LM5030: PWM outputs not working

Hi Amalachukwu Okeke,
Welcome to E2E.
I am not able to give a complete answer without your schematic. Fortunately I can give some pointers based upon the details that you have provided.
Having no outputs is likely due to the low COMP pin voltage. Please refer to the Block diagram given in Section 7.2 of the datasheet. You will notice that if the COMP pin voltage is 0.8V then the voltage on the '-' input of the PWM comparator will be at or below GND. In other words the TON period will be terminated immediately. Hence the outputs will be high for zero time.
The next question that we must ask is why is the COMP pin voltage low when Vfb = 0. Normally if Vfb is pulled low the COMP pin voltage should be high (5V). Hence I need to know what you have connected to the Vfb and COMP pins that may cause this unexpected operation.
I hope these comments assist you.
Thanks
Joe Leisten

The above is the schematic. The second IC is an optocoupler ( FOD2712AR2) the COMP pin is connected to pin 2 of the opto coupler which is the collector of the phototransistor. On the PCB, the Vfb pin is directly connected to the GND pin (pin 7). I am just confused as to why the COMP pin isn't going to 5v despite Vfb being 0 ( 0.05V as measured with an oscilloscope, same voltage with GND). Note that other components of the push-pull converter have been connected. On the second IC which I soldered to compare performance. The values are same as above, but the SS =0.8V and the COMP = 1.34V (on this PCB, I have not connected other parts of the converter, just the timing resistor and capacitor for VCC and VIN)

Hi Amalachukwu Okeke,
This part of the schematic looks fine and the COMP pin voltage should be around 5V.
May I suggest that you try disconnecting the opto-coupler from the COMP pin. It may be that the opto-coupler circuit is damaged and is pulling down the COMP pin.
May I also suggest that you add a 1k resistor between your current sense resistor and the CS pin. This should help to protect the controller in the case of poor layout. Could you please advise the value of resistor that you currently have connected between the CS pin and GND (we normally call this the CS or current sense resistor).
I did happen to notice it looks like the primary of your transformer is not connected correctly for a push-pull configuration. The input voltage should connect to the center tap of the primary and the two MOSFETS should connect to the upper and lower taps.
I hope this helps
Joe Leisten

Hi, do you have any idea why the MOSFETs could get hot even without any load at the output. Judging by the schematic I sent.

Thanks

Hi Amalachukwu Okeke,
Sorry for the delayed response. Your additional question got lost because the post was marked as resolved.
Could you please give me some idea how much input power is flowing when the output power is zero?
This should help to give some idea of the magnitude of the issue.
Thanks
Joe Leisten

Hi Amalachukwu Okeke,
It would be great if you could send some waveforms showing both of these operating cases (loaded and unloaded).
The best waveforms to show would be:
1) OUT1 voltage
2) CS pin voltage
3) COMP pin voltage
4) Drain voltage of Q1
Our office is on holiday until 3rd Jan so there will be a short delay. If this is a problem for you please post a new question which will be answered by one of our other offices.
Thanks
Joe Leisten

Hi Joe, the attached file contains the requested waveforms. I am also thinking if an undersized filter inductor and capacitor could cause that. The output voltage drops from 120V to about 10V when a load is connected, even the smallest of loads. I use FOD2712AR2 chip for feedback, which is an optoisolator and voltage reference combined.

waveforms.pdf

Hi Amalachukwu Okeke,
Thanks for the waveforms.
It is clear from them that there is something very wrong with the power stage.
I know this because the ON periods of the two switches should be equal.
If you imagine the transformer to be ideal then regardless of which switch is ON the same voltage (VIN/N) should be applied at the left of the output inductor. This should cause the inductor current to ramp up.
The inductor current /N should be detected as a voltage across the current sense resistor and the ON period of the pulse should be terminated when the inductor current reaches the required level. This process should be the same for both switches giving the same ON time. You are showing very short ON periods for OUT2 and long ON periods for OUT1.
If the actual ON periods applied to the transformer are like this then the transformer must saturate.
Please check your power stage very carefully. Look for things like:
1) One of the transistors Q1 or Q2 is not being turned ON or is damaged.
2) Transformer phasing is not correct. When each transistor is turned ON it should give a positive voltage at the left of the output inductor.
3) On of your output diodes is damaged
You will know when the circuit is working properly because the ON periods of both outputs should be approximately equal.
Hope this helps.
Joe Leisten