Delta DPS 386AP for HP plotter fuses blown

[Slawkos]

Hi there,
I have a problem with my DPS386AP Power Supply, it blew up the fuses after turning on, the same scenario after replacing them. Have found
faulty MOSFET IRFP450 marked as Q1 on PCB (have checked all of them and all Diodes). Not sure about the rectifier bridge (KBU8K) shows drop of 0.507V and 0.505V on "-" and "~" and 0.944V on "-" and "+" (the last one I'm not sure about). What else can cause the MOSFET burnout? Any suggestions very welcomed. Thanks

[R_J]

Re: Delta DPS 386AP for HP plotter fuses blown

The bridge is just 4 diodes in a single package, check each diode on its own. If the reading across + & - is low, its likely something else not the bridge, If the man cap has any voltage on it, it can throw off the reading.
If the fets were shorted drain to gate, check the gate drive components, It looks like the gate drive ic is on the small sub board, it may have been damaged.
Also check this thread https://www.badcaps.net/forum/showthread.php?t=17779

[Slawkos]

Re: Delta DPS 386AP for HP plotter fuses blown

Hi, thanks for your help I have seeing the topic you've suggested, I have removed the bridge and checked as I have mentioned in first post. Drop voltage of 0.507V and 0.505V across "-" and "~" and 0.944V across "-" and "+", checked rest of the components on the hi side, they're looking ok. How to check the ic chip I just measured the drop voltage between Ucc and GND and it seems to be ok.
Is there anything else I can do?

[R_J]

Re: Delta DPS 386AP for HP plotter fuses blown

But what is the VCC voltage? Also check the voltage on pin 14 Vref, it should be 7.5 volts.
You can check the voltage on the output pin that feeds the fet, but to really check if the ic is ok you need to scope the output to see if it is producing an output pulse

Download the ML4841CP pdf it may help since there is no schematic of this power supply.
I found this schematic at danyk.cz/s_atx_en.html, it uses a similar drive ic, It might help?

[Slawkos]

Re: Delta DPS 386AP for HP plotter fuses blown

Hi, thank you for the schematics just have got time and the parts now, after replacement of the fet the supply is back to life the Vcc is 12.54 and Vref 7.49, PFC OUT 2.73V, PWM OUT 3.42V the source on the Fet is 124V, what else can I diagnose and it could lead to the fet failure. I have just got scope but don’t want to brake it as I heard about Isolating transformers need to be used wit it, cheers

[R_J]

Re: Delta DPS 386AP for HP plotter fuses blown

Anytime you use a scope on the primary side of the power supply, you need an isolation transformer, the reason is that the scope ground needs to be connected to the hot ground, and if you do that without isolation, you will blow the power supply.
There is no problem using the scope on the secondary side if you need to without an isolation transformer.
As for the voltages, they are likely ok if the supply is working, You may want to either check or replace some of the small electrolytic caps in that circuit. The can go bad (open) and not show any signs like bulging or leaking, they just dry up due to heat over time.

[Slawkos]

Re: Delta DPS 386AP for HP plotter fuses blown

Hi, thanks for the tips I have check some tutorials on YouTube. I have checked the small caps and they're ok but the big one had big resistance so I've replaced it hope it will help for long, big thank you for the help