Here is a summary of the material covered in this chapter:
An ideal voltage regulator would provide a constant output voltage regardless of any variations in input (line) voltage or load current demand. In practice, the output from a voltage regulator remains relatively constant over a limited range of line voltage and load current values.
The line regulation rating of a voltage regulator is defined as the change in output voltage per unit change in input voltage. By formula:
Line regulation =
Example 21.1 of the text demonstrates the concept of line regulation. In practical circuits, is usually in millivolts or microvolts and is usually in volts.
An ideal voltage regulator would have a line regulation value of zero. In practice, the lower the value of line regulation, the higher the quality of the regulator circuit. The units used for line regulation vary. Table 21-1 lists and defines the commonly used line regulation units.
The load regulation rating of a voltage regulator indicates the change in regulator output voltage per unit change in load current demand. By formula:
Another way that load regulation can be expressed would be
Example 21.2 of the text illustrates the concept of load regulation.
An ideal voltage regulator would have a load regulation value of zero (as would equal ). In practice, the lower the load regulation rating the better. Like line regulation, load regulation can be expressed in a variety of units. Table 21-2 lists and defines some of the commonly used load regulation units.
It should be noted that some manufacturers combine line regulation and load regulation into a single rating. This rating indicates the maximum change in regulator output voltage when both input voltage and load current are varied over their entire rated ranges. For example, a regulator may have the following ratings:
This rating indicates that the output from the regulator will change by no more than 0.33% (from its rated value) as long as the input is between 12 V and 24 V, and the load current is held to a maximum of 40 mA.
Note that the units used to quantify regulation can be used as an indicator of the quality of the regulator. Refer to the end of Section 21.1 of the text for a discussion of this concept.
Figure 21-1 shows the two basic types of discrete voltage regulators. As shown, the series regulator is placed in series with the load while the shunt regulator is placed in parallel with the load. Both types of circuits are discussed in this chapter.
Figure 21-1. Series and shunt regulator configurations.
The pass-transistor regulator uses a series transistor to regulate load voltage. The term pass-transistor is used because the load current passes through the transistor (as illustrated in Figure 21-2). The key to this circuit is that the transistor base is held to a relatively constant voltage by the zener diode (). As shown in Figure 21-2, the load voltage is found as:
and the base-emitter voltage can be defined using
This equation can be used to demonstrate what happens when load demand changes. If load resistance increases, will initially increase. Assuming that the zener voltage remains constant, decreases (as demonstrated in the equation). The drop in causes a decrease in collector (and therefore load) current, which causes to decrease.
If load resistance decreases, will initially decrease. Since the zener voltage remains constant, increases. The rise in causes an increase in collector (and therefore load) current, which causes to increase. In both cases, a change in the load voltage results in a compensating response from the transistor.
Figure 21-2. Pass-transistor regulator.
The pass-transistor regulator has good line and load regulation characteristics, but it does have one problem. When either or increase, zener conduction increases. As a result, the zener power dissipation also increases. The Darlington pass-transistor regulator can reduce this problem.
A Darlington pass-transistor regulator is illustrated in Figure 21-3. The use of the Darlington pair ( and ) means that the load voltage is found as:
This relationship reflects the fact that there are two base-emitter junctions. Because the gain of the Darlington pair is so high, any increase in load current results in very little increase in zener current.
Figure 21-3. Darlington pass-transistor regulator
Unfortunately, zener current is affected by temperature, and any change in zener current is amplified by the high-gain Darlington pair. That is why this regulator design must be operated at a relatively constant temperature.
Series Feedback Regulator
The series feedback regulator uses an error detector to improve regulation. A series regulator block diagram is provided in Figure 21.6 of the text. The error detector receives two inputs:
The error detector compares the two inputs and provides an output that is proportional to the difference between the two. This difference voltage is amplified and is used to drive the pass-transistor regulator. This results in the series feedback regulator being able to respond very quickly to any changes in line voltage or load demand.
A basic series feedback regulator is shown in Figure 21-4.
Figure 21-4. A basic series feedback regulator
One last word about series voltage regulators: The pass-transistor in a basic series regulator can be destroyed by excessive current if the load is shorted. This problem can be prevented by the addition of a current-limiting circuit like the one in Figure 21.8 of the text.
Shunt Voltage Regulators
The simplest of the shunt regulators is the zener regulator introduced in Chapter 3. Unfortunately, the zener wastes too much power for most applications. The shunt feedback regulator is a far more practical circuit.
The shunt feedback regulator shown in Figure 21-5 uses an error detector to control the conduction of a shunt transistor. Here is a very brief description of the circuits response to a change in load demand:
If load resistance increases, the load and sample voltage (at the base of ) increase. The increase in sample voltage causes and conduction to increase, decreasing the current to the load. (In effect, is shunting current away from the load). As a result, load voltage decreases (offsetting the initial increase).
Figure 21-5. A shunt feedback regulator.
