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Old 11th July 2013, 05:20 PM   #1
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Default TIP35/36C maximum power at one pair

Hi.
Can one pair of this transistors survive 80W/8R and 120W/4R at 47V supply?
I'm building AB class amp, potentially TIP35C is good for this aplication by SOA, but I don't want to make impredictable amp.


I can use 2SC5200/2SA1943 if TIP35/36C are too weak, but in my Country Toshiba transistors are often fake.
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Old 11th July 2013, 05:34 PM   #2
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The absolute maximum power dissipation of TIP35 is 125W, that is assuming there is sufficient heat dissipation. So good for 70% of total power I would say.
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Old 11th July 2013, 06:03 PM   #3
jaycee is offline jaycee  United Kingdom
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I'd say just one pair of transistors on this voltage is pushing it. Remember a typical speaker is an inductive load. Even with 1943/5200's i'd say use two pairs.
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Old 11th July 2013, 06:31 PM   #4
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Such calculations are too general, I have counted it like under:
Output voltage for 120W/4R-->22V
Current -->5.5A
Vce=47V-22V=25V
Dissipated power --> 137.5W
For one transistor from pair -->68.75W

Did I do it right?
Probably there will be no secondary breakdown, but I don't know whether this is counted well.

btw Sorry for my English...

EDIT:

For example Technics SU-V85a use one pair 2SC3280/2SA1301 at 57V with 150W/4R, 100W/8R.

Last edited by elektryk; 11th July 2013 at 06:41 PM.
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Old 11th July 2013, 06:40 PM   #5
jaycee is offline jaycee  United Kingdom
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You are only calculating resistive loading. Speakers are inductive. This means current and voltage will be out of phase, the worst is a 90 degree phase shift.

Rod Elliot has an excellent article on this: Phase Angle Vs. Transistor Dissipation
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Old 11th July 2013, 06:54 PM   #6
jaycee is offline jaycee  United Kingdom
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Don't judge it by commercial amps - these often use undersized output stages, together with an under-rated power supply that will collapse if the load is too severe.. this protects the output transistors, but also sounds like sh*t.
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Old 11th July 2013, 07:01 PM   #7
wg_ski is offline wg_ski  United States
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Peak dissipation with resistive load = (23.5^2)/4 = 138W (half voltage on load, half on transistor). For 60 degree reactive load, double it. That's 276W @ Vce=47V. That's outside the SOA for even a pair, but you can get away with a pair in parallel because the supply will drop enough under load to keep it under 250. The 276 is (47^2)/8 for 4 ohm load (half the peak current at full VCE).

In reality it's much more complicated than this.You may (probably will) run into a worse load, and there is temperature derating to consider. On the plus side you're not operating at DC conditions, and for practical purposes the 25C DC condition is a wash with pulsed conditions derated for about 60 or 70C. This is pretty much on par with what they assume for store-bought amplifiers.
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Old 11th July 2013, 08:13 PM   #8
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That's right, but people listen music power and dissipated power will be smaller than with continuous sine wave. In non continuous load this rules applicable too?

I believe that 400VA transformer will be good for this (home) amp, but I'm only worry about transistors, 'cause I have already PCB design, and want to make this device cheap.
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Old 11th July 2013, 09:22 PM   #9
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by elektryk View Post
'cause I have already PCB design
If you've got a PCB that's only got room for one output pair and you're committed to it, use the MJL21193/21194 pair. Those are about as strong as you're going to get. Anything else I would use two pair. It doesn't have anything to do with continuous vs. non continuous load - if the transistors are insufficient you can blow them without ever getting them hot.

400VA trafo is plenty. Unlike transistors, a trafo can handle about a 10X overload for a few minutes and it won't hurt anything.
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Old 11th July 2013, 09:28 PM   #10
jaycee is offline jaycee  United Kingdom
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With 400VA of power supply, all you'd need is to have a party, turn it up a bit, and you'd quite easily risk blowing transistors.

You might consider TO-3 package parts if you only want to use one pair - these typically have better SOA due to the better thermal profile of the casing. Alternatively, use MJL21193/4 as recommended. You might be able to get them directly from On Semiconductor as a sample part.
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