Is duty cycle related to output voltage?

D

dddvvv

New member
hi everyone.

my first post here.

here's my setup; a microchip pic16f1783 outputs a push-pull pulse train from its psmc module (programmable switch mode controller). at the moment, its on breadboard for testing purposes first.

2013-07-24 22.10.48.jpg

input is 12vdc, into the etd29 size transformer, (or etd19 ?) yanked out of a old computer power supply. its rewounded with 2 x 12 turns primary, and 1 x 9 turns secondary 0.7mm wire.

i can change/programme different values of duty cycles, from 1% to 25%. output from pic is fed into a tc4424 dual mosfet driver, driving 2 x irfz44v, connected to the push pull primary windings. frequency is ~ 45khz. here are the scope images:

View attachment 2222.bmp View attachment 3333.bmp

4.61% duty produces 9.6 volts.
25.3% duty produces 10.0 volts

my question; is this tiny variation in voltage (0.4v) ,between the two different duty cycles(4.6% and 25.3%) normal?

i was expecting a much bigger range/difference in output voltage. at least 5v, since im trying to incorporate voltage feedback.

this is just an experiment to get my feet wet, before i jump into the mains voltages smps. current limiting is working great.

thanks for reading.
 
K

KX36

New member
I have only skimmed your post, but if I understand correctly it's a push pull converter, but I don't see an output inductor and I only see 1 diode for rectification.

Without these components, it won't work properly, the secondary side will just be a peak detector type rectifier and, as the name suggests, that'd output the peak voltage on the secondary regardless of duty cycle. The small difference in voltage would be just due to the load discharging the capacitor during the off time.

You can't just take the secondary circuit from a flyback converter and think it will work in buck-derived topologies as it looks like you've done. They're completely different. The flyback "transformer" is a linked inductor which stores energy, the transformer in a push-pull is an actual transformer, operating on different principles.

Then again, I might be completely wrong, I only read about 2 lines of the post and looked at the pic for 5 seconds :D
 
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D

dddvvv

New member
Thanks for the replies.

Its a push pull and only 1 diode for rectification, on output, with a 5kohm resistor as load.

I will try full wave rectification and filtering, and a bigger load. Thanks for pointing me to the right direction.
 
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