Vo is found as Equation ( ) explains the response of the pass-transistor to a change in load resistance as follows:- If load resistance increases, load voltage also increases.Since the Zener voltage is constant, the increase in Vocauses VBE to decrease.The decrease in VBE reduces conduction through the pass-transistor, so load current decreases.This offsets the increase in load resistance, and a relativelyconstant load voltage is maintained.
18 Fig. 6.7 Op-amp series regulator circuit. Basic op-amp Series RegulatorControl ElementError DetectorSample CircuitVREFFig Op-amp series regulator circuit.The resistor R1 and R2 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.
19 Control ElementError DetectorSample CircuitVREFThe resulting difference voltage causes the transistor Q1 controls the conduction to compensate the variation of the output voltage. The output voltage will be maintained at a constant value of:(6-5)
20 Exercise ; The output voltage for the series regulator circuit is: (a) What is the output voltage for the series regulator?(b) If the load current is 200 mA, what is the power dissipated by Q1?(a)18 V4.7 kW100 kW3.9 V(b)P = VI= (18 V – 12.2 V)(0.2 A)47 kW
21 Series Regulator with constant-current limiting Current limiting prevents excessive load current. Q2 will conduct when the current through R4 develops 0.7 V across Q2’s VBE. This reduces base current to Q1, limiting the load current.The current limit is:For example, a 1.4 W resistor, limits current to about 0.5 A.
22 Regulator with Fold-back current limiting Fold-back current limiting drops the load current well below the peak during overload conditions. Q2 conducts when VR5 +VBE = VR4 and begins current limiting. VR5 is found by applying the voltage-divider rule:An overload causes VR5 to drop because VOUT drops. This means that less current is needed to maintain conduction in Q2 and the load current drops.