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SPD3303X Catastrophic Fault (Design Flaw?) - (Edit: Nope! User Error!)

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christopher.robot:
Hey folks - looking for something of a sanity check here...

I recently had to teardown & repair my SPD3303X because of what appears to be an absurd design oversight.

I was charging a large lithium ion battery (yes, through a BMS & active balancer) when we had a brownout in my building. 

(Before someone shouts at me - it wasn't just banal charging, I was performing charging tests which utilized some features of the PSU).

When the PSU re-powered it went through an internal relay cycling which immediately flooded the room with the stench of conflagrated FR4.

Upon inspection, the supply had shorted the inputs on power-up (which seems an intended part of the process) and turned two PCB traces into fusible links and took out a triac (UB19, a BTA08-600C).

I've never had any other bench PSU do something like this.  That includes 4 Agilents, 2 Tenmas, and a host of other cheapie uprights (I'm not saying it couldn't happen, I'm just saying it hasn't).

What on earth is the point of this shorting?  Some kind of self test for the relays?  Why would it not check for voltage on the output first?  Why wouldn't there be fuses somewhere?  (Crowbar circuits typically have fuses, no?)

Am I crazy?  Is this a normal implementation and I should always externally fuse or Schottky any high-current capable sources connected to the output?  Would it be silly to modify the circuit to include some chunky SMT fuses (higher than the output rating, but below "blowing traces off the board" current)?  I get that adding components to a calibrated output is usually a no-no, but some beefy fuses right at the output lugs aren't likely to change the specs, I'd wager...

This seems like madness to me, and cost me a day of repairs - and I'm frankly afraid to use this thing for anything with batteries or big capacitors connected to the output.

Forgive the somewhat scrubby repairs, I went back and cleaned them up after I verified the supply was again functional, but I didn't take any pics.

jjoonathan:
Yeah, the "social contract" around power supplies can have a lot of caveats. One way or another, we're all familiar with "current limit doesn't account for output cap," leading to first-time EE lab students exploding LEDs everywhere  :D . "If anything happens to the input power, all bets are off" is another common for cheap supplies, though I'm surprised to see it from Siglent. My GPP-4323 doesn't do this, fwiw -- there's a reverse protection diode and a 500 ohm resistor, but no momentary short on boot.

My first power supply was a KKMoon that dumped its 48V intermediate voltage caps into the output if you removed power before disabling output first, so it *can* get even sillier.

AVGresponding:
One should always use a diode when using a psu to charge batteries, to prevent this exact situation. Many psus have the capability to sink a certain amount of current from the DUT (two or four quadrant operation, basically operating as a load) and might well default to 0V after a mains failure, and with a large battery this might easily exceed the unit ratings.

christopher.robot:

--- Quote from: AVGresponding on December 20, 2023, 06:12:17 pm ---One should always use a diode when using a psu to charge batteries, to prevent this exact situation. Many psus have the capability to sink a certain amount of current from the DUT (two or four quadrant operation, basically operating as a load) and might well default to 0V after a mains failure, and with a large battery this might easily exceed the unit ratings.

--- End quote ---

Yes yes, sure sure, I could have been "more careful" - but the point is that this seems like it was easily preventable in the design.  It seems like when implementing the shorting behavior, they didn't consider or didn't care about potential failure modes.  A simple chunky SMT fuse (which I guess the PCB trace sorta is, albeit unpredictable) would suffice - though it's a PSU with output sensing - it seems a simple firmware solution to have it check for voltage on the output before shorting during power up...

nctnico:

--- Quote from: AVGresponding on December 20, 2023, 06:12:17 pm ---One should always use a diode when using a psu to charge batteries, to prevent this exact situation. Many psus have the capability to sink a certain amount of current from the DUT

--- End quote ---
I agree 100% ! Some PSUs even have a crowbar (thyristor) which shorts the output if the output voltage is higher compared to the set voltage to prevent a failure inside the PSU to cause the DUT to be destroyed. I've seen the damage that can happen if you charge a battery using such a PSU when the PSU decides it is overheating and shuts down...

Bottom line: never ever charge a battery from a lab PSU without a series diode.

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