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PWM signal Error(unwanted PWM signal)



 HI.

I use 28335 DSP for 3-level inverter application and now I have trouble with PWM out put signal..

In my system I use epwm interrupt routine for control(main control program is in this routine).

The problam is that an error PWM pulse is generated at the PWM signal output, 

it looks like the refernce is changed from low level to high level just in a sample period(20kHz).

Fig 1, Fig 2 show that  the experimental phenomenon.

Figure 01

Figure 02

In Figure02

Green : control period(20kHz)-interrupt period

Brown : S1(top switch) PWM signal (in Figure 01, green)

Red : S2(middle switch) PWM signal