current limiting smps

leeps wrote: (snip)

An SMPS is either completely on or completely off, with a passive filter averaging the on and off times to produce the effective output. SMPS current limit acts by sensing the output current and reducing the on time when the current reaches the limiting value.

Switch mode sounds like the way to go. you might start by reviewing some basic switch mode tutorials like:

I didnt want to design my own power supply, unless i could cheaper. I did make a regulated charge pump voltage doubler. It put out regulated

14 volts from 12 and adjusted switching frequency to maintain the voltage, i just limited the switching frequency when the transistors got hot. But i was planning on buying an open frame unit from jameco and was under the impression that if you overload it you fry it so i was wondering if i could current limit it without losing efficiency. Actually i have an idea if the voltage is adjustable could i replace the potentiometer that adjusts the voltage with a digital unit use a one milliohm resister on the output use an opamp to amplify the voltage dropped across the resistor and then adjust output current with the voltage. Being that i want to use this thing as a battery charger the current would be a function of voltage. what do you think?

arrange to hve the switcher reduce power when the output is loaaded,

it seems for that application you really want a regulated current with a voltage limit.

design the switcher for constant current output...

--

Bye.
   Jasen

CCCV chargers are common for Li+/NiMH, but not usually at the voltage you want. I am not personally aware of any COTS supplies that have a constant current constant voltage mode control, so you would probably have to make your own to get a true CCCV charger.

Switch mode is, as you surmise, the way to go, but to implement this properly you would probably need a PWM controller with an external microcontroller to so you can detect the point at which to change from CC to CV mode (plus the other parts you need to make a SMPS).

Your other option is, as Jasen said, to see if you can do constant current with a voltage limiter on the charger, or to see if a different charge regime might work.

Cheers

PeteS

Being that most of the switch modes i found in jameco have a 10% voltage adjustment range i was hoping that replacing the potentiometer that adjusts the voltage with a digital unit hooked up to a micrcocontroller would allow me to have computer controlled (i like saying that) control of the voltage and in turn get control over the current. The current drawn would be a function of the battery voltage and the impedance of the battery, these arent constant by any stretch of the imagination thus the microcontroller. I have a love affair with the pic microcontrollers.

The kind of power supply you'll be getting will have overload protection built in. I went to Jameco's website and found a 15 volt 10 amp open frame power supply, the Meanwell LPP-150-15 (they mean well, they really do). Clicked on the data sheet, and voila, just about the first words I see are "overload protection." Even if you wanted to execute that plan with the sense resistor and feedback to the voltage adjustment, the 10% voltage adjustment available to you isn't going to mean much to a big, thirsty battery. If it can suck up 20 amps at 15 volts it's going to suck up almost that much at 14.85 volts. Know what I'm sayin?

Duh, my bad (arithmetic). Guess I can't tell the difference between

10% and 1%.

If what you want to do is charge batteries, why not just buy a battery charger?

If you really insist on building your own, why not get a switcher that already has a current limit? I definitely would NOT want to hack an off- the-shelf switcher.

Good Luck! Rich

YES, thank you, I love you guys, i went to the meanwell website and they said that this specific supply is good for charging batteries because when overloaded it goes into constant current mode, good stuff thank you

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