Feeding a SMPS with DC instead of mains

[jmaf]

(I guess this qualifies as circuit bending? I came to post this under the lounge or offtopic stuff. Please move if wrong? Thanks!)

A friend of mine built a 220 V DC battery. He's experimenting with some solar power stuff. (I've warned him about safety and 220 VDC with no circuit breaker at the terminals, he says he'll be careful.)

So i had the idea that a SPMS' first thing is always a rectifier (right?), so....the 220V mains fed into it is immediately rectified before being oscillated?

Is there any problem with feeding a switch mode PS with 220 VDC directly instead of 220 VAC RMS mains? Will it damage anything if used continuously with DC instead of mains?

[nickb]

There are some things to think about when feeding DC. Don't forget a fuse, a common mode filter and some filter caps. The filter caps are partially to give a low impedance source at the switch and also to prevent EMI. Another consideration is the 220VAC will rectify to give about 300V DC. In other words 220V DC isn't enough and that can cause major problem if the switcher has not been designed to operate at that voltage.

Another consideration is that some SMPS's (mainly lower voltage input ~120VAC types AFAIK) use a voltage doubler and that requires an AC input.

[jmaf]

There are some things to think about when feeding DC. Don't forget a fuse, a common mode filter and some filter caps. The filter caps are partially to give a low impedance source at the switch and also to prevent EMI. Another consideration is the 220VAC will rectify to give about 330V DC. In other words 220V DC isn't enough and that can cause major problem if the switcher has not been designed to operate at that voltage.

Another consideration is that some SMPS's (mainly lower voltage input ~120VAC types AFAIK) use a voltage doubler and that requires an AC input.

All great points indeed. Thank you.

[glebert]

I actually think of SMPS as working MOSTLY on DC, but then again I was working in the PC industry where we pretty much assumed everthing to be DC. Don't know about at 220VDC though.

[Enzo]

Yes, an SMPS is DC after the rectifier, but every PC I worked with plugged into the AC mains. Lap tops run on low DC.

[glebert]

This is classic SMPS DC-DC conversion: https://en.wikipedia.org/wiki/Buck_converter

[Enzo]

Ah, we may be talking past one another.

Yes, that buck takes a DC voltage and works with it. But where did that DC come from? In our amps, the DC supply is switched off and on through the primary of a transformer. But that DC is made by rectifying the mains AC.

[jmaf]

Yes, an SMPS is DC after the rectifier, but every PC I worked with plugged into the AC mains. Lap tops run on low DC.

Thanks Enzo. Yeah this is definitely unconventional. I've never done it, though should be simple to test at the bench, problem is I'm not near the shop now. Can't test. So I wanted to bounce the idea off you guys before I mushroom-clouded a SMPS.

This is classic SMPS DC-DC conversion: https://en.wikipedia.org/wiki/Buck_converter

This definitely a good analogy. What I'm asking about is just a buck converter after all, duh!

I just gotta look for what nickb said about it having some AC-only circuit at the input, doubler and so on. Other than that should be cookbook buck converter.

I got off the phone with some more feedback, he said it's a 90 to 240 VAC automatic SMPS, so the other problem nickb said about it working on lower voltage won't be an issue I guess, probably has a voltage controlled oscillator that'll increase PWM frequency for higher voltages or something like that.

[jmaf]

Ah, we may be talking past one another.

Yes, that buck takes a DC voltage and works with it. But where did that DC come from? In our amps, the DC supply is switched off and on through the primary of a transformer. But that DC is made by rectifying the mains AC.

Exactly. I'm thinking of injecting straight DC where the AC would be....where mains would be... Instead of pushing in 220 VAC I'd give it 220 VDC, the rectifier would be redundant...

Sorry for double post, Enzo replied while I typed.

[J M Fahey]

220V DC and 220V AC are completely different.
111V different to be precise.
Now that I gave you the clue, YOU tell me why
Proof you actually read and understood it

I suggest you read a little on how Offline SMPS work, to save you reinventing the wheel time and time again.

(I've warned him about safety and 220 VDC with no circuit breaker at the terminals, he says he'll be careful.)

famous last words

[jmaf]

220V DC and 220V AC are completely different.
108V different to be precise.
Now that I gave you the clue, YOU tell me why
Proof you actually read and understood it

Thanks for your input. nickb mentioned this too. But this supply has a voltage-controlled oscillator, it'll work with any voltage from 90 to 240 VAC.

famous last words

LOL

But yeah I'm concerned. I told him so many times.

[cerrem]

You should be fine with DC input if this SMPS is less than 65W.......
Over 65W then you have Power Factor Correction on the front end of the switcher....depending on circuit used you should be OK..
Best to get schematic..
I recently designed a 5kW SMPS for a MIL application....
One requirement was that it needed to accept both AC or DC input...
The AC input frequency range had to be huge...DC to 100kHz ...
The 90V to 240V range is standard for commercial SMPS... For Dell I designed them from 80 to 300V to account for India's wild grid voltage swings...
The frequency usually is fixed on most commercial SMPS....It's the Duty cycle range that changes...
It becomes tricky at very low duty cycle , less than 10%, to control the inner current loop, so instead the Off-Time ramp is used to predict the On-Time slope ....
Best to get a schematic....
Can you vary this 220V DC you have planned to use ??

[glebert]

I think the 220VDC is a battery bank.

[jmaf]

You should be fine with DC input if this SMPS is less than 65W.......
Over 65W then you have Power Factor Correction on the front end of the switcher....depending on circuit used you should be OK..
Best to get schematic..
I recently designed a 5kW SMPS for a MIL application....
One requirement was that it needed to accept both AC or DC input...
The AC input frequency range had to be huge...DC to 100kHz ...
The 90V to 240V range is standard for commercial SMPS... For Dell I designed them from 80 to 300V to account for India's wild grid voltage swings...
The frequency usually is fixed on most commercial SMPS....It's the Duty cycle range that changes...
It becomes tricky at very low duty cycle , less than 10%, to control the inner current loop, so instead the Off-Time ramp is used to predict the On-Time slope ....
Best to get a schematic....
Can you vary this 220V DC you have planned to use ??

That's awesome info, thank you.

I think he spot-soldered a ton of batteries together, I haven't seen it in person. I guess he could derive taps along the way if several DC levels are needed, assuming he didn't pot them in gook. Would lower voltage taps be useful somehow?

[nickb]

Thanks for your input. nickb mentioned this too. But this supply has a voltage-controlled oscillator, it'll work with any voltage from 90 to 240 VAC.

That does not mean there is not a doubler that is switched in as needed. Measure twice, cut once...