High current DC/DC isolating stepdown

[Oznog]

I was talking to some electric vehicle guys who have large battery packs which are not grounded to the frame who still need a 12v supply to accessories.

Some have used commercial converters intended to be line-driven. I'm not sure what the failure modes are but I'm hearing they didn't work all that well. The source voltage is not well matched to begin with and it can be quite noisy.

It would need to take in 80-225 vdc depending on pack design and put out around 12v-14v, and the input and output need to be isolated. The input may see large spikes due to being hooked up to a PWM converter driving a huge motor. Output current was something like 50-100 amps.

So a simple buck converter with an inductor would be out of the question since we require isolation. Buck converters would be pretty rough with a high input/output ratio anyways.

I'm thinking an SMPS power supply. 225v isn't much higher than a normal 120v SMPS rectifies to and is lower than a 220v rectifies to. 80v of course is lower.

The range of possible voltages seems to be a problem. My first thought would be to have multiple taps on the transformer that could be reconfigured according to the battery configuration. The batt voltage will of course have a high and low voltage due to load and charge state but it's not as wide as the 80v-225v configuration variance.

Another thought is we only use one tap and the SMPS controller modifies the pulse width. I think this is a good idea. The only problem is the entire input winding would need to be of a gauge which can handle 16 amps for the 80v input situation. With multiple taps the tap for 225v would have wire rated for 6 amps, so it would go in series with the 80v tap so about 2/3's of the total turns could be much smaller making it fit on the core much better.

What's out there for SMPS controllers? What about ones that can accept feedback from an isolated output stage through an optocoupler?

 

[Russlk]

What kind of accessory takes 100 amps at 12 volts? It does not make any sense to derive that from the main battery pack, it need a battery pack of its own.

 

[Oznog]

What kind of accessory takes 100 amps at 12 volts? It does not make any sense to derive that from the main battery pack, it need a battery pack of its own.

I am unsure of that as well. We can chalk up 15 amps for headlights and maybe a couple amps for some other lights, 5-10 amps for the wiper motor, a couple of amps for a normal stereo, etc, so I'm picturing more like 50 amps. Perhaps they want to do a big stereo too.

Then electric power steering was mentioned. Since there's no rotation at idle, and variable speed power steering pumps are inefficient, they may use electric power steering and the 12v units need like 12 amps continuous and 60 amp peak. If there's not a large battery to make up the peak difference then the converter needs to have that sort of output. Power seats, power windows, etc also have substantial peak demand needs that can't be covered by a capacitor.

 

[Glyph]

Yeah, that 80v-225v range is going to make things difficult.

I'm not an expert in SMPS, i've only made about a dozen or so and usually pretty shotty but here's my two cents.

To get the universal input and the isolation you could try first putting a boost regulator to push the voltage up to say 240v or more. Chips for handling boost topology are easy to come by. Use the 240v and send it through a full-bridge or half-bridge isolated buck regulator. Because the 240v is already regulated you'll get smoother output at the 12v end. Although you'll have to synchronize them to get best results.

Since the 240v is a design specification, you can tune your transformer to be perfectly suited for 240v : 12v operation.

Spikes are mitigated by the boost inductor and storage capacitor, giving your OVP circuit more microseconds to respond.

if you want a single circuit rather than the two independent circuits above, you can try a SEPIC converter or CUK converter, you can make them isolated. I personally haven't done anything like this so i don't know if it can handle the current.


Alternatively, if you like messing with transformers (I sure don't) you can drive the full/half-bridge circuit directly with the battery back and simply alter the transformer tap to suit the voltage range. It's still isolated. and you only need one control circuit.



As for optocouplers, they'll work but you got the problem of transient response. I've heard optocoupers just aren't fast enough to give you the extreme noise mitigation you demand, although i could be wrong on that point.

what i've seen done is the control circuit is actually placed on the secondary side and samples the voltage directly, the gate drive signals are transferred to the primary side by a gate drive transformer. Responds much faster than optocouplers, and doesn't exhibit aging that optocouplers do. The drawback of placing the control circuit on the secondary side is that you need an auxilary supply to start up the control circuit. But the auxiliary supply can be MUCH smaller, simpler and not as well regulated as the main supply.

oh well, my two cents. i'm probably wrong but i hope i atleast point ya in the right direction.


P.S. To get really smooth, highly efficient supplies, you can try looking into ZVS or ZCS tolopogies. Really tough to build but are the latest thing in SMPS design.