I have designed a Flyback using UC3845, PCB and components assembling are done. But I have some problem in transformer. My primary winding is 40 turns ( 2 paralell 0.40mm), and secondary winding is 9 turns(3 paralell 0.65mm). When I apply around 100kHz square wave in primary windings and observe secondary, the results are in shown below in picture. It is saturated? Can any body comment this issue? It can some problem?

Thanksenter image description here

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    \$\begingroup\$ Are you applying the signal from a function generator? \$\endgroup\$ Mar 22, 2020 at 16:11
  • \$\begingroup\$ yes,from a function generator \$\endgroup\$ Mar 22, 2020 at 16:21
  • \$\begingroup\$ What’s your primary inductance and core cross sectional area? Have you tried simulating your circuit? \$\endgroup\$
    – winny
    Mar 22, 2020 at 18:32

1 Answer 1


Shortly, it's the effect of \$V_L=L\ di/dt\$.

Here's the equivalent diagram of your setup (neglecting the real model of the inductor):


simulate this circuit – Schematic created using CircuitLab

The inductor sees a positive DC voltage for the half of the period and a negative DC voltage for the rest of the period. So, from \$i=\frac{1}{L}\int{V_L\ dt}\$, the current changes linearly. This linearly-changing current drops a linearly-changing voltage across the output impedance of the function generator (probably 50 Ohms).

These drops show themselves as ramp-tops as you see on the scope. So, nothing to worry about. It's naturally expected.

  • \$\begingroup\$ Now, I apply to 10V to UC3845, and I observe a oscillation above PWM signal. \$\endgroup\$ Mar 24, 2020 at 19:59
  • \$\begingroup\$ @alimutlu that is not related to neither your question nor my answer. \$\endgroup\$ Mar 24, 2020 at 21:21

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