A voltage regulator provides a constant voltage to specified loads from a limited range of input voltages. Since most applications require the series-type regulator, only the series type will be discussed, although a shunt regulator may be used where the load is relatively constant.
General design procedure may be divided into five elements as shown in the block diagram of Fig. 9.1. Note that regulation is performed by comparing a sample of the output voltage with a reference; any error present is amplified and used to control a series element. The sampling element of the block diagram is usually a simple voltage divider across the regulated output, as shown in Fig. 9.2 with its Thevenin equivalent circuit. For this sampling element, the voltage to the com-parison element is
where and
ATVO
=
VO(R2+
Rp2)Rl
+
R2+
RpAT
=
voltage division of the resistive divider=
R2+
Rp2Rl
+
R2+
RpVo
=
regulated output voltage+ I Control I
I I +
I
AmplifierI I I
(in)
I
Comparisonl
I I SampleI
V(o ut)v:
I
ReferenceI I
1
Fig. 9.1. Block diagram of a series or emitter-follower d-c voltage regulator.
145
(1) (2)
+ B
Il~ Rl A(RI +Rpd
+
B Rpl
A V(out)
'"
To Rp2 Rp V(out)
comparison
12~ R2
Fig. 9.2. Sampling element and its Thevenin equivalent circuit.
Equation (1) is valid when resistors R1, R2, and Rp are at the same temperature and have the same temperature coefficient, i.e., are of the same type material.
Silicon breakdown diodes are generally used as voltage references in transistor regulators because their breakdown voltage is relatively constant over a wide range of reverse current. The effects of temperature, reverse current, and diode resist-ance on breakdown voltage are characteristics that must be considered in the selec-tion of a reference diode.
Silicon diodes with low breakdown voltages (on the order of 5 volts or lower) usually have a negative temperature coefficient. As the breakdown voltage in-creases, the temperature coefficient becomes increasingly positive. Figure 9.3 shows how the temperature coefficient depends upon the nominal breakdown voltage and the diode reverse current of typical breakdown diodes.
The doc resistance of breakdown diodes is also a function of reverse current and breakdown voltage as shown in Fig. 9.4. The doc diode resistance of a breakdown diode can be calculated, using
~Vz . fj
Rd-c
=
Ra_c+
~T V zv (3)where Ra_c
=
dynamic diode resistance, ()=
thermal resistance of the diode, and Vz and ~Vz/~T are as shown in Fig. 9.3.Because of the characteristics shown in Figs. 9.3 and 9.4, a series combination of low-voltage diodes is usually preferred over one high-voltage diode if a high reference voltage is needed. A series string of low-voltage breakdown diodes can be made to have a lower net temperature coefficient and total diode resistance than a single diode used to provide the same reference voltage. The combined tem-perature coefficient can also be changed with very little change in reference voltage by using forward-biased diodes (either general-purpose or breakdown diodes) in series with breakdown diodes.
The combined temperature coefficient of a series string of diodes can be deter-mined from the following equation:
~VT ~VZl ~VZn ~VFl ~VFn
~T = ~T + ~T + ~T + ~T
(4)Voltage Regulators 147
Fig. 9.3. Temperature coefficient vs. breakdown voltage: 1 N746 series.
where IJ.. V T/ IJ..T
=
combined voltage change per COIJ.. Vz/ IJ..T = voltage change per CO for each breakdown diode IJ.. Vp / IJ..T
=
voltage change per CO for the forward diodesThe temperature coefficient of each diode carries its own sign in Eq. (4). The combined temperature coefficient computed from Eq. (4) is valid at only one cur-rent value; therefore a constant diode curcur-rent must be provided to obtain a stable reference element. A constant diode current is also required to keep the break-down voltage and the diode resistance constant.
80r---r---,---~----~---~----~
o----... - - - Q + 0 - - - 1 - - - 0 +
t - - - - o Q + ~----o+
Fig. 9.5. A reference element and its equivalent circuit.
