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A replacement proposal for e.g. LNB200-0.12-A


✈ Motivation

If you need a high voltage power supply (e.g. 200 V for driving piezos) you mayst be attracted by the power supplies from XP Power. But those devices suffer from starting when driving a capacitive load. They fall into that Foldback Current Limiting. You mayst get away by setting the max. current to the maximum value, but sometimes, this is not enough.

As we do need a lot of high voltage power supplies, we decided to design our own which does not have this Foldback Current Limiting. It is inspired by our replacement circuit for the VB408 circuit.

✈ Circuit Description

The main regulating circuit is the transistor T1 (IRF840 or STP12NK60Z). It's gate is biased by R1 (47kΩ, 11W). The gate to source path is bypassed by a 10 V zener diode to keep T1 in the safe operating area. At the output, there is a voltage divider consisting of R6 and R10. If the output voltage gets too high, the down-divided value is larger than 2.5 V and then IC2 (TL431) starts conducting, lowering the source resistance of T3 (IRF840) and therefore increasing the drain-source current of T3. By that, the gate of T1 is pulled down.

In case the current is larger than Imax * R2 ≈ 0.6 V the transitor T2 (BC546) starts conducting and therefore reduces the gate to source voltage of T1. By that, the main transistor reduces conducting and so, the output voltage drops and the current is limited.

The circuit is powered by an isolation transformer 230V / 230 V.

✈ Circuit Performance


Warm-up ... "stable" after about 12 minutes

The power supply is loaded with an 1 kΩ resistor. This will draw approx. 210 mA resulting in a (317 V - 210 V) * 210 mA = 22.5 W power dissipation. The temperatures are shown below :

Heatsink 51 °C @ 22.5 W load (flir lies)
Transistor 85 °C @ 22.5 W load (flir lies)

The setup indicates very high temperatures. But that is not true. FLIR lies at you. We measured the temperature with a RTD / Multimeter.

✈ Downloads


The main transistor is mounted on a large heatsink (SK 92/100 SA) with 2.15 K/W

✈ Share your thoughts

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t1 = 6585 d

t2 = 200 ms

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