500W inverter problem

[yusrie]

regards...
i need a little help here....
i'm designing a 500w inverter for my school project..
I'm using a full bridge topology (4 MOSFET connected H-bridge)...
and i'm using an IC SG3524 as a switching element for the MOSFET..
as you know, in H-Bridge there cannot be all 4 switch operating at the same time but that just what happen to my current design...
thanks in advance...

 

[audioguru]

The Mosfets on the high side are N-channel source-followers. The Mosfets need their gate voltages to be 10V higher than their source voltages for them to turn on so the high side gates must be +22V for them to turn on but they have only +12V which will not turn them on.

A Mosfet driver IC has bootstrap capacitors that boost the high side gate voltages to +22V.

 

[Hero999]

It's also squarewave which isn't much use.

Modified sinewave is better, pure sinewave is better still.

 

[audioguru]

I notice that your supply is only 8V so the Mosfets probably will not turn on completely.
You shorted together both opposing outputs of the SG3524.
Output A is the +8V supply.

The max allowed value for the timing capacitor is 0.1uF but yours is 47 times too high.

 

[yusrie]

I notice that your supply is only 8V so the Mosfets probably will not turn on completely.
You shorted together both opposing outputs of the SG3524.
Output A is the +8V supply.

The max allowed value for the timing capacitor is 0.1uF but yours is 47 times too high.

thanks for your quick reply...
i need a 50Hz switching and one of the capacitor and resistor combination is such...
is there any effect using an out of range capacitor?...because the circuits works well except for a dead time problem...
any suggestion on what should i do??...maybe using astable multivibrators as an alternatives....because i'm stuck with this switching problem like a week...i cannot proceed until i solve this switching problem..

 

[audioguru]

The SG3524 is spec'd with a 20kHz switching frequency. Its frequency is 130Hz with its max value capacitor (0.1uF) and max value resistor (100k).
It has push-pull outputs that you shorted together.
It does have a small amount of dead-time (3.3us when its timing capacitor is 0.1uF).

I think you are trying to make a simple 50Hz square-wave inverter like were sold 30 years ago.
Modern inexpensive inverters use a voltage step-up high frequency switching converter that has a small lightweight ferrite core transformer and high frequency PWM to make a sine-wave. They have voltage regulation.

 

[yusrie]

no, i'm going for a pure sinusoidal...240V ac output...
is it posible with the current circuit?...please help this newbie..

 

[audioguru]

Your square-wave inverter will work if you use an oscillator with opposing outputs like a CD4047 that drives two pairs of complementary transistors used to supply high currents to the gates of your Mosfets to switch them quickly. Use a 12V supply.
Gating can be added for some dead-time or the gates can make the circuit into a modified sine-wave inverter which has a square-wave with two steps. The steps have almost the same peak voltage as a sine-wave.

You can use the SG3524 or a TL494 PWM controller IC to make a high frequency carrier and have a 50Hz sine-wave PWM modulate it. It drives a Mosfet driver IC that drives Mosfets. Since it is a high frequency then a small lightweight ferrite-core stepup transformer can be use. The high frequency carrier can be filtered out with a small lightweight LC filter and the output is a sine-wave at 50hz. Negative feedback can be used for voltage regulation.

Another way is to make a fixed high frequency oscillator drive a Mosfet driver IC that drives Mosfets driving a small high frequency stepup transformer. Its high voltage output is rectified and filtered into DC that powers a high voltage pair of Mosfets that are driven with PWM.

Both circuits are pretty complicated.

 

[yusrie]

"Both circuits are pretty complicated"

as complicated as they sounds like....
can you provide a schematic to helps me understand better...

 

[Nigel Goodwin]

"Both circuits are pretty complicated"

as complicated as they sounds like....
can you provide a schematic to helps me understand better...

I've never seen anyone post a circuit for them, ready made ones are available at little cost (far less than the parts would cost), so there's little incentive to build one.

You also have the problem of making the transformer, which is a crucial part of the design.

 

[yusrie]

I've never seen anyone post a circuit for them, ready made ones are available at little cost (far less than the parts would cost), so there's little incentive to build one.

You also have the problem of making the transformer, which is a crucial part of the design.

if it's not a school project i'll just buy one...

 

[blueroomelectronics]

if it's not a school project i'll just buy one...

Does your school care if it's a 500W inverter? A small 5W might be more reasonable and affordable, after all it's just a school project.

 

[audioguru]

A few years ago I was going to design a simple pure sine-wave inverter using National Semi's class-D audio amplifier IC. Then they discontinued the IC because too many blew up.