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BUP49 transistor - Max frequency?

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Patrick

Sat Apr 16 2011, 01:21AM

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

Ash Small wrote ...

Here is a photo of the cores I plan to use, with an 'average' flyback core for comparison. Rated for 1.5 kW in flyback mode by the manufacturer. 12 V @ 90 Amps = ~1kW.

you found a manufacturer who will rate their cores by watts? really, I have never seen this before. whow is the maker for that large core?

Arcstarter

Sat Apr 16 2011, 01:24AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well you couldn't run 90 amps RMS, that would be well over the devices SOA. Even if the die could handle the current, the max dissipation would likely be exceeded. An irfp450 would be a lot cheaper, and a lot better. What do you think makes the bup49 more suitable than a mosfet?

And as for the 12v 90 amps = 1kw, 7 amps and 151 volts is also 1kw. The difference is, you could actually run 151v and 7 amps RMS without exceeding the devises SOA with the IRFP450.

James

Sat Apr 16 2011, 01:25AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

You will still need a high voltage rating, even with a snubber, and you will need a BIG snubber. To get a little perspective, my IRFP450 based driver with ~50V feeding it was dissipating ~100W in the snubber and the spikes were still dangerously close to the limit.

I've never heard of the flyback topology being used for anything larger than about 100W, it makes no sense. You would be far better off to use some sort of push-pull drive.

I'm a bit baffled though. You came asking questions, said that you're a newbie, and then have shot down all the advice you have been given by knowledgeable folks who are experienced with this stuff. If you've already made up your mind then why are you asking? Seems like it would sting each time you blow a Â£25 transistor, and believe me you will. Whether mosfet, IGBT or BJT, one spike of voltage or current beyond the absolute max rating and zap, your device is instantly toasted. Go ahead and try it though, best of luck.

Oh and forget RS, if you don't have any local surplus sources look on ebay, I've bought piles of mosfets and IGBTs from various sellers, usually less than $2-$3 each, sometimes much less. I like to buy at least 10 at a time because I invariably blow a few while I'm working the kinks out of any new design.

Ash Small

Sat Apr 16 2011, 01:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

.you found a manufacturer who will rate their cores by watts? really, I have never seen this before. whow is the maker for that large core?

EPCOS (Seimens-Matsushita).....I've posted the details elsewhere, I'll see if I can find a link and add it as an edit.

EDIT: James, I'm not 'shooting down' advice, I'm questioning it. I apologise if I come across as being confrontational, that is not my intention. And I appreciate your (and Arcstarter's) input.

My original question was 'will a BUP49 be fast enough?'......I've only ever blown one 2N3055, and that's when I ran it without a heatsink.

EDIT: Patrick, this should be the info you require: (1492 watts @ 25 kilohertz in flyback mode)

Ash Small

Sat Apr 16 2011, 01:50AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Arcstarter wrote ...

.
And as for the 12v 90 amps = 1kw, 7 amps and 151 volts is also 1kw. The difference is, you could actually run 151v and 7 amps RMS without exceeding the devises SOA with the IRFP450.

Firstly, I want to keep the voltage as low as possible, and secondly, My 'basic' understanding of flyback circuits suggests they rely on current (ie they do not multiply voltage in the ratio of primary to secondary windings).

James

Sat Apr 16 2011, 02:16AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

While my understanding is admittedly less than perfect, here's what I know. If anyone can correct me or add more, please do.

Flyback topology works by the principal that if you pass current through a winding, you cause a magnetic flux to build up. When the switch opens, the field collapses and the energy has to go somewhere, so it converts back into a voltage in the winding. The amplitude of this voltage depends on how quickly the field collapses, hence the spike can be far greater than the input voltage.

A flyback transformer is effectively two coupled inductors. The spike created in the primary winding is induced in the secondary and stepped up according to the turns ratio. It's the spike created by the flux in the primary inductor collapsing that is responsible for both the very high output relative to the total turns involved, and the very large spike created across the switching device. There are some good application notes out there regarding flyback topology SMPS design which will apply here. It is a very simple topology, but as sizes get much over 100W things begin to get tricky, hence push-pull or bridge topologies are used almost invariably in commercial designs.

Patrick

Sat Apr 16 2011, 02:18AM

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

Ash Small wrote ...

EDIT: Patrick, this should be the info you require: (1492 watts @ 25 kilohertz in flyback mode)

I retract my previous statement, i too have seen this manufacturers datasheets, but forgot. though youll still have to do some math on the magnetic physics, im doubtful youll get 90 amps in to that core with a few turns, and avoid saturation.

Ash Small

Sat Apr 16 2011, 02:29AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

James wrote ...

While my understanding is admittedly less than perfect, here's what I know. If anyone can correct me or add more, please do.

Flyback topology works by the principal that if you pass current through a winding, you cause a magnetic flux to build up. When the switch opens, the field collapses and the energy has to go somewhere, so it converts back into a voltage in the winding. The amplitude of this voltage depends on how quickly the field collapses, hence the spike can be far greater than the input voltage.

A flyback transformer is effectively two coupled inductors. The spike created in the primary winding is induced in the secondary and stepped up according to the turns ratio. It's the spike created by the flux in the primary inductor collapsing that is responsible for both the very high output relative to the total turns involved, and the very large spike created across the switching device. There are some good application notes out there regarding flyback topology SMPS design which will apply here. It is a very simple topology, but as sizes get much over 100W things begin to get tricky, hence push-pull or bridge topologies are used almost invariably in commercial designs.

That's pretty much how I understand it James, but I understood it was the 'change in current' or 'rate of change in current' that induces the voltage spike.

I understand that BJT's take longer to switch off than MOSFET's, so the rate of change in current (time taken for 'field collapse' may be longer, but no-one has mentioned this, (this is something I'm not sure about)

Won't a larger current in fewer primary windings induce a higher voltage in the secondary? (according to your reasoning above)

Ash Small

Sat Apr 16 2011, 02:33AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

. I retract my previous statement, i too have seen this manufacturers datasheets, but forgot.

Probably last time I posted them here

Patrick wrote ...

though youll still have to do some math on the magnetic physics, im doubtful youll get 90 amps in to that core with a few turns, and avoid saturation.

I doubt I'll get 90 Amps in.....I want SOME safety margin for the transistor.....Can I avoid saturation by gapping the cores?

James

Sat Apr 16 2011, 02:50AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

As far as I know, the transformer HAS to be gapped for flyback converter to work. Increasing the gap reduces the permeability of the core and increases leakage inductance.

Here's some info you might find useful.

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