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4hv.org :: Forums :: General Science and Electronics

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Grid tie inverter

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LAN_403

Ash Small

Fri Nov 28 2014, 05:47PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Mains transformer (for reference)==>>class D amp==>>bridge driver==>>bridge (and SMPS)==>>filter circiut

The class D changes it to a digital representation of a sine wave, the filter changes it back to analogue.

hen918

Fri Nov 28 2014, 06:19PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

furnace wrote ...

So basically you can you use any high kHz frequincy at your gate/base. As long as your secondary's output comes to a clear sine wave?

Well, I have successfully (to my surprise) used an arduino to give an H-bridge driver IC a PWM "sine" wave using the arduino's PWM pins.

This is the code I used:

float value = sin((micros() / timing) / 6.283185) * 255.00;
if(value >= 0) {
outvalue1 = value;
outvalue2 = 0;
}
else {
outvalue1 = 0;
outvalue2 = value * -1;
}
analogWrite(PWMpin1, outvalue1);
analogWrite(PWMpin2, outvalue2);
delayMicroseconds(100);

Because it uses the sine function the output is a clean sine wave, I fed that into the driver IC, and then, after it came out of the H-Bridge I used an Low Pass LC filter to smooth it out.

furnace

Fri Nov 28 2014, 06:34PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

My induction heater uses the same circuit, I just thought that you needed to have a 50 hz output at your secondary for driving ac motors.
This makes everything a lot easier. I haven't used the ardiuno yet but I guess I'm heading to that direction very soon.

Shrad

Fri Nov 28 2014, 06:36PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

here is the picture I missed to include in my post, sorry

if you input a sine wave to a class D amp it will deliver a signal like the one in red before the filter

you're supposed, in this case, to replace the filter with an AC transformer so that you get the blue signal at the output

dexter

Fri Nov 28 2014, 07:32PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195

so a mains transformer (laminated iron core) can be used without the need of any other output filter? because i looked into some UPS's and got me wondering

i know a UPS is different because it require a mains transformer/autotransformer due to its bidirectional usage but my question still stands...

ofc a ferrite transformer + output LC filter takes far less space and is less expensive

BigBad

Sat Nov 29 2014, 04:49PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Yes, should basically work I would think.

However, the best kind of filter for these type of circuits would be almost totally reactive, but the transformer may be fairly resistive due to skin effect and stuff like that.

You should check the power dissipation; the transformer may need a heat sink and you may want to add extra low resistance reactive components to filter off the high frequency currents from flowing through the transformer.

It also helps to run the PWM cycle as far above the 50 hz as you can. that pushes the harmonics way up. On the other hand frequent switching loses more power in your fets or whatever so there's a tradeoff.

Shrad

Sat Nov 29 2014, 07:36PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

that leaves place to a very nice set of experimentations

hen918

Sun Nov 30 2014, 11:07AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

BigBad wrote ...

Yes, should basically work I would think.

However, the best kind of filter for these type of circuits would be almost totally reactive, but the transformer may be fairly resistive due to skin effect and stuff like that.

You should check the power dissipation; the transformer may need a heat sink and you may want to add extra low resistance reactive components to filter off the high frequency currents from flowing through the transformer.

It also helps to run the PWM cycle as far above the 50 hz as you can. that pushes the harmonics way up. On the other hand frequent switching loses more power in your fets or whatever so there's a tradeoff.

Audiophile grade PP caps work well for the filter and the larger the inductor the better!
Running the frequency high also makes it easier to filter out and leaves the filtered sine wave smoother. I would use a frequency in the order of 10s of KHz

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