I am assuming you are talking about the Tiny-3 spreadsheet? You can lock the secondary turns NS by manually entering the desired NS. Fixing the NS will recalculate your NP based on other parameters you change (VOR/VO). You can do the same exercise for the wire gauge by fixing the OD and then changing parameters of interest. Let me know if I have addressed your query.

Yes, sorry it is TinySwitch-3 not TOPSwitch.

I am able to manually set NS but not NP. What I am really looking to do is lock all transformer parameters (NP, NS,  AWG, etc) and then vary the input parameters (input voltage range, output current) to see how the design behaves under different conditions. In particular I want to see how my 24-277VAC transformer works in the range 100-277VAC.

Thanks.

If you manually lock NS, the only way the NP will change is if you change the VOR or VO. Assuming VOR and VO is fixed, changing the line voltage will affect the inductance LP. Now let us try to address your issue.

1. You have a specific transformer with a fixed inductance and fixed turns ratio. This transformer was designed for 24-277VAC. The VACMIN=24V seems to be small: I assume you have a really low power design and PIXls flags the VMIN. Right? 

2.  If you have already designed for 24-277VAC, then your transformer should also work in the 100-277VAC range as well: Right?

3. What behavior are you trying to observe?

1. Yes, PIXls flags Vmin and the input capacitor size. However, we know the design works and these are not a concern.

2. The design works at 100-277VAC because it was designed for 24-277VAC. The available power in the range 100-277VAC is much greater than at 24VAC. I wanted to use PIXls to find the actual power capacity in the range 100-277VAC.

3. Since I have not been able to find a way to lock the transformer parameters, our fallback strategy is currently to apply 100VAC to the actual physical supply and adjust the load to find when it breaks down. This method is less than ideal of course.

To answer #3, PIXls will perform a full load calculation only. This implies that if you have created a design for 85-265VAC line voltage and a 12V, 1A load, then PIXls will provide you the magnetics and electrical parameters for the full load case only. It will not let you sweep through intermediate load conditions and tell you what happens with the duty cycle, peak current ..etc.

Yes, I understand that PIXls only performs a full load calculation. What I would like to do is lock the transformer parameters and then manually adjust the load to determine what load the 24-277VAC supply can handle in the range 100-277VAC. The problem is that when I change the input, PIXls automatically changes the transformer. I need some way to prevent it from doing that.

You are trying to evaluate the gain in output power when you increase the line voltage for the 24-277VAC design, right?

If yes, I would suggest the following.

1. To fix the transformer, fix the NS and VOR to whatever corresponds to the 24-277VAC design. Also pick the right core and manually overwrite all core parameters. This fixes all the physical parameters. At this point you have a value of LP that corresponds to the 24-277VAC design. Note this value and lets call it LP24. If you now change the input voltage, the KP will change and hence, the LP will change as well.

2. At this point, go to Tools->Goal Seek... in the PIXls menu. Select LP from the 'Parameter Name To Set' menu and enter the value of LP24 in the 'Parameter Target Value' input box. Now select IO from the 'By Parameter Name' box and hit OK. You will see that IO has increased, consequently increasing the load power. Hopefully this method is suitable from what you wish to do. Let me know if you have any further questions.

Yes, that is exactly what I want to do. I will try out your method. Thanks.

Andy