Drain Voltage Reported Incorrectly

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I did the design in the attached Sample3.uds file.  In the Design Results, it reports an Actual Estimated Drain Voltage of 666.69 volts.

I then changed VOR, in the Key Parameters tab, from the default value of 108.7 volts to 100 volts, and saved this as Sample4.uds.  I would expect the VDRAIN Estimated value to go down accordingly.  However, the Design Results still reports 666.69 volts!

AttachmentSize
Sample3.uds224 KB
Sample4.uds224 KB

Hi

 

You are using a zener clamp, therefore changes in the VOR will theoretically not change your peak drain voltage once the mosfet turns off. Only the plateau while the secondary side is conducting will change, but the initial spike will stay the same. 

I realize now that what I really should have changed is the "Target Clamping Voltage" in Clamp -> Components.

However, when I do this, it has no effect.

Hi

 

The Target Calmping Voltage field is really meant to help with the RCD clamp. If you want to change the clamping voltage with a zener clamp, simply change the zener diode manually by clicking on it with the left mouse button on the schematic screen, or in the Clamp -> Components dialog by clicking on the calculator symbol inside the "TVS Zener Clamp" section.

 

Turth be told though, it would be logical to have the zener clamp also linked to the "Target Clamping Voltage" in order to automate the process. Thank you for bringing this to our attention. We will update this in the future

Thank you.  Manually changing the diode does indeed change the reported Drain Voltage.

However, please note the following:

1) Changing the Zener does not change the reported Clamp Dissipation.

This is true, even if I select a Zener diode with a voltage less than VOR.

2)  Selecting a Zener with a voltage less than VOR does not trigger an error report.

Actually, according to information in PI's App Notes, there should be an error or warning reported if the selected Zener is less than VOR * 1.5.

3) According to PI's App Notes, the worst case Drain Voltage is 1.4 times the zener voltage (plus 20 volts).

This 1.4 factor does not seem to be taken into account when reporting the Drain Voltage.  (It appears that the 20 volts is included.)

There seem to be some issues with the RCD + Zener clamp, similar to those described in my last posting for the Zener Clamp.  (I have not examined this in detail.)

I hope you can address these in the next release, as well.

Thank you.

Hi

 

Thank you for checking this. You actually found a bug. It seems that if the user tries to change the "Target clamping voltage" an error will indeed appear if the user goes below the VOR, however if the user changes the clamp directly, then no error will pop-up.

 

Regarding your concerns with dissipated power, and ignoring the zener below VOR (as this would not allow us to get power on the output), I can say that theoretically it does not matter what zener you select. In theory the dissipated power is always the same (it's basically the energy stored in the leakage inductance). In practice this is not true because  energy  is dissipated in the primary winding, and other parasitics play a role too however we only try to estimate the dissipated power, therefore we don't take this into account.

 

As for our appnotes they tend to give practical guidelines, so choosing a low voltage zener does have some disadvantages due to increased EMI, increased dissipation (because of the difference between practice and theory).

 

Once again, thank you for calling the points to our attention though, I really appreciate it. We will correct the bug you pointed out in one of our next releases.