Maybe there's a follow-up video to be had here?
Hook up a scope to the MOSFET and measure Vgs and Vds.
Then, apply various conditions at the output terminals, and see what the MOSFET is subjected to in each case. Maybe stick a differential probe across that current sense resistor too, so we can see the drain current and calculate the instantaneous power dissipation in the die.
Then, see if there are, say, transient spikes on the gate which exceed its Vgs rating. See what the maximum Vds is, and what the power dissipation would be. Try and identify if there are load conditions which place undue stress on the MOSFET, and perhaps come up with a modification to protect it better.
Dave,
did you notice that Channel 1 was displaying "UR" as the mode instead of "CC" or "CV"at the start of the video? Could this have been a factor in the failure or was it a symptom of it having failed? I haven't seen this before and the DP832 Manual doesn't say much about it, just a one-liner.
Dave,
did you notice that Channel 1 was displaying "UR" as the mode instead of "CC" or "CV"at the start of the video? Could this have been a factor in the failure or was it a symptom of it having failed? I haven't seen this before and the DP832 Manual doesn't say much about it, just a one-liner.
I didn't notice that.
I presume that UR means Under Regulation?
In that case I would expect that based on the fault. I'm surprised it would even display anything for that, that's a good thing though.
Dave Says:
"...I would not expect a lab power supply of this price and grade to blow a pass transistor..."
Really? It's a $450 power supply from Rigol. That is pretty much the supply price and grade I would expect to randomly blow a pass transistor.
I'm dumbfounded at your annoyance by the hand-soldering around the pass transistor. How else are they to attach it to the heatsink?
Name one.
I've shorted countless cheap linear supplies over the years, never killed one that I can recall.
Only exception would be the Korad, but that wasn't a short with leads, it was oscillatory constant power mode on a load.
I wouldn't say you can't of course, but IME it's not common.
Hook up a scope to the MOSFET and measure Vgs and Vds.
Then, apply various conditions at the output terminals, and see what the MOSFET is subjected to in each case. Maybe stick a differential probe across that current sense resistor too, so we can see the drain current and calculate the instantaneous power dissipation in the die.
Then, see if there are, say, transient spikes on the gate which exceed its Vgs rating. See what the maximum Vds is, and what the power dissipation would be. Try and identify if there are load conditions which place undue stress on the MOSFET, and perhaps come up with a modification to protect it better.
I wouldn't say MOSFETs are more prone to fail in power supplies. It all comes down to how good the protection is done.
But all things being equal, MOSFET's do have an inherent probability of being partially damaged by ESD perhaps leading to more field failures.
High density means it is less suited for linear operation, because the die is small.
It looks like the designer of the power supply simply trusted the specs in the datasheet, without thinking about if it is plausible.
There is nothing wrong with MOSFETs for linear operation, but you need to understand the limits, and it looks like the Rigol engineers did not.
Dave,
did you notice that Channel 1 was displaying "UR" as the mode instead of "CC" or "CV"at the start of the video? Could this have been a factor in the failure or was it a symptom of it having failed? I haven't seen this before and the DP832 Manual doesn't say much about it, just a one-liner.
I didn't notice that.
I presume that UR means Under Regulation?
In that case I would expect that based on the fault. I'm surprised it would even display anything for that, that's a good thing though.
I dont think this Rigol supply is rated correctly. 30V 3A is 90W dissipated in that FET. It is very on the limit. I did not allow more than 75W in a TO220 package, when it was put on an anodised water cooled heatsink. With a 220, you have best case 0.5K/W thermal resistance junction to case, plus case to sink. With a black anodised heatsink like this, with screw, that is like another 0.5K/W. So the FET is running 90+ degrees above heatsink temperature. Not a safe margin. I bet if Dave would short the output, put it on the sun, the supply would die on its own after a while.
They should just upgrade it with a TO247. That has approximately twice the surface area to the heatsink, and 0.2K/W junction to case for the best devices.
And dont get me started on the lack of output relay.
So this is how many thermal issue in this PSU? Already 3? I guess chinese watts are smaller.
But all things being equal, MOSFET's do have an inherent probability of being partially damaged by ESD perhaps leading to more field failures.Do you have any reference for that?
I have never heard it. The gate is sensitive to ESD damage, that is well known.
Dave, how often/hours have you used this supply? Is it still in its 'running in ' period?
I would be leaning toward a pre-insertion static damage, (?hand soldered).
According to the Rigol DP832 User Manual:
"DP800 series power supply provides three output modes: constant voltage output (CV), constant current output (CC) and critical mode (UR). In CV mode, the output voltage equals the voltage setting value and the output current is determined by the load; in CC mode, the output current equals the current setting value and the output voltage is determined by the load; UR is the critical mode between CV and CC."
Which doesn't say much. This is the only sentence in the Manual that mentions UR at all.
According to the Rigol DP832 User Manual:
"DP800 series power supply provides three output modes: constant voltage output (CV), constant current output (CC) and critical mode (UR). In CV mode, the output voltage equals the voltage setting value and the output current is determined by the load; in CC mode, the output current equals the current setting value and the output voltage is determined by the load; UR is the critical mode between CV and CC."
Which doesn't say much. This is the only sentence in the Manual that mentions UR at all.
Interesting. Surely it would only be in UR "mode" during the very short time it's transitioning CV <> CC. So basically just the response time of the regulator loop.
I'm surprised they even detect and display this. Perhaps that shows they know it's loop response is pretty poor?