QW-MS3010D 30V 10A Variable PSU Blown MOSFET

[RukyCon]

Hello again, I have this cheap 30v 10A variable power supply that died about a year ago, the issue with it is that one of the MOSFETs (IRF840) Blew its guts. Now despite this thing being cheap, the design may have saved the control electronics from being blown up, as they appear to be in good shape.

I may also replace some other components while at it and fix some of the bodge jobs so this thing looks nicer.

Being that this is my first time replacing a blown MOSFET, I may need a bit of help, so any help will be appreciated.
And hopefully, i don't make a dumbass out of myself again.

[RukyCon]

I was hoping for a bit of help, but i guess i'll try fixing it on my own. I think i'm going to replace both MOSFETs with IRF840BPBF parts, on top of that, i think i'm going to do a recap because, why not, as all of the caps are generic crap anyway (and doing this will also give me more parts to throw into the Digikey order so that most of the cost isn't shipping).

[R_J]

Many times when a mosfet blows it shorts Drain to gate, this can damage the gate drive components and the drive ic. I would get the power supply working before replacing any caps. Once the power supply is operational, then replace the caps a few at a time, checking operation between replacing caps.

[RukyCon]

How do i test the drive circuitry? as it appears the drive circuitry is driven off its own separate power supply.

The main driver IC is a KA7500B (U4), the other two chips are LM324N (low power OP-AMP) (U6) and DK112 (low power SMPS driver)(U1).

[R_J]

If you don't have a scope to check the ic output, then replace the mosfets and hope they don't blow up, or replace the driver ic along with the mosfets.

[RukyCon]

It does appear the gate and drain pins are shorted, out of curiosity, how does this kill the chip?, as i'm looking at this thing and it looks like the chip is driving the MOSFETs through T1, which i would think would isolate the chip and MOSFET.

On top of that, could i use a different MOSFET/Transistor for testing this thing, I have a ton of salvaged MOSFETs from PSUs over the years. So if i install a used one and the chip kills it, then i will know if the driver is bad?

[CapLeaker]

Because you get full power shorted to gate pin. The gate drive can't handle that kind of power. The PWM doesn't have to necessarily blow up, but it could be dead, or stuck on. If it is stuck on, well it will short out the new mosfet. There could be other devices in the gate drive circuit that could be bad.

Specifically for testing power supplies there is this "60W or 100W incandescent light bulb in series trick"

https://www.badcaps.net/forum/showthread.php?t=32748

[RukyCon]

I guess i'll add a replacement driver chip to the shopping cart, just in case, will a KA7500C do as Digikey will only sell KA7500B chips in cases of 1500 (from what i can tell, the C variant is just a newer design of the same thing).

Between the chip and MOSFET appears to be 4 pairs of 1K resistors (in two parallel pairs), 4 pairs of 22 Ohm resistors (in two parallel pairs) 2 J3Y NPN Transistors, 2 2TY PNP Transistors, and 4 Diodes (and a transformer).

I'm very aware of the incandescent bulb trick and have used it in the past for testing.

[RukyCon]

So i decided to try swapping the MOSFETS with what i have for testing (which in this case, were 2 K3568's (which i know may not be ideal in this thing)) and the PSU almost worked, the only issue is that it only outputs 500mV max (which is roughly 29.5V short of its rated max output). So i'm wondering now is it still the control chip or do i need to get the right MOSFETS and this thing will work fine?

A couple of things to note: the voltage control knobs were able the control the output (but due to the low max voltage, it wasn't any major change), and i was testing this thing in series with a 53W halogen bulb, but the bulb did not glow for most of the testing.

[CapLeaker]

what is on the other side of the heat sink?

[redwire]

The power supply is a half-bridge configuration with transformer drive to the power mosfets. So shorted mosfets cannot damage the KA7500 (TL494). But the gate-drive resistors, R2 50R? looks be blown and there is another for Q3's gate to check. They are the fuse sometimes. In-circuit you would read a short G-S due to the drive transformer to the IRFP840's.

