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BUP49 transistor - Max frequency?

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Ash Small

Fri Apr 15 2011, 10:52PM

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

Anyone know the maximum operating frequency of a BUP49?

(Basically I want to know if it will work as a flyback driver)

The ''short' datasheet I downloaded says to E-mail Semilab for more info.

Also, I'm considering using two paralleled BUV20's instead, (only 50 amps each instead of 90 amps, but 125 V instead of 80 V) but the dissipated power figures look a bit disappointing, any comments?

(Two BUV20's work out cheaper than one BUP49 from RS, but then there is the problem of synchronization, although there are solutions like using two flybacks driven by an astable multivibrator, etc.)

ScotchTapeLord

Fri Apr 15 2011, 11:33PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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I go by a general rule that states, if the device is good at something, it will brag about it in the datasheet...
I would avoid something with such barren information.

Why do you need 90A current capability for a flyback driver?

Is there a reason why you want to use a BJT instead of a MOSFET?

Ash Small

Fri Apr 15 2011, 11:56PM

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

ScotchTapeLord wrote ...

.
Is there a reason why you want to use a BJT instead of a MOSFET?

I knew someone would say that.

Because it is simpler to drive and I don't need the speed of a MOSFET for a flyback driver, 20-30kHz is enough.

The datasheet mentions 100 kHz, but it also mentions 8 kHz.....I'm a relative newbie but I'd imagine any BJT would cope with 20-30 kHz, but I don't know for certain.

I also like TO3 packages....makes sense to me....all metal = more heat dissipation....but I'm not an expert.

Maybe I'll just get one and try to blow it up....~Â£25 including postage.

Arcstarter

Sat Apr 16 2011, 12:25AM

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

If you are sure you don't need the speed of mosfets, then why are you asking if the transistor is fast enough?

Transistors are not easier to drive than mosfets, they are the same. As a matter of fact i would consider a mosfet easier to drive, considering it does not require a gate resistor. A transistor *does* need one to limit the current on the base. As long as the drive voltage for the mosfet is not too high, you can drive it directly. Not to mention, it requires less power to drive.

Depending on your driver and what kind of performance you expect, this transistor may be too low voltage. Flyback and pushpull both require a switch with around 3.3 times the supply voltage. That would limit you to 24v input if you use the BUP49.

The dissipation is not purely reliant on the mating surface area. It mostly has to do with the die's thermal resistance to the mating surface. The datasheet will tell you the max dissipation.

To answer your question, unless the quality is incredibly terrible, the transistor could handle 20-30KHz.

James

Sat Apr 16 2011, 12:26AM

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

How is a BJT easier to drive? One of the main advantages of mosfets is that they tend to be easier to drive than BJTs. That's also the point behind IGBTs. The gain on large BJTs is usually quite low, so you need a beefy transistor to drive it.

Â£25 is obscenely expensive for a flyback driver. I'm using IRFP450s that I bought on sale for $0.69 each.

Adding response to Arcstarter's post that came in while I was typing mine.

BJTs are current driven, while mosfets are voltage driven. A BJT requires a continuous B-E current flow to stay switched on, while a mosfet or IGBT draws essentially nothing once the gate capacitance has been charged up.

Flyback topologies can produce a lot more than 3.3 times the input voltage. I was seeing ~1kV spikes at the switching device from a 40V input. One of the biggest challenges of a robust flyback driver is dealing with the enormous kickback spikes.

Arcstarter

Sat Apr 16 2011, 12:38AM

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

James wrote ...

Flyback topologies can produce a lot more than 3.3 times the input voltage. I was seeing ~1kV spikes at the switching device from a 40V input. One of the biggest challenges of a robust flyback driver is dealing with the enormous kickback spikes.

That is why a shunt capacitor should be added across the drain and source. Or some sort of snubber. I say that if you are going to bother making a flyback topology driver, you might as well make it quasi-resonant. An incredible increase in efficiency can be had, considering it only requires one extra component.

Ash Small

Sat Apr 16 2011, 12:40AM

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

James wrote ...

.
Â£25 is obscenely expensive for a flyback driver. I'm using IRFP450s that I bought on sale for $0.69 each.

.

IRFP450's are around Â£4 +Â£5 odd for postage here (from RS).....and they are only 14 Amp. (they are 500 V, though) Amp for amp, the BJT's work out better value.

Ash Small

Sat Apr 16 2011, 12:45AM

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

Arcstarter wrote ...

.That is why a shunt capacitor should be added across the drain and source. Or some sort of snubber. I say that if you are going to bother making a flyback topology driver, you might as well make it quasi-resonant. An incredible increase in efficiency can be had, considering it only requires one extra component.

I've added a shunt capacitor and diode to my present 2N3055 driver (15 Amp max, ~Â£2.00 here), I'm considering putting a capacitor (and possibly a diode) across the primary as well, but I'm still trying to understand the full implications.

Arcstarter

Sat Apr 16 2011, 12:54AM

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

14 amps is plenty, using a device with higher current but lower voltage makes no sense to me. 14 amps is plenty, and 500v is as well. Considering you are driving a flyback which cannot handle all that much power, you dont need a transistor with 90 amp rating, and 80v is cutting it really close for pretty much any driver. Unless you want to drive your flyback with 12 volts and really high current to get the same wattage.

Ash Small

Sat Apr 16 2011, 01:09AM

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

Arcstarter wrote ...

14 amps is plenty, using a device with higher current but lower voltage makes no sense to me. 14 amps is plenty, and 500v is as well. Considering you are driving a flyback which cannot handle all that much power, you dont need a transistor with 90 amp rating, and 80v is cutting it really close for pretty much any driver. Unless you want to drive your flyback with 12 volts and really high current to get the same wattage.

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.

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