Hi all!

I'm after some help with a bench power supply that I've recently purchased from china.
It's an LW K3010D 30V 10A switch mode power supply.

It worked fine out of the box. It wasn't until I tried to do some current limiting with it something give and I was suddenly getting no output at all other than about 0.02V. I originally thought the mosfets went due to not turning on/off fully because of the very low current control.
I've had the unit open and checked for burns and scorches but there's nothing obvious. Now, if I turn the unit on and start winding up the voltage control, the mosfets inside just get hot and the fan turns on and I swiftly have to turn it off.

The unit still turns on and the display is shown but there isn't any output. The main control circuit has it's own separate switching power supply hence why it still works. I've checked the power output side and it's incredibly simple so I can't see what the issue is. It's just a LV side of a transformer, rectifier and a few filtering caps/inductors.

The output side has a large high freq transformer which is connected to 2 caps in series so the trafo see's ~350V DC. That goes to a pair of mosfets in a totem pole configuration with the source of the bottom mosfet returning to the -ve of the rectifier.

It looks like the mosfets do the switching of the HV side of the trafo. A smaller isolation transformer electrically isolates the switching of the control circuit to the HV mosfets gates. On the HV side on the isolation trafo, there is 2 windings, there's a small set of circuitry for each winding, a few transistors and a diode which seem to only rectify and regulate the output of the isolation transformer to the gates of the mosfets.

There is a 30khz signal being fed to the mosfets gates from the isolation transformer.

It would appear that there's no reason for the power supply not to work, there's a switching signal, the output is monitored but nothing appears on the output side of the transformer and before you ask, the windings are fine!

Now the thing that's REALLY cooking my noodle is this, the bottom mosfet gets a voltage to the gate of approx ~12v at 30khz. However, the top mosfet has a voltage on the gate equal to about ~330v with a 30khz signal piggy backed onto it. When the voltage is turned down, the switching stops and there's about ~300v on the gate/source. This dissipates through the probe eventually so it's capacitive at least. I have tried installing a 150k ohm resistor from the gate of the top mosfet to -ve to try and remove any capacitive effects from the mosfet but it only dropped the voltage a small amount, I dare not go much less than that for the fear of it blowing (although it's only like 0.2mA)

Further more to this, I've pulled both mosfets and tested them electrically and know that they both work and switch off/on fine. I have also tested the power supply without the mosfets to test the isolation transformer signal to the gates. They both output about 30khz at ~12v p/p however the top mosfet gate signal doesn't return to ground, it sort of floats above ground.

It's got me entirely stumped because:
1. the mosfets are fine
2. there's switching on the gates
3. the source voltage of the top mosfet appears at the gate despite the mosfets being fine

I'm currently in a dispute with the seller over the item and hopefully I'll get my money back but if I don't, then I really need to get this fixed.

Any input would be massively helpful!

 

It sounds like the problem may be a short circuit in the output circuitry - the rectifier or capacitors. Check them and see what you get.

 

By Jove! you're right.

See, I checked the output earlier and saw it was shorted and for some reason thought it was normal.

Would appear that the rectifier has gone as it's shorted across both of the diodes.

it's an MBR20200CT.

Just pulled the diode bridge and replaced it with something from my spares drawer and it sprang into life!

Luckily enough I had something of a similar spec. Works a treat now.

Time to modify it even more now, to replace the current pot with a multi turn pot.

Thanks for your help!

 

Well that's your warranty and refund out of the window

 

Well, I'm not fussed. It cost £45 and it came from aliexpress on China so to return to China would cost more than its worth and warranty? Well, I don't expect something that cost as little as that which is built like it is to have a worthwhile warranty anyway. I'll still push on with the dispute because I should still be entitled to a partial refund.

I'm just happy it's working

 

I should still be entitled to a partial refund.

3 hours of your time to repair it, at £15 per hour, I reckon you're entitled to a full refund .