Topology Federmann, HQQF-55 ...

[anatech]

Hi Federmann,
I have a lot of actual experience with paralleled mosfets with no source resistors. Believe me, this doesn't work at +/- 50 VDC and it works much less well at +/- 70 VDC. If you do not perform exhaustive matching (two days and over 40 devices of each type for me), the amplifier will fail spectacularly! Any damping resistors will flame, as will most of the drive section.

High impedance gate drive doesn't work very well either. High distortion results.

This is why I am concerned with the design here. Did you say you actually have one working now?

-Chris

[megajocke]

Originally Posted by Federmann

Transistors are next to each other, it is not possible that the first had a temperature of 25°C and the second 150°C.

It does not matter. Actually there was a typo in the last post. Currents should be 0.5A and 2.5A (not 25A). So how much positive feedback is there then? Have you calculated it? Current increases by 0.016A for every celsius increase in temperature, except for very low currents.

This means any mismatch in current will be amplified by about 1.8. This will in turn be amplified by 1.8 and so on... (0.016A/deg * 90V * 1.2deg/W) = 1.8. Assumed was 90V rails and Rth(j-s) = 1.2degC/W, 0.4 degC/W from the insulating pad.

1 + 1.8 + 1.8^2 + 1.8^3 + ... (infinite geometric series) does not converge. Any mismatch will be amplified by infinity.

At lower rail voltages it might share correctly if device matching is very tight. Somewhere around 30-40V might be OK.

anatech: What amplifier are you referring to that did not employ source resistors? The matching you speak of sounds like a major PITA...

[anatech]

Hi megajocke,

At lower rail voltages it might share correctly if device matching is very tight.

Don't count on it! The level of matching is much closer than you want to know about. Bias will change very slowly and never really settle down. So just setting it up will be a problem. I've worked on both 2 and 4 pair type outputs. Not fun.

-Chris

[Upupa Epops]

One treatment exist...but F. is the last, to which I can to tell....

[megajocke]

Don't worry anatech: I would never ever even think about designing something that way considering the large spread in mosfet parameters. The matching nightmare would probably make me insane.

In an active load for PSU testing I made with mosfets each one got its own current regulator loop with an op-amp.

[Lumba Ogir]

Michael,
you are apparently filled with a genuine sense of profound excitement for an underdeveloped idea, based on circulating marginated stereotype templates of random concepts, exacerbated by poorly substantiated technical suppositions and a neglectful, fragmentary cross-functional sequence analysis, lacking applicative-type structure norms and a reliable symptomatic long-term specificity, obscuring the outcome. You should, however, show your overwhelming but unfounded admiration in a calm and collected manner in oder to avoid finding yourself in the cumbersome situation of being threatened with a stiff bin time sentence.

[anatech]

Hi megajocke,

I would never ever even think about designing something that way considering the large spread in mosfet parameters. The matching nightmare would probably make me insane.

Exactly!

I figured you knew, my comment was actually for people who have not tried to dance with this particular devil.

Hi Lumba,
You crack me right up!!

Nicely put sir!

-Chris

[Lumba Ogir]

anatech,

[Ultima Thule]

Originally Posted by Lumba Ogir

Michael,
you are apparently filled with a genuine sense of profound excitement for an underdeveloped idea, based on circulating marginated stereotype templates of random concepts, exacerbated by poorly substantiated technical suppositions and a neglectful, fragmentary cross-functional sequence analysis, lacking applicative-type structure norms and a reliable symptomatic long-term specificity, obscuring the outcome. You should, however, show your overwhelming but unfounded admiration in a calm and collected manner in oder to avoid finding yourself in the cumbersome situation of being threatened with a stiff bin time sentence.

The pop on a stick has spoken!

[Federmann]

Originally Posted by Ultima Thule

Thank you Federmann for your latest simulation presented!

Could also show a simulation with only gate voltage of both IRFP240 and IRFP9240 and output voltage of the amplifier, thank you!

Simulators do not make mistakes, just people awarded incorrect data.
Then another haunted explosions.

I will return back to their topology. Voltage ±32V and load 6Ω.





Voltage ±32V, load 3Ω, at 20°C.



Voltage ±32V, load 3Ω, at 80°C.



Voltage ±32V, load 3Ω, at 20°C ÷ 80°C. I see that increasing temperature and decreasing current!


More shows temperature dependence of the RDS. Less temperature dependence is reflected VGS

Nice day