Designing a high-voltage regulator can pose difficulties associated with component stress, reliability, and costs.

This Design Idea offers a solution: a simple unregulated high-voltage supply in series with a low-voltage switched-mode power supply (SMPS).

Figure 1 The LTC1625 regulates the combined HV output.

The sum of VHigh and VLow is fed back for regulation.

At minimum load, when VHigh is maximum, the SMPS outputs a small voltage. As the load increases and VHigh drops, the SMPS outputs an increasing voltage, and also compensates for ripple.

D6 and the fuse offer some short-circuit protection, and can be adjusted to suit your application. Without them, an output short will force C1 to discharge into the switcher’s output, possibly destroying it.

Figure 2 VReg and VHigh (Y-axis) vs. load in milliamps (X-axis).

At around 200 mA, the circuit ceases to regulate, as the SMPS output reaches its upper limit.


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Gheorghe Plasoianu has a Masters degree in electrical engineering from the Polytechnics Institute of Bucharest.