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Is a BUX348 a suitable replacement for a 2N3055 in the single transistor flyback driver?

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Ash Small

Thu Jan 13 2011, 04:47AM

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

Is the BUX348 suitable for use in the single transistor flyback circuit?

BUX348:
Polarity: NPN
VCEO: 450V
VEBO: 7V
Cont Collector Current: 45A
Power Dissipation: 300W

2N3055:
Polarity: NPN
VCEO: 60V
VEBO: 7V
Cont Collector Current: 15A
Power Dissipation: 115W

Steve Conner

Thu Jan 13 2011, 11:49AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What is the current gain of the BUX348?

Ash Small

Thu Jan 13 2011, 04:15PM

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

Steve McConner wrote ...

What is the current gain of the BUX348?

8, minimum, if I remember correctly. Same as 2N3055 (minimum 8, nominal 15 ?)

I'm assuming it will be ok to run it at a higher voltage CE, but maybe keep the BE voltage down (bigger resistors)?

They're almost Â£15 from RS though (ST Microelectronics), so I don't want to blow too many. (TO3 package)

EDIT: I've checked, and I think these Hfe figures are incorrect. Apparently the 2N3055 is min 20, max 70, can't confirm what the BUX348 is. The BUR52 is min 20, max 100, though, (not that that is much help, but they all appear to be similar in other respects (BUR52 is 350 Watts but lower max voltage, 350 instead of 450, can't find any vendors for BUR52, though. All these are TO3)

Looks like I might have to get one and measure it. (I'm trying to find a cheaper vendor than RS)

James

Fri Jan 14 2011, 12:50AM

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

That is a really old circuit, around since at least the mid 70s. If you want a self oscillating design I suggest trying the Mazzilli driver. Otherwise use a 555 or a PWM chip like the TL494 or SG3524 driving a modern mosfet. I built a couple of the ones designed by 'uzzors2k' and have been impressed. I added an outboard snubber and can easily push 50V into flybacks and get incredible output.

Arcstarter

Fri Jan 14 2011, 01:52AM

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

While i agree with james, i also understand why some would like to make this circuit. Anyway, most transistors will work, i bet almost anything this one will too.

That is so odd, nowhere can i find the gain! But, with this circuit, gain does not really matter. If the gain was too low, you would just use smaller resistors for higher current while still giving the required bias voltage.

Ash Small

Fri Jan 14 2011, 02:38AM

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

I appreciate your point, James. I probably won't use the single transistor self-oscillating circuit anyway. I'm currently running a 2N3055 (with snubber cap and diode) from a signal generator, so I can play around with the frequency. I'm running the primary from a 4 amp 12 volt battery charger with 5000 microfarads smoothing, and I'm planning to put a snubber cap and diode across the primary.

I'm keeping the power low at the moment to ensure I don't damage the sig. gen. if the transistor blows.

This circuit only seems to operate at 30kHz (The output falls off very quickly either side of 30 kHz)

I have a 48 volt 300 watt supply that I'm planning to use with either a BUX348 or BUR52 and I'm planning to switch the transistor using an astable multivibrator, which will have variable caps and resistors so I can alter the frequency. (Thus not having to risk the sig. gen.)

This may seem to be a strange approach, but I don't want to use IC's, and using a 300 or 350 watt TO3 appeals to me.

I also understand the theory that this circuit uses.

I have some large N27 ferrite 'C' cores and I'm planning to wind secondaries for them, then, once it's working, I plan to modify the circuit from 300 watts to 3 kilowatts, but I want to be able to evaluate each modification as I go, so that I gain a full understanding of what exactly is going on.

The first stage of this will be to get the astable multivibrator to switch two transistors alternately, each one switching a transformer.

I've not yet seen any other circuits that will run two transformers in this way, although I appreciate it could be done with IC's, but these will still need additional components to make them work.

The cores I have are EPCOS B67345.

Proud Mary

Fri Jan 14 2011, 09:27AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Arcstarter wrote ...

That is so odd, nowhere can i find the gain!

That's because the the device is primarily intended as a two state switch - on or off.

James

Fri Jan 14 2011, 09:47AM

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

That's a very expensive transistor! I recommend sticking with much cheaper parts until you get the hang of what you're doing. You *will* blow a few, especially if you're considering pushing into the kW range. It only takes one incident that pushes something outside of the safe operating area for a few nanoseconds and your transistor is an expensive paperweight. High power switchmode supplies are not trivial. Simple in concept, but the devil's in the details.

Have a look at these:

The MK II circuit is the one I've built a couple of, there's even a tidy PCB layout available. All the parts to make it can be had for less than US$10, you can hardly go wrong.

When you want more power, explore the site, he's got some clever designs that look like they'll fit the bill. The TL494 is a push-pull PWM driver that does exactly what you want. The SG3524 is a similar IC that I'm also a fan of.

Ash Small

Sat Jan 15 2011, 05:32AM

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

James wrote ...

That's a very expensive transistor! .

It's cheaper than an IRFP460, and handles more power. (marginally, on both counts)

I appreciate it doesn't switch off as fast, though, which may prove, ultimately, to be the deciding factor here.

Steve Conner

Sat Jan 15 2011, 12:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Since when did an IRFP460 cost Â£15? You should be able to get them for Â£3-5 each.

Yes, it should work better than the 2N3055, almost anything will.

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