Small, simple, high-voltage supply features single IC
An output toroidal transformer with a low-voltage feedback winding enables a small, high-voltage supply that uses a single IC.
Alfredo H Saab and Tina Alikahi, Maxim Integrated Products, Sunnyvale, CA; Edited by Martin Rowe and Fran Granville -- EDN, June 26, 2008
Sensors, electrostatic traps, and other applications require regulated, high-voltage power supplies that deliver modest amounts of output current. Simplicity, low quiescent current, and compactness 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. A separate winding that generates a feedback voltage proportional to the output voltage, but lower, enables the floating output. This arrangement eliminates the need for high-value resistors in a resistive-feedback divider, which the circuit would otherwise require for direct sampling of the high-voltage output. This low-voltage divider contains resistors with much lower values, which dissipate much less power.
The MAX1605 IC from Maxim contains the necessary switching regulator, modulator, error amplifier, and power switches (Reference 1). It drives the primary of a toroidal transformer that includes a feedback secondary and several output windings. With the component values in the figure, the circuit can generate 500V (figure 2 and figure 3). 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. The BAV21 is a high-voltage, low-reverse-current, general-purpose diode.
As with all switching converters, EMI (electromagnetic interference) and circuit parasitics can present problems. The circuit needs careful PCB (printed-circuit-board) layout, along with filtering, decoupling, and shielding. The high-voltage output has approximately 1% ripple. You can add an RC or an LC filter in series with the output to achieve lower output ripple.
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