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PWM Sine Wave Inverter output problem

Started by Myauk February 28, 2012
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