DC/DC converters, parallel and series questions

[jonescg]

Hi all,

I know that if you want to double the power output of a DC/DC converter, you can parallel the outputs with limited success.

What I want to know is can you parallel the outputs while putting the inputs in series? I've always been planning on running my race bike with a separate auxiliary battery, and probably will for some time. But is it possible to run one DC/DC converter on one half of the pack, and a second DC/DC on the other half of the the pack, (300-350 V and 300-350 V) and parallel the outputs? Basically a 700 V input DC/DC is not readily available.

When I eventually get the race bike road registered (hehe, yeah that's gonna be awesome) I'd like a way of keeping the auxiliary pack topped up. Any thoughts on this?

 

[Holocene]

Hi Casey,

This is basically the same problem faced on the Swiss Federal Railroad. The standard for their overhead lines is 15 kV, 16.7 Hz AC. So some smart engineer has already solved this, no doubt involving an onboard transformer.

But here's another approach: Drive a 12v generator off the wheel, ala a SON hub. Or a DD hub motor if you want lots of amps :lol:

http://www.nabendynamo.de/english/

Oh BTW: 700 volts!!

 

[major]

I did kind of the opposite. I used 2 boost converters in parallel from a 12V source to get 192V from the series connected outputs of 96V each. It worked great. So I am pretty sure your scheme would work with good quality isolated converters like Vicor. They have a gate-out and in signal so you can synchronize the two PWM.

 

[jonescg]

Thanks Major, I might try to get in touch with the application engineers at Vicor to see what they recommend. I think synchronising the PWMs is a smart move, and the Vicors give you that option. I will sort of be running them with a centre tap, however there's nothing stopping me from running the inputs completely in series... I guess. How's the inrush current with these guys? Is there a way of turning them on after the main contactor closes so as to not impact on precharge?

 

[major]

It's been a few years since I used them. Never had a problem with inrush. And, IIRC, you pull the _gate-in_ pin low to turn them off, so it is simple to turn it on after main precharge. They have good application notes. A bit pricey product but top quality.

BTW, we used the Vicor 300 to 12V converter on the Lightning bike, so race proven :wink:

 

[dnmun]

if you can determine that the output of the dc converter is isolated from the input then you could use the two sources from each half of the battery pack and combine the outputs together in parallel.

you can check for isolation with the ohmeter on your DVM. look at the ground side which is where they would be connected if true.

i just found a lifepo4 charger where they carried the earth ground from the AC input to the - output terminal. bad luck.

 

[jonescg]

I have been in conversation with Vicor and they were not keen on the idea, however once they learned that I had a centre-tap available they warmed to the idea. I sent through this diagram:



But they don't seem to be happy with the fact that the pack can be isolated into four lower voltages.

Hi Chris,

Thanks for the clarification.
However, we request that the midpoint must be solidly referenced. Making it essentially a ±350V feed to the converters.
Otherwise a potentially lethal condition could occur if a module fails as the input would be connected to the output.

Thanks and regards
Felix

Any thoughts?

 

[Frank]

I don't understand what they are saying (no surprise there) but wouldn't fusing protect against this?

 

[major]

Any thoughts?

I assume the circles on your diagram are fuses. Just move the positive input on the lower converter to join the negative of the upper converter above that center big fuse. Then fuse that mid-tap before it splits to the 2 converters. That way upper converter negative input is always tied to lower converter positive input and the lethal condition is avoided, whatever that was.

 

[dnmun]

they didn't tell him anything he didn't already know.

 

[Holocene]

Hi again,

The railroads solved this problem in the past with motor generators, where a generator with the desired output is driven by an electric motor capable of running on the available input power:

In modern times, these have mostly been replaced by solid-state solutions.

For example, Schaefer Power sells a 600/750 V to 24 V DC-DC converter. Their model MHVC250 provides 250 W output @ 27 VDC, weighs 2.5 kg, and requires no active cooling fan. Input power can vary from 420V to 1100V DC. From their product page:

The MHVC250 design features a wide input range of 420VDC to 975VDC which converts voltage from the third rail to 24VDC for supplying points, signal lamps and electronic monitoring.

MHVC250 product specs are in this PDF:

http://www.schaeferpower.com/pdf/mhvc250.pdf

Good luck with your project. Cheers! 8)

 

[jonescg]

Any thoughts?

I assume the circles on your diagram are fuses. Just move the positive input on the lower converter to join the negative of the upper converter above that center big fuse. Then fuse that mid-tap before it splits to the 2 converters. That way upper converter negative input is always tied to lower converter positive input and the lethal condition is avoided, whatever that was.

No, they are gigavac contactors. There's fuses in there too, I just didn't draw them.

I guess I can do as you've suggested, and it will ensure that it won't suffer any floating reference. But Travis reckons they could make a 700 V DC input converter. That would be ace.

Holocene - thanks for the history lesson I know that the naval base here in Perth has a couple of huge motor-generator sets for servicing the US vessels as they all run on 60 Hz. So a huge motor converts our 50 Hz power into their 60 Hz. Sloppy, but effective. Oh, and that DC/DC pictured is way too big. I need something the size of a deck of cards (or two).