bn44-00604F No Secondary Power

[afnan]

Initially, the FUSE was blown and on further diagnosis, I found that 9NM60N transistor was short as well. Ordered it and replaced it. Did not get any power. Then saw the power resistor right next to the MOSFET was bad and did not show any value. Bypassed it as well considering it was providing GND to the source of MOSFET. Now I get 311 v at the bridge and on the can see voltage pretty much everywhere on the hot side but no voltage at any pin on the secondary side. Nothing gets hot and I have tested diodes, caps, and resistors.

One interesting thing is that on there are three SB5150 diodes. When I check them on beep settings of my DVM they beep for one or two seconds and then act normally.

Any hints on diagnosing the problem is helpful.

The primary side of the transformer has four inputs. If I check those, let's say first and fourth pin, I can see the same voltage that is at the bridge, is it normal?

Board bn44-00604F
TV: UE32F4000

[nomoresonys]

Re: bn44-00604F No Secondary Power

Seems a little strange, maybe check a diode out of circuit, you can use diode mode or resistance/ohm mode.

[dick_barton]

Re: bn44-00604F No Secondary Power

How did you bypass it. It may have been a current sensing resistor providing feedback to the PFC circuitry.
Looking at this from Spare2repair I think it may be a 0.3 ohm resistor.

https://spares2repair.co.uk/samsung-...0e-dhs-rev-1.0

To check the diodes it's best to lift one of the legs on the one to test.

[afnan]

Re: bn44-00604F No Secondary Power

How did you bypass it. It may have been a current sensing resistor providing feedback to the PFC circuitry.
Looking at this from Spare2repair I think it may be a 0.3 ohm resistor.

https://spares2repair.co.uk/samsung-...0e-dhs-rev-1.0

To check the diodes it's best to lift one of the legs on the one to test.

I just removed it and put a jumper there. Yes it was 0.3 Ohm resistor.
I have investigated each diode out of the circuit and they have 0.3v forward based which is consistent with all three. What should be my next thing to focus?

I rekon when these power supplies are running I should hear buzzing sound which I don't.

The board you have referred to is correct.

I have removed JP802 and JP805 and manually given 5v to see if there is a short. The current draw is in mAs.

In your opinion, how do I see if MOSFET is working fine? and if Transformer is not dead

[dick_barton]

Re: bn44-00604F No Secondary Power

You cannot just bypass it and expect the circuit to work. It probably monitors the current through the mosfet and is used by the mosfet driver IC to control its output.
This is just off the top of my head without the circuit diagram available.

What is the part number of the IC driving the mosfet you replaced?
Did you replace the blown mosfet with the same part number 9NM60N

[afnan]

Re: bn44-00604F No Secondary Power

You cannot just bypass it and expect the circuit to work. It probably monitors the current through the mosfet and is used by the mosfet driver IC to control its output.
This is just off the top of my head without the circuit diagram available.

What is the part number of the IC driving the mosfet you replaced?
Did you replace the blown mosfet with the same part number 9NM60N

you are correct. If this was a sense resistor then it would be required for the driver. I did not replace the driver IC. I just replaced the mosfet with same number.

So I will put a sense resistor and then try again. Anything else I should look at ? is there a way to ensure mosfet driver works?

Thanks

[dick_barton]

Re: bn44-00604F No Secondary Power

What is the part number of the IC driving the Mosfet?

[afnan]

Re: bn44-00604F No Secondary Power

what is the part number of the ic driving the mosfet?

3s111

[dick_barton]

Re: bn44-00604F No Secondary Power

With the set powered off check the resistance between ST and GND and also between DRV and GND and VCC and GND

[afnan]

Re: bn44-00604F No Secondary Power

With the set powered off check the resistance between ST and GND and also between DRV and GND and VCC and GND

ST - GND = 0.5 MOhm
DRV - GND = 1 kOhm
VCC - GND = 170 Ohm

[dick_barton]

Re: bn44-00604F No Secondary Power

VCC - GND looks a little low but it's not a short circuit. If you power up can you check the voltage between VCC - GND

[afnan]

Re: bn44-00604F No Secondary Power

VCC - GND looks a little low but it's not a short circuit. If you power up can you check the volage between VCC - GND

VCC and GND is 0.49V DC

[dick_barton]

Re: bn44-00604F No Secondary Power

I would check your mosfet again. If there was no current sense resistor in circuit it may have blown again.

