Output transistor safe operating area

[Bitrex]

I have been trying to learn how to appropriately select appropriate output transistors for power amp designs to keep them within their maximum ratings, unfortunately with little success. From the material I have read on this site it seems to be an extraordinarily complicated matter with much disagreement! I have read the material at Semiconductor Safe Operating Area and while I understand the issues involved, the site doesn't seem to actually go into an actual process that a designer could use. I guess what I'm looking for is an "example problem" to show the thought process a designer goes through when making the calculations for a particular requirement. Does anyone have a reference for something like that?

[Bob Cordell]

Originally Posted by Bitrex

I have been trying to learn how to appropriately select appropriate output transistors for power amp designs to keep them within their maximum ratings, unfortunately with little success. From the material I have read on this site it seems to be an extraordinarily complicated matter with much disagreement! I have read the material at Semiconductor Safe Operating Area and while I understand the issues involved, the site doesn't seem to actually go into an actual process that a designer could use. I guess what I'm looking for is an "example problem" to show the thought process a designer goes through when making the calculations for a particular requirement. Does anyone have a reference for something like that?

You are correct, this is never an easy problem. First, you have to decide what minimum impedance and worst phase angle the amplifier must operate into without having to go into protection. You next have to decide how agressive the protection will need to be to keep the transistots safe.

If you use modern transistors with good safe area, like MJL21193/4, a semi-reasonable rule of thumb for sizing the output stage is as follows: divide rated power into 8 ohms resistive load by 75 and round up to next integer. This is the number of output pairs to use. This is only a very rough suggestion and does not address the details of how much protection you need when this number of transistors is used. For this number of output pairs, the necessary protection will not have to be overly agressive for most reasonable speaker loads. This rule of thumb will get you to about 150W/8 ohms with two output pairs for an amplifier rated for continuous duty into 4 ohms if a heat sink is used that will never be allowed to get above 60C (which is the highest temperature you can keep your finger on indefinitely).

Again, this is a tough problem involving many tradeoffs, including risk, and I'm sorry I can't be more specific with details here. It would take a chapter in a book.

Cheers,
Bob

[PMA]

Originally Posted by Bob Cordell

This rule of thumb will get you to about 150W/8 ohms with two output pairs for an amplifier rated for continuous duty into 4 ohms if a heat sink is used that will never be allowed to get above 60C (which is the highest temperature you can keep your finger on indefinitely).

Bob, I would suggest 4 pairs of MJL21193/94 for the power amp rated at 150W/8ohm, 300W/4ohm. I would take into account numerous speakers with impedance dips to 2ohm, and complex load, to feel "safe".

Regards,

[Bob Cordell]

Originally Posted by PMA

Bob, I would suggest 4 pairs of MJL21193/94 for the power amp rated at 150W/8ohm, 300W/4ohm. I would take into account numerous speakers with impedance dips to 2ohm, and complex load, to feel "safe".

Regards,

No strong disagreement here, but four pairs for 150W is pretty generous. Remember, what I said is that it involves a tradeoff of protection scheme agressiveness vs number of pairs. I did not say that two pairs for a 150W amplifier did not require a protection circuit.

However, apart from SOA, four pairs for a 150W/8-ohm amplifier is nice because it reduces beta droop at high current and makes the amplifier more capable of delivering high current.

Remember, even driving a low-Z resistive load requires a certain amount of safe area, usually peaking at some power level below max (but not necessarily 1/3 power where average dissipation peaks). The introduction of phase angle not zero increases the needed amount of safe area for handling of a load that has a given minimum impedance (the phase angle is close to zero at the minimum impedance, so the worst case safe area need occurs at an impedance magnitude that is greater than the minimum impedance).

So, for example, if your worst case 4-ohm speaker has a minimum impedance that dips to 2 ohms (often Re of the woofer, but not always), the load impedance that imposes the greatest safe area need will have a magnitude greater than 2 ohms and will have a phase angle usually in the 40-60 degree range.

Again, one very rough rule of thumb emerges: If an amplifier has enough safe area for a 2 ohm resistive load at the worst-case power level, it probably has enough safe area for a 4-ohm-rated reactive load.

Keith Howard wrote a good article in Stereophile a year or two back that helps shed some light on this.

BTW, it is possible to use LTspice simulations with a variety of different loads to display the safe area excursions when a sinewave is applied.

Finally, a nearly pure low-z reactive load as sometimes presented by electrostatics can be difficult to deal with.

Cheers,
Bob

[wahab]

at 300W /4 R , that make 600W peak , alternatively on
each rail, supposing that the load is purely resistive...
with a load that would be reactive and dipping at the same time,
even 4 x 200W devices/rail would be at pain if the amp is pushed to
its limits for a long time, say a live performance, in a overheated stage,
that is, continous duty..
4 pairs are a minimum for such an amp...

[Rafael L]

With music we use ~50% the RMS power, but has income class AB which is ~60%, I dimencion of according to the RMS power. Dips on the low impedance are peaks, I do not know if should include.
I know that care must be greater with professional audio amplifiers that are taken to the limit in operation.

[Bob Cordell]

Originally Posted by wahab

at 300W /4 R , that make 600W peak , alternatively on
each rail, supposing that the load is purely resistive...
with a load that would be reactive and dipping at the same time,
even 4 x 200W devices/rail would be at pain if the amp is pushed to
its limits for a long time, say a live performance, in a overheated stage,
that is, continous duty..
4 pairs are a minimum for such an amp...

Bear in mind that the power delivered to the load is not the same as the power dissipated in the output stage.

Also bear in mind that I said that my rule of thumb was for when the heat sink never goes above 60C. In many cases, this can mean a very large heat sink, especially if the ambient is well above 25C.

Cheers,
Bob

[PMA]

Originally Posted by Bob Cordell

However, apart from SOA, four pairs for a 150W/8-ohm amplifier is nice because it reduces beta droop at high current and makes the amplifier more capable of delivering high current.

Exactly. I made quite a deep study concerning simulations into complex loads, and 4 pairs seem (about 25mV at every Re of 0R22) to be a good solution, with total O/P stage idle current about 450mA.

Regards,

[nigelwright7557]

Originally Posted by Bitrex

I have been trying to learn how to appropriately select appropriate output transistors for power amp designs to keep them within their maximum ratings, unfortunately with little success. From the material I have read on this site it seems to be an extraordinarily complicated matter with much disagreement! I have read the material at Semiconductor Safe Operating Area and while I understand the issues involved, the site doesn't seem to actually go into an actual process that a designer could use. I guess what I'm looking for is an "example problem" to show the thought process a designer goes through when making the calculations for a particular requirement. Does anyone have a reference for something like that?

One point to remember is the output transistors dissipate most with an AC signal that is 2/3rds of B+. This is to do with the function of voltage/current through the output transistors and speaker(s).

[wahab]

Originally Posted by Bob Cordell

Bear in mind that the power delivered to the load is not the same as the power dissipated in the output stage.

Also bear in mind that I said that my rule of thumb was for when the heat sink never goes above 60C. In many cases, this can mean a very large heat sink, especially if the ambient is well above 25C.

Cheers,
Bob

right, bob, i was pointing the fact that these 600W
must pass across the (fully) conducting devices...
sure that power dissipation is another thing, but
the amp wll dissipate more than the 150w theorical, because
when the output voltage is at half the value (in rms term) of the
power supply , this latter will not be as collapsed that at full power,
making the 300w amp worst case dissipation the one of a 350 to 400
theorical one (one which would have fixed supply voltage for the
convenience of the maths)..

regards,

wahab
,