Transistor Duty Cycle

Guest
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
 
ecmcn@yahoo.com wrote:
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
Click to expand...
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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ecmcn@yahoo.com wrote:
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

The devices may well stand the abuse, but the main reason for a 600 mA
Click to expand...
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:
http://www.zetex.com/3.0/pdf/ZTX789A.pdf

Look at the gain and saturation voltage at .98A, compared to the 2N2907:
http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/2N2905A,%202N2907A.pdf
 
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
 
On 11 Jul 2005 12:09:47 -0700, ecmcn@yahoo.com wrote:

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
Click to expand...

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
 
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
 
On 11 Jul 2005 17:07:03 -0700, ecmcn@yahoo.com wrote:

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.
Click to expand...
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
 
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