Isolating DC/DC supplies have always been quite costly in my designs. They seem to make up the bulk of the end BOM cost. When I need isolated supplies I generally require upwards of 3 kV isolation because of the applications the equipment is used for. I generally always end up selecting the medical grade DC/DC as isolation deteriorates with time (short and long term). I also wanted the isolated supply to be able to delivery upwards of 1A.
Recently while designing a 1kV, 100mA switch mode power supply I required four isolated supplies. These supplies are to drive the FETs (H bridge configuration). Due to the design the supplies have to be isolated and with the output voltage being 1kV i wanted at least 2kV minimum preferable 3kV isolation.
This time I was determined not to pay the price for an isolated DC/DC. I wanted to tackle this issue, finding a solution that fits the following:
- Low component count
- Off the shelf
- 3kV isolation minimum
- Reasonable efficient
- Small form factor
Solution 1: Astable Multivibrator
One method is to have a custom transformer wound and drive it with a simple astable multivibrator. I like this method due to its low component count. However I don’t want to have to wind another transformer or pay for one to be wound. The equipment I design is low quantity, meaning custom transformer are expensive. What I want is an off the self solution.
See the diagram below:
Solution 2: H-Bridge Transformer Driver
There are a few single chip transformer drivers on the market. These are really nifty devices with built in drivers and even settable current limits. The chip I choose was Maxim’s MAX13256 (£4.67 from Farnell) due to its higher voltage rating of 36V and its ability to drive up to a 10W transformer.
The advantage of using these chips is that there are a range of transformers, from multiple manufactures designed specifically for the chip. I chose to use one from Pulse Electronics (PH9184.011NL) which has 4kV isolation and can be bought from Mouse at £2.85 each.
On the design below there are two snubbers (R57, C39, R58, C40) which are not required unless at VDD is above 27V. There is also a fault LED which is not required (L1, R54). I also choose to limit the output current to 600mA in this example. As you can see you can really keep the component count low with off the shelf products.
I ended up choosing this method and I’ll perform some tests on it soon. Hope this helps someone.