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Query for use of bias-winding

Posted by: GADV1975 on Fri, 07/30/2021

Hello,

I've used PI Expert (10.3.5) to develop a flyback supply using a TNY285.

I would like to use the bias winding to supply low-power circuitry (< 100mA at about 8V) on the primary side of the transformer. The circuitry has nothing to do with the TNY switcher and is also referenced to mains neutral N and not the 'S' side of the TNY.

The only way I could create an extra primary side winding was to enable the Use Bias Winding in the project options and adjusted the voltage and current setting of the bias winding to 8V and 100mA. The circuit design optimised without any errors.

My question is: would it be ok if I disconnected the bias winding completely from the TNY (BP/M and S pins), leaving the current-select 10uF cap in place and used the bias winding with diode-cap rectifier to drive my circuitry instead? The attached pic shows intended use.

Thanks,

Gary

A suggestion for addition to PI Expert: it would be very convenient if PI Expert would allow a designer to add auxiliary coils to the primary side of the transformer to be used to power circuitry un-related to the PI switcher. Perhaps the in the Outputs menu, an option could be added to allow a designer to assign an output to the primary or secondary side of the transformer?

I initially created a floating coil on the secondary side, but then there's no way to 'force' this coil's connections onto the primary side in the transformer design. This is also not ideal as the isolation between the floating coil (which, in my case, would be referenced to the primary circuit) and secondary coils also then becomes an issue.

 

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Submitted by PI-Ayel on Fri, 07/30/2021

Hi Gary,

Yes, you can do that as an alternative. However, take note of the voltage levels that you want. The bias winding in this engine is designed to supply the BP/M pin at 5.85V so you may want to tweak it slightly to power up your external circuit.

As of the moment, the PI Expert engines are designed to at least generate the fundamental circuit requirements. As for your suggestion, that's a good feature to consider!