I'm pretty familiar with using MOSFET H-bridge circuits. I've built several 48 VDC to 120/240 VAC transformer based power inverters. I perfected a very robust isolated gate drive circuit that I think works very well. Now I want to build a full bridge DC-DC step down converter. It seems like I could just make basically the same system as I did for the inverter but wind the transformer for step down instead of step up, and rectify and filter the output for DC.

The basic use is to take up to 500 VDC (but likely less) and convert it to 48 VDC at up to 120 amps. There are many circuits I've studied but none of them at quite as high a primary voltage or as high of a secondary current. Besides electrical safety, which I am familiar with as an electrician, what are some things I should be looking at that make this application different from the inverter design I'm familiar with?

 

Phase shifted full bridge is probably the best high power circuit.
This device does the job for you, but I've not checked that it can handle the power you need.
Thermal management (i.e. heatsinking) will be the biggest challenge.

 

Phase shifted full bridge is probably the best high power circuit.
This device does the job for you, but I've not checked that it can handle the power you need.
Thermal management (i.e. heatsinking) will be the biggest challenge.

Is that type of soft switching circuit a must for this application or is it just preferred? What is the advantage? I can see how it is more efficient and easier on the FETs. But will a more brute force method of hard switching work?

 

Is that type of soft switching circuit a must for this application or is it just preferred? What is the advantage? I can see how it is more efficient and easier on the FETs. But will a more brute force method of hard switching work?

It is optional, but it reduces switching losses. It probably reduces interference as well.

 

What is the purpose of conversion?? I have read a lot of stuff telling us how great 48 volts will be but everyone who is telling me that is part of the companies that sell the components that would be used for the converters. So the stink of fallacy is rather strong in the arguments.

 

What is the purpose of conversion?? I have read a lot of stuff telling us how great 48 volts will be but everyone who is telling me that is part of the companies that sell the components that would be used for the converters. So the stink of fallacy is rather strong in the arguments.

That might be true, but in my case it is to charge a 48 volt forklift battery.

 

The welder thread had me looking at inverter welder repair videos. I realized that the output stage of those cheap Amazon inverters is exactly what I'm looking for. For $100 or less I can gut the power stage out of one of those and be 80% done.

 

OK. Update on the plan. I'm going to get a couple of $70, 100 amp stick welders on Amazon. Believe it or not, the basic output stage is very beefy. I actually have the Harbor Freight portable stick welder and it will truly do its very best to deliver all it can until the thermal trips. I'm going to split my 500 VDC in half and make a +/- 250 VDC three-wire system with a neutral. If I put one welder between neutral and +250 VDC and the other between neutral and -250 VDC, I can parallel the outputs and not have to make a new 500 VDC front side. The rectified 240 VAC is 339 VDC after filtering, already a perfect range out-of-the-box. I just have to do a little hacking on the current sensing side. If I give the 100 amp rating a conservative 50% derating, then paralleling the outputs of two units could deliver 100 amps comfortably.

 

WOW!! It seems that you have a plan that should work. PLEASE be sure to share the details with us when it is done.
AND charging a forklift battery is one of the good uses for 48volts. I was commenting on the highly touted automotive proposals.

 

That might be true, but in my case it is to charge a 48 volt forklift battery.

Assuming lead-acid, the fully charged voltage will be around 55V while charging (based on 13.8V for a 12V auto battery). Your charger needs to limit the charging current to safe levels, and be able to float up to that voltage. You need to check the specs on your forklift battery to see what the mfr says about max charging current, which will be much less than max discharge current.

 

A forklift battery should be charged to 57.6V absorption charge with a maximum current of C/5, then given an equalisation charge at constant current of C/20 for a specified time, or until the voltage reaches 65V.