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Omron 24 VDC power supply Mosfet and diode replacement

jadao

Nov 12, 2017
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Nov 12, 2017
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Omron power supply 6.5A 24vdc. Model number S8JX-15024CD. Used in industrial field.

History:
there was a direct short between V+ and V-, the short was from the wiring on a +24v distribution terminal block. It was used in a biscuit factory.

Since then, power supply was showing 24vdc without load but it was dropping to 17-16v as soon as it was hooked to a load.

I decided to repair the PS and use it on my bench. Inspected power supply, hooked it to 220vac, and nothing visually obvious, output green led was on, but a tic tic tic sound was coming from pwm controller. As if it was entering over current or overload protection.

Didn't noticed any obvious short on dc output.
Unfortunately I didn't had time to push further investigation and the power supply just exploded because of a small staple that fly directly into the pcb. This is dumb and dangerous I know :(

Damage:
-KA7552 PWM controller exploded
-A diode (D5) sitting between GND and the controlled power mosfet gate, not sure witch type, labeled 4 79 was full shorted (not burned).
-Power mosfet 2Sk3875 exploded
-Fuse 5A 240VAC blown
-A couple of resistors on the path looked burned.

What I did:
-Restored all blown components with original one except power Mosfet.
-Replaced any suspicious resistors
-Checked all diodes and they are fine except the one that shorted.
-Removed the shorted D5 diode.
-Checked for any burned traces, all ok.
-Checked the AC stage with the bridge rectifier and the main 400v Cap.
-Checked the (regulated ?) 24vdc stage (after transformer)
There is no more short.

My questions:
1)
I have a power Mosfet STW15NK90Z it looks pretty same than original 2Sk3875. Main specs look very close.

Any advice/toughts if it is suitable? Am I missing other important details?
Here is both datasheet links:

Replacement : http://www.st.com/resource/en/datasheet/stw15nk90z.pdf
Original : http://pdf.datasheetcatalog.com/datasheets/105/499735_DS.pdf

2)
D5, the shorted diode between gnd and power mosfet's gate has this marking 4 79. "4" on the radial orientation and "7" followed by "9" on the axial.

I googled 479, 4-79 and 79 and found BA479 PIN diode and schottky barrier diode 30v 1.5a. But not sure about the results accuracy.

The output controller is rated at +/-1.5A. And vgs on the replacement Mosfet is 30v.
What suitable diode can I fit?

Also, the replacement power mosfet datasheet states that it already has an integrated Gate-Source protection zener diode and no need of external protection component.
Am I lucky on this one and could forget about replacing the diode?

Please find the following pictures:
-A partial circuit draft (hope no mistake there!)
-D5 shorted diode pictures
-PCB pictures

Thanks in advance
 

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kellys_eye

Jun 25, 2010
6,514
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Jun 25, 2010
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6,514
When looking to repair SMPS devices it is always useful to find the data sheet for the controlling device AND the application sheet - as shown here.

ka7552 application.gif

I also found the block diagram for the PSU too - it sometimes helps:

block diagram.png

Based on your description of the part location and comparing it to the application sheet you can see that it is a zener diode (20V) protecting the MOSFET gate so fitting a suitable replacement would be deemed necessary even though your replacement device has Vgs rated at 30V - there may be spiking issues (dunno?).
 

jadao

Nov 12, 2017
7
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Nov 12, 2017
Messages
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Thanks kellys_eye,

I already have the block diagram in the Omron manual and it helped me for sure.
The application sheet you included is handy and I didn't find it.
I found another application sheet (please see attachment) for a samsung DVD905 player with same controller KA7552 but doesn't seem to be a good example because it doesn't have a mosfet gate-source protection...

KA7552_application.jpg

In the application sheet that you included there is a 20V zener diode, PVZD1, protection for a Vgs=20V N power Mosfet "K793".

My actual concern is adding a zener diode while knowing that the replacement Mosfet STW15NK90Z (available in my parts) has already an integrated back to back zener gate-source diode.

Zener protected STW15NK90Z Mosfet.jpg


The original 2Sk3875 Mosfet didn't have a "built-in" zener diode so I understand in that case the need of an external diode.

Don't know what would be the effect of adding a zener in parallel to the built-in one Mosfet especially if they don't exactly match...

