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Old 3rd June 2012, 03:35 AM   #1
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Lightbulb Using power BJT's beyond their rated (but below their measured) Vceo?

I'm looking for comments and feedback about the following idea:

What are the possibilities of using power BJT's closer to their measured Vce (or Vceo) breakdown voltage, but beyond their rated breakdown voltages? Naturally, they have to be used within their other stated parameters - namely their SOA. What are some of the potential pit-falls or concerns I should be aware of?

Here's how I came upon this:

I designed and built a DC junction breakdown tester (I could post a schematic and/or design information if there is interest) so I could measure the specs of unknown devices. It measures at either 25 or 100mA with a breakdown potential at either 160 or 320VDC. I then began measuring lots of parts. While measuring power transistors, there was no surprise to see that many parts measured within 10 to 20% of their rated voltage. A couple percent of devices unfortunately measured below their rated voltages too. But a lot of power BJT's measured significantly higher breakdown voltages than their ratings. Some were as high as 250% above their ratings! Plus, when I added some resistance from base to emitter, the breakdown voltage naturally increased. I was also surprised that when I heated the device, approaching 80C, the breakdown voltage further increased slightly.

This is certainly no surprise given how semiconductor manufacturers measure and rate their devices offering a spectrum of similar parts with the higher breakdown devices costing more. This is also why the data sheets state that some devices' parameters are listed as "guaranteed minimum..." with their actual voltages being some margin of error higher to compensate for real-world variations in manufacturing.

Here's some examples of parts I measured vs. their measured breakdowns:
TIP35C/TIP36C - rated 100Vceo, measured at up to 210V
MJE15030/MJE15031 - rated 150Vceo, measured at up to 220V
MJH11021/MJH11022 - rated 250Vceo, measured at up to >320V
And the list goes on.

My thoughts aren't to build monster amps and push the devices right to their hairy edge of breakdown, but to build amps with rails closer to the devices' ratings knowing there is a margin of error beyond its rating. It would also allow a wider range devices to work as a cross. Still, when using TIP35/36C output devices, I could use +/-50V and still be safe instead of +/-35, like I've done in the past.

I think I'll experiment with some devices and really push them to see how they perform.

I'd appreciate your comments, thoughts, experience, and suggestions.

Thanks,
Paul
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Old 3rd June 2012, 05:26 AM   #2
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Remember that the secondary-breakdown curves on the TIP35/36 start folding back above 30V, the MJH11021/22 fold back above 40V.
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Old 3rd June 2012, 06:07 AM   #3
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@ djk

No. I measured Vceo and Vce with the base shorted to the emitter. Would you mind explaining what the significance of SOA curves "folding back?" It's been quite a few years since my Advanced Linear class in college. LOL Looking at the SOA curves on the data sheets, I would naturally extrapolate the acceptable current at the higher Vce making sure I'm under the curve. And, for output devices, parallel them as necessary to ensure dependable operation even under worst-case reactive loading.

@ CBS240

Regarding the % of fails, I would only use devices beyond their rated voltage after I hand-measured each one and new what it could handle. Sorry if I wasn't clear about that. But just like hand-matching paralleled output devices or differential pairs for hfe and Vbe, I would also hand-measure the actual Vceo breakdown to ensure a good margin-of-error above my desired rails.

Regarding selecting the proper device for the job, that too would be the case for an OEM or new design - which I've done many times. The specific devices I listed I already have a good number of in stock. I'm just a DIY'er doing what I can, with what I have, and like most of us, always trying to push the boundaries of quality, performance, creativity, and function. I'm also trying to learn, help others, and get my hands dirty building audio gear (and robotics, but that's for a different forum).
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Old 3rd June 2012, 06:18 AM   #4
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I don't think that would work. SOA has to do with current bunching on the die at a specific actual Vce. The fact that a particular device would survive a higher-than-advertised Vce doesn't increase the SOA.

jan didden
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Old 3rd June 2012, 06:27 AM   #5
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@ janneman

Understood. Naturally at higher Vce the current would drop by the square of the increased voltage. Is this what you meant? It's why I mentioned the need for additional paralleled devices for the same load.
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Old 3rd June 2012, 06:54 AM   #6
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"Would you mind explaining what the significance of SOA curves "folding back?" "

MJH11021 is 150W at 40V but only 40W at 100V, the power rating starts to fold-back above 40V.

As janneman points out the hot-spots on the die get worse at higher voltages.

Vceo is kind of meaningless in an audio amplifier when it is forward-biased under no-signal conditions.
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Old 3rd June 2012, 07:07 AM   #7
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well ... here is my 2 cents

as about small devices ....lets say in a line of amplifier from input till drivers it normally pays to run semis as close to rated voltage .. most of them perform the best when working on the edge
seen goldmund amps running 100 v semis to 93 volts seen hardman cardon running drivers also too close to the maximum and since power wasn't enough use 2 of them in parallel to achieve the target .

Point is that in DC conditions you might get away with it ,... If the duty is steady and the conditions stabilized then you also might get away with it .

Point also is that in audio amplifiers there is way too many variables that actually decrease SOA of the all project ...number one variable is abuse .....

Also when it comes to long run you need also to take in mind thermals ,,,seen many of these amps with thermal/pcb/soldering issues when the choice is to run semis hot. Add to this the stabilization issues when semis are in parallel .

Finally .... to my understanding high quality audio amp ( with bjt class AB ) transistors can only be made with a couple of transistors only ( preferably sziklai ha ha ha ) increasing the number of outputs to get more power will degrade the quality ... to a small but existing ratio ......

Kind regards
sakis
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Old 3rd June 2012, 07:21 AM   #8
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Quote:
Originally Posted by DCPreamp View Post
@ janneman

Understood. Naturally at higher Vce the current would drop by the square of the increased voltage. Is this what you meant?
No, I meant the SOA limitation.
Like djk mentioned.

jan
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Old 3rd June 2012, 07:23 AM   #9
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Quote:
Originally Posted by sakis View Post
it normally pays to run semis as close to rated voltage .. most of them perform the best when working on the edge
No this is incorrect. The fact that many manufacturers do that has to do with economics not with better performance. Most semi parameters deteriorate with higher voltage. Datasheets show that.

jan
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Old 3rd June 2012, 07:25 AM   #10
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Quote:
Originally Posted by sakis View Post
increasing the number of outputs to get more power will degrade the quality ... to a small but existing ratio ......

Kind regards
sakis
I think it is just the opposite, with possibly the exception of input C everything else improves - bandwidth, distortion, damping factor, etc.

jan
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