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switch mode mains inverter questions

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Avalanche

Mon Aug 21 2006, 09:54PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

wrote ...
If you like to isolate low-frequency drive you can rectify a free winding on the HF transformer.

That's a good idea

I was thinking though, if I'm pwm'ing the output for true sine, I should be able to use a couple of GDTs perhaps, because of the higher frequency carrier? The only problem is that two of the opposing MOSFETs would need to be held off during each 'half-cycle' of the mains... would the shorting of the gate to source through the unused GDT windings be sufficient?

Hope this makes sense, sponsored by JD+coke

Marko

Mon Aug 21 2006, 10:09PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Output PWM is done at low frequency, in the way that you add deadtime between output cycles.

If you just chop 320VDC rms you will get square wave wich is also 320V rms, and to get that to 230V you need duty cycle percentage that is (230/320)*100% = about 70%.

When you feed that to LC filter you will get nice sinewave 230Vrms output.

Simplest way to do that is just another SG3525 or etc. with common bootstrapped driver like IR2112.
Maybe you could even desing your own bootstrap circuit out of discrete components for fun, since frequency is low and switching times aren't paramount.

Steve Conner

Mon Aug 21 2006, 11:53PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Avalanche: If the PWM waveform is going to reconstruct into a 50Hz sine wave, it has to have a big 50Hz component that will saturate an ordinary GDT. In other words, the duty cycle has to vary between 0 and 100% at a 50Hz rate, and GDTs struggle with duty cycles far from 50%. (The bipolar gate drive used in PC power supplies doesn't count because they turn every other cycle upside down. It's not what you want for an inverter.)

That's what I believe anyway. You can pull fancy tricks with diodes and reset networks to let a GDT pass wide ranges of duty cycles, or you can wuss out and use something like the IR2181 or TLP250.

Avalanche

Tue Aug 22 2006, 12:33AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

wrote ...
...and GDTs struggle with duty cycles far from 50%.

You probably saved me a few days of my life there, I was thinking I had everything ready to go

Hmm onto plan B... I'll grab an isolated 12v supply from the transformer, to provide drive to the 2 upper MOSFETs, and opto-isolate the drive signals as well. I have the sine generation sorted anyway, I'll be using a PIC to generate the sine wave using 1 6 bit port, and the other port can host a bit of trickery maybe. I'll post a full (so far) schematic soon, to see what you guys think.

Steve Conner

Tue Aug 22 2006, 09:34AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sounds good!

If you want a copy of the PIC code I used for my original inverter, just let me know. It was designed to phase lock to the mains and sell power back to the grid, but you should be able to wedge it easily so that it runs standalone. It was an open-source academic project so you guys can blag any bits you like

Avalanche

Tue Aug 22 2006, 04:47PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

sure, if I could have a copy of the code that would be awesome, thanks

What PIC did you use? I'll probably have to modify it to work on a 16F684, but it's mainly the sine generation i'm interested in, ie how you get the right shape.

schematic (incomplete and full of bugs) will be posted soon

Marko

Tue Aug 22 2006, 05:02PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

May be a bit more complex but sounds interesting anyway..

If I get it right pic again uses high-frequency PWM that models sinewave using a H bridge?

I wonder if you could then simply eliminate low-voltage HF oscillator and use pic to generate 50-HZ envelope on transformer output (ala bridge rectifier envelope) and then just put it into sinewave by asquarewave-driven H bridge?

Altough I don't know wich is harder from that.

If you ask me I would just use another SG3525, set correct rms voltage and stick a big LC filter on output.. :p

Avalanche

Tue Aug 22 2006, 06:45PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I think I know what you mean, but anything that interfered with the DC-DC stage like that would make it impossible to achieve regulation... if i'm on the right tracks.

I'm just using the PIC to synthesise a nice sine wave at 50Hz, this is then used to modulate the duty cycle of a higher frequency carrier (triangle) through a comparator. The result is that the duty cycle of the high frequency goes from high to low in time with the 50Hz sine wave, and this drives your fullbridge. hope that makes sense

The problem with just driving the output with another PWM chip and using a filter is that the power factor can be all wrong depending on what your powering from the inverter, ie there could be an impedence mismatch to the load which would waste power, so that's the reason to use true sine. It should also be quieter...

I think this is all mostly correct, but I am trying to learn as I build this thing

Marko

Tue Aug 22 2006, 07:04PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

DRSSTC seems to work fine similar way, just on higher frequency.

Afer thinking again deadtiming of output probably isn't even needed when you can regulate your DC voltage - simply set the DC-DC converter for 230VDC, chop it at 50Hz and filter it.

Low frequency bridge would be fully soft switching that way and actually less power lost (?)

Steve Conner

Wed Aug 23 2006, 10:20AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The way Avalanche described the sine wave generation is the way my inverter worked. The PIC generated the 50Hz sine wave that modulated an ordinary PWM generator. It used a PIC16F84 and the sine wave was stored in a lookup table. I described it all in the appendix of my thesis

Firkragg just mentioned another method of true sine wave generation that I have seen used. You modulate the DC-DC converter stage to produce a full rectified sine wave at 320V peak instead of a steady DC voltage, and then you use a MOSFET H-bridge switched at 50Hz as an "Unrectifier"

(it's called an unwrapper in the literature)

The advantage of this method is lower drive and switching losses, since the high frequency PWM is applied to the DC-DC converter that was switching at high frequency anyway. The four output MOSFETs just plod along at 50Hz.

The disadvantage is that you can't drive reactive loads without the feedback losing control and the waveshape going crazy. (Unless you make your DC-DC converter bidirectional so that it can feed current either in or out of the batteries under control of the feedback loop.) PWMing the four output devices gives full four quadrant operation that can drive any reactive load.

I'll dig out the code later this week :P I'd love to see this project get a new lease of life. Eventually I'd like to see it grow into an all-purpose bidirectional inverter that can be a standalone power source with batteries, or recharge the batteries from the mains, or sell extra power back to the mains if you have solar panels charging your batteries.

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