The importance of maintaining a nearly constant temperature coefficient is apparent. The reference element and equivalent circuit in Fig. 9.5 are used to show why it is also desirable to keep the resistance of the breakdown diode as low as possible.
Assume AIl is small so that ARR ~
o.
Assume 11 ~ 12 •
l1R 3
+
VR=
T(out)AI1R 3
+
AVR=
AT(out) AVR ~ AI1RRAV~3
~+ AVR ~AV(out)
AVR ~ RR A T(out) = R3
+
RR(5) (6) (7) (8)
(9) Equation (9) shows that the change in reference voltage for a change in output voltage can be made very small by selecting a breakdown diode with low resistance and by holding the reverse current nearly constant. Ideally, the reference voltage in a regulator should not change for any normal output voltage change.
The temperature characteristics needed in the reference and comparison elements are usually determined simultaneously, because each element affects the tempera-ture requirements of the other.
9.1. COMPARISON ELEMENT
The comparison element takes a sample of the output voltage, compares it with the reference voltage, and produces a signal that is proportional to the difference.
A single common-emitter stage or an emitter-coupled differential amplifier may be used for the comparison element. The choice depends on the degree of regu-lation and temperature stability required. The common-emitter stage (Fig. 9.6) is discussed first.
The potentiometers in Fig. 9.6 are used to adjust the output sample to match the reference voltage at specified output voltages. The current from the potenti-ometer wiper into the comparison element must be kept much smaller than that through the divider so that the sample voltage remains an accurate portion of the
Voltage Regulators 149
output. Is in Fig. 9.6a or 14 in Fig. 9.6b must be considerably larger than the emitter current of Ql or Q2 so that the diode reverse current can be kept nearly constant.
The sample voltage is applied to the base of Ql, and the reference voltage is applied to the emitter in Fig. 9.6a. If the output voltage tends to increase, the base-emitter voltage of Ql will increase and cause more collector current to flow.
A drop in output voltage causes the collector current of Ql to decrease. The change in Ql collector current and the change in the input current to the control element are out of phase. This means that if the output voltage begins to rise, the amplified difference voltage will decrease the current into the control element and the output voltage will be corrected.
The reference element may be used as shown in Fig. 9.6b for high output volt-ages. This enables transistors in the comparison element to operate at low voltage levels, regardless of the regulator output voltage.
The collector currents of Fig. 9.6a and 9.6b are of opposite phase because of the different reference-element positions. If Fig. 9.6b is used, a d-c amplifier is required between the comparison element and the series control element to provide the cor-rect phase relationship.
Temperature compensation for the common-emitter comparison element: Since the reference voltage plus the base-emitter voltage of the comparison transistor is equal to the output sample, the following equation applies for Fig. 9.6a:
where V(out)
=
regulated output voltage VR=
reference voltageVBE
=
base-emitter voltage of comparison element If R1, R2 , and Rp are of the same material,Rl
+
R2+
Rp~V(out)
=
R R (~VR+
~VBE)2
+
p2(10)
(11) and any change in output voltage is the result of ~ V R and ~ V BE. Since ~ V BE/ ~T
...---_---0+ ...---... - - . q +
To d·c amplifier 1-t--+---CRp V(out)
(a) (b)
Fig. 9.6. Common-emitter comparison element with sampling and reference elements: (a) low output voltage; (b) high output voltage.
is usually negative, the temperature problem can be solved by using a breakdown diode which has a positive temperature coefficient that exactly cancels ~ VBE/ ~T
of Qi in Fig. 9.6a. If the sampling resistors are of the same material, the breakdown diode must be chosen with a negative temperature coefficient equal to ~ VBE/ ~T of Q2 in Fig. 9.6b.
Temperature compensation can be provided with little difficulty for temperatures below about lOOoe, using a single silicon common-emitter stage for the compari-son element. The effects from leBo are usually not critical below this temperature in silicon transistors; therefore, the problems of compensation are due primarily to changes in the base-emitter voltage, VBE, with temperature.