I have this Mastech HY3030E schematic which I believe is similar enough to understand the circuit.
PWM IC drives 4 transistors which drive the gate-drive transformer.

The low voltage SMPS IC DK112 looks discoloured, I wonder if it works and you have any low voltage DC present?
Note the (Mastech) power suplpy has an SCR which supplies power to the power mosfets only after the little SMPS turns it on.

[RukyCon]

Originally Posted by CapLeaker

what is on the other side of the heat sink?

On the other heatsink is a P75NF75 and an STTH1602CT, i haven't tested either of them since i don't think they would have been damaged.

Originally Posted by redwire

The power supply is a half-bridge configuration with transformer drive to the power mosfets. So shorted mosfets cannot damage the KA7500 (TL494). But the gate-drive resistors, R2 50R? looks be blown and there is another for Q3's gate to check. They are the fuse sometimes. In-circuit you would read a short G-S due to the drive transformer to the IRFP840's.

I did not notice R2, but it's definitely blown, the first numbers on it appear to be 47, but the multiplier is missing, but i'm guessing it must be a fairly low value (maybe 47R).
Edit: appon closer inspection, i found another resistor (R5, or in your schematic, R203?) thats also blown, the resistors marking are still readable and say 47R3.

Originally Posted by redwire

I have this Mastech HY3030E schematic which I believe is similar enough to understand the circuit.
PWM IC drives 4 transistors which drive the gate-drive transformer.

Okay, i'll take a look at it to help me figure this out (hopefully)

Originally Posted by redwire

The low voltage SMPS IC DK112 looks discoloured, I wonder if it works and you have any low voltage DC present?
Note the (Mastech) power suplpy has an SCR which supplies power to the power mosfets only after the little SMPS turns it on.

It still works, as stuff like the voltage/current displays and controls still work, it also looks fine so it may have been poor lighting that made it look bad.

[R_J]

R2 and R5 are likely 47Ω, they should be the same value. Check the voltage before Q2 (p75nf75), the output of the dual diode (STTH1602CT) or across C11,16 or 22

[RukyCon]

Originally Posted by R_J

R2 and R5 are likely 47Ω, they should be the same value. Check the voltage before Q2 (p75nf75), the output of the dual diode (STTH1602CT) or across C11,16 or 22

Tested, and it's roughly 500mV, or roughly the same voltage as i was able to get on the output with both IRF840's replaced (see post #9 for more info).

[RukyCon]

If i were to replace both 47Ω resistors, will this thing start working again? As far as i can tell, they are the only components that are still blown. So if i were to replace both of them, this should work again, or is there anything else i should check?

[R_J]

Well the mosfets won't work if the resistors are open, so will it work after? If the resistors are good, then that's not the problem.

[RukyCon]

The resistors are both blown open, or at least have a resistance greater than 10MΩ. I'm going to see if my local electronic surplus store has any 47Ω SMD resistors and IRF840 MOSFETs (to replace the placeholder K3568's) today, if not, then i'll try ordering them from Digikey.

[RukyCon]

So i went and found some 51Ω resistors and some IRF840A's from the surplus store, threw them into the power supply, tested it, and it appears to be fully working as now when its set to full power it outputs 31V instead of 500mV. I also found some 470uf 50v KZEs that i plan to put on the output, but i think they're NOS so i may have to reform them first (they still test within spec).

Once i'm done with testing, i'll post photos of the repair

[RukyCon]

Here are the final repair photos...

[sam_sam_sam]

Originally Posted by RukyCon

I also found some 470uf 50v KZEs that i plan to put on the output, but i think they're NOS so i may have to reform them first (they still test within spec).

How long did you have them in storage because I have had some that I have bought at least 5 years or more and have had no issues using them I have values from 0.47UF to 47000UF

One thing that I do power on the device at low load and leave it powered on for hours to at least 24 hours before turning the device off

Then use the device normally after this