I don't know what the VCC should be for that IC but would expect it to be between 10 - 20 V. Once again I would check the components around that IC and Mosfet.

[afnan]

Re: bn44-00604F No Secondary Power

I would check your mosfet again. If there was no current sense resistor in circuit it may have blown again.

I don't know what the VCC should be for that IC but would expect it to be between 10 - 20 V. Once again I would check the components around that IC and Mosfet.

I will try to source sense resistor and change the MOSFET to see if it does the job. All components in that area report OK.

What I can't understand is that the transformer has 4 pins on the primary. One is connected to + from the bridge, second to MOSFET drain, third is to Bridge - and 4th to the MOSFET driver VCC.

I guess attached is a typical schematic for it. Now on ST, I have 124 v or something. Since I have good ground and ST voltage, I believe the transformer is not turning on to give feedback and constant Vcc. The reason could be missing Rsense. I have tested MOSFET its fine. I have other new ones I will swap anyway.

Am I proceeding in correct direction?

[R_J]

Re: bn44-00604F No Secondary Power

If the mosfet was shorted and the sense resistor was open, then the ic is bad. When the mosfet shorts and the sense resistor goes open, then the 311vdc connects to ic101 via R104 damaging it. Maximum voltage on pin 6 (ocp) is 6 volts
The transformer has a primary winding connected to B+ and mosfet drain, the second winding is to supply VCC voltage once the circuit starts working. The ic has an internal startup cell that supplies a small amount of voltage to start the ic, but once it starts it requires that the vcc be supplied externally.

[afnan]

Re: bn44-00604F No Secondary Power

If the mosfet was shorted and the sense resistor was open, then the ic is bad. When the mosfet shorts and the sense resistor goes open, then the 311vdc connects to ic101 via R104 damaging it. Maximum voltage on pin 6 (ocp) is 6 volts
The transformer has a primary winding connected to B+ and mosfet drain, the second winding is to supply VCC voltage once the circuit starts working. The ic has an internal startup cell that supplies a small amount of voltage to start the ic, but once it starts it requires that the vcc be supplied externally.

I find that in this board the R sense is connected to ground side and if it gets short, worst that can happen is it gives B- to the pin, which is ground in this case. Thanks for explaining transformer ckt.

[afnan]

Re: bn44-00604F No Secondary Power

https://www.semicon.sanken-ele.co.jp...311a_ds_en.pdfDatasheet for the driver

[dick_barton]

Re: bn44-00604F No Secondary Power

According to the datasheet and the circuit shown in post #14 above, capacitor C103 should charge up to 15V before the control circuit starts operation. (Taken from datasheet) This voltage is derived via R103 and the zener diode connected to ST input.

I would check both and if OK then I would tend to agree at this point with RJ that the IC may well be damaged.

[afnan]

Re: bn44-00604F No Secondary Power

According to the datasheet and the circuit shown in post #14 above, capacitor C103 should charge up to 15V before the control circuit starts operation. (Taken from datasheet) This voltage is derived via R103 and the zener diode connected to ST input.

I would check both and if OK then I would tend to agree at this point with RJ that the IC may well be damaged.

The Schematic of the board is different to those typical ckts. In the case of the board, there is no Zener. There are three 24kOhm resistors 72k total in series after B+. The voltage at ST is 130V. ST can handle up to 600Vdc. Since Vcc for the IC is isolated from B+ I wonder how shorted sense resistor can kill the IC. I am asking these questions as a student.

If Rsense shortens it will ground the IC the VRcop will be zero and the Vsc generated from FB pin will make the comparator (sec 8.2 datasheet) try even harder. Other than that I wonder how it can die. The IC costs 30$ and I can get power board in relative price.

[dick_barton]

Re: bn44-00604F No Secondary Power

Without the exact diagram it is difficult to say what is wrong other than the capacitor on the VCC pin should initially charge to 15V and this is supplied via the voltage at the ST pin.
If there is no kick start voltage across the capacitor connected to the VCC pin then either there's a problem with the path supplying the voltage to the ST pin, the VCC capacitor is faulty or the IC itself is faulty.
You mention earlier that VCC - GND = 170 ohms which does seem to be low. Have you checked the components that feed back from the transformer to VCC.

Would be interested in what others think since it's all guesswork on my part.