I am trying to design a DC-DC flyback converter.

Input voltage is 110V and output voltage is 24V. I have used NCP1252A PWM controller.

Flyback transformer windings:

  • Primary -> 24 turns

  • Secondary -> 12 turns

  • Aux -> 6 turns

Switching frequency is 470 kHz (for very small ferrite core)

When I tried to apply 110V, although the MOSFET is robust, gate signal is not created. I can not understand why it is not working. When I removed the MOSFET, the PWM controller created gate signal. Why?

I wonder if the transformer is in saturation. What happened when the flyback transformer is in saturation?

I can not understand how to calculate Rcomp resistor. I examined the NCP1252 datasheet and I calculated as 0R. (Because I am using lower duty cycle than 50%. As I understand We needn't it if using lower duty. But I'm not sure this info !!! This matter is explained at slope compensation section at the datasheet. NCP1252 datasheet

Circuit schematic is below [added in 23.07.2020]

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  • \$\begingroup\$ Maybe you wired up the MOSFET incorrectly. Show a circuit please. \$\endgroup\$
    – Andy aka
    Jul 22, 2020 at 10:01
  • 3
    \$\begingroup\$ First, schematic please. Second, if the gate signal is not present then this means that the circuit cannot start. This is not related to saturation. \$\endgroup\$ Jul 22, 2020 at 10:02
  • 1
    \$\begingroup\$ XY problem. Saturation has probably nothing to do with it. Show schematic. "robust" = not destroyed? \$\endgroup\$
    – winny
    Jul 22, 2020 at 10:24
  • \$\begingroup\$ Is your bootstrap working? You are deepening the XY problem by shifting focus to Rb calculation. Do you even have Vcc? Show Vcc and DRV waveforms.100 ohm pull-down on the gate seems very excessive. \$\endgroup\$
    – winny
    Jul 23, 2020 at 9:22
  • \$\begingroup\$ I used an external supply source for VCC to avoid the VCC problem. Unfortunatelly the problem continues. I'm sure there is no VCC problem. Also I replaced 100 ohm pull-down resistor with higher one (1k, 10k, higher one ... ) \$\endgroup\$
    – Bunyamin
    Jul 23, 2020 at 10:02

1 Answer 1


If the core saturates, the CS pin will exceed 1V and cause the controller to latch off after 15ms or something like that, until the power is cycled.

You can monitor the voltage on that pin and see if that is occurring. Also, measure the primary inductance and see if it is as designed (and double-check your design), preferably under bias if your LCR bridge allows that.

Make sure your transformer air gap was calculated and manufactured correctly.


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