Distortion in single phase inverter

[fadhel93]

Greetings.
Trying to make a sine wave inverter.
Sinusoidal PWM, an mbed driving an H-bridge with two IR2186 at 20KHz




the outputs from the mbed are clear half sinewaves but the output without the filter has distortions every cirtain time



(the code)

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/*
*********************************************************
*               Fadhel Ali 201102569                    *                                      
*                  DC to AC Inverter                    *
*********************************************************
*/
#include "mbed.h"
#include "QEI.h"
#define size 512
#define ph50Hz 4400
#define pi 3.14159265
 
 
 
PwmOut out(p21);
PwmOut out1(p22);
PwmOut out2(p23);
PwmOut out3(p24);
QEI wheel (p15, p16, NC, 16);
DigitalIn  switch1 (p19);
DigitalOut base(p30);
 
        
        
int main() 
{
    out.period_us(50);
    
    float sin1[size],sin2[size],sin3[size],sin4[size];
    float omega;
    int x=0;
    int y;
    int n, z=0;
    base = 0;
    
 
        for (int i=0; i<size; i++)
    {
        //fill the arrays with the sine wave's amplitude values
        omega= 2*pi*i/float(size); 
 
        sin1[i]=(sin(omega));//half-wave rectified signal     
        sin2[i]=(sin(omega+pi));//180 deg phase so each phase will not work togother       
        sin3[i]=(sin(omega));    
        sin4[i]=(sin(omega+pi)); 
        
 
    }
   wheel.reset();
 
    n=ph50Hz;
      
       
    while (1)
    {
    z=wheel.getPulses()*100;   //read the state of the shaft encoder
    x+=n+z;
    y=(x>>14) & (size-1);  //controls the frequency
    //call the sine wave arrays
    out=sin1[y];   
    out1=sin2[y];
    out2=sin3[y]; 
    out3=sin4[y]; 
    }
}

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what could be the problem

 

[Warpspeed]

What type of load is there on the output ?
It may not be happy running without any load at all.

 

[fadhel93]

thank you for replying,
its a resistive load, about 17watts

 

[BradtheRad]

It appears to go into high frequency oscillation, during those parts of the cycle where the amplitude drops.

 

[fadhel93]

yes in the DC to DC converter, there are some ripples. but I checked the dc to dc converter seperatly and it was fine withoud any voltage drop or regulation but only when it is connected to the h-bridge the voltage drops.

 

[BradtheRad]

See the circled areas below. The mosfets are not fully turning on or off during those parts of the cycle.



The switching is too rapid for them to act. This is typical of parasitic (high-frequency) oscillations.