Switchers as DC-DC converter

ZoNiE · July 2014

Hi Folks, I have a few Solyndra panels I am looking to make a battery charging system with. I have been reading that switching power supplies can also take a DC input although they are usually not labeled as such.

I have so many laptop supplies and other open frame supplies laying around and I was thinking of trying one to reduce the DC voltage to something I can use.

Voc is around 118VDC for each panel, the input on the supplies are 100-240VAC, and some of the power supplies are 16-19VDC out, so with a charge controller, this seems to work in my head. I may wire the inputs after the rectifier in the PWS to eliminate that component entirely.

Most of these supplies are Taiwan/China so I have no Idea about their efficiency, they are Flyback types, so there may be some electrical noise, but since they are made for computers, probably "clean enough".

Has anyone here done anything like this?

BB. · July 2014

Re: Switchers as DC-DC converter

It depends is the answer on powering your power supplies directly from AC.

For a typical 120/240 VAC power supply--Set the switch to 240 VAC (if manual switch) and feed it with approximately:

240 VAC * sqrt(2) for AC peak voltage = 339 VDC from your solar panel array

If you have "Power Factor Corrected" (PFC) input supplies, just feed them with ~100-373 VDC (you do not want to bypass any of the AC input section of these PFC supplies).

As always, there are probably a million different AC to DC power supplies out there--Which ones you have, I don't have a clue and without further information/schematic/model number, etc.), I could not tell you which will work best for you--Or even if they will work reliably (or safely) for your application.

One thing to remember--Solar panels are (more or less) constant current sources. And any of these power supplies you attach are expecting to be connected to an AC (and/or DC) voltage source. Basically, if the power demand of the power supply exceeds the current capability of the solar array (of course, dependent on angle of sun, weather, etc.), the Solar Array voltage will collapse and cause the power supply to go through its restart proceedure (i.e., don't power anything that expects clean power--Like a computer without battery backup).

Take all necessary safety precautions with your experiments. It is easy to explode a capacitor/fry wiring&electronics when using equipment outside its rated/design specifications.

-Bill

jonr · July 2014

Re: Switchers as DC-DC converter

Two panels in series should work with a PFC supply.

Computer supplies I've seen have a voltage sense wire that can be used to adjust the output voltage. But I found regulation surprisingly poor. So you might want to build your own charge controller/regulator that also senses panel voltage and doesn't allow it to drop below ~Vmpp.

Cariboocoot · July 2014

Re: Switchers as DC-DC converter

Bill;

I don't think he's talking about running anything off AC. He's asking about using a switching supply as found on many laptops as a DC to DC buck converter (which is basically what they are). Feeding DC output from PV's into the 'AC side' to get lower DC output which would then be regulated to charge batteries. A sort of poor man's MPPT, except it will utterly lack the ability to track the available input power.

Which is why jonr warns about maintaining panel Voltage. These power supplies operate over a fairly wide range of Voltage, but from a Voltage-based supply not a current-based one.

I think it will be rather hit-and-miss getting a power supply that will do what ZoNiE is after.

BB. · July 2014

Re: Switchers as DC-DC converter

Hi Marc,

I was talking about powering AC input rated power supplies with DC from solar array... And he wants to charge batteries with the output.

As long as we are going into the details (aka "weeds")--Might as well add that many switching power supplies for computers/etc. (not battery chargers) can have a "crow bar" circuit on the output. The idea being if the power supply regulation went bad, a second independent circuit would turn on a transistor on the output of the power supply to "crow bar" the output and prevent damage to the downstream electronics.

Obviously, a "crow bar" circuit on a battery charger's output is not a good thing--Where the battery bank can feed lots of current back into the power supply and smoke your wiring/power supply (and possible fire).

So, look at both the AC input and the DC output of your switching power supply to ensure that everything is safe and compatible.

-Bill

bill von novak · July 2014

Re: Switchers as DC-DC converter

ZoNiE wrote: »

Voc is around 118VDC for each panel, the input on the supplies are 100-240VAC, and some of the power supplies are 16-19VDC out, so with a charge controller, this seems to work in my head.

Might work, but the low voltage is going to be something of a problem. A 100 volt input means they will want to see about 130 volts DC for reliable startup. (Even a supply with a PFC front stage relies on that peak to start up.)

I have done this with Mean-Well power supplies (a common Asian supplier.) They work but there are some caveats:

-You need to keep the voltage within the rating of the supply. For the most common Mean-Well supplies, that's about 130-370 volts for one of their 120/240 volt supplies.

-The adjust range is limited so make sure you find an output range that can cover your battery voltage.

-You will see power supply collapse if the available power is less than the rating of the supply. The power supply will pull as much current as it needs to maintain its output (up to the conversion limit of the power supply) and if the array cannot supply the current the voltage will drop, the LVD will shut it down, voltage will rise, it will restart etc etc.

RCinFLA · July 2014

Re: Switchers as DC-DC converter

A normal switching power supply is designed to work from a low impedance input voltage source not a variable high impedance current source (solar panel). With a big enough input filter capacitor and ensuring the power supply load never exceeds the panels momentary illumination output current peak it will sort of work but not very reliable since the minimum output current from the PV panel is zero and variable up to rated current based on it momentary illumination (as clouds go by).

Normal switching power supplies feedback control system will not handle a high impedance input source. They will just oscillate.

A MPPT controller is a switching power supply that has its feedback control system based not only on output voltage and current but also on input voltage. When the input voltage starts to collapse they adjust back their output current sourcing to keep the input voltage from collapsing.

ZoNiE · July 2014

Re: Switchers as DC-DC converter

Thanks guys, for all the comments. I hadn't considered the VRMS conversion (I'm a little rusty). I have considered a low voltage cutoff relay on the input that also cuts the output basically pulling the PWS out of the circuit completely.

Probably more trouble than it is worth.

Anyone know of a good Inverter for these panels? I need four to run a 600V inverter, I only have three, but want to use one on my trailer and two on the house. I was at intersolar and found a few but on second look, they do not do what I need.

One company CyboEnergy had one that can run when the grid is offline and with no battery. Was running some household appliances. Their website doesn't show the 120V unit although the flyer I picked up does. The rep was talking like I could tie the inputs in series to allow the higher voltage, but I don't think he knew what I needed or didn't know what he was talking about.

They call theirs a mini inverter. Has four separate MPPT inputs so it handles each panel independently to keep the 'trons flowing even if one is shaded and shuts down. You can also mix-n-match DC inputs like two panels and a battery for example. The battery can be charged by the AC output through a separate charger. Interesting, but didn't help me as the input DC voltage was too low.

I wish I could cut the voltage by cutting the rack in half and rewiring in parallel, but each tube generates 118VDC. GAHH!

Switchers as DC-DC converter

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