12 V to 250 V (DC) 5 mA step-up converter

Hi,

am am thinking about building a small supply of regulated DC 250V 5 mA
from a 12V DC input. Does not have to be isolated, but the supply should be filtered and regulated (does not have to be very well regulated though). Probably flyback converter, I think it is a too big voltage difference (ratio) for a simple boost converter with one inductor. I have found specialized ICs and design wizards but they do not manage with such a high output voltage, usually common flyback designs are downconverters for off-line switched power supplies.

So any idea for a simple design ? especially the flyback transformer design ?

Thanks,

Petr

The circuit described in the following website might suit your requirement. It is for a 180V low current Nixie clock supply from 12V.

The author also provide detail information and method of testing/winding the HV transformer.

Circuit diagram of the Flyback converter

That's overkill if you ask me. You can build a simple step-up switching regulator with a couple of transistors.

Here's an idea, it's pretty crude and will need some modifications but it will work.

**broken link removed**
https://www.electro-tech-online.com/attachments/switch-pdf.9974/

I think I'll try the 1st version - I do not love 555 but better then inconvenient dual primary transformer ....
My concern is for protection of the driving transistor (MOSFET) from both overvoltage (if the output is not enough loaded) and from excessive current (if the transformer by chance saturates ....)
The overvoltage protection is more of the concern for me because I intend to drive the flyback converter from a source that will not provide more than 2A and I'll use transistor able to handle even shorting of such a power supply but the overvoltage can still be there so I'd like to add some kind of clamping device to protect the MOSFET.

Nixie Power Supplies

Here are some nixie boost supplies I've used in the past:

**broken link removed**

They are shown for 180V output, but can be altered for 250V by changing the feedback path. For example:

MC34063.gif change R3, from 3.3K to 2.4K
MC34072op.gif change R7, from 15K to 10K
UC3843_PS.gif change R5, from 6.98K to 4.99K

Parts can found at Digikey or Mouser. Just google them.

When picking a coil, find one that can handle a peak current of 500mA or more. This is the critical part in every boost type switching supply.