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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I've been reading your thesis Steve, that's some interesting stuff! That's the kind of thing I hope to do one day for my final project, when I eventually go to Loughborough to study EE (future plans).
I suppose when the mains lock is lost, you could switch the 'software pll' onto using an internally generated 50Hz reference... I dunno, you'd still need to keep an eye out for the mains coming back on. This is the kind of stuff that needs to get noticed if distributed / local power generation is ever going to get the go ahead. I think grid-tying my inverter will be a bit beyond me, plus I have no real way of testing it. I'll probably use a cut down version of your code in my inverter, or just bodge a 50Hz 555 in to make it work
After the 29th August I'll have all the time I want to work on this inverter...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I think you can get pretty nice sinewave output with just chopping the output with NE555, sg3525 or whatever you like and filtering it.
For filter I would use some large value caps (470uF caps with diodes) and 21 milihenri choke to get it resoannt at 50.65Hz :p
I would wind the coil on a stem core (like ignition coil) to be able pushing more power trough less weighty inductor (while keeping smaller number of turns). If you want sinewave.
If you want to sinchronyze with mains I would do it rather directly, by turning output of a small transformer into squarewave and driving the bridge with it (you need to watch phasing).
It would be practically same circuit that is used for synchronous rectifying at mains frequency:
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I guess you could do something like that, but the only problem I can see is that it would be too in phase with the grid. I think you would need to be slightly out of phase, so that there would be a load for the inverter... not sure which way it should be though. I guess if it lags behind the grid, it'd be easy to stick a delay in. Must resist making it grid tied for now though, anything I connect directly to the mains seems to result in powercut and angry parents
Anyway trawling through Steve's code at 4.30am is probably not a good idea
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Ok, time for a schematic
First plan is to get the DC-DC stage working, and most of all the control loop. I haven't quite cracked that part yet, I can't decide whether or not I should tie the grounds together on the low voltage and high voltage side, then do the feedback bit with a simple potential divider on the output of the 325VDC, or use an opto-iso. I'm considering just tying the grounds together, to save myself hassle
The schematic also shows the isolated gate drive sections (2 of the same circuit, I think it should work) and the PWM circuit. I'm running the TL494 in common emitter config, and using one of the error amplifiers for the feedback. I think efficiency might suffer if I don't optimise things like excessive gate drives, but I want to get something that works first...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I'm a bit confused though: I never seen those MC35151 drivers before, do you have a datasheet
Also, if you are planing to isolate the gate drives thgat way, you are going to need 4 of them for fullbridge.
Steve Conner wrote ...
..you need at least 3 separate power supplies. Sure, the collectors of the top two IGBTs are connected together, but the gate drive has to be referenced to the emitters, which are connected to the RF outputs.
You can use a common power supply for the two bottom gate drivers, but like most people, I used 4 completely isolated supplies and drive signals. It's not really that much more hassle and you don't have to worry about the L*di/dt voltages that tend to appear between things that are supposed to be "connected together" in power electronics.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Sounds interesting Is this a similar way to what Steve has done, ie store the shape of a half cycle in eeprom, then on every timer interrupt increment the pointer and read the contents of the eeprom onto the port...
wrote ... I'm a bit confused though: I never seen those MC35151 drivers before, do you have a datasheet
here's a datasheet. I have loads of those drivers, nice fast little dual inverting drivers. That schematic is out of date now though, I've switched to UCCs on the DC-DC stage now, because they have the enable pin that I need for the remote control. I figured the easiest and most efficient way to but the thing into standby was to disable the gate drive on the DC-DC converter.
It's just a good job this thing isn't going to be mass produced!!
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switch mode mains inverter questions
