How do you determine what voltage is safe for your output transistors? - diyAudio
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Old 10th November 2004, 04:04 AM   #1
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Default How do you determine what voltage is safe for your output transistors?

How do you determine what voltage is safe for your output transistors? I couldn't find a definite answer in a search.

The reason I ask, is because a typical audio amp has each output sharing half the total supply voltage with no signal being applied to the amp. But then with a signal, the voltage swings to each rail.

For example, say an amp power supply uses +- 50V supplies

Do the transistors have to be rated for 50V, or 100V to be safe?
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Old 10th November 2004, 04:31 AM   #2
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for most manufacturers there is a chart in the product PDF which describes the "Safe Operating Area" -- violate these bounds and you will toast your devices.
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Old 10th November 2004, 04:33 AM   #3
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Default how brave are you?

Hi,

If you want to be conservative you need to use the sum of the rails, plus a margin for safety, just in case the mains voltage surges, 10% is not unusual. I'd go for 30%, and not just on your transistors, every PSU component needs to have a similar margin or there will be a lot of magic smoke released occasionally...big caps sometimes have their surge rating explicitly listed on the package...

Stuart
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Old 10th November 2004, 06:07 AM   #4
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Default Safe voltage rating for transistors

At full power conditions the amplifier's output will swing to almost the rail voltages.

In class A and A/B amplifiers (90+% of solid state amps in the world) the output devices sit across the rails and output.

This means that under full power conditions, when the positive rail transistor has pulled the output close to the positive rail, the negative rail output transistor will have the total rail to rail voltage across it. The reverse is true when the output swings to near the negative rail.

So yes. Under all circumstances, the voltage capability of the output transistors must exceed the rail to rail voltage.

Cheers
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Old 11th November 2004, 12:48 AM   #5
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Thanks for all your replies, that really cleared things up. I plan on going to On Semi to get some nice, high voltage transistors.

It makes me wonder why I see schematics on the web with 60V 2N3055/MJ2955 transistors used with an amp with +-35V rails

I guess it would blow up if you clipped it hard
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Old 11th November 2004, 02:00 AM   #6
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Default safety margins...

Hi,

The process used for making transistors is not an exact process, so there is a little margin in the specs, and sometimes a lot. 3055/2955 transistors were first created decades ago at the limit of the current production process. Since then methodologies have made big strides in quality and the variability of the parts has improved, allowing them to work in places that at first sight would be marginal. From what I understand think +-35 volts is 'safe', pushing them to +-45 would be asking for failures, and much beyond that would be a spectacular, if brief experiment.

Stuart
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Old 11th November 2004, 03:18 AM   #7
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Some nice general purpose TO3 power transistors from them are the MJ15003 or the MJ15024. Those examples are from a family of power transistors. I work in pro audio and we use those types quite a lot. Look into them as choices.

The 2N3055 was very popular 25-30 years ago, it was the big happening power transistor. We have come a long way since then.
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