Ideally, the output of most power supplies should be a constant voltage. Unfortunately, this is difficult to achieve. There are two factors that can cause the output voltage to change. First, the AC line voltage is not constant. The so-called 120 volts AC (used in the United states) can vary from about 114 volts to 126 volts. This means that the peak AC voltage to which the rectifier responds can vary from about 161 volts to 178 volts. The AC line voltage alone can be responsible for a 10 percent change in the DC output voltage. The second factor that can change the DC output voltage is a change in the load resistance. In complex electronic equipment, the load can change as circuits are switched in and out. In a television receiver, the load on a particular power supply may depend on the brightness of the screen, the control settings, or even the channel selected.
These variations in load resistance tend to change the applied DC voltage because the power supply has a fixed internal resistance. If the load resistance decreases, the internal resistance of the power supply drops more voltage. This causes a decrease in the voltage across the load.
Many circuits are designed to operate with a particular supply voltage. When the supply voltage changes, the operation of the circuit may be adversely affected. Consequently, some types of equipment must have power supplies that produce the same output voltage regardless of changes in the load resistance or changes in the AC line voltage. This constant output voltage may be achieved by adding a circuit called the voltage regulator at the output of the filter. There are many different types of regulators in use today and to discuss all of them would be beyond the scope of this section.
A commonly used figure of merit for a power supply is its percent of regulation. The figure of merit gives us an indication of how much the output voltage changes over a range of load resistance values. The percent of regulation aids in the determination of the type of load regulation needed. Percent of regulation is determined by the equation:
This equation compares the change in output voltage at the two loading extremes to the voltage produced at full loading (VfL). For example, assume that a power supply produces 12 volts when the load current is zero (VnL). If the output voltage drops to 10 volts when full load current flows, then the percent of regulation is:
Ideally, the output voltage should not change over the full range of operation. That is, a 12-volt power supply should produce 12 volts at no load, at full load, and at all points in between. In this case, the percent of regulation would be:
Thus, zero-percent load regulation is the ideal situation. It means that the output voltage is constant under all load conditions. While you should strive for zero percent load regulation, in practical circuits you must settle for something less ideal. Even so, by using a voltage regulator, you can hold the percent of regulation to a very low value.
There are two basic types of voltage regulators. Basic voltage regulators are classified as either series or shunt, depending on the location or position of the regulating element(s) in relation to the circuit load resistance.
The shunt regulator, while one of the simplest semiconductor regulators, is usually the least efficient. It may be used to provide a regulated output where the load is relatively constant, the voltage low to medium, and the output current high. The shunt regulator utilizes the voltage-divider principle to obtain regulation of the output voltage.
The figure below shows the shunt regulator reduced to its fundamental form. It is called a shunt-type regulator because the regulating device is connected in parallel with the load resistance. The fixed resistor Rs is in series with the parallel combination of the load resistor, RL, and the variable resistor, Rreg, and forms a voltage divider across the input circuit.
A brief operational description of the basic shunt regulator will serve to explain the manner in which regulation of the output voltage is achieved.
All current that flows in the complete circuit passes through the series resistor, Rs. The magnitude of this current and thus the value of the voltage drop across Rs are controlled by variable resistance Rreg. The voltage across Rs is equal to the difference between the larger voltage of the DC source and the output voltage across load resistance RL. The difference voltage across Rs is varied by action of resistance Rreg, as required, to compensate for circuit changes and maintain the output voltage to the load constant at the desired value.
If the input voltage to the regulator circuit decreases, the voltage across load resistor, RL, and the variable resistance, Rreg, tends to decrease. To counteract this decrease, the resistance of Rreg is increased, which reduces the total current flow through Rs an thereby the voltage drop across it. Thus, by decreasing the difference voltage of Rs to compensate for the decrease in the input voltage, the output voltage remains constant at its nominal value. Conversely, if the input voltage increases, the voltage across RL and Rreg tends to increase. To counteract the increase, the resistance of Rreg is decreased. This results in more current through Rs and thus an increase in the voltage developed across it. The increase in the difference voltage compensates for the increase in the input voltage, and again, the output voltage remains constant at the regulated value.
