Transformer library and # of secondaries

You will need to create a duplicate transformer (name it EF20 for DPA or something like that) to the EF20 transformer. Only difference is that when you add this transformer be sure to check the "High frequency transformer core" checkbox. see the "Adding a Custom Transformer into the Component Library" video from the Pi Expert website for details http://www.powerint.com/en/design-support/pi-expert-design-software

The bias winding is essential for the operation of DPAswitch and TOPSwitch designs. You cannot design without a bias winding. Only the Tinyswitch and LinkSwitch products have the optional bias winding.

PI-Spock, Thanks for the reply. Why is the bias winding essential? Do you mean that PIExpert doesn't know how to deal with a design that doesn't have a bias winding, or do you mean that it's not possible to operate the DPA without a bias winding? I don't think the latter case is true, as I believe it should be possible in some cases to derive the bias voltage from the input voltage instead of the output voltage, but I'd like to hear your explanation. Thanks Joe

For the PWM devices and also some of the larger MOSFETs in the on-off control devices (like PeakSwitch) the Control pin capacitor is too small to act as the Vcc for the chip. Since the controller sinks about 6 mA of current continuously, this capacitor needs to be held around 5.8 V continuously, and the only way this can be done is through an external bias winding.
For TinySwitch, the current requirement is fairly low (1 mA max), and the BP pin capacitor can be charged through the chips internal current source
So to answer your question, its not a PI Expert limitation, its just the way the chips were designed.

PI_SPOCK, I think you don't understand my request. I know that the DPA control-pin needs a power source for the driver that operates it, but I don't understand why PIExpert forces me to use a seperate bias winding to power that circuit. In my control circuit, the control pin is driven from the emitter of an optocoupler transistor. It seems to me that the collector of this transistor can be powered from any voltage source that is referenced to the same 'common' as the DPA, as long as the absolute max voltage, current and dissipation ratings of the optocoupler transistor are not violated. I want to obtain that power source from the DC input of the power supply and dispense with the seperate bias winding. There is absolutely no hardware limitation I can see that should prevent me from doing this, and in fact, the data sheet mentions alternate bias supply considerations. So why does PIExpert force me to have a winding that I'm not going to need? Joe

Ah! Finally I understand now.
Yes you can provide bias power in the way that you have suggested, provided that the bias voltage is large enough to keep the optocoupler biassed on and mall enough so that it does not exceed the max voltage rating.
However it is a limitaton of the software that it will not allow you to do away with a bias winding... Software was designed to always have a separate bias winding. Sorry about that!

Thanks. Not what I was hoping to hear, but I think you understand the intention.

Why isn't the design of the bias winding done merely by adding a primary-referenced winding in the output configuration dialog? This way, if you don't want it, then you just don't add it. This is something to consider in a future release.

Joe

I'll pass that feedback on to the guys on the PI Expert team.