An emitter-coupled differential amplifier is ideal as a comparison element if the regulator is to perform over a wide temperature range or at very high temperatures (Fig. 9.7).
The symmetrical arrangement of the differential amplifier tends to make it self-compensating for temperature effects. Self-compensation can be improved by selecting well-matched transistors and mounting them on a common heat sink.
The degree of matching needed is determined by the temperature compensation required. The position of the reference diode, the associated phase shift, and a slight gain variation are the only differences between the high- and low-output stages in Fig. 9.7. The output of the differential amplifier is usually taken from only one side unless cascaded amplifiers are used. The side chosen for the output is determined by the number of phase shifts between the comparison element and the control element.
The currents through the reference element and through the divider must again be much larger than the base currents of the differential amplifier.
If the differential amplifier is perfectly temperature-compensated, the following equations apply:
Equations (l3) and (14) show that an output voltage change with temperature variation is due to a change in the reference voltage if the divider resistors are of the same material. In both cases the breakdown diode should be chosen with a temperature coefficient near zero. Equations (l3) and (14) were obtained assum-ing that the base-emitter voltages of Qi and Q2 are equal and also that the base-emitter voltage changes with temperature of Qi and Q2 are equal.
It should be noted that the performance of the voltage regulator with changing
Voltage Regulators 1 51
O-~~----~-'---1---O+ +
(a) (b)
Fig. 9.7. Differential amplifier comparison elements with sampling and reference elements:
(a) low output voltage; (b) high output voltage.
temperature is primarily determined in the design of the sampling, reference, and comparison elements. The operation of the other elements is not critically depend-ent on temperature.
9.2. D-C AMPLIFIER ELEMENT
The d-c amplifier must raise the difference signal from the comparison element to a level sufficient to drive the control element. Because the amplifier is within a strong feedback loop, very critical d-c amplifier design is not necessary. The only requirement in most cases is that a gain be provided that is large enough to supply the required current to the control element and small enough to retain circuit stability.
In many cases, a single transistor or stage functions as both the comparison element and d-c amplifier. Additional amplifier stages may be required for higher loop gain to further improve the regulation and decrease the output resistance of the regulator. The usual d-c amplifier element is similar to the common-emitter comparison element in Fig. 9.7. A breakdown diode is used in the emitter circuit to improve the voltage gain of the amplifier. Temperature compensation is not critical in this portion of the regulator, but the breakdown diode should be chosen with a temperature coefficient that will tend to cancel ~ VBE/ ~T of the amplifier transistor.
9.3. CONTROL ELEMENT
The control element interprets the signal from the d-c amplifier and makes the adjustment necessary to maintain a constant output voltage. The basic control elements used in the three regulator types are shown in Fig. 9.8.
The control elements of series and emitter-follower regulators are basically the same except for the base drive, which comes from the d-c amplifier in a series regu-lator and from the reference element in an emitter-follower reguregu-lator. Series and emitter-follower control elements must be capable of carrying the full load current
of the regulator, but during normal operation the collector-emitter voltage can be much less than the output voltage.
The shunt control element must be capable of withstanding the entire output voltage; however, it does not have to carry the full load current unless required to regulate from no load to full load. Since the series dropping resistor used with the shunt regulator has high dissipation, total efficiency of the regulator is degraded.
The preceding observations indicate that a series regulator is preferable for high voltage and medium current outputs with variable loads. The shunt regulator can be used for medium to low voltages and high output currents with relatively con-stant loads. Application of the emitter-follower regulator is usually limited to low output voltages. It has poor ripple suppression and poor regulation with respect to input variation, compared to the other regulator types.
