[Moved] need help on 2 Diode D1 and D2 250 to 5000 watts PWM Dc/Ac power Inverter

[Rambo55]

Need help on the 2 Diode D1 and D2 in the Schematic

 

[FvM]

Re: need help on 2 Diode D1 and D2 250 to 5000 watts PWM Dc/Ac power Inverter

Need help on the 2 Diode D1 and D2 in the Schematic

They are shorting the inverter and must not be there. Who has invented this circuit?

The designer probably meaned to limit transistor overvoltages. There are two possible means:
- anti-parallel diodes to the transistors to cut negative voltages, that can be brought up by inductive loads
- using (power) z-diodes instead of regular diodes anti-parallel to the transistors can additionally cut positive overvoltages above 2 -3 times the battery voltage

 

[OBIALOR]

Re: need help on 2 Diode D1 and D2 250 to 5000 watts PWM Dc/Ac power Inverter

The diode are IN5407

 

[sahu]

Re: need help on 2 Diode D1 and D2 250 to 5000 watts PWM Dc/Ac power Inverter

The diode are IN5407

yes it is IN5407\IN5406

 

[FvM]

Re: need help on 2 Diode D1 and D2 250 to 5000 watts PWM Dc/Ac power Inverter

The diode are IN5407

yes it is IN5407\IN5406

You mean, it's correct to connect diodes across a push-pull transformer?

 

[murali_dece]

Re: need help on 2 Diode D1 and D2 250 to 5000 watts PWM Dc/Ac power Inverter

You mean, it's correct to connect diodes across a push-pull transformer?

Yes those diodes are correct they are used to protect transistors from regenerative currents from Transformer during pulse of time. these diodes will short the reverse current and capable to withstand and protect remaining circuit.

 

[FvM]

Do you understand how a transformer works?

There are two options to protect the transistors against inductive kickback:
- connect reverse diodes between collector and emitter of the output transistors
- connect a Z-diode + rectifier diode series circuit across the transformer windings. Z diode voltage should be at least equal to the supply voltage.

wrong:

right:

 

[betwixt]

Trust FvM. his solution is best !

Consider what happens when one side of the transformer is connected to ground via Q7 or Q8, see what the polaity of the OTHER winding does at the same time and how the diode across it conducts.

Incidentally, the driver circuit is VERY prone to leakage current, it effectively has four bipolar transistors in cascade, the slightest leakage in Q3/Q4 will be magnified many times by the overall gain. If Q3/Q4 were mounted on the same heatsink as Q7/Q8 there could be serious thermal runaway problems.

Brian.

 

[FvM]

Incidentally, the driver circuit is VERY prone to leakage current, it effectively has four bipolar transistors in cascade, the slightest leakage in Q3/Q4 will be magnified many times by the overall gain.

I agree, that the design has other issues besides misplaced diodes.