About Safe Operation Area

[qwertyqwq]

Can someone please explain what is SOA(Safe Operation Area) curve? Please explain with an example.
THANKS..

 

[rjenkinsgb]

Basically, it's keeping the dissipation (watts) within the permitted rating under all combinations of voltage across a device and current through it.

eg. a 50A 200V FET rated for 100W dissipation. Fully on and passing eg. 20A, or fully off, V * A is low or zero.

Partly on with eg. 100V source to drain, you can only allow 1A continuous current without exceeding the 100W rating.
Or 10A at 10V..

 

[qwertyqwq]

Basically, it's keeping the dissipation (watts) within the permitted rating under all combinations of voltage across a device and current through it.

eg. a 50A 200V FET rated for 100W dissipation. Fully on and passing eg. 20A, or fully off, V * A is low or zero.

Partly on with eg. 100V source to drain, you can only allow 1A continuous current without exceeding the 100W rating.
Or 10A at 10V..

Hmmm. Thanks.

What about switching freq. There are few lines like 1mS ,100uS or 10mS in the curve. What they are means ??
THANKS.

 

[rjenkinsgb]

Without specific examples from data that could mean various things.

Low speed switching is generally not a great problem; if properly driven the device is either fully on or fully off.

With high-speed switching such as PWM or switch mode PSUs, you have to allow for the device turn-on and turn-off times.
It's during those transitions that the devices may be dissipating a relatively large amount of power, so the percentage of time they are in that state should be small relative to the interval between switching transitions, for things to work properly and efficiently.

 

[dr pepper]

Also a consideration with Soa is change in voltage with time and change in current with time.
Back in the 80's it was trendy for audio amps to have safe operating area protection for the o/p devices.

 

[shortbus=]

@ qwertyqwq Here is the best thing I've found for understanding mosfet data sheets.

FET Current Ratings -- Chuck's Robotics Notebook

 

[simonbramble]

Basically, a FET can withstand a certain amount of heat. This heat is generated by the FET passing current at the same time as a voltage is across the FET (from the Drain to the Source). If the voltage and current are there constantly (like with a linear regulator) it can only handle a small amount of heat. However, if the voltage and/or current is pulsed, it can handle a lot more power (since the average power dissipation is dependent on the total power multiplied by the ON time divided by the ON + OFF time). The shorter the ON time for a given period, the higher the pulse power can be... to a limit. There is still a finite amount of time for the heat to get out of the package and the silicon could be heating up inside the package well before the package has had a chance to get rid of the heat. The curves help explain this

 

[ronsimpson]

What about switching freq. There are few lines like 1mS ,100uS or 10mS in the curve. What they are means ??

I disagree with the advice you were given. The lines are about time not frequency. The time is for a one time occurrence. Not like in a PWM.
The 1mS line shows what you can do if you power the part for 1mS, then power down and let it cool off. The 100uS curve shows that you can have more power in the part but for a shorter time.
This has to do with how fast the heat travels from the silicon to the metal case and out to the heat sink. Look at the fine print. Many of the "absolute max numbers" are based on a "infinite heat sink". Quick some one post a picture of a infinite heat sink. I have always want to see one.

 

[atferrari]

Quick some one post a picture of a infinite heat sink. I have always want to see one.

Picture too big. Exceeds maximum size for this forum. Sorry.

 

[unclejed613]

Back in the 80's it was trendy for audio amps to have safe operating area protection for the o/p devices.

here's a good explanation on how they work, as well as some information about transistor SOA in general:
https://sound-au.com/soa.htm

 

[dr pepper]

Esp is a good site for all things audio.
Linear amps are making a slow exit, a bit like tubes 30 years back.
Maybe they'll be trendy for guitarists in another 20 years.

 

[ronsimpson]

SOA(Safe Operation Area) curve?

This reminds me about high school and dating. "You can go this far and no farther"
Cross the line and pow, bang, boom you are dead!

 

[unclejed613]

Linear amps are making a slow exit,

i'm not so sure of that... except for the very high end stuff using chips like B&O's ICE chipsets, class D still has a few downsides. especially the "budget home theater" market and boomboxes, etc.... even with the high end stuff (like Pioneer's ELITE Series) an amp failure requires a board replacement. class D also requires a high parts count, often including many custom made inductors and ultra low ESR electrolytic caps. Pioner's ELITE Series receivers have ICE amp chipsets, but the power supply is still "heavy iron"... apparently a lot of the "ultralights" didn't sell as well as the marketers thought they would. even with pro-audio equipment, sound crews didn't like what felt like "flimsy" amplifiers.

Cross the line and pow, bang, boom you are dead!

yeah, second (or secondary depending on who you talk to) breakdown can ruin your day in less than a heartbeat.

 

[dr pepper]

I'm glad I'm not involved in the sound industry anymore.