Some of the quantities that must be considered when selecting a control element are the maximum voltage, current, and power ratings of the transistor. The limi-tations for a single transistor series or emitter-follower control element can be determined from the following:
VCE(ma:c) > V(inma:c) - V(outmin) I C(ma:c) > I(out ma:c)
P C(ma:c) > (V(in ma:c) - V(out min»I(out ma:c)
where V(in ma:c)
=
maximum unregulated input voltageI(out ma:c)
=
maximum load currentV(out min)
=
minimum output voltageVCE(ma:c)
=
maximum allowable collector-emitter voltageI C(ma:c)
=
maximum allowable collector currentP C(ma:c)
=
maximum allowable collector dissipation(15) (16) (17)
The shunt control element must satisfy the following requirements: The series dropping resistor, Rs, should be chosen such that the current through the control element can be held to a minimum.
R < V(in min) - V(out ma:c) The control element and regulator type can be chosen based upon the preceding limitations if the performance requirements of the regulator are known. The maximum power-dissipation ratings must be observed at all operating tempera-tures. Power derating at elevated temperatures is given on the transistor data sheet.
The control-element drive current and the collector current of the d-c amplifier are supplied from a common shunt current source.
Voltage Regulators 153
Fig. 9.8. Regulator control elements: (a) series regulator; (b) emitter-follower regulator;
(c) shunt regulator.
The series and shunt regulators should be designed so that the collector current of the d-c amplifier is equal to or greater than the maximum current needed for the base drive of the control element. This design consideration is needed to ensure that the control element will have enough base drive current available to maintain the required output current.
Because the current supplied to the base of the control element is usually small, a compound connection is often used to provide the current gain necessary to maintain a required load current. A compound connection, sometimes called a beta multiplier, is shown in Fig. 9.9.
Assuming hpE ~ 1,
I(out) ~ (hpElhpE2hpE3 . . • hPEn)IBn (22) Each transistor in the compound connection must withstand a voltage equal to the maximum unregulated input voltage minus the sum of the output voltage and the base-emitter voltage of each preceding transistor. The collector current require-ments of each transistor are decreased by the corresponding hPE(n-l), going from Ql to Qn. The power requirements for each transistor can be determined from this information, and the best-suited transistors can be chosen for the compound connection.
Fig. 9.9. Compound connection used as series control element.
+ o---..---~
. .
. .
[(out)
--~----_o+
V (out)
~ •
• To d·c amplifier Qn and currentsource
9.4. PREREGULATOR
The preregulator should be included as a functional element if the regulator is to perform to its full capability. The preregulator provides a constant current to the collector of the d-c amplifier and the base of the control element. If point A in Fig. 9.10 is returned through a resistor directly to the positive terminal of the unregulated supply, ripple current caused by the unregulated voltage variations will be injected into the base of Q2. The ripple will be amplified by the series con-trol element and appear in the output. A preregulated d-c supply obtained from R1 , R2 , and Dl helps eliminate the ripple current. The breakdown diode tends to keep a constant voltage across R2 and a constant current to Q2 and Q3. The breakdown voltage of Dl can be any value less than V(in) - V(out) that will supply sufficient current to Q2 and Q3. If possible, the breakdown voltage of Dl should be approximately equal to four times the normal change in base-emitter voltage of the control element. The values of this voltage may be calculated as follows:
where
IB2 and 103 will have been previously determined by the selection of the control element, the comparison element, and the d-c amplifier. Therefore, lz is a known value.
R2 = VZ1 - V BE1 - V BE2
IB2
+
103 (26)Knowing the nominal voltage of the breakdown diode, the current through Rz, the input and output voltages, and the maximum allowable current through the diode, the resistance of Rl can be determined. power-dissipation ratings of the breakdown diode.
In addition to improving input regulation, the preregulator reduces the output resistance of the regulator. If the current supply to Q2 and Q3 were shunted through only a resistor, a load current increase would tend to cause the output voltage to drop. The drop is caused by an increase in base-emitter voltage of the control element and by a higher voltage drop through the internal resistance of the unregulated supply. When this occurs, the comparison element is forced to com-pensate for both changes, and the resistance of the unregulated supply as well as the emitter resistance of the control element will contribute to the output resistance of the regulator. Depending upon the source resistance, the output resistance can be reduced by as much as one order of magnitude if the preregulator is used. If the voltage across R2 is constant with small load changes, the output resistance is independent of the internal resistance of the unregulated supply.
Voltage Regulators 1 SS Dl
+Q---+-,<: ~+----~-~+
VCout)
Fig. 9.10. The preregulator.
Improved performance for a preregulator can be obtained by using the circuit of Fig. 9.11 in place of the two resistors (Rl and R2 ) and the breakdown diode Dl of Fig. 9.l0. In the circuit of Fig. 9.ll, the collector current of Q4 is independent of changes in VBE of Q2 caused by temperature or load variation. In Fig. 9.10, the current 12 is a function of the combined VBE of Q2 and Ql, due to either tempera-ture or regulator load, or both.
From Fig. 9.l1,
A low breakdown voltage for D3 will fulilil two requirements: It will allow this type of preregulator to operate with low unregulated input voltages, and will also pro-vide the necessary negative temperature coefficient to cancel ~ VEBI ~T of Q4. If the ~ VEBI ~T of Q4 is greater than the available negative temperature coefficient of D3, a small silicon diode may be added in series with D3 to provide additional compensation.
Fig. 9.11. Transistor preregulator.
1B2 and 103 have been previously determined; now a transistor is selected which will handle the sum of 1B2 and 103. A low-voltage breakdown diode is selected for D3 with lz3 determined from its power-dissipation rating. R5 can now be found from
R5
=
V(inmin) - VZ3 - lz3RRIZ3
+
IB49.5. FILLING IN THE BLOCKS
(33)
The elements just described can now be connected to show a complete regulator.
Figure 9.12 shows a regulator using a single common-emitter stage as comparison element and doc amplifier element, and the resistor-breakdown diode-type preregulator.
Figure 9.13 shows a typical regulator using a differential amplifier comparison element, a single-stage doc amplifier, cmd the single transistor preregulator of Fig.
9.11. Note that this type of circuit uses both of the best methods of temperature compensation: i.e., the differential amplifier and the temperature-compensated transistor current source.
Fig. 9.12. Typical series regulator using single common-emitter stage comparison: doc amplifier element and resistor-breakdown diode preregulator.
r - - - l r----'
+0 ,
• f I I I I +
•
I I I II I
I
IR;~
fR; I Rs R5 I II
C1 I
I
I Rl II I I
I I
I I I I I I I I
*C
2I I I
I
I L _ _ _ _ _ _ _ ...J
I
I
I II Control I I
I I I I
I
I I I I
I
V(in) I I I I I V(out)
I I I I Rp I
I I I I I
I * Current sources I I I
R; I for Dlo D2, I I I
I and Da
,--
--l I I II I
I
I I II I I I I I
I B I B I I I I
I I Dl
I
I I I <.R2I D2 I I I I
I I I I I I
I I I I I I
I I I I I I
I -
-
I I I I I IL _ _ _ _ _ _ _ _ ..J L _ _ _ _ J L _ _ _ _ _ _ _ _ _ ...J L _ _ _ _ ..J
t;; Preregulator Reference Comparison Sample
~
Fig. 9.13. Typical series regulator using differential amplifier comparison element, single-stage d-c amplifier, and single transistor constant-current source.
9.6. TYPICAL VOLTAGE-REGULATOR DESIGN
In this section a voltage regulator will be designed from a typical set of regulator requirements. The regulator requirements must be specified before any attempt is made to design the circuit. The problem is then reduced to determining a circuit configuration adaptable to the application and solving for the component values.
In this section a voltage regulator will be designed from a typical set of regulator requirements. The regulator requirements must be specified before any attempt is made to design the circuit. The problem is then reduced to determining a circuit configuration adaptable to the application and solving for the component values.