20 Vdc to 230 Vac Inverter Design Approach

[audioguru]

Please explain what is SPWM.

The TL494 has its duty-cycle controled by a DC input voltage.
The output AC voltage of the inverter can be rectified, attenuated and opto-coupled to the duty-cycle input of the voltage stepup TL494 for output voltage regulation.

 

[Ubergeek63]

Oh.. I will need a pure sinewave for my design. If I need lightweight than I can't use isolation transformer right as from what i know is that isolation transformer is very heavy. Am I right?

Can an opto_isolator be used to do the isolation between the two circuit?

By the way.. I have already done my DC DC Boost Converter (20VDC to 325VDC with 94% duty cycle) and DC AC Inverter but the problem i'm facing now is the isolation part and also the short circuit protection.

Another problem i'm facing is the deadtime problem for my DC AC Inverter. when i'm input the supply voltage for the inverter to below 20Vdc, it works well. But when i increase the supply voltage, its keep giving me short circuit problem. Luckily my supply current is not very high to the extend of burning the MOS switches. Is there a good way to implement deadtime delay for rising edge.

Another possibility i'm suspecting is that it might be noise that create the short circuit.

Any recommendation or suggestion?

Using what circuit and FET? It is not typically a good idea to run high duty cycles.

 

[mneary]

Oh... US$20 Inverters? Can I have the model number so that I can take a look at the specification?

They are sold at various electronics stores. They are "modified sine wave". (Which aren't really a sine wave, they are a modified square wave.)

Example at frys.com: VAKOSS 75W POWER INVERTER $14.99.

 

[audioguru]

I read 3 reviews of a cheap small 100W inverter by people who bought it.
Two people say it is junk. One died after powering a 10W load for 2 minutes and one overheated and was returned (but not replaced). The 3rd one still works after 6 months.

 

[skyflyer54]

Please explain what is SPWM.

The TL494 has its duty-cycle controled by a DC input voltage.
The output AC voltage of the inverter can be rectified, attenuated and opto-coupled to the duty-cycle input of the voltage stepup TL494 for output voltage regulation.

Hi,

Take a look at the attached JPEG file. It clearly explain what is SPWM. The bottom waveform is a SPWM where u can see that the duty cycle varies. Unlike PWM the duty cycle is fixed just like a square wave.

 

[skyflyer54]

Using what circuit and FET? It is not typically a good idea to run high duty cycles.

Oh... I'm using MOSFET for my DC DC Boost Converter and IGBT for my DC AC Inverter.

My problem now is on the DC AC Inverter. It is a fact that the top and bottom switches cannot be turned on at the same time else short circuit will occurs due to deadtime. I have tried to put a slight delay for every turning on but it still give me the same deadtime problem when ever i increase the supply voltage.

Is there a good recommended way to implement the deadtime circuit?

Attached JPEG are the inverter design.

 

[skyflyer54]

I read 3 reviews of a cheap small 100W inverter by people who bought it.
Two people say it is junk. One died after powering a 10W load for 2 minutes and one overheated and was returned (but not replaced). The 3rd one still works after 6 months.

Oh.. so do you have a better one to recommend?

 

[Ubergeek63]

Oh... I'm using MOSFET for my DC DC Boost Converter and IGBT for my DC AC Inverter.

My problem now is on the DC AC Inverter. It is a fact that the top and bottom switches cannot be turned on at the same time else short circuit will occurs due to deadtime. I have tried to put a slight delay for every turning on but it still give me the same deadtime problem when ever i increase the supply voltage.

Is there a good recommended way to implement the deadtime circuit?

Attached JPEG are the inverter design.

What frequency? most IGBTs are slow so you would need to make sure you have fast ones like IRFs warps. And at what load? They are even slower at low currents.

 

[audioguru]

You like talking with letters so I looked up SPWM.
A TL494 PWM IC can make SPWM by inputting a sine-wave. It uses a triangle wave internally and comparators to create the vaying pulse widths exactly like your circuit does except it has more functions like adjustable dead time control and feedback.

 

[skyflyer54]

What frequency? most IGBTs are slow so you would need to make sure you have fast ones like IRFs warps. And at what load? They are even slower at low currents.

Oh.. I'm using STGW19NC60WD_IGBT and it datasheet states that it is an ultrafast powerMESH IGBT and it turning on/off time is just a few nanosec.

Actually, no matter how fast the IGBT is switching.. it will have a crossing point between the upper and lower switch when turning on/off. Thus, deadtime occurs...

 

[skyflyer54]

You like talking with letters so I looked up SPWM.
A TL494 PWM IC can make SPWM by inputting a sine-wave. It uses a triangle wave internally and comparators to create the vaying pulse widths exactly like your circuit does except it has more functions like adjustable dead time control and feedback.

Oh... so you means that TL494 can address the deadtime issues?

 

[audioguru]

Oh... so you means that TL494 can address the deadtime issues?

You are confusing "shoot-through" with "deadtime".

Shoot-through is when the upper and lower transistors are turned on at the same time when one is turning off and the other is turning on. A very high current shoots through from the power supply to ground.

Deadtime is the fix for shoot-through because deatime is a pause when both transistors are turned off for a moment so that one can completely turn off before the other turns on.

Look at the datasheet for the TL494 and it is shown. All PWM ICs have a deadtime.
Are you trying to make your own PWM controller IC?

 

[skyflyer54]

You are confusing "shoot-through" with "deadtime".

Shoot-through is when the upper and lower transistors are turned on at the same time when one is turning off and the other is turning on. A very high current shoots through from the power supply to ground.

Deadtime is the fix for shoot-through because deatime is a pause when both transistors are turned off for a moment so that one can completely turn off before the other turns on.

Look at the datasheet for the TL494 and it is shown. All PWM ICs have a deadtime.
Are you trying to make your own PWM controller IC?

lolx.. thanks for telling me the term "shoot-through"... In our sch, the lecturer always said deadtime that why I said it deadtime... I'm designing PWM circuit not IC but I think it good to give it a try on just a single IC as the power consumption and noise will greatly reduced.