Switching Mode Power Supply half output problem

Thread Starter

wardjk

Joined Jun 20, 2019
10
@Dodgydave: The smps is working; details are in my post on 26-June. I didn't design the board; I just drew what I saw but didn't consider it wrong. A distinction that I didn't notice before my original post is that the switcher in example in figure 44 was a TOP256Y rather than the TOP245PN as is on my board which has no pin L. I began troubleshooting by searching for TOP245PN which lead me to the datasheet. From the examples, I settled on the one that appeared most similar to mine, suspecting that the designer wouldn't bother to significantly deviate from the example other than to eliminate the outputs for voltages not needed. None of the examples use the P or G package as on my board although I didn't notice that at first. When I later discovered the significant deviation from the example, I considered the possibility that the board had been designed for a predecessor of the TOP245PN and was based on an example in a different data sheet so I pored through other data sheets and application notes from the manufacturer but didn't find anything that more closely resembled the board I had. That's when I decided I needed to undertake the tedious task of drawing the schematic. The image below shows what I used. The bottom is a mirror image. As far as I know, the schematic I posted on 26-June resembles it more closely than any of the examples we've cited.
 

Thread Starter

wardjk

Joined Jun 20, 2019
10
I should have mentioned that I was also curious when I saw that the 12V winding is connected to the 5V diode cathode on both the board and the partially applicable figure 44 schematic in my initial post. The data sheet explained it:
"DC stacking (the secondary winding reference for the other output voltages is connected to the cathode of D10 rather than the anode) is used to minimize the voltage error for the higher voltage outputs."
@AlbertHall also confirmed this in his reply:
It is quite common for one output to 'sit on top of' another output like that. For example see the schematic in post #1.
 

Thread Starter

wardjk

Joined Jun 20, 2019
10
After my initial repair by replacing the obviously faulty capacitor didn't fully restore the PS, I ordered replacements for all other electrolytic caps just as a best practice. I wasn't sure that would solve the problem so I continued to troubleshoot the problem in search of a more definitive answer while waiting on the replacements which is when I discovered the symptom in my initial post. Several people listed the true cause of the problem among suggestions.
capacitors faulty
Perhaps a bad capacitor in the feedback loop.
possible feedback loop fault
usually the caps.
Let us know how it turns out.
Other suggestions were also logical and the right approach. I wanted to give credit where credit is due. Thanks everyone.
 
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