Transistor Duty Cycle

Can you overdrive transistors in the same way you can overdrive LEDs, if you keep the duty cycle low?

I need to drive 16 LEDs through one PNP transistor, with each LED connected to a constant-current sink of up to 55mA per LED (using Maxim Max6978's to sink), for a total of 880mA through each PNP transistor. But there's a 1/16 duty cycle (this is a 16x16 matrix), so the average power disipation won't be too high.

I'd like to use 2N2907's (TO-18 case) or similar, but the datasheets say the max continuous current allowed is 600mA. The word "continuous" makes me think I could have higher peak currents, but I don't see anything in the datasheets as to what that might be (like I do with LED datasheets, where it's assumed you'll be overdriving them in multiplexed apps). Think it's ok to use the 2N2907 here, or do I need to move up to a higher-power transistor?

thanks in advance,

Eric

Reply to
ecmcn
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When designing circuits, it's customary to DERATE devices rather than overstress them. If you're only gonna build one and you don't care if it fails, go right ahead. If you're building more than one, you're asking for a nightmare.

Do some research on secondary breakdown. You don't say what voltages are involved, so it may or may not apply. Saw some research 30 years ago showing the effects of thermal cycling. Transistor chips have a thermal time constant that depends on the package, heat sink, etc. If you drive 'em within the spec at a rate that maximizes the thermal excursion, few Hz rate as I recall, you can kill 'em in short order...like a few hundred hours.

I'd vote for the bigger transistor. mike

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Reply to
mike

Thanks John and Mike. I'll go with a larger transistor. The ZTX789A definitely looks interesting. One of my reasons for not wanting to just go with a typical 3A transistor was that I didn't want to give up the space that 16 TO-220's would take up.

Eric

Reply to
ecmcn

It'll be OK as long as you provide lots of base current, and some software stall doesn't leave it on full-blast.

The Hfe may not be spec'd at 800 mA, so enough base drive will have to be a guess. But a fatter transistor would simplify life.

Power FETS are great; some are spec'd to handle hundreds of times their DC power dissipation for short pulses.

John

Reply to
John Larkin

The Max6978 has a watchdog that kicks in after 1 sec if my program hangs and stops sending data to the chip, so I'm not too worried about the continuous current.

Is there a common Power FET you'd recommend for this type of application? Anything in a DIP package? Really all I need is a switch. My LEDs drop 4V at 55mA.

thanks, Eric

Reply to
ecmcn

The devices may well stand the abuse, but the main reason for a 600 mA limit is that, above that, the current gain falls off dramatically, so that you need lots more base drive for a little more collector current, and the on saturation voltage goes to hell.

You will be much happier with a ~3A device.

I suggest something like one of these:

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Look at the gain and saturation voltage at .98A, compared to the 2N2907:

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Reply to
John Popelish

Just peek at a Digikey or Mouser catalog. You could use a P-channel, low-threshold ("logic") mosfet, like Zetex or IR or whatever.

John

Reply to
John Larkin

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