I have a device that requires approximately 500 mA at 5 VDC, and I am required to get this power from two legs of a three phase power supply. The voltage between these legs can be between 110 and 700 Vrms. This device and its power supply will be enclosed in a small, hot area which will be exposed to fairly large amounts of vibration. There is no need for isolation, and the transformers I have found are too large anyways.
I had previously used this design: http://www.ti.com/lit/an/slua721/slua721.pdf as a starting point for a supply that operated up to 525 Vrms. With some small modifications and a simple DC/DC buck supply at the output I was able to get the required 500 mA at 5V DC, but now of course the bosses say we need to push the input range up a bit. I went back to try and redesign this to work with 700 Vrms, and I have run into some problems.
For one, my rectified DC Voltage is going to be up to 1000 V. Since this design uses BJTs, this seems to limit my choice in transistors to those which can only withstand 200 mA max or are too large for my application.
I am thinking that an IBGT would probably be a better choice, but I am unsure of the implications of replacing the BJT in this circuit:
Can anyone tell me if it would be possible to replace Q1 with an IGBT to handle higher voltages?
If there are better suggestions on how to get from 700 Vrms to 5 VDC I am open to suggestions.
[Edit 1:] This device will be located in the same physical location as the high voltage load that is being power by the three phase supply. It is essentially inside of the casing of a three phase motor. There is no realistic possibility of a human coming close to this device while it is powered. I have isolated DC/DC supplies coming off of the 5 VDC output for powering the microcontroller and sensors. Given this, is isolation really necessary?
[Edit 2:] I have not been able to located any potential transformers that might be small enough for this application. Could someone point me in the right direction?
Best Answer
Is the supply power at a consistent AC frequency? If so, use a small transformer to take the worst of the bite out of the high voltage. You need a transformer certified for near the actual voltages you'll encounter, that is also commercially available at sane prices, such as the 575V popular in Canada.
You need like 3 VA ( not 3 KVA, just 3 VA) so any transformer will do.
So for instance if you can find a 575V to 48V transformer, that will re-range your voltage to 9-60VAC give or take.
Sorry, didn't realize I essentially duplicated Transistor's comment, but I'm saying don't brew your own transformer, buy one that is UL/CSA/CE listed for at least near the high voltage.
And you really, really want the isolation. Seriously. And that's why to use listed parts on the HV side.