Thanks again for your help and advices.
 

kellys_eye

Jun 25, 2010
6,514
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The replacement device with built-in protection seems more than capable. Use it.
 

jadao

Nov 12, 2017
7
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Nov 12, 2017
Messages
7
Ok so I verified all caps (ESR) and double checked for easy and obvious short.
Plugged in the power supply and got the dc output green light (V+ / V-) for few seconds and pshhh!

The typical white fume and very nasty smelling comming out from electrolyte capacitor:

V+ and V- were totally shorted, it was one of the 3 caps (1500uF, 35V) on the dc output stage that shorted out and bulged. Removed the cap and no more short back to square one.

The shorted cap didn't show any signs of buldging before the test, except if it randomly get stressed with initial failure...
I am more affraid the +V line is maybe providing more than 35Vdc the rated voltage capacitor.

IMG_6380.JPG IMG_6381.JPG IMG_6382.jpg

I have actually 280ohms between V+ and V- (0.160 diode mode both ways) witch doesn't seems too low to me compared with other power supply.

I may try to capture briefly what is the dc output voltage before any other cap pops out...
Any ideas before I replace the capacitor?

Thank you
 

kellys_eye

Jun 25, 2010
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Do you have the facility to apply AC to the input as a variable supply?

Such a source is very useful when it comes to SMPS repair work. If not then perhaps you have a transformer that has a couple of low(ish) secondaries you can series-up....?

Have you checked the voltage divider on the secondary side that provides the feedback (via opto coupler) - the schematic I posted (application note) showed a TL431 (or equivalent) as a reference. Is there such thing on your board and if so, is it ok?
 

jadao

Nov 12, 2017
7
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Nov 12, 2017
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7
Unfortunately, I don't have yet a variac.
But I do have a couple of transformers some of them with a multitap switch.

For the feedback voltage divider I've already replaced the resistors R9 R10 they were in spec but they looked like they overheated.

image.png

Tomorrow I will confirm continuity of feedback.

Yes I have a photocoupler in my board (P421) not sure how to test it on the board. I just checked pin 3 pin 4 if the diode is good. And checked if there is no short on the transistor side pins 1 and 2. I can replace it thought i have a lot of spares.

Thanks
 

jadao

Nov 12, 2017
7
Joined
Nov 12, 2017
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I Checked the optocoupler outside the pcb with diode mode and found diode side reads only one way 1.035 and phototransistor side pins reads o/l both ways witch seems ok. But optocoupler doesn't look good on transistor side pins...
image.jpeg image.jpeg
I will replace it to avoid any additional unknown variable to my problem.

Little bit concerned about using a variable ac to monitor dc output because I don't know for sure if the power supply will allow me that. I mean will the pwm controller work normally if I start with low ac voltage?

Besides the optocoupler that I will replace, I would like to monitor dc output because I suspect a high output voltage (I had an electrolyte cap that instantly shorted out) probably because a reference/feedback circuit issue.
Feedback resistors are ok, even bipolar reference transistor c1093 has been replaced.

My V+ dc output is generated from a fast recovery dual rectifier image.png , I have the idea to unsolder the middle V+ leg to monitor it, but not sure if this will lead to catastrophic failure if the feedback circuitry doesn't see any V+ output maybe it will tell the controller to continually try to reach it?
 
Last edited:

jadao

Nov 12, 2017
7
Joined
Nov 12, 2017
Messages
7
Finally I decided just to replace the optocoupler and the shorted capacitor and give it a try by hooking my dmm to v+ and v- so I can monitor dc output voltage.

This time no more short, but output was unstable and fluctuating between 14 and 17vdc with the green output led fluctuating.

I double checked the feedback circuit and discovered the V+ ADJ potentiometer was stuck at 300ohms, replaced it with another one and put it at 75 ohms, now I have a steady light and stable 24vdc.

Checked again after 1 hour and there was no heat neither any abnormal condition.

I will test power supply tomorrow under load and maybe call it solved :)
 

jadao

Nov 12, 2017
7
Joined
Nov 12, 2017
Messages
7
Just to let you know that the power supply works fine under load, thanks for the help and advice. This was worth the 11€ repair
 
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