If load resistance decreases, the load and sample voltage decrease. The decrease in sample voltage causes and conduction to decrease, increasing the load current. As a result, load voltage increases (offsetting the initial decrease).
Just as the series regulator must be protected from a shorted load, the shunt regulator must be protected from any excessive voltage that can result from an open load.
Linear IC Voltage Regulators
A linear IC voltage regulator is a device that is designed to hold the voltage from a dc power supply relatively constant over a specified range of line and load variations. The most common style is a three-terminal IC though some have more than three terminals. Figure 21-6 shows the schematic symbol for a three-terminal regulator.
Figure 21-6. Schematic symbol for a three-terminal regulator
There are four basic types of IC voltage regulators.
Regardless of the type of regulator, the input voltage must be the same polarity as the devices rated output polarity. For instance, a +5 V regulator must have a positive input voltage. IC voltage regulators are series regulators. Their internal circuitry is similar to the series regulators we looked at earlier in this chapter.
Figure 21.11 of the text shows the spec sheet for the LM317. Here are some of the basic specifications:
A voltage divider, such as the one illustrated in Figure 21.12 of the text, is commonly used to control the output voltage of the LM317. The spec sheet for the device gives us the following equation for determining the dc output voltage:
The use of this equation is demonstrated in Example 21.4 of the text. The spec sheet for any adjustable voltage regulator provides the equation for determining output voltage.
Figure 21-7 illustrates a complete dual-polarity power supply. Matched fixed-positive and fixed-negative regulators are used to provide positive and negative outputs. Note the use of the center-tapped transformer. This is used to prevent an imbalance if the load current demands vary. Also note the filter capacitors at both inputs and outputs. They are used to further reduce the ripple rejection of the circuit.
Figure 21-7. Complete dual-polarity power supply
There are several modifications that can be made to the basic IC regulator circuit to increase maximum load current. Figures 21.15 and 21.16 of the text illustrate two possible variations. Figure 21.17 provides a summary illustration of IC voltage regulators.
The two fundamental types of regulators are linear regulators and switching regulators. The linear regulator provides a continuous path for current between the regulator input and the load. The switching regulator does not. The pass-transistor in a series-switching regulator is continually being switched between cutoff and saturation. The pass-transistor in such a circuit is often called a power switch. The benefit of a switching regulator is that it offers higher regulator efficiency and higher power-handling capability. Figure 21.18 of the text provides a block-diagram comparison of series-linear and series-switching regulators.
A switching voltage regulator can be divided into four basic subsystems. As shown in Figure 21-8, these subsystems are:
Figure 21-8. A basic switching regulator.
The control circuit is used to control the output of the switch driver. It senses any change in output voltage and sends a signal to the switch driver. The switch driver output varies according to what changes are needed to compensate for the change in output voltage. If the output is too high, the switch driver decreases the conduction of the power switch. If the output is too low, the driver causes the power switch conduction to increase.
From the above description, it should be obvious that the conduction of the power switch controls output voltage. Remember, the power switch is not operating in a linear mode. It is constantly switching back and forth between cutoff and saturation.
Controlling Power Switch Conduction
The average (dc) value of the waveform at the emitter of the power switch can be found as
Example 21.5 of the text demonstrates the use of this equation. As the equation shows, controlling the conduction of the power switch controls the output voltage.
One way to control the conduction of the power switch is to control the amount of time that the switch is in saturation. This technique, called pulse-width modulation (PWM), is illustrated in Figure 21-9. A gated latch receives two inputs:
The output from the latch goes high only when the oscillator input is greater than or equal to the error voltage input. If the load voltage increases, the following events occur:
Note that the cycle time has not changed. If load voltage decreases, an opposite set of events occurs: error voltage decreases, increasing the duration of the pulse width produced by the gated latch and the power switch. This increases the average load voltage (offsetting the initial decrease).
Figure 21-9. Pulse-width modulation
Another means of controlling the conduction of the power switch is called variable off-time modulation. In this technique, the pulse width is constant and the cycle time is varied, as shown in Figure 21-10. The gated latch receives inputs from the error detector and an astable multivibrator. The square-wave output from the multivibrator is coupled to the power switch (by the gated latch) only when the error voltage is high. Thus the level of the error voltage determines the average output voltage of the power switch.
Figure 21-10. Variable off-time modulation
A step-down regulator provides a value of . It is also possible to configure a switching regulator so that:
Both of these circuits are illustrated in Figure 21.22 of the text. Because switching regulators can be designed for different input/output voltage relationships, these circuits are sometimes called dc-to-dc converters.
Switching regulators are available on a single IC. The MC34063 is illustrated in Figure 21.23 of the text. Figure 21.24 illustrates the MC34063 wired in a step-down configuration. Note that the internal circuitry of the IC contains all the components of a switching regulator that we have used in the discrete regulators.
Switching regulators have both strengths and weaknesses: The strengths are as follows:
The weaknesses are as follows: