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March 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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A linear series-pass voltage
regulator is about as simple a circuit as you can devise that will provide a reasonable
degree of consistency, with as little as a Zener diode, a transistor, and a biasing
resistor. The small parts count does not allow for temperature compensation, variable
output voltage adjustment, overcurrent protection, etc., but for most instances
it gets the job done. The disadvantage of a linear series-pass voltage regulator
is that the larger the difference between the input and output voltage, the greater
the inefficiency. That is because the total power dissipated by the circuit is the
product of the input voltage and current through the load (Vin x ILoad)
Since the series pass transistor must drop the voltage difference between the input
and output, and since all the current to the load also flows through the transistor,
it dissipates (Vin - Vout) x ILoad. The load dissipates
Vout x ILoad, so the total is [Vout x ILoad]+
[(Vin - Vout) x ILoad] = Vin x ILoad.
Modern-day switching voltage regulators (aka "switchers") use pulsewidth modulation
that feeds a hold-up capacitor to do the job with very high efficiencies even for
large differences in input and output voltage. The disadvantage of a switcher is
the digital noise that is generated in the process. That buzzing noise you hear
on an AM radio when turning on a CFL bulb is due to the el cheapo switching power
supply in its base.
Design of a 11- to 32-volt, 700-mA power supply which has good
regulation and built-in overcurrent protection.
Fig. 1 - In the simple series-pass fixed-voltage regulator
transistor Q1 controls the regulated output voltage, Vo.
By G. V. Fay / Applications Engineering, Motorola Semiconductor Products Inc.
An ideal voltage-regulated power supply has zero output impedance so that the
output voltage remains constant for any load current requirements. But, zero output
impedances cannot be achieved, although supplies with impedance levels of a milliohm
can be constructed. And, too, semiconductor device characteristics place a limit
on the current and voltage that can be supplied to a load.
Two simple series-pass voltage regulators are shown in Figs. 1 and 2. Transistor
Q1 in each of these circuits controls the output voltage; thus the collector-emitter
voltage rating of Q1 determines the maximum input-to-output voltage. All the current
delivered to the load passes through Q1. Therefore, the power dissipated in Q1 is
(Vin-Vout) times ILoad. If high input voltages
and low output voltages and/or large load currents are necessary, it is possible
that Q1 will have to dissipate large amounts of power. Obviously, under these conditions
the regulator efficiency is low and the power rating (for the particular heat sinking)
limits regulating capacity. For limited load variations, Q1 can be shunted by a
resistor, thus shifting some power from Q1 to the resistor. The output voltage of
the circuit (Fig. 1) is the Zener voltage minus the base-emitter voltage of
Q1. R1 must provide enough current to the base of Q1 and to D1 to keep the voltage
of D1 above its knee at all times. If the temperature characteristic of D1 is equal
and opposite that of the base-emitter junction of Q1, the output voltage will remain
constant with temperature.
When variable output voltages are required, a circuit similar to that of Fig. 2
can be used. Here, the lowest possible output voltage is the breakdown voltage of
D1 plus the base-emitter voltage of Q2.
Fig. 2 - In this variable-voltage series-pass regulator,
transistor Q2 and a potentiometer are used to set voltage level.
Fig. 3 - This variable-voltage supply has overcurrent protection.
Maximum output voltage is limited by the changes in the unregulated input plus
the drive requirements of Q1. R1 provides the bias current for D1 whose voltage
is compared to that at the potentiometer arm. If the load increases and causes the
output voltage to drop, Q2 conducts less and Q1 conducts more, restoring the voltage
to the original level.
Basically, series-pass regulators are current, voltage, and power limited; and
the circuit's control transistor must always be run in the safe operating area -
region where there is no danger of secondary breakdown. However, series-pass regulators
do provide extremely good regulation, fast response time, low output impedance,
and low ripple. Their performance is almost independent of input frequency fluctuations
and they have excellent dynamic response. They also provide variable output voltages
and are readily adaptable to remote voltage sensing, remote programming, and current
limiting or current regulating applications. The circuit output is virtually transient
free.
Overcurrent Protection
The circuit shown in Fig. 3 is a regulated power supply with overcurrent
protection and an output of 11 to 32 volts d.c. The maximum output current with
the specified circuit components is 700 mA. The diode bridge consisting of D1 through
D4 provides full-wave rectification of the 25-volt secondary of T1. Capacitor C1
is used to filter the rectified current and keep a positive voltage on the collectors
of Q1 and Q2. Resistor R1 and capacitor C2 further reduce the ripple to maintain
a stable d.c. voltage as a collector supply for Q3 and Q4, and for driving the base
of Q1. Transistor Q2 is the series-pass device which regulates the output voltage.
The resistor divider, R4-R5-R6, forms a sensing network. The voltage at the arm
of potentiometer R5 is applied to the base of Q4, which compares the voltage to
that of Zener diode D5. (Continued on page 78)
The difference between the two voltages determines how much Q4 conducts. If the
output voltage increases, the base voltage of Q4 increases and pushes it farther
into the conduction region. This reduces the base current of Q1, which in turn reduces
the current through Q2, lowering the output voltage. If the output voltage drops,
Q4 moves toward cut off and Q1 and Q2 turn on. This increases the output voltage.
In essence, the circuit is a feedback amplifier which tries to maintain the output
voltage at a constant level independent of load condition.
Fig. 4 - Voltage output versus load current for the circuit
of Fig. 3.
Fig. 5 - Regulation versus load current for the circuit
shown in Fig. 3.
The output voltage is determined by the potentiometer setting according to the
following equation:
If R5 is set at the low end (A), the output is maximum and is given by:
The minimum output voltage (11.2 V) occurs when the arm of R5 is set to the high
side (B).
The regulation is a function of the potentiometer setting since this determines
the amount of feedback. The closer the base of Q4 is to the positive output the
greater the feedback.
Overcurrent protection is provided by R2 and Q3. Since R2 is in series with the
output, the voltage across it is proportional to the output current. This voltage
is used to drive Q3 so that the larger the output current becomes, the more Q3 conducts.
When Q3 is turned on, the base drive is removed from Q1 and this turns off power
regulator Q2, thus limiting the output current. In this example, component values
were chosen to limit the maximum current to approximately 700 mA. Fig. 4 shows
the effectiveness of the current limiter.
Load regulation at several output voltage levels is shown in Fig. 5. The
overcurrent actually degrades the performance of the regulator circuit, particularly
at the higher current levels, since it decreases the base drive of Q1.
The power stage transistors, Q1 and Q2, were selected for high gain at -55°C.
This maximizes the voltage and current regulator's control. At maximum output voltage
and at low temperatures, Q1 requires a base current of about 1 mA. R1 was selected
to deliver approximately 2.5 mA. This sets the minimum voltage regulator current
in Q4 (MPS6531) at 1.5 mA, when the current regulator is not working. When the current
regulator Q3 (2N4921) comes on, it must handle this current. For the opposite condition
at high temperatures and low output voltages, R1 provides about 25 mA maximum current.
The drive requirement for the power stage is not critical here, but Q3 and Q4 must
be able to handle this current for effective control.
The maximum temperature at which the circuit will operate is set by the heat
sinking of Q1 and Q2. If these transistors are mounted directly to a heat sink and
a good silicone grease is used, then the worst-case temperature rise above the temperature
of the heat sink for Q2 is 25°C, while that for Q1 is about 20°C. Since
the maximum operating temperature of Q1 is 150°C, this means the heat-sink temperature
must be kept less than 130°C.
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