Hello,
I am working on a power converter that uses four phase shifted 50% duty ratio signals, i.e. phase shift of three signals has to be controlled with respect to the first one. I composed and successfully tested the required code for generating these signals on the F2808. Configuration of the EPWM modules is shown in the diagram below (up count only with CMPA=Period/2, Active high complementary outputs, phase loading enabled on EPWM2,3,4). I need the hokey sync scheme below to ensure that the phase shifts remain within bounds. In the simplified version alphaP = alphaS =0 , which leads to :
EPWM2A 180 out of phase with EPWM1A
EPWM3A lagging EPWM1A by Phi_f
EPWM4A 180 deg out of phase with EPWM3A
[Note: pi = Period/2; 3pi/2 = 3/4*Period etc]
Now, I am beginning to use the F28335 and the same code does not work as expected. I get two phase shifts correct (EPWM1,2,3A/B) while the third one (EPWM4AB) is off. After significant trial and error with the EPWM settings I came to the following strange conclusion about the F28335 EPWM modules. Setting the SYNCOSEL for EPWM2 to CTR=CMPB sets the SYNCIN of EPWM4 to SYNCO of EPWM1.
So I reconfigured the EPWMs to the following scheme assuming that SYNCIN of EPWM4 is the same as SYNCO of EPWM1.
Now the output of EPW5 is what I expect it to be. Essentially, in the new scheme I am using EPWM4 to generate the phase shift I was expecting from EPWM3 in the old scheme and then I am using EPWM5 to generate the actual PWM signals for S3s and S4s.
I find it hard to believe that the DSP actually behaves like this. What am I missing? Are there any changes in the EPWM modules going from 2808 to 28335?
Best Regards,
Amit
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Amit Kumar Jain, PhD
Senior Engineer, Power Electronics
Peregrine Power LLC
27350 SW 95th Avenue, Suite 3022
Wilsonville, OR 97070.
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