HELP! SMPS low output voltage!

[gojunnyo]

I have problems with this Philips FWP3100 mini Hi-Fi sound system because it does not produce any sound at all.
I dismantled the said unit, and come across the AMP+Power Board, the said board should have the following voltage rails, 5V, 12V and 32V.
 (According to the schematic) Unsurprisingly, the 32V rail is to be fed to the amplifier but as I voltage test across the filter caps, it only shows around 1V.

Here's what I've tried so far:

• Connected the 5V rail to the optocoupler (IC700) responsible to feedback "ON" (to turn on the oscillator chip SSC6200)
• Removed the voltage feedback optocoupler (IC702) to test if it is working, and after knowing it is functional, I returned it
• Resistance across the FB pin of the oscillator chip and the ground is 63.5 Ohms, the same resistance across the other side of the optocoupler (IC702)
• Visual inspecting all the electrolytic caps, no bulging sighted
• Changed the TL431 as an assurance maybe it could be the problem
• Resistance across the 32V electrolytic caps is 99.6 Ohms (maybe this could be the problem?)

Could it be the feedback circuit causing the problem? or could it be the oscillator chip is bad?
Btw the datasheet of the oscillator chip (SSC6200) is japanese, and I am a noob at looking at specifications.

Any help would be much appreciated! 

Here's the link for the service manual, it contains the schematic diagram of the said unit
https://elektrotanya.com/philips_fwp3100_05_55_sm.pdf/download.html


Here's the link for the oscillator chip responsible for driving the bigger chopper transformer
https://datasheetspdf.com/pdf-file/822984/Sanken/SSC620D/1

[xavier60]

63.5 Ohms is too low.
I have occasionally found spiked FB pins on SMPS controller ICs.

[gojunnyo]

Thanks for the reply! So its broken then? and If it is, any recommendations of substitute ICs, I cannot find these specific chips anywhere

[PKTKS]

If you have a  ~30V proper PSU with auxiliary
5/3/12V  whatever to replace whole board..

remove board and power via controlled external PSU

If your "LOAD" is not faulty you can diagnosis
"ONLY" the power section

It may eventually require to strip some power rails...
Whatever. You need to discard faulty load from faulty SMPS

And you have 2 supervisors with 3 opto coupled sensors

Paul

[gojunnyo]

Sadly, I do not have any ~30V PSU lying around, and the AMP and Power supply section lies on the same PCB. Although I already tried powering the 32V rail with a 19V laptop charger, the amplifier worked but not to its full potential. My goal is to find the bad component to be replaced. I hope it would be worth the hassle.

[PKTKS]

Sadly, I do not have any ~30V PSU lying around, and the AMP and Power supply section lies on the same PCB. Although I already tried powering the 32V rail with a 19V laptop charger, the amplifier worked but not to its full potential. My goal is to find the bad component to be replaced. I hope it would be worth the hassle.

Two UPS batteries in series would do the trick about 24V.

But you  still need some CC/CV bench gizmo to work

And probably "CUT" some traces

Paul

[gojunnyo]

Thank you for your inputs!

Due to quarantine protocols, all electronics shops are closed and the original IC is not readily available, Can I use pwm oscillator IC from other smps circuits? (forgive me if its a noob question, I don't have a vast experience on electronics yet)

Also I want to utilize the existing NPN Mosfet and Transformer so if I were to design my own +32V smps, where do I start?

Any help would be much appreciated!

[gojunnyo]

Please don't mess with mains, especially Her Majesty's 240V mains.

FYI, the controller datasheet is in traditional Chinese, with only the last page, contact info, in Japanese. I checked the pinout, and it doesn't match the common UC384x family. You might have to find the exact one. Other than simply swapping a part, I don't recommend any sort of messing with mains powered SMPS.

I'm a professional SMPS engineer both by education and by work. As intrestupid as I am, I'm not messing with mains without proper minimum tools like isolation transformer, current limiter, variac, differential probes and another person watching my back.

Sir, assuming if I don't die, can you help me design a driver circuit for the original MOSFET and Transformer? Not just for repair actually, I wanted to know more!

[wraper]

Thanks for the reply! So its broken then? and If it is, any recommendations of substitute ICs, I cannot find these specific chips anywhere

Order on aliexpress or ebay. Search for sc6200

[xavier60]

Thank you for your inputs!

Due to quarantine protocols, all electronics shops are closed and the original IC is not readily available, Can I use pwm oscillator IC from other smps circuits? (forgive me if its a noob question, I don't have a vast experience on electronics yet)

Also I want to utilize the existing NPN Mosfet and Transformer so if I were to design my own +32V smps, where do I start?

Any help would be much appreciated!

I should have looked at the schematic earlier. There is a zener from the FB pin to ground. Has it been ruled out?

[gojunnyo]

I should have looked at the schematic earlier. There is a zener from the FB pin to ground. Has it been ruled out?

The schematic mentioned a zener diode, but there is no zener diode on the actual PCB

[xavier60]

What appears to be the SMD version,SSC620S, is abundantly available on ebay and Aliexpress.
Fakes are always possible, I don't suspect it though.
Maybe there is a good reason for the zener to be present.

[gojunnyo]

It's not just driving something. The chip integrates driving, sensing and controlling all in one.
The converter employs what we call flyback topology, where when the primary switch turns on, current rises in transformer primary winding, converting electrical energy to magnetic energy.
When the switch turns off, the primary coil current collapses, so the magnetic field has to maintain (remember, inductor current can't change suddenly), so it has to go somewhere else -- be dumped to secondary side, thus transferring energy from primary side, through transformer, to secondary side.

In this process, the primary winding can only hold a certain amount of energy, which is defined as Ipeak^2*L/2, where Ipeak is peak allowable current (saturation current), and L is primary inductance.
Passing that point, L drops drastically, and you are essentially switching a shorted load, and tings will blow up.
Therefore, it is crucial to make sure current never goes beyond Ipeak, and that's where current sensing comes to the play.
IS pin of the controller chip senses voltage on current sensing resistor, thus it can know what is the source current of the MOSFET, which is also drain current of MOSFET, which is also primary winding current.
The controller will turn off the MOSFET at a current below Ipeak, protecting the system. This control method is called peak current mode.

To regulate output voltage, there is another control loop called average voltage loop, which takes a reference voltage input (defined by TL431 on the secondary side), output voltage, and outputs an error signal, which then gets compensated for loop response (in this case, a type III compensator).
The error signal gets passed back to primary side through the optocoupler, setting reference current (it has to be below Ipeak, but it can be lower, the lower the current, the less energy transfers to the secondary side, thus the lower voltage on the secondary side).

The primary IC at least controls an inner peak current loop, handles current sensing (and leading edge blanking and more tricks), and handles gate driving. You can design something to replace it, but it will be a very hard task.

Thank you for enlightening me how the IC works, I am stuck here in Philippines and shipping time normally takes 1 month as I inquired, but if I were to design, any tips where to start? or the IC to start from? ICs that are readily found. I'll just remove the transformer and NPN and start building from there.

[gojunnyo]

Thank you all for your inputs! I will update you all whatever happens. I hope the risk would be worth it as much as it provides valuable experience. Keep safe!