TTi CPX400A teardown/repair

[ovnr]

So I accidentally blew up my trusty TTi CPX400A PSU (2x 20A/60V limited to 420W). Was playing with a DIY 3-phase motor drive, and ended up regeneratively braking into the PSU. With a motor that - when connected to a 240V SLA bank - regens at around 1 kW. Ended up with an overvoltage trip, and upon restarting, it failed to regulate at anything over 15-20V at zero current, and substantially lower if there was a load connected.


Oops.


So off I went trying to find the fault. Surprisingly the pass transistor was fine; the topology tripped me up for a good while. It's a bit unusual (even for a SMPSU with a linear post-regulator):

It consists of a PFC stage with a 390V output, which is fed into two forward converters (for isolation) running at a fixed duty cycle. These do not regulate based on the set output voltage; unloaded they output 350+ volts. That is then fed into a magamp.

"What's a magamp?" I said.

Turns out it's a magic inductor that essentially acts as a buck regulator (it's the large toroidal inductor w/ stranded wire in the pictures). There are no switches driving it; it takes the raw switching signal from the forward converter, and by adjusting the reset current, regulates it to whatever you end up setting the PSU to (plus a volt overhead for the linear post-reg). Hadn't even heard of one before this; it seems like a nifty technology, but it's weird.

Anyway, it's then rectified, filtered, and the final regulation is done by a beefy NMOS. Nothing unusual here.




Once I stopped looking for a broken feedback path, I figured that the magamp wasn't being driven correctly. Turns out the control transistor - a MJE350 - was toast, as was most of the overvoltage shutdown components. Ended up replacing Q8, D22, R27, R28 and R34. Works perfectly well now; total cost was something like $5/channel to repair it.



Anyhow, attached photos because yay teardowns, right? Also, here's the service manual (PDF, 5.8 MB).

[ovnr]

More pictures. Damaged components highlighted in #2.

[Vgkid]

Thsnks for the teardown. Interesting failure, parts even look ok. Expected the resistor to look crispy.

[ovnr]

Thsnks for the teardown. Interesting failure, parts even look ok. Expected the resistor to look crispy.

Mmh. Should maybe point out that the parts had already been replaced by the time I bothered getting the camera.


But yeah, the original parts had no visible signs of damage; the fusible resistors were all open circuit, the zener was shorted, and the transistor was shorted in some configuration too (it's been a while since I actually fixed it).

[imamophead]

Hi,
Really sorry to resurrect an old post, but i have one of these bad boys and one of the outputs keeps going straight into CI mode (constant current). Just wondered if you or anyone had any clues on this? Its auto switching so i may try to isolate that circuit as a next step.

Cheers.

[ovnr]

Hmm. Question time!

* Is it always in constant current mode, even with no load attached?
* When the output channel is off, does the current display show a sane value when you adjust the current limit pot?
* What's the output voltage when it's in constant current mode? Measured with a DMM on the output, not the built-in meter.
* When in CC mode, does adjusting the voltage or current pots do anything at all?
* If it's not always in CC mode, does it regulate voltage properly?

Its auto switching so i may try to isolate that circuit as a next step.

By auto-switching, do you mean between 110/220V input? Because this is a true universal power supply; it doesn't so much "switch" between modes as not give a shit about the input voltage (within reason).
Also, note that there's +350V DC present on everything but the front panel board at all times, even when the power switch on the front is off.

Again, the topology consists of a boost converter straight off of mains (probably PFC, can't recall), going into a forward converter for isolation. The output of the forward converter is dumped into a magamp, acting as a buck converter, which is then post-filtered by a linear regulator. If something is wrong, it's very likely on the post-regulator or magamp controller side.

[imamophead]

wow thanks for the swift reply!

* Is it always in constant current mode, even with no load attached?
* When the output channel is off, does the current display show a sane value when you adjust the current limit pot?
* What's the output voltage when it's in constant current mode? Measured with a DMM on the output, not the built-in meter.
* When in CC mode, does adjusting the voltage or current pots do anything at all?
* If it's not always in CC mode, does it regulate voltage properly?

- Yes always in constant current mode. (im assuming here CI is constant current mode? im usually used to CC)
- Yes current pot shows what you would expect when adjusting with the output off and on.
- Measured output 0.00, 0.01 if i crank the current up a bit (but didnt want to go too high).
- As aforementioned voltage pots do something current does nothing.
- Its always in
So as writing this response i thought id check for shorts directly across the outputs....There IS A dead short somewhere. Could be the issue or just a symptom not sure.

Thanks again for the help

[ovnr]

So your issue is something else, not the same as I had.

Unplug it, wait a couple minutes, then open it up. Disconnect the output channel that's acting up from the front panel by unscrewing these lugs and separating the wires:


Measure the front panel jacks again. If it's still a dead short, your issue is likely that the reverse protection diode has blown up:



If the front panel isn't shorted and it just reads like you'd expect a capacitor to (steadily increasing resistance), the issue is on the power board. Measure the drain-source resistance on the output MOSFET; it's accessible from the top while assembled:


(I really don't expect that to be the issue, tbh.)