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Old 10th January 2005, 01:38 AM   #1
TroelsM is online now TroelsM  Denmark
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Default safe operating area

Correct me if Im wrong: When designing class A/B amps I need to be within the SOA curves, eventhough each transistor is only conducting the time?

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Old 10th January 2005, 07:34 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
YES stay within the SOA curves which are usually based on a case temperature (Tc) of 25deg C.
But there are many curves and all the faster ones require non-repeating pulses and these will not suit hi level bass signals.
I use DC values or 100mSec at worst.
I think you also need to reduce SOA values by the temperature reduction factor.
Combining these two factors can reduce a 250v device by upto 80% to 90% at 100v but much more favourable at 50v.
Do you want bullet proof / commercial / general domestic / Hifi?
regards Andrew T.
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Old 10th January 2005, 11:31 PM   #3
Stocker is offline Stocker  United States
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Agreed. Worst case is a shorted output or a speaker-out signal crossing zero out of phase with max. current at 0V output... You will likely regret it more if you blow one up than if you spend an extra $10.
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Old 10th January 2005, 11:50 PM   #4
TroelsM is online now TroelsM  Denmark
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Default The project

Im building a bridge amp with 6* TIP33C/34C in each half-bridge.

To make a stereo-amp that gives 24 TIPs backed up by a 2*24V/620W transformer and the result should be around 2*150-200W/8ohm.

I would like the amp to be able to drive 8ohm speakers for PA use, but more likely it will only be used in a HIfi/Home-setup.

What do you think of this? Are the TIPs to small or will it work?

As I see it the problem is the the current-capacity at 36V. Its only 3A for the combined 3 TIPs.

The complete schematics will be posted later, - I hope..
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Old 11th January 2005, 02:42 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
If your supply rails are running +-36v then to safeguard your output stage your short circuit protection must be less than 3A. To allow for imbalance between transistors and lack of accuracy in the set point I suggest you aim for 2A to 2.5A. This will strangle your bridged output!
Have you already bought the TIP's?
Buy something with a better 36v capability.
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Old 11th January 2005, 03:19 PM   #6
TroelsM is online now TroelsM  Denmark
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Default you've got a point but..

The short-curcuiet will be a VI-limiter-type. The currentlimit will be made dependent of the output voltage and therefore the current-limit will rise as the output voltage rise.

In this way the protection curcuit will keep the TIPs within the SOA and not "just" limit the current.

The simulation shows that it should be possible to drive a 4ohm load with the bridge, but that would push the TIP's to the limit.

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Old 11th January 2005, 10:45 PM   #7
TroelsM is online now TroelsM  Denmark
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Default New thread

Please see:
Please comment "my" design

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Old 11th January 2005, 10:59 PM   #8
AKSA is offline AKSA  Australia
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If you are designing down to the wire, using cheapest possible components, then paradoxically SOAR constraints are very important.

Consider worst case with a 4R nominal load. With 36V rail, 4A would imply 16V at the load, and thus 20V across the device. These days, this is not so unrealistic, and if you stick with your 36V rails, use 230V transistors like the 12A 2SC5200/2SA1943, then SOAR failures are extremely unusual and you can often avoid the complexity of protection, which frequently has intrusive sonic effect. In the eighties manufacturers eschewed protection at their risk, but these days it's fairly common, and even practical when aiming for highest possible fidelity.

Cheers,

Hugh
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Old 11th January 2005, 11:18 PM   #9
TroelsM is online now TroelsM  Denmark
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Default I know but..

The TIP's are free (to me, at least) and that's the main reason for using them. I know they are not very good at this "high" voltage, but still it should work.

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Old 12th January 2005, 05:18 PM   #10
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I am using "TIP141/146" devices in a class G amp and have to use 3 pair (as top drivers) to drive a pair of high current outputs. They were cheap and I expected I might "experiment" and blow a few. The SOA is not so great on these devices. In addition to that, the TIP141's that I got are fakes. and have an even lower SOA, obviously. So I had to triple them up, temporarily. I will get some better parts in the future and use just two paralell. Until then, it still cranks a LOT louder than I will listen. (and that's just one channel) , just doesn't look as pretty.
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