CCS/TMDSHVRESLLCKIT: How to get the value of FEM

Hi Aiden,

You're right, it doesn't look like FEM = 65 in Figure 23 of the user guide.

1) My understanding for this reference design is that FEM is fixed, unless the user changes it. The user can change the value of FEM through the watch window in CCS. The user should be able to see how the switching waveforms change as the PWM values are changed.

For example, in build 1 the SR signal timing depends on only the PWM counter:

HVLLC-SWGuide, p.17 - Build 1

15. Carefully adjust “Period” and verify that “Gui_Vout” increases when you decrease “Period” and
that it decreases when you increase “Period”.
16. Carefully adjust “RED” and “FED” to see how they affect the Half-Bridge PWM signals.
17. Carefully adjust “REM{X}” and “FEM{X}” to see how they affect the Rectifier PWM signals.

In build 3, the SR signal timing depends on the trip zone event and the PWM counter:

HVLLC-SWGuide, p.22, 23 - Build 3

14. On the oscilloscope, make a note of where the SR PWM signal’s falling edge is relative to where
the SR Current drops back to 0.
15. Set “Comp_Enable” to 1. Observe how the SR PWM signal’s falling edge has shifted. The SR
PWM signal’s falling edge should now be closer to where the SR Current drops back to 0.
16. Now experiment with changing the load. Notice that the SR PWM signal’s falling edge tracks
where the SR Current drops back to 0.
17. Now experiment with changing the “COMP{X}” value. This affects the current level at which the
analog comparator will allow the SR PWM to turn-off.

...

Note: The analog comparators is used to delay the SR PWM’s falling edge and will not advance the
falling edge beyond that specified by “FEM{X}”.

2) The default value assigned to FEM at initialization is 65 from the HVLLC-Settings.h file, but as you've pointed out, the value of FEM in figure 23 likely isn't equal to the default value. I don't see a description in the user guide that says what the conditions were when that figure was captured, so it was probably after the step instructing the user to "carefully adjust REM{x} and FEM{x} to see how they affect the Rectifier PWM signals", or even in build 3 with the comparator action active.

If you find line 343 in the HVLLC-Main.c file and then you right click "FallingEdgeMargin1" and select "Open Declaration" (Keyboard shortcut F3), CCS will take you to the place where the default value is defined.

When the code is running on the device, you can open the watch window and read/modify the value of FEM1, FEM2. You could also inspect the DBFED register of the PWM2, PWM3 to get the current value of FEM.

Best,

Clayton Greenbaum