The shunt regulator must be capable of withstanding the entire output voltage of the DC source; however, it does not have to carry the full load current unless it is required to regulate from the no-load to the full-load condition. Since series-dropping resistor Rs, used with the shunt regulator, has relatively high power dissipation, the overall efficiency of this type of regulator may be less than that of other types. One advantage of the shunt regulator is the inherent overload and short-circuit protection offered. The series resistor, Rs, is between the DC source and the load; and thus, a short circuit or overload merely decreases the output voltage from the regulator circuit. Note that under no-load conditions, however, the shunt regulating device must dissipate the full output; therefore, the shunt regulator is most often used in constant-load applications.
From the general discussion given in the preceding paragraphs, it can be seen that the shunt voltage regulator is essentially a voltage divider circuit, with the output voltage across the load being held essentially constant, regardless of input voltage or load current variations. The control action required to vary the resistance of Rreg and, consequently, to develop a variable-voltage drop, is completely automatic. This basic principle of voltage regulation is used in the transistorized, shunt-type voltage regulators to be described later in this section.
The series regulator, as the name implies, places the regulating device in series with the load; regulation occurs as the result of varying the voltage developed across the series device, The series regulator is preferable for high voltage and medium output current applications where the load may be subject to considerable variation. Most critical semiconductor applications require that a regulated voltage source utilizes the series regulator; and as a result, there are many regulator circuit configurations. These circuit configurations vary from one application to another, depending upon the regulation required to be maintained over a given temperature range.
The series regulator can be compared with a variable resistor in series with the DC source and the load, thus forming a voltage divider. The variable-resistance action of the series regulating device maintains the output voltage across the load resistance at a constant value.
A simple series voltage-regulator circuit is shown in the figure below to help explain this principle of voltage regulation. The variable resistor, Rs, is in series with the load resistance, RL; thus, the two resistances in series form a voltage divider across the input voltage. The load current passes through Rs and develops a voltage across it. The voltage developed across Rs depends upon the value of resistance of Rs and the load current through it. Since the input voltage to the regulator circuit is always greater than the desired output voltage, the voltage developed across series resistor Rs is varied to obtain the desired value of output across the load resistance RL.
If the input voltage to the regulator circuit decreases, the voltage across load resistor RL and variable resistor Rs also decreases. To counteract this voltage decrease, the resistance of variable resistor Rs is decreased so that a smaller voltage is developed across Rs, and the voltage across the load resistor returns to its former value. Conversely, if the input voltage to the regulator circuit increases, the voltage across load resistor RL also increases. To counteract this voltage increase, the resistance of Rs is increased so that a larger voltage drop occurs across Rs, and the voltage across the load returns to its former value.
From the analysis in the preceding paragraphs, it is obvious that the series-type (as well as the shunt-type) voltage regulator is essentially a voltage-divider circuit, with the output voltage produced across the load being essentially constant, regardless of input voltage or load current variations. The control action required to vary the series regulating device and, consequently, to produce a corresponding variable-voltage across Rs is completely automatic.
Zener Diode Shunt-Type Regulator
The Zener diode, shunt regulator is used as a voltage regulator where the load is relatively constant. This circuit is frequently used in more complex regulator circuits as a reference-voltage source and as a preregulator in transistorized series regulators.
- Uses a Zener diode as a shunt regulating device.
- Regulated output voltage to load is nearly constant, even though changes in input voltage or changes in load current occur.
- Voltage-divider principle is employed, using a fixed resistor and a Zener diode in series; regulated load is taken from across the diode.
- Variation in the basic circuit permits positive or negative voltages to be regulated.
The Zener-diode regulator is the simplest form of shunt regulator. The regulator circuit consists of a fixed resistor in series with a Zener diode. The regulated output voltage is developed across the diode; therefore, the load is connected across the diode. The regulator circuit develops a definite output voltage that is dependent upon the characteristics of the particular Zener diode.
The Zener diode is a PN junction which has been modified during its manufacture to produce a specific breakdown voltage level; it operates with a relatively close voltage tolerance over a considerable range of reverse current. The Zener diode is subject to a variation in resistance with a change in temperature of the diode.
In the figure above, schematics "A" and "B" illustrate a Zener diode used in a basic voltage-regulator circuit. Resistor R1 is the series resistor; semiconductor D1 is a Zener diode. The circuit in "A" provides regulation of a positive input voltage, while the circuit in "B" provides regulation of a negative input voltage.
