Linear regulator acts weird

Thread Starter

Dods

Joined Jun 11, 2019
7
Hello,
I'm trying to regulate 800V input to about 300V output, using the regulator below:
upload_2019-6-11_15-11-26.png

It "works" without load, but when i attach the PSU on the output, the whole thing goes nuts, transistor starts to vibrate, connected multi-meters start to beep, power supply tries to turn on and fails about every 2 seconds.
I'm controlling the input voltage from 0 to 1000V, so I've tried multiple input voltages.
Without load, output voltage is ~303V (if input>output ofc).
With load, voltage sags to ~30, then starts (slowly) rising to ~90 before dropping again.
Does anyone have any idea why the circuit would behave like that?
 

BobaMosfet

Joined Jul 1, 2009
1,208
Hello,
I'm trying to regulate 800V input to about 300V output, using the regulator below:
View attachment 179468

It "works" without load, but when i attach the PSU on the output, the whole thing goes nuts, transistor starts to vibrate, connected multi-meters start to beep, power supply tries to turn on and fails about every 2 seconds.
I'm controlling the input voltage from 0 to 1000V, so I've tried multiple input voltages.
Without load, output voltage is ~303V (if input>output ofc).
With load, voltage sags to ~30, then starts (slowly) rising to ~90 before dropping again.
Does anyone have any idea why the circuit would behave like that?
Input AC/DC, output AC/DC??? C1 makes me think DC input...?
 

Sensacell

Joined Jun 19, 2012
2,598
The Traco Power Top 60 is going to look like a wildly non-linear load.
It looks like a big capacitor with a negative resistance across it.
As the voltage input rises, the current decreases. (it's a constant power load)

Additionally, it probably doesn't draw any current until the startup threshold is reached, then it draws a bunch as it starts up.

Your regulator's output impedance is too high.

I would imagine that's the cause of your troubles.
 

Thread Starter

Dods

Joined Jun 11, 2019
7
Input is rectified AC (so, "DC")

When on load, does the input voltage remain constant?
Yes, input is always stable.

hi Dods,
Welcome to AAC.
Have you measured the Gate to Source voltage, Off load and On Load.?
E

Update:
Added Traco d/s

Thank you! Didn't know about this forum!
I haven't measured it because it's comming from a high power high voltage supply so i refrained from picking while it's on. Looks like i'll have to, though.

The Traco Power Top 60 is going to look like a wildly non-linear load.
It looks like a big capacitor with a negative resistance across it.
As the voltage input rises, the current decreases. (it's a constant power load)

I would imagine that's the cause of your troubles.
Yeah, but the thing is, i had used this circuit before, with cheaper and weaker transistor and bigger PSU i had (i think it was XP Power ECS130W), and it worked as intended.
For some reason, this one doesn't, and it should, because the transistor is far better and the load is weaker. That's why I'm baffled. The 'old' one had been tested with ~100W load on the PSU and it was fine.
I built both of them, and i had to buff up the transistor because of the load, it was running toasty.
No other changes, even the same PCB.
 

Thread Starter

Dods

Joined Jun 11, 2019
7
Also, I'm okay with changing the design, but I'm not sure how i could help with the turn-on, and even when it does turn on, like i said, it goes absolutely bonkers
 
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