1
\$\begingroup\$

IMAGE

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.

\$\endgroup\$
  • 1
    \$\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
4
\$\begingroup\$

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.

\$\endgroup\$
  • 1
    \$\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
0
\$\begingroup\$

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

\$\endgroup\$
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.