Regulate a 0 to 500V, 10-mA power supply in a different way

Contemporary power supplies use switching techniques to achieve the desired output voltage from the primary source. Switching power supplies, however, are often too noisy to be used in sensitive analog circuits. You may find linear power supplies to be preferable in these cases.

A standard practice for a linear voltage regulator is shown in Figure 1. A higher-than-desired, unstable voltage is connected to the input, V_IN, and the series-pass transistor, Q₁, reduces the voltage to the desired level at output V_OUT. An error amplifier, IC₁, compares a fraction of V_OUT with a reference voltage, V_R, and controls Q₁ to keep the output fixed regardless of the load current, I_OUT, and variations of V_IN. Such a circuit is suitable only for a small range of output voltages.

Figure 1 A standard linear voltage regulator connects the feedback directly to the
series-pass transistor, Q₁.

When a wide range of output settings is desired, as in laboratory power supplies, the value of resistor R_Q1 must be small enough to allow sufficient base current for transistor Q₁ at the high end of the output voltage range, but excessive power is dissipated at this resistor and transistor Q₃ when output voltage is reduced. Additionally, Q₃ must withstand the maximum V_IN.

You can use the circuit in Figure 2 to overcome these problems. Two standard transformers, T₁ and T₂ (220V ac to 6V ac, 10W), are used to make an isolated replica of the mains supply, V_M. This replica is doubled and rectified using D₁, D₂, C₁, and C₂ to get about 560V at V_IN from 220V ac at V_M. As in the standard Figure 1 connection, a series-pass transistor, Q₁ (BU508A), is used to reduce the unstable V_IN down to a fixed V_OUT, and IC₁ compares the divided V_OUT with V_R. Potentiometer R₃ sets V_R to allow for the adjustment of V_OUT, as given by the following equation: V_OUT=V_R×[(R_FG+R_F)/R_FG], where R_F=R_F1+R_F2 … R_Fn.

Figure 2 Optical coupling isolates the high voltage at Q₁ from the op-amp output.

With 10 resistors (1 MΩ each) connected in series to form R_F and a maximum reference voltage of 5V, the output voltage can be set from 0 to 505V. The OPA364 operational amplifier is a rail-to-rail input type to allow proper operation, with V_R ranging from 0 to 5V, and is able to source a current of up to 40 mA.

To reduce the power dissipation caused by driving a series-pass transistor and expand the output voltage range, the driving of transistor Q₁ is done in an unconventional way using optical isolation. Two photodiodes, FD₁ and FD₂, operating in photovoltaic mode, provide the driving current for the base of transistor Q₁. Light falling on the photodiodes causes a current flow into the base of Q₁.

The maximum voltage from a single photodiode working in photovoltaic mode is not sufficient to drive the base; therefore, two photodiodes are connected in series. Photodiodes for infrared light at 870 to 950 nm are used, and two IR LEDs, LD₁ and LD₂, illuminate them. The LEDs are standard 5-mm, plastic-encapsulated types. To improve the transfer ratio of the current through the LED versus the current generated by the photodiode, cut off the tops of the LEDs and polish them to form a flat surface. Place the photodiodes in proximity to the surfaces obtained. The transfer ratio of this homemade optocoupler is about 0.05. (The current of 20 mA through the LED causes a current of 1 mA through the photodiode.) Alternatively, you can use a commercially available linear optocoupler—for example, an IL300, which houses two photodiodes. Its current transfer ratio is only about 0.007, so you should use several such components in parallel.

The current-limiting circuit formed by Q₂ and R₂ simply shorts the FD₁ and FD₂ photodiodes when the output current exceeds Q₂’s turn-on threshold, and the limit is independent of the output voltage. Capacitor C₆ is added for compensation, and transistor Q₁ should be fitted with a heat sink of at least 5°C/W. The power supply for the operational amplifier and the reference voltage is provided from the ac signal between the two transformers using bridge rectifier BR₁ (50V, 1A); two filtering capacitors, C₇ and C₈; and voltage regulator IC₂ (LM7805). A shutdown of the output voltage can be made by a simple short circuit across capacitor C₅, making V_R equal to 0.

Those living in a 110V ac region can use locally available transformers, but they should modify the circuit to achieve 500V by adding yet another transformer, T₃ (the same as T₁ and T₂, all 110V ac to 6V ac, 10W), in such a way that the low-voltage windings are connected in parallel, while the high-voltage windings of transformers T₂ and T₃ are connected in series. The operation of the high-voltage windings can be verified using an ac voltmeter; if the voltmeter reads zero, the ends of the windings from T₃ must be exchanged. Alternatively, if a 220V/6V transformer is available, keep T₂ as 220V/6V and use 110V/6V at T₁.

Editor’s note: High voltages of 500V and the available current of several milliamps can be lethal; exercise caution when building, testing, and using this circuit.