The series resistor, R1, needs only to stabilize the load; it compensates for any difference between the diode operating voltage and the unregulated input voltage. The value of the series resistor depends upon the combined currents of the Zener diode and the load. The series resistor is generally chosen with the following factors in mind: the minimum value of input voltage (unregulated), the maximum value of load current, the minimum value of Zener diode current, and (knowing the diode characteristics) the value of the highest voltage to be developed across the Zener diode and its parallel load resistance. Once the value of series resistor R1 is determined, the maximum power dissipation in the diode can be determined by considering the maximum value of input voltage (unregulated), the minimum value of load current, and the minimum value of voltage developed across the diode (using the value of series resistance established for R1). In order to obtain stable operation, the Zener diode must be operated so that its reverse current falls within its minimum and maximum ratings for the specified voltage. It is important to note that in no-load conditions, the Zener diode must dissipate the full output power.
If the input voltage to the regulator circuit decreases, the voltage decrease appears across the Zener diode, D1, and immediately the current through the diode decreases. Thus, the total current through series resistor R1 decreases, and the voltage developed across R1 decreases proportionately, so that for all practical purposes the output voltage across the load resistance (and Zener diode) remains the same. Conversely, if the input voltage to the regulator circuit increases, the voltage increase appears across the Zener diode, and immediately the current through the diode increases. Thus, the total current through series resistor R1 increases, and the voltage developed across R1 increases proportionately, so that for all practical purposes the output voltage across the load resistance (and Zener diode) remains the same.
If the current drawn by the load resistance decreases or increases, the total current drawn from the input source does not change. Instead, a corresponding change in current through the Zener diode occurs and the current drawn from the source remains constant, so that the output voltage across the load resistance remains constant.
The figure below shows simplified drawings of the series-transistor regulator. In this figure, schematic "A" shows a regulator for a positive supply voltage, and schematic "B" shows a regulator for a negative supply voltage. Notice that this regulator has a transistor (Q1) in the place of the variable resistor (potentiometer) found in the basic series regulator. The polarity of the supply to be regulated will determine the type of transistor to be used. Because the total load current passes through this transistor, it is sometimes called a "pass transistor". Other components which make up the circuits are the current limiting resistor R1 and the Zener diode D1.
The positive regulator in "A" uses an NPN transistor as the regulator. The collector of the regulating transistor is connected to the unregulated power supply. For proper bias on an NPN transistor, a positive potential must be applied to the collector. The base must be negative in relation to the collector (or less positive). The emitter must be the most negative (or least positive) potential on the transistor. A constant (reference) potential is maintained on the base using the Zener diode. As a result, the transistor has forward bias, emitter to base, and reverse bias, collector to base. Reversing the applied polarities to the PNP transistor in schematic "B" of the figure above will apply the proper polarities for correct biasing to that transistor.
To understand the regulating action, think of the transistor as replacing resistor Rs shown in the basic series regulator. With forward bias applied to the emitter-base junction, the transistor conducts, causing part of the unregulated supply voltage to be developed from collector to emitter across the transistor. The rest of the unregulated supply voltage is developed across the load. The voltage developed across the load is the regulated voltage. To change the conducting resistance of the transistor, it is necessary to change the forward bias. An increase in forward bias causes an increase in conduction and therefore a decrease in conducting resistance. A decrease in forward bias causes an increase in conducting resistance. Since the base potential is held constant by the Zener diode, the only change in bias can be caused by an attempted change in load potential, or the regulated supply potential at the emitter.
A change in forward bias, then, gives the same result as turning the potentiometer knob in the basic series regulator. To illustrate this point, consider an increase in load current. This increase is caused by a decrease in load resistance (as when switching in another parallel path for current). Load voltage tends to decrease with load resistance. This is seen as a change in forward bias on the regulator transistor. Since the emitter voltage is decreased, the forward bias is increased. As a result, the transistor (in series with the load) conducts the new higher load current, and the conducting resistance of the transistor decreases. The decrease in resistance causes less of the supply voltage to be developed across the transistor, leaving nearly the same voltage available to the load that it had prior to the load change.
Now consider an increase in unregulated supply voltage. It has been shown by transistor characteristics in previous lessons that a change in collector voltage has negligible effect on collector current. The regulated voltage, as a result of no change in current through the collector (therefore, through the transistor), will not be varied.
The transistor used as a regulator must be capable of handling the load current safely. Generally, a power transistor is used because of the need to handle the high load currents. If a single transistor will not handle all the current, transistors may be placed in parallel.