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Push Pull Converter Problem (Switch Mode PSU)

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aliasgherman

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Dear Experts,

I am having problems in driving a center tapped transformer via a push pull converter. No matter what turn ratio I use, (with switching frequency = 58kHz) the output voltages are always too high. And the input current is extremely high (at very low output loads the current >= 9A).

I suspect that the issue is related to transformer (toroidal core transformer with turns ration 60:30, 20:10, 40:20 are tried but to no success). My reason for suspecting transformer is that if I use a simple 50Hz transformer, I get a smooth output voltage (similar to the pwm - based inverters just that I dont get a sin wav output) and am able to drive little bulbs.

I would like to know if transformer winding is the problem OR if the problem is known and someone can help me troubleshoot.

I have attached the circuit diagram (taken from source **broken link removed**) but some filtering components were removed.
 

You will have to provide information about the type core material and core size as well. This is needed for calculation.
 

Dear E-design,

Thanks for your reply. The core used is toroidal Ferrite with approximate dia 1.4 inch
 

It is important to know the core material type as this will determine the peak flux swing allowed @ operating frequency, and from this the v/t and turns ratio.

Without this information it becomes a bit of trial and error to design the transformer as you don't know when core saturation can occur.
 

Dear,

The toroidal core is salvaged from an ATX power supply (the one used for coupled inductors in the supply). The wire used is 26 Guage, with number of turns 20:10, 10:20, 60:30 ................ etc etc etc. All the times, I have seen that usually output is much higher than expected.

Also with very minute loads, the input current is increased too much with the switching MOSFETs heating a lot.
 

It won't be possible to do the required calculations without the data I mentioned. Trial and error is the only other option. Output toroid chokes often are made using powdered iron material which the manufacturing characteristics can vary over a wide range. Impossible to know which type you have used.
 

An uncontrolled push pull can be quite efficient with little or no output inductor needed but the output voltage will rise and fall in unison with the input voltage (times the turns ratio), the primary windings need to be closely coupled to reduce leakage inductance between the two, else you will need RC snubbers across the drain to source of each fet to absorb the energy stored in the leakage inductance every switch cycle, for high leakage these snubbers may need to dissipate 0.5 - 5W in the resistor and the caps have to be able to handle the peak currents. For a controlled PWM push pull the above still applies but you really need to run current mode control (with slope compensation) for duty cycles above 50% otherwise you may well get subharmonic oscillations - which are well covered in industry literature - a good size o/p choke is required with PWM control ideally to keep the full power current ripple below 20%, the bigger the choke the less slope compensation need also. You may also need RC snubbers on the o/p diodes to limit the peak voltage overshoot on these. Hope this helps, Orson Cart
 
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