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Need help with current limiting.

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hen918

Mon Jan 11 2016, 06:22PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

That voltage control loop in your latest post is the standard isolated feedback loop in almost anything that needs one, from 3000W step up DC/DC converters, to 300mW current ticklers. There are many variants, most with capacitors to improve the start up characteristics, but what you have in that picture will work. (Buy some TL431s; after the 555 timer, I would vote them the second most useful IC. Ever. Just look at the datasheet...)

Patrick

Mon Jan 11 2016, 08:17PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I realize how valuable the TL431's are. I just which I had some, ill put them in my next order to Mouser. Is there a simple way to replace it with a 5.1 Zener ?

I'm thinking like this one, but with a Zener for now.

EDIT:
I'm planning to use a 5.1 zener into the PS2561-1. ic, so with 12 volts in and 5.1 + 1.25 Vf = 6.35v / 0.03mA = 220 ohms in series to current limit it. not sure how all this works, I'm just randomly hoping here.

Patrick

Wed Jan 13 2016, 08:33PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ive pushed up the Mouser order.

Shunt - TL431CZT @ 20 pcs
Mosfet - FQP19N20C @ 12 pcs
Caps - REA100M1HBK0511P @ 100 pcs
Caps - RS1HH101MPD @ 30 pcs
Caps - QYX1H103KTP @ 20 pcs

I couldn't find a TVS to protect this mosfet at my operating conditions.

DerAlbi

Thu Jan 14 2016, 05:06AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Heeey! Cool that you went fort the Mosfet! I hope it serves you well (and the specifications i based my search on was actually correct)
The mofset is, as you can see in the datasheet, repetetive avalance rated. So you dont need any form of protection compared to an IGBT. As long as your gate driver is safe (<30V) of course...

Btw: your foil capacitors (QYX1H103KTP) datasheet says on its first line: "Inductive construction". What?? Do you really want that?
100x 10uF: The energy density increases usually when you order bigger caps. Or just the right cap. You dont need to parallel many electrolytics to have better performance. Their inherent ESR and ESL are better compensated by the parallel use of a different capacitor technology. This is of course only relevant if my assumption is correct that you plan to parallel them and not just to have them laying around for general purpose. I couldnt find anything with "RS1HH101MPD" on google. If thats for your supply please excuse my lecturing.

Patrick

Thu Jan 14 2016, 07:39AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

don't feel you are lecturing, I need help to get this one right as it runs off 120 VAC, and is high power.

ill look at the caps.

DerAlbi

Thu Jan 14 2016, 07:56AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Good you mentioned the 120Vac.... are you actually sure you have only 170V on your mosfets when they are off? I am not up to date with the current schematic however i am feeling if you have a Vin=170V you often need to switch more than that. In your initial post the Push-Pull-Converter requires 2x Vin Voltage-capability on the switches. Am i wrong?

The foil capacitors are hillarious. Not that you chose them, but its basically the first time that a manufacturer is just honest^^. Usually they advertise "noninductive construction" but they never just say "inductive" when its not noninductive...

Patrick

Thu Jan 14 2016, 08:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

yes 170 DC input.
so that means 340 ?

let me look at topology. From here :

if I remember right some of the voltage gets added at switch off, I forgot that.

DerAlbi

Thu Jan 14 2016, 05:06PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Well.. its right there in the picture... "Max Switch voltage = 2xVin." So with a 200V mosfet you are toast. Go look for 400V.

Edit: ..or let me just look...
Have fun with:
IPP50R190CE
FDP26N40 <--- i prefer
FDP20N50
FDP24N40
FDP19N40

sorted by price @Digikey. Mouser might be different. Sorry to say.. but double voltage is inherently more expensive. Searched for THT, Vds=>400V, Rdson <250mOhm.

if you want to go cheap (rdson<500mOhm which is bad, then)
IPP50R500CE (500mOhm)
IPP50R380CE (380mOhm)

The one i makred "prefered" i have checked the Datasheet. Its fully avalanche rated, so its ok. But all the others migt be ok too.. i just wont check every Datasheet for now.. but if you tell me what you decide...

Patrick

Thu Jan 14 2016, 06:23PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ill use the FDP26N40 mosfet, its $2.01, ill get 12.

Ill get some 100 volt schottky's too.

hen918

Thu Jan 14 2016, 07:41PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

What output voltage are you going for? Remember that the output diodes will have to withstand at least the maximum input times the winding ratio, plus a good safety margin.

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