Hello forum,
I am thinking to make an isolated convertr using pushpull as shown below schematic.Please advise is there any drawback in using such a topology . I am making my own because board space / cost is constraint beside it has to meet explosion proof standard .The voltage I try is 3V to 3.25 volt isolation . I can see the regulation is rather poor because of no feed back . Is there any other problems that could arise with such a topology.

Thanks for your time.
PS: topology shown for reference only

 

Here's a simpler version that has the advantage of being self-oscillating.

Another problem with these simple circuits is high voltage pulses generated by the coil destroying the transistors, especially on lighter loads.

Attached Thumbnails

 

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Thanks for reply , can you please provide link to transformer design for these , and how to derate these converters.

-Adi

 

Hi,

Sometimes transistors explode in converters because something goes
wrong and the transistor draws a huge current. Because of this a fuse
is needed to cap the max energy available to the transistors in case
of a failure. It also prevents circuit traces from blowing off the board
because of excess current too.

You need to study transformer design a bit as you need some leakage
inductance to help with the flux 'ratcheting' effect.
If you cant find anything else, try OnSemiconductor's site. They have
excellent power converter app notes and other documentation.

 

Thanks , I will check the Onsemi site . Can one replace a EI core transformer with a Potcore ,I wonder are these compatible if we get similar inductances? .what to look for .

-Adi

 

Hi,

Potcores work pretty nicely for converters. I would definitely look into that.
I like the ease in which the windings can be made too, compared to a
toroid which is difficult.

 

The basic push pull oscillator circuit is very adaptable and can take surprisingly rough service if the components are properly sized.
I have used that basic circuit with the addition of one or two additional capacitors in parallel with the primary and or secondary windings.
It can work with 50 -60 cycle iron core transformers or toroids but the ferrite or potted type cores are better for higher frequency use and do help pick up some added efficiency too.

When done right I have been able to get that circuit to actually work reliably with air core coil setups at several 10's of megahertz using cheap transistors!
And using vacuum tubes it will easily do 100's of Mhz.

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Hi,

I understand these type of converters has to be treated well , I plan to oscillate it at around 50KHz , Can you please advise how to select toroids for this and how to keep turn ratios. Application notes specific to this will be very help full.

Thanks

 

The transformer in this circuit I posted is not critical. It will always oscillate at the optimum frequency. If you use a low frequency mains transformer it will oscillate at a low frequency and if you use a high frequency ferrite it will oscillate at a much higher frequency.

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1. How many milliamp do you require?
2. With your +5v -5v design, both outputs must be loaded for the outputs to deliver current.

 

Does the output NEED to be isolated? Or do you just need a -5v supply?

Since you seem to be daunted by winding the transformer there are very simple ways to get a -5v supply using a cheap 2-wire pre-wound inductor.

 

Hi,

But I guess at higher frequency we require less turns?


I require around 250mA . Do you say if the o/p isnt loaded the converter will fail as soon as its powewred up.


Yes , Thats the issue . The O/P has to be isolated and I require isolated supplies .


Thanks

 

Yes at higher frequencies less turns are required.

The disadvantage with higher frequencies is that switching loss becomes more of a factor since there's a limit to how fast the transistors can switch. Hysteresis and eddy losses in thre transformer also contribute.

The advantage is higher frequency is generally better, smaller transformers cost less and fewer turns mean lower resistive losses.

The circuit also oscillates at a higher frequency at lower loads.

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Do you need 250mA total or 250mA for each output.

 

Sorry for not explaining correctly, 200mA for +ve and 50mA for the -Ve supply.