Hi guys
This PSU is actually in an amplifier, but it is the PSU itself that has a problem
I have spent quite a lot of time trying to diagnose this and the more I get into it the more puzzled I become
It has a Standby, or Startup supply (15V) generated by a TOP245GN and this is working
The main SMPS is controlled by a SG3525A and it does not start up
According to the datasheet
https://pdf1.alldatasheet.com/datash...A/SG3525A.html
This SG3525A will shut down if pin 10 is high, or pin 9 or pin 8 are low. Further it says that pin 9 & 8 have an internal pull up and can be taken low (disable) by drawing as little as 100uA externally.
OK so I have pin 10 low, Pin 9 high (this pad does not go anywhere) and pin 8 low - should be high
First I changed the capacitor from pin 8 to ground in case it was leaky (just tagged a 4.7uF electrolytic on there as the example circuit showed 5uF) but it made no difference
Then I changed the SG3525A - no difference
I then reverse engineered the circuit connecting to pin 8 (a couple of G1 SMD transistors and the opto-isolator)
From what I worked out (see attached pic) pin 8 is held low because the transistor connected to it is turned on (base connects to the 15V Standby via a resistor and has 0.47V which seems a bit low but I guess it is enough).
There is also the opto-isolator connecting here to pin 8 but the LED side has no voltage supply so it must be off.
The only way I can see to turn this transistor off is to turn the other transistor on, this transistor connects via 3x 150K resistors and a diode to 240V AC (not the +ve of the bridge rectifier - one lead of the actual AC cable!)
However it does not seem to have enough base voltage (0.35V) to turn on.
There is a resistor divider 150K-150K-150K-1K5 that supplies the base, I removed the transistor and checked the divider. I removed the 150K resistors they all check OK, I measured the voltages across each resistor and applying ohms law this should indeed supply 0.34V to the base (see pic).
** Note I originally had 0.26V on the transistor base, and tried strapping a 10uF electrolytic across the capacitor in parallel with the 1K5 resistor and the base voltage went up to 0.34V
The 1K5 to hot ground reads correct. I thought the capacitor across the 1K5 may be faulty so I removed it. It wasn't leaky but only measured 0.1uF so I replaced it with a 10uF. That gave me 0.34V on the transistor base.
So now I just can't figure out why the circuit does not work.
My best guess of the intentional operation of this circuit is that for the main PSU to power up it needs:
1. 15V standby supply
2. Sufficient voltage on the AC lead (is it intended detect a brown out as a fault condition and shut down the main supply?)
3. The opto-isolator is off - I assume this is driven by some error detection circuit in the amplifier section.
Anyway, could anyone confirm this is the intended operation of the circuit, and explain why it doesn't work or what to check next? Because it is frying my brain LOL!
Rich
This PSU is actually in an amplifier, but it is the PSU itself that has a problem
I have spent quite a lot of time trying to diagnose this and the more I get into it the more puzzled I become
It has a Standby, or Startup supply (15V) generated by a TOP245GN and this is working
The main SMPS is controlled by a SG3525A and it does not start up
According to the datasheet
https://pdf1.alldatasheet.com/datash...A/SG3525A.html
This SG3525A will shut down if pin 10 is high, or pin 9 or pin 8 are low. Further it says that pin 9 & 8 have an internal pull up and can be taken low (disable) by drawing as little as 100uA externally.
OK so I have pin 10 low, Pin 9 high (this pad does not go anywhere) and pin 8 low - should be high
First I changed the capacitor from pin 8 to ground in case it was leaky (just tagged a 4.7uF electrolytic on there as the example circuit showed 5uF) but it made no difference
Then I changed the SG3525A - no difference
I then reverse engineered the circuit connecting to pin 8 (a couple of G1 SMD transistors and the opto-isolator)
From what I worked out (see attached pic) pin 8 is held low because the transistor connected to it is turned on (base connects to the 15V Standby via a resistor and has 0.47V which seems a bit low but I guess it is enough).
There is also the opto-isolator connecting here to pin 8 but the LED side has no voltage supply so it must be off.
The only way I can see to turn this transistor off is to turn the other transistor on, this transistor connects via 3x 150K resistors and a diode to 240V AC (not the +ve of the bridge rectifier - one lead of the actual AC cable!)
However it does not seem to have enough base voltage (0.35V) to turn on.
There is a resistor divider 150K-150K-150K-1K5 that supplies the base, I removed the transistor and checked the divider. I removed the 150K resistors they all check OK, I measured the voltages across each resistor and applying ohms law this should indeed supply 0.34V to the base (see pic).
** Note I originally had 0.26V on the transistor base, and tried strapping a 10uF electrolytic across the capacitor in parallel with the 1K5 resistor and the base voltage went up to 0.34V
The 1K5 to hot ground reads correct. I thought the capacitor across the 1K5 may be faulty so I removed it. It wasn't leaky but only measured 0.1uF so I replaced it with a 10uF. That gave me 0.34V on the transistor base.
So now I just can't figure out why the circuit does not work.
My best guess of the intentional operation of this circuit is that for the main PSU to power up it needs:
1. 15V standby supply
2. Sufficient voltage on the AC lead (is it intended detect a brown out as a fault condition and shut down the main supply?)
3. The opto-isolator is off - I assume this is driven by some error detection circuit in the amplifier section.
Anyway, could anyone confirm this is the intended operation of the circuit, and explain why it doesn't work or what to check next? Because it is frying my brain LOL!
Rich
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