The PWM signal is from microcontroller (STM32F051***) about 500KHz (48MHz/96)=500KHz . note : control register for PWM - CCR : 0-65000

When I connect battery and PWM signal : CCR = 1, i get output about 1.4v. but changing duty cycle little bit more (CCR = 2) , i get drop in voltage : 0.87v. WHY? for CCR = 3 , no output 0 .

CAN anyone explain why I get that behavior? I don't have oscilloscope nor henerymeter.

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    \$\begingroup\$ D1 diode "kills" all induced voltage. You need RCD snubber network instead or a Zener clamp. dos4ever.com/flyback/flyback.html \$\endgroup\$ – G36 Oct 25 '17 at 19:47
  • \$\begingroup\$ ok . What values do you recommend for R and C (Calmping circuit) \$\endgroup\$ – omar wahab Oct 25 '17 at 19:50

You can't have a flyback diode on the primary of the transformer - it will take all the energy and cause saturation of the core.

You have what is called a "Forward converter" here and there needs to be specific arrangements to reset the flux in the transformer.

It is a "Forward Converter" because the polarity of the windings is such that the diode D2 conducts at the same time as T2 is conducting.

Did you intend to make a flyback converter?

A darlington transistor such as you are showing is probably too slow to function in this circuit. At a minimum you need a resistor from T2 base to ground to quickly make T2 stop conducting. I would recommend using a MOSFET or an integrated PWM controller such as a MC34063. There are many alternatives available.

It is very difficult to develop such a circuit without an oscilloscope to see what is happening.

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    \$\begingroup\$ If it's a forward, there really ought to be a filter inductor after D2, among other things... \$\endgroup\$ – Adam Lawrence Oct 25 '17 at 19:58

It seems that you are trying to implement a flyback converter but

1) topology is wrong 2) component selection is dubious

To recover look for flyback application notes / simulations, for example on texas instruments home page.

However I think you may see some voltage if 1) remove D1, 2) swap pin 6 and 7 of TR1 3) lower the switching frequency to a few kHz or tens of kHz

  • \$\begingroup\$ The three transistor switch will have miserable turn-off speed which causes enormous losses at normal swithing frequencies. The idea to lower the frequency in theory can reduce the losses, but the transformer get bulky. Worse: If you place a switcher which has audible frequency into a space where people live or work, your health is in a danger. Your nose will extreme easily collide against someone's fist. \$\endgroup\$ – user287001 Oct 25 '17 at 21:22

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