Heavy-duty power supply regulates either voltage, current, or power

By combining switching and series-pass techniques, this high-voltage supply's designer achieved 0.01% regulation at power levels to 100W.

Jim Williams, Teledyne Philbrick -- EDN, June 16, 2011

Regulated high-voltage power supplies, while common, generally offer only constant-current and constant-voltage modes of operation. This one adds a constant-power (EI product) mode.
Regulator + converter = amplifier


Considered as a unit, the pass regulator and converter function as an amplifier within the servo amplifier's feedback loop. When feedback is taken from the "voltage sense" network, a constant-voltage output is produced. Taking it from the "current sense" network results in a constant current through the load. Lastly, when inputs from the voltage-sensing and current-sensing networks are multiplied by the multiplier circuitry, the load receives constant power.

The pre-chopper maintains a small fixed voltage across the pass regulator regardless of inverter output setting. It does this by synchronously chopping the 120-Hz peaks from a full-wave rectifier in a manner similar to a lamp dimmer. This limits the pass regulator dissipation to an acceptable level. Had it not been done, dissipation would have been excessive, especially at low-voltage output settings.

Crowbarring prevents overload by shutting down the supply when it senses either too much current flowing in the load or a load dropout.

A Cook's tour of the circuit

Details of the circuit will now be discussed with the aid of a detailed schematic (Fig. 2). Looking at the SERVO AMPLIFIER section, reference stability results from using a 1N944B temperature-compensated zener diode (D7) with its output scaled to 10.000V across the Kelvin-Varley potentiometer. The potentiometer's output biases the 1023 FET amplifier A1 which functions as a precision servo amplifier. Its 20-pA bias current insures negligible loading error on the potentiometer. A1's output drives the Q6/Q7 pair, a 2N2102-2N3442 Darlington pass regulator, via the 2N2102 pull-down transistor Q5. Q7's collector is supplied dc power from the output of the prechopper, which will be described later. Q7's emitter drives the toroid transformer, T1.

The wide dynamic range of the inverter is due to the 2N2528 transistors (Q8, Q9) that featured low saturation voltages, good beta linearity and reasonable speed. They permit the inverter to run at low output voltages with no resultant sacrifice in performance at high output potentials. Output of the transformer is rectified by a full-wave bridge employing two 1N5061,s in each leg. The stacking allows use of diodes rated at only 800V. Filtering, provided by the 1-vF capacitor, is adequate for the square-wave output.


Voltage feedback is derived by a 99 to 1 division of the filtered output. The current-feedback signal, on the other hand, is split into four separate switch-selectable ranges. This promotes ease of setting and keeps the current-feedback signal at high levels—and therefore easy to work with. The "shorting-switch" selection scheme insures feedback even during the switching operation.


As might be suspected, the servo loop is very prone to oscillation. C3 and C4 were included to insure loop stability, but slow it down as well. Loop response is about 75 msec (no load to full load), so transient response clearly is not this circuit's forte.

Pre-chopper keeps a constant drop

The pre-chopper is essentially a servo that keeps the drop across pays transistor Q7 at a constant, low voltage, regardless of inverter demand conditions. This lowers dissipation and insures reliability. A4 looks differentially across the Q7 pass element. A4's negative input is biased through the 10V zener, D5, and its output voltage is compared to a 120-Hz line-synchronized ramp by amplifier, A5. This op amp functions as a pulsewidth modulator, and drives the Q3, Q4 combination that delivers phase-controlled power to C2 and the collector of Q7. Diode D4 insures that Q4 will not be reverse biased when the 120-Hz signal is below the dc across the capacitor.

Since A4's negative input is routed through the 10V zener, Q7's emitter will always be 10V below the collector, despite the required inverter input power. This value, 10V, is low enough to keep dissipation down, yet high enough to insure good regulation characteristics.

Loop inside a loop

The battle-scarred veterans among those reading this article will realize the unpleasant surprises that can be encountered by running a servo loop within a servo loop. Here, these embarrassments have been avoided by giving the pre-chopper slower response time than the main servo loop. C1, the 2.2-μF capacitor across A4, satisfies this condition.

The 120-Hz reference ramp arrives at A5 via the 2N2646 unijunction transistor Q2. Q2, in turn, is driven by Q1, the 2N2907 current source. SCR1, D1 and R1—which is connected to –15V—assure a true zero-volt reset for the ramp. D2 and D3 provide the synchronizing signal, which cannot be taken from the bridge rectifier because the bridge output waveform is heavily influenced by the phase angle at which Q8 and Q9 fire.

Carry a crowbar for protection

A 1339 amplifier (A6) helps protect the supply from excessive output current. The amplifier looks at the current-feedback signal and will swing its output to positive saturation if that signal exceeds 10V. In turn, SCR2 is triggered and grounds the inverter drive signal, resulting in a supply shutdown. The "overload indicate light (I1) will come on to alert the operator to the situation. To reset, the "overload reset" button is pressed, commutating the SCR and enabling the inverter to again receive bias. D6, a 1N914 in the base line of Q6, assures a clean turn-off when the SCR comes on.

Overvoltage protection is provided by D10, D9 and D8, the 10V zener diode and the 1N914's, which are connected between the "voltage" output signal and SCR2. This arrangement prevents the supply from running away in the event of a load dropout when in the "current°' or "power" regulation modes.


Physical layout of the supply is not critical except for the point grounding considerations common to any precision circuit. The inverter ground return (from Q8 and Q9 collectors) contains fast, high-current spikes and should be returned directly to supply common. Returns from the reference diode, its potentiometer and the amplifiers are also critical. They also should be connected directly to the supply ground.

Particularly insidious failures can result from a malfunction in the pre-chopper circuitry. As an example, assume an emitter-to-collector short in Q4. All of the 120-Hz waveform will then be supplied to the 3500-μF integrating capacitor, and the dc potential at Q7's collector will rise to maximum voltage. The power supply will, however, continue to function in an apparently normal fashion—that is, until Q7 achieves its molten state. This most unwelcome state of affairs is prevented by the 175°F thermal switch (S5) mounted next to Q7. Closing of the switch will blow the fuse at the transformer primary.

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