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I designed a dc-dc flyback converter that is expected to reach 30 W output power. But it seems there are problems with reaching maximum power. Firstly I tested without the input filter and with non-isolated feedback. Applied 25 V input voltage and 8.6 Ω load. I used a current probe to measure input and output currents. The current probes are set to 1 V/A.

Schematic
schematic

Layout
PCB layout Assembly Plot

Converter Specs
Input Voltage: 9-36 V
Output Voltage: 15 V
Output Power: 30 W
Switching Frequency: 200 kHz
Controller: LT3758A
MOSFET: ON Semi FDMS86181
Diode: SS8PH10HM3_A/H
Transformer Lp = 2.4 uH
Transformer Ls = 2.4 uH
Transformer Core: EF20H55

Test Results:

(Output Caps. 2 x 10 uF parallel with 1 x 220 uF)
Input Voltage: 25 V (Given in the figure.)
Avg. Input Current: 1.4 A
Output Voltage: 15 V (Oscillating ~1 Vp-p)
Output Current: 1.6 A (Given in the figure.)
Primary MOSFET getting warmer.

Input Current: Avg. 1.4 A (In: 25 V)
Input Current: Avg. 1.4 A (In: 25 V)

Output Current: (Out. Voltage: 15 V Load: 8.6 Ω)
Output Current: (Out. Voltage: 15 V Load: 8.6 Ω)


I attached the schematic and layout. I am not an expert, but output current oscillations are kind of problematic. And MOSFET getting hot while reducing input power (I guess related to RCD snub). How can I fix these oscillations and heat problems?

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    \$\begingroup\$ I see 3 volts p-p and nothing that tells me that it's current. You haven't said/shown so many things that it's worrying. \$\endgroup\$
    – Andy aka
    Nov 6, 2021 at 17:28
  • \$\begingroup\$ Please show you probe setup. \$\endgroup\$
    – winny
    Nov 6, 2021 at 18:00
  • \$\begingroup\$ What's the total value of the output capacitance? At glance, it seems there's no adequate ripple filtering at the output. Post your schematics because there are many flyback topologies. \$\endgroup\$ Nov 6, 2021 at 18:01
  • \$\begingroup\$ @SamGibson Thanks for your interest. Yes, the current probes are set to 1 V/A. The physical photos are not available right now. \$\endgroup\$
    – alperuzum
    Nov 7, 2021 at 17:53
  • \$\begingroup\$ @Enrico Migliore Total output capacitance is 240uF. 2 x 10 uF parallel with 1 x 220 uF. \$\endgroup\$
    – alperuzum
    Nov 7, 2021 at 19:50

2 Answers 2

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An R+C snubber across the MOSFET, diode or both (Q1, D3) will dampen the ringing. Whether this is manifest at the input or output (or both) due to common mode coupling or strays of the components, isn't clear.

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Try doubling the output capacitance. If the ripple lowers than the problem is that your capacitors have a high ESR and aren't good at filtering a 200 KHz PWM. Try TH aluminum electrolytic caps. SMD is no good.

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