Controlling Inrush Current in a Power Supply

"D from BC"

** 180 uF and 85 ohms is not a good combination.

The ripple voltage will be huge - about 50% or 80 volts p-p.

I = C dv/dt

........ Phil

oops...maybe I oversimplified that schematic....

+----(high side lim)--+--------+ | | | \\/\\/\\/170VDCpk 180uF reg | | | +-----(low side lim)--+--------+ | | Com1 Com2

reg = DC-DC convertor acting kinda like an 85ohm resistor.

But yeah...there's lots of ripple but the convertor doesn't drop out. D from BC

"D from BC"

** Have some consideration for the poor, 180uF cap.

A 2 to 5 ohm NTC is all you need for inrush surge limiting - one rated for about 4 to 5 amps max will be fine.

....... Phil

oh crap....took awhile to sink in.. Not 85ohms....there's a load of 260ohms across the cap. Anyways...I don't think it'll affect the surge current issue too much.

Either way, there's still going to be a big current spike when power is applied. D from BC

Yeah...high ripple current---> self heating--->shorter lifespan I goofed the load value and posted that prior to this post.

About the thermistor.. Wow.. only 2 to 5 ohm R @25C (for the original posted circuit)

I was thinking if I clicked on the power switch just at the mains peak then the current spikes to:

170V/5ohm = 34 amps

However...I suspect that's about as good as it gets...

D from BC

NTC thermistor makes a good inrush current limiter, but it only limits it when its cold, as it heats up it disipates less energy, its best to insulate it so it doesnt wats emuch enrgy, however it then takes a long time to cool down.

when its hot a short power interruption will give no protection against inrush.

a smart way is to have a mosfet acros the NTC, well actually we used relay back in the days ....

ofc if you have a mosfet there it could control the inrush all on its own anyway.

Colin =^.^=

"D from BC"

** Most folk make the mistake of thinking inrush surges ought to be completely eliminated.

Why ?????

A 250 watt incandescent lamp surge is up to 35 amps at switch on.

A 300 VA transformer surge is up to 100 amps at switch on.

A TV set or CRT monitor surges up to 50 - 100 amps at switch on, due to the de-gauss circuit.

Each of the above are household items PLUS the inrush surge duration they generate is ** waaaaay ** longer than that of a SMPS cap charging.

The most insidious example around now are CFLs ( ie compact fluoros) where the inrush surge is anything up to 50 amps for a lamp that draws only 20 watts when running !!

Put 10 or 20 on the same domestic lighting circuit and see what happens to the switch and cct breaker.

....... Phil

It's certainly on the flip side to thermally insulate a part.. Usually I'm bolting semi's to heatsinks..

NTC across the mosfet?? Like:

+---------------------+-----+ | | | \\/\\/\\/170VDCpk 180uF Rreg | | | +-----NTC----+--------+-----+ | | | +-----S[_D---+ Com2 | G | | |--?----| Com1

I'm not sure what the control is like... I'm guessing the NTC reaches max conduction and then the mosfet turns on with a latch circuit. This lets the NTC to cool down to R 25C. A mains power disruption resets the latch...When power is back on..the surge current is limited by the NTC.

Is that the idea? D from BC

"colin"

** Using a relay (or mosfet ?? ) gives no protection against inrush surges if the interruption is briefer than the time to reset the device to its open condition and AC supply interruptions can be as brief as only 1 cycle.

If the inrush surge current of some item is so * horrendous * that it must never happen, then a missing cycle detector is essential, followed by instant shut down and eventual re-start of the equipment.

See my other post re widespread inrush surge paranoia.

BTW

Use a damn spell checker - there are no less that six errors in the above quote from you.

....... Phil

I didn't see the trees in the forest :)

Thanks

D from BC

yeah thats the idea, the mosfet just turns on as soon as the caps have fully charged. maybe just a short delay. the mosfet will ofc see the full 170v. the ntc will disipate whatever power it needs to reach a certain temperature, so bolting it to a heatsink will just make it disipate loads of power.

insulating it will not make it run much hotter. you can get them poted in an insulating capsule.

the mosfet just makes it so it limits inrush during a short power interuption, and saves a little bit of power, most equipment that claims to be inrush protected just has the NTC.

Colin =^.^=

note...

limiting

If you have the room, great big inductors are kind of nice for inrush limiting. You put a low resistance in parallel with them so that the resistance charges the capacitors before the inductor saturates.

I have seen a truely huge case where the capacitors were charged up in

3 steps with relays shorting out resistances. The relays were closed as the voltage neared fully charged.

note...

limiting

Nah... The inrush problem is in the front end of an offline smps. I did alot of work to reduce the power inductors for the smps... Not gonna ruin the design with a relatively massive inductor. It's like a tumor.

I going to use a thermistor.

But I'm still interested in alternative ideas...

D from BC

How much do you expect the cap voltage to drop in one cycle? If it's only

10's of volts, it's not going to be the full inrush.

If it's what I think it is.. its zero crossing turn on inrush control.

The inrush limiter can kick in on the next zero or the zero after the power is turned back on.

But yeah...A brief power disruption with that type of inrush control won't let the bulk caps discharge much and there will be a lesser surge upon power up. Assuming the load goes hi-Z during bulk capacitor undervoltage. D from BC

"Matthew Beasley" "Phil Allison"

** The above applies to transformer isolated PSUs where the major source of inrush current is due to magnetic saturation of the iron core.

With a 1 kVA transformer, the initial urge peak can be many hundreds of amps, subsequent peaks are of all of the same polarity and extend for about

20 cycles before the event is over.

Typical "soft start" schemes seen involve high power resistors, NTCs, relays, triacs etc but normally fail to work if the interruption is only a few cycles. A blown AC fuse is then the result.

...... Phil

It's basically cold resistance, maximum current rating and maximum energy rating.

You're looking for a >2.5A, >5R part with 'minimal' energy limit requirements.(often stated as capacitance at operating voltage). If ripple current is high, get an accurate rms measurement and act accordingly.

Energy handling capacity determines thermistor diameter, so anything over 11mm rated for inrush limiting will likely serve in this case. The smaller parts have the advantage of diissipating less energy to reach their quiescent operating temperature (ie improved efficiency).

Active inrush limiting is another subject.

RL

note...

limiting

Extra heat! Ick poo ick poo ick poo :)

A resistor and an SCR in parallel, I think, is one of the better ways to go. You delay the start on the DC-DC until the capacitor is certain to be charged. An extra winding on the transformer triggers the SCR when the DC-DC starts. It is a low parts count way to go.