PWM Sine Wave Inverter output problem

Dear All,

I am building a PWM Sine Wave Inverter using H-Bridge IGBT
Configuration.

The inverter has the following stages:
1. A Sine Wave Oscillator
2. An unity gain inverting amplifier to generate inverted sine wave
3. A Triangular wave generator
4. Two Sets of Comparators to generate PWM waveform for positive and
negative cycles
5. Two Half Bridge IGBT Drivers to generate A and A' signals for each
of the half bridge with death time considered.
6. IGBT H Bridge
7. Output filter.

I have calculated the pass band for output filter using 1/2 * pi *
sqrt{LC} to get 20.6mH inductance and 30uF capacitance with a pass
band of 100 Hz. Feeding the H Bridge output to LC filter, to my
surprise, the output is almost zero, as I use the scope, I have found
a very small sine signal with peak to peak voltage of 100mV! (The H
Bridge output has a peak to peak of 24V)

Any suggestions?

I am just thinking that it might have something to do with the
modulation index when mixing the sine waves to the triangular wave
using the comparator.

Regards

On Feb 29, 1:17=A0am, Myauk <aungkokot...@gmail.com> wrote:
> Dear All,
>
> I am building a PWM Sine Wave Inverter using H-Bridge IGBT
> Configuration.
>
> The inverter has the following stages:
> 1. A Sine Wave Oscillator
> 2. An unity gain inverting amplifier to generate inverted sine wave
> 3. A Triangular wave generator
> 4. Two Sets of Comparators to generate PWM waveform for positive and
> negative cycles
> 5. Two Half Bridge IGBT Drivers to generate A and A' signals for each
> of the half bridge with death time considered.
> 6. IGBT H Bridge
> 7. Output filter.
>
> I have calculated the pass band for output filter using 1/2 * pi *
> sqrt{LC} to get 20.6mH inductance and 30uF capacitance with a pass
> band of 100 Hz. Feeding the H Bridge output to LC filter, to my
> surprise, the output is almost zero, as I use the scope, I have found
> a very small sine signal with peak to peak voltage of 100mV! (The H
> Bridge output has a peak to peak of 24V)
>
> Any suggestions?
>
> I am just thinking that it might have something to do with the
> modulation index when mixing the sine waves to the triangular wave
> using the comparator.
>
> Regards

** 1/ pi * sqrt(LC) i mean, sorry the equation was wrong!

Myauk wrote:

> Dear All,
> 
> I am building a PWM Sine Wave Inverter using H-Bridge IGBT
> Configuration.
> 
> The inverter has the following stages:
> 1. A Sine Wave Oscillator
> 2. An unity gain inverting amplifier to generate inverted sine wave
> 3. A Triangular wave generator
> 4. Two Sets of Comparators to generate PWM waveform for positive and
> negative cycles
> 5. Two Half Bridge IGBT Drivers to generate A and A' signals for each
> of the half bridge with death time considered.
> 6. IGBT H Bridge
> 7. Output filter.
> 
> I have calculated the pass band for output filter using 1/2 * pi *
> sqrt{LC} to get 20.6mH inductance and 30uF capacitance with a pass
> band of 100 Hz. Feeding the H Bridge output to LC filter, to my
> surprise, the output is almost zero, as I use the scope, I have found
> a very small sine signal with peak to peak voltage of 100mV! (The H
> Bridge output has a peak to peak of 24V)
30 uF????  That sounds HUGE!  I think you have to move the knee way above
the desired output frequency.  I use a filter with a knee around 20 KHz
and a 47 uH inductor and a .1 uF cap in some servo drives.

Your 30 uF cap has an impedance of j53 Ohms, your inductor is j12 Ohms,
at 100 Hz.

Jon

On Feb 29, 5:39=A0am, Jon Elson <jmel...@wustl.edu> wrote:
> Myauk wrote:
> > Dear All,
>
> > I am building a PWM Sine Wave Inverter using H-Bridge IGBT
> > Configuration.
>
> > The inverter has the following stages:
> > 1. A Sine Wave Oscillator
> > 2. An unity gain inverting amplifier to generate inverted sine wave
> > 3. A Triangular wave generator
> > 4. Two Sets of Comparators to generate PWM waveform for positive and
> > negative cycles
> > 5. Two Half Bridge IGBT Drivers to generate A and A' signals for each
> > of the half bridge with death time considered.
> > 6. IGBT H Bridge
> > 7. Output filter.
>
> > I have calculated the pass band for output filter using 1/2 * pi *
> > sqrt{LC} to get 20.6mH inductance and 30uF capacitance with a pass
> > band of 100 Hz. Feeding the H Bridge output to LC filter, to my
> > surprise, the output is almost zero, as I use the scope, I have found
> > a very small sine signal with peak to peak voltage of 100mV! (The H
> > Bridge output has a peak to peak of 24V)
>
> 30 uF???? =A0That sounds HUGE! =A0I think you have to move the knee way a=
bove
> the desired output frequency. =A0I use a filter with a knee around 20 KHz
> and a 47 uH inductor and a .1 uF cap in some servo drives.
>
> Your 30 uF cap has an impedance of j53 Ohms, your inductor is j12 Ohms,
> at 100 Hz.
>
> Jon

Finally, I have found the problem, I am using IR2111 as IGBT Driver, I
used two of them for each Half Bridge of the H-Bridge. I tested it
without the bootstrap capacitor between Vb pin and Vs pin. (http://
www.irf.com/technical-info/appnotes/an-978.pdf). I put 33uF capacitor
there and now I am getting the desired output.

I am using 50mH and 50uF for this moment in time with fc of 100 Hz.
The carrier frequency is 5kHz and it is working fine now.

Regards