A single chip with small size and high voltage power supply function

A single chip with small size and high voltage power supply function

Summary: This circuit includes magnetic isolation, allowing you to configure a positive, negative, or floating output. The floating output is enabled by a separate winding that produces a feedback voltage proportional to, but lower than, the output voltage, thus eliminating the need for large value resistors in the resistive feedback divider.

Certain sensors, electrostatic traps, and other applications require a regulated high-voltage supply that provides a modest amount of output current. Simplicity, low quiescent current, and small size are desirable for 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.

Figure 1. Taking feedback from the low voltage secondary winding, this high voltage produces 500V with low quiescent current.

The floating output is enabled by a separate winding that produces a feedback voltage proportional to the output voltage, but lower. This arrangement reduces the need for large value resistors in the resistive feedback divider, which is otherwise necessary if the high voltage (HV) output is sampled directly. As shown, the low voltage divider contains much lower value resistors, which dissipate less power.

One IC (MAX1605) contains the necessary switching regulator, modulator, error amplifier, and power switch. It drives several outputs with a toroidal transformer that includes a feedback winding. With the component values ​​shown, the circuit can produce 500V. You can vary the output voltage by ±30% by adjusting the resistor feedback divider ratio. You can also increase or decrease the output voltage in steps by adding or removing capacitors/rectifiers from the output winding module (BAV21). The input current and output voltage vary as shown with input voltage (Figure 2) and load current (Figure 3).

Figure 2. Output voltage (upper trace) and input current (lower trace) versus input voltage for the Figure 1 circuit.

Figure 3. Output voltage (upper trace) and input current (lower trace) versus current for the circuit of Figure 1 loaded.

As with all switching converters, EMI and circuit parasitics can present problems. The board requires careful layout, as well as filtering, decoupling and shielding requirements. The high voltage output ripples around 1%. You can add an RC or LC filter in series with the output to achieve reduced output ripple.