inverter output transformer saturation

[franticEB]

Hi,
i'm testing an inverter with these features:
Fswitching=220KHz
Fout=3KHz-10Khz
Vout=150Vac
Pout=150W
Current feedback.
Sinusoidal output is obtained with transformer leakage inductor+output capacitor.
In the figure below there is the schematic.



The problem is that a little DC component (about 100mA) put transformer into saturation and the output waveform has a distorsion as in figure



I don't understand from where it can come this DC component. Could you help me?

A capacitor could solve my problem? what features should have this capacitor? Ceramic or film cap? I had thought of 47uF@25volt as the voltage across it should never be too high.

Let me know.

 

[andre_luis]

You should add something at the circuit so that you could be able to tune the delay on each path of the switches. In most of the cases, the saturation is due to a unbalanced conduction on the positive/negative flow of the current on the transformer winding.

 

[franticEB]

You should add something at the circuit so that you could be able to tune the delay on each path of the switches. In most of the cases, the saturation is due to a unbalanced conduction on the positive/negative flow of the current on the transformer winding.

Could you give me a more detailed example of what you have in mind? Thanks

 

[crutschow]

A 47µF cap in series with the transformer primary will block any DC current.
You could use two back-to-back 100µF electrolytics for that purpose.

 

[Audioguru]

As I said on the other website forum, the upper Mosfets are missing a boost circuit for them to fully turn on that boosts the gate voltage to 10V higher than the 70V supply voltage. A Mosfet driver IC does this.

 

[dick_freebird]

Specifically, a bootstrap type power MOSFET driver.
Low side driver types won't. Many half-bridge and
full-bridge drivers, do. They may need a supplemental
bootstrap diode and will need a source-connected
fly cap, per leg.

 

[franticEB]

As I said on the other website forum, the upper Mosfets are missing a boost circuit for them to fully turn on that boosts the gate voltage to 10V higher than the 70V supply voltage. A Mosfet driver IC does this.

Actually a board has 2 half-bridge drivers with 4 isolated power supplies, one for each mosfet. So there is no need of bootstrap cap.

 

[FvM]

DC component in transformer will be brought up due to the bad considered design:

- PI-controller generates unknown DC component
- transformer blocks the DC component
- current sensor can't sense it

Need a circuit means that assures DC-free bridge control signal, or place current sensor in transformer primary

 

[Audioguru]

Actually a board has 2 half-bridge drivers with 4 isolated power supplies, one for each mosfet. So there is no need of bootstrap cap.

Your schematic showed a completely different circuit with only one 70V power supply.
I guess I am wasting my time looking at your schematic of a different circuit than you are talking about.