LM5002: Flyback for high voltage supply

Hi Husain,

Thank for reaching out with your question and for considering the LM5002 in your design.

Can you please send the schematic? It looks like it did not attach in the last post. Can you please let answer the following question as it will help me review the circuit after it is posted

1. What is the input voltage range (Minimum and Maximum)

2. Target output voltage: 126 V

3. Maximum output current? What is the load that the circuit is being tested at now?

Please also provide the waveform of the output voltage that was described above.

Thanks,

Garrett 

Hi, sorry about that. I'll try again with the schematic.

1. Input voltage range is 5-8V.

2. Target output is ~126V for now. This is a test circuit to see how well this works. We will eventually have different voltages, as high as 330V. (We will add more output filtering etc to reduce noise but this is just a test circuit).

3. Max output current is 0.5mA. We are currently not applying a load to the test circuit.

Here's the schematic:

Here is a screenshot of the waveform at SW at 2us/ and 1us/.  HVIN is 5.4V in this case, and the measured output at TP8 is 77.8V.  The sine wave seems like some kind of ringing, at ~500kHz and the spike happens every ~6.6us, so that corresponds with the 150kHz frequency we set at Rt.

Hi Muzafar,

Thank you for posting the waveforms. It looks like the LM5002 is providing minimum on-time pulses, which is why the pulses occur at a frequency of 150kHz. The ringing occurs when the secondary current reaches 0A. The ringing happens between the primary inductance of the transformer and the parasitic capacitance on the switch node. This is normal for a flyback converter that is operating in discontinuous conduction mode.

As this is a test circuit is the circuit being implemented on a PCB or is this being designed on a breadboard? If it is on a breadboard the layout might not be tight enough and this could be causing the strange operation. Can you please probe the COMP pin voltage?

Thanks,

Garrett

Hi Garret, thanks for the reply. It's a PCB but we have some jumpers so we can manipulate values. It should be tight but we'll check the board for errors. Here's the signal at COMP, sitting at ~4.39V. 

Hi Muzafar,

Is the COMP pin voltage really that noisy? Typically the COMP pin voltage should be pretty smooth. Are the compensation components populated correctly? Is the signal being probed with a short ground lead?

Is the FB pin voltage below the 1.26V? 

Also can you provide the layout for the circuit? It looks like the LM5002 is issuing a pulse that is close to the correct value based on the application parameters. It is not clear to my why the output voltage is still below the 126V margin. 

Thanks,

Garrett

Hi Garrett,

Thanks for the reply.

Yeah that's the comp pin. That noise matches what's on the HV output and proportionally on the FB. The FB gets to about 800mV or so. The LM5002 seems to be trying to turn the switch on and then turns it off immediately.

Can we continue this on email and I can show you the layout etc?

Hi Muzafar,

I sent you a connection request. This way we can review the layout through direct messages.

I am going to close this thread as we will be communicating over direct messages.

Thanks,

Garrett