This circuit includes magnetic isolation that allows you to configure a positive, negative, or floating output. The floating output is enabled by a separate winding that generates a feedback voltage proportional to, but lower than the output voltage, thereby eliminating the need for large-valued resistors in a resistive feedback divider.
A similar version of this article appeared in the June 26, 2008 issue of EDN magazine.
Certain sensors, electrostatic traps, and other applications require a regulated high-voltage power supply that delivers a modest amount of output current. Simplicity, low quiescent current, and small size are desirable in such supplies. The circuit of Figure 1 meets these requirements, and its magnetically isolated output allows you to configure a positive, negative, or floating output.
The floating output is enabled by a separate winding that generates a feedback voltage proportional to the output voltage, but lower. That arrangement eliminates the need for large-valued resistors in a resistive feedback divider, which is otherwise required if the high-voltage (HV) output is sampled directly. As shown, the low-voltage divider contains resistors with much lower values, which dissipate much less power.
A single IC (MAX1605) contains the necessary switching regulator, modulator, error amplifier, and power switches. It drives the primary of a toroidal transformer that includes a feedback secondary and several output windings. With component values as shown, the circuit can generate 500V. You can vary the output voltage ±30% by adjusting the ratio of the resistive feedback divider. You can also increase or decrease the output voltage in steps, by adding or removing the rectifier/capacitor/ output-winding modules (BAV21). Input current and output voltage vary as shown with input voltage (Figure 2) and load current (Figure 3).
As for all switching converters, EMI and circuit parasitics can present problems. The circuit needs careful board layout, along with filtering, decoupling, and shielding as required. The HV output has about 1% ripple. You can add an RC or LC filter in series with the output to achieve lower output ripple.