Induction Cooking Table : IGBT keeping to short !

In article , cLx

On Mon, 19 Nov 2012 19:50:17 +0100, cLx

I thought about that, but i'm afraid to choose something which would present more gate capacitance than the originals IRGP4068D. I've closely inspected the Kapton isolation, but new thermal paste under it already.

cLx

Thanks for your advice. It's polypropylene caps I guess. There is a picture :

Any recommended source ?

I'll also change the two "snubber like" 47nF MKP capacitors (mounted from the common node of transistors and coil to both supply rails).

On Tue, 20 Nov 2012 21:28:14 +0100, cLx There is a picture :

This appears to be the manufacturer's product page:

Curiously, the construction is "MFP" which is Metallized Polypropylene Film, but the datasheet for the type, B32669, lists it as "MKP", and the lowest capacitance is 1uF.

I'd try Mouser, Digikey, Farnell, RS Components.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

Sometimes a call to the company or the service center will get an answer like, " oh ya, you need to replace #%&#@ or it will keep shorting the bottom one. Mikek

On Nov 19, 11:50 am, cLx

The big problem with IGBTs is they are a parallel structure of many bipolar transistors, with POSITIVE temperature coefficient. So, they do not balance current across the die well. When driven hard into saturation, they balance better. So, the secret an IR app engineer told me years ago is you have to drive them hard into saturation, and never allow them to stay in the linear region for more than a few ns, if possible. (You can only do so much of this during turnoff, however.)

Jon

their

was

hold

in

Is there any rules for substituting powerFETs for IGBTs in such situations that you had there, assuming you are not averse to going against the designer's wishes?

their

Thanks for the insight, I'll try to remember the warning

I did not designed the gate control circuit on that applicance... I hope there is no problem there.

The main reason is that they're considerably cheaper when you need both high voltage rating _and_ high current rating. That's because they use less silicon die area. On consumer products the cheapest component that will do the job acceptably well usually gets designed in. Try pricing a 30A 800V MOSFET vs. a similar rating in a IGBT.. the MOSFET will probably be 5x the price.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Assuming there is space to put 2x powerFETs in place and the switching f is not beyond sensible powerFET territory, other than adjusting the gate drive , any other considerations ?

available

reward"

is

drive

Since IGBTs tend to be rather slow, power fet speed is not normally the issue. Just the same IGBTs almost may be called IG-GTOs.

?-)

Below 400 V there is probably no benefit to an IGBT. At 400 V and above, MOSFETs start to show a higher on-resistance. The higher the breakdown voltage, the higher the on resistance. IGBTs have a constant Vce drop of about 2V or better, up to several times continuous rated current. That starts to look real good in serious power devices like motor drives.

IGBTs in an audio amp sounds really odd to me, I'll bet a set of FETs could have been dropped into it perfectly.

Jon

"Jon Elson"

** Switching PSUs in many high powered audio amps use IGBTs - often in parallel groups. The topology is a driven, square wave inverter - no feedback or regulation is needed.

The dodgiest time is at first switch on, when the filter electros present a dead short to the DC output. Usually, the drive wave begins with a very low duty cycle ramping slowly up to full square wave. Drive frequencies are in the order of 100kHz to 150kHz.

Readily available, low cost mosfets are not in the game.

.... Phil

OK, in the off-line DC power supply section, this makes perfect sense! Not in the audio amp section. (You can probably tell I rarely get into late-model consumer audio gear.)

Thanks,

Jon

** But no body said that they were used there, you misread Arthur's post.

" A while ago, I had the misfortune to work on a switcher that was in a Yammy powered speaker."

The "switcher " is the PSU.

.... Phil

I am not responding to you here Phil, but the whole bunch.

Everyone wants to get into all this esoteric shit here and really an induct ion top doesn't run at microwave speeds. Simple RF. I mean television SMPS RF.

The bottom keeps shorting, well lessee here, what can cause one transistor in a totem type pole circuit to short ? This is a switcher. Can an imbalanc e cause a problem ?

First of all the drive. Now you might not be able to compare the drive to t he top transistor without diff inputs or all that, but yuo should stil be a ble to tell. If the duty cycle is 50/50 then the E-B or S-G voltage will re ad the same on a voltmeter, which can be floated easily.

Once a 50 % duty cycle is conformed then the DC reading confirms equal driv e. I ASSUME both transistors were changed at the same time. If not, shame o n you.

There will be some snubber caps, and then there are the coupling caps. If i t ain't drive it is load, PERIOD.

You got the bottom Xstr shorting, look at the TOP coupling cap. Leakage the re would never bother the top Xstr. But the bottom might have a problem wit h it. The capacitor decides to be a resistor. C'mon you old folks, tell eve ryone about it.

Know what else ? If you can't get the same rating caps, so what ? Most like ly they were chosen by price. Engineers might even make adjustments in the operating frequency to accomodate lower value caps, if it saves the company money.

You got 0.68uF and you are afraid to use 1uFs ? Just use them as long as th e current capabilities are up to snuff. The value does not mean shit as lon g as it doesn't go too low.

Use whatever, within reason, just make them equal. They must be equal, othe rwise the engineer would have just used one cap instead of two.

Kapeesh ? Think of it from their end.

J