A small and cheap 5V/400V DC/DC converter can be useful in many DIY projects, e.g Geiger–Müller counters. I will present here one of such DC/DC converter based on popular MC34063 chip in step-up configuration.
One big limitation of this device is little output power, but for many applications this won't be a problem. Another problem is poor temperature stability, but this could probably be improved by using low tolerance C1, R7-R11.
Q1, L1, D2 are coil-based step-up converter. Q1 should be a power MOSFET rated a bit more than the output voltage. Empirical tests have shown that Q1 didn't heat up, so I didn't mount it on a radiator. D2 and C4 rectifie output voltage, they also should be rated for bigger voltage than the output. To improve performance, D2 can be replaced by a Schottky diode, capacitance C4 can be increased. R3-R11, C3 provide output feedback to the MC34063. S1 is used to select output voltage if needed.
Idle power consumption is 85mW. Below presented fluctuation in output voltage at constant temperature and constant load.
Such high output voltage can be dangerous (even while the maximum curent iof this converter s small), so if you make the same or similar device, please be careful. If you are interested in higher voltage, you ma take a look on High voltage supply (10-30kV) made from CRT television flyback transformer.
Eagle files for this project can be downloaded from my GitHub repository.