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Registered Member #1506
Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18
Greetings,
Please see the attached extra rough mspaint schematic. Note that the 555 stuff isn't shown (real basic astable oscillator with a potentiometer to adjust frequency, running at very close to 50% duty cycle), nor is the transformer. The half bridge is being driven by two IBGTs (1200V, 30A) across a ten turn primary, and I haven't decided on capacitors for the other end of the primary yet. Planned to have a frequency range of 350Hz to 350kHz.
In the picture below, the two (highly tentative) GDTs (1:2 to get 24V to IBGT gate) are being driven from pin 3 of the 555 via 2 mosfets (link to digikey below) and the simplest of inverter circuits. The 12V supply is a computer PSU rated for 30A on the 12v rail.
I have several questions though, that I was hoping to get some help with.
1. The value of R1. Obviously I want a little dead time between pulses to prevent shoot-through, but if R1 is too high then the right mosfet might take a little to long to shut off, resulting in shoot-through anyway. Suggestions?
2. I've never understood why with GDT secondaries are always connected to the higher voltage DC side (other than perhaps full electrical isolation). Why doesnt this cause the first IBGT to fire to be in an always on state, or blow the hell out of the gate from the excessive voltage? Thus, I tentatively grounded it back to the 12v ground. Th only reason I am thinking of using a GDT since it wont be providing electrical isolation, is the fact that it (probably) switches a lot softer than driving the gate directly (the data file on my IBGT suggests that they are 'ideal for soft switching applications), and yet will provide a higher overall voltage (24V) to better drive the IBGT.
3. I decided not to go with a GDT set up with a single mosfet, because I felt that I couldnt be sure of the accuracy and timing of having capacitors discharging through a resistor for the second half of the cycle. Am I just being foolish in this regard?
4. Finally, I would be welcome to any opinions/suggestions as well.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Here's the simplest way to do what you want. Adjust the 555 section to suit your frequency requirement.
The high side switch has to "float" for a bridge to work properly, this implies no stable ground reference is available. The simplest way to counter this is to use a transformer which is always referenced to the source/emitter, with an output that swings +/- XX volts. An IGBT or mosfet only cares what it's gate voltage is compared to it's emitter/source, and if you look closely you'll see the gate will never sees more than the GDT itself provides. When using the source as ground of course, compared to the power ground the gate will swing from ground to V/2.
Registered Member #1506
Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18
First, allow me to say thanks to Uzzors for the simple design schematic, as well as the easy to understand explanation of why the gate drive has to be wired that way on a half bridge circuit. I also would like to apologize, as I had at one point typed out a thank you, but apparently I did not get it posted.
So, based on "Erik's Flyback Driver MkIII", I contructed the control portion of the circuit on a breadboard, wound my GDT on a toroid from an old ATX power supply, mounted my diode bridge and two IGBTs on an old athlon heatsink (w/ fan), and all the misc. wiring.
(Prescript: Pics to follow post.)
I decided not to try and step up the line voltage (120v), as I dont have a good core to wind a step-up transformer on at the moment.
However, first light ended badly, possibly for several reasons.
1. I had no real load on the primary...secondary is not yet wound. 2. I was using a 600W cieling fan dimmer as a means of regulating current into the system.
I'm not sure which of theme caused the problem, but it sounded kinduv like the system was switching very hard at 60hZ for about two seconds, and then something blew. There was no sign of damage to the IBGTs or diode bridge, or an excessive amount of heat. I therfor assumed that the cieling dimmer is what had blown. Even more foolishly, I just now realized that I threw the dimmer away without even trying to test it with my multimeter. Damn.
What was odd about the dimmer is that it only had one wire in, one wire out. Other wall dimmers I have worked with had the usual white/black lines in AND out.
I then tried hooking up a 300W light dimmer, and then a 100W light bulb in series, neither of which worked (the actual behaviors in a sec). Finally tried hooking up the line voltage directly to the diode bridge, and got an extremely odd result. Halfway down the line from the IGBT to the negative terminal on the diode bridge, I got what appeared to be a shorting arc. On closer inspection however, the insulation on the line appears to be intact, and whats even wierder is that after surrounding the line with 5 or 6 wraps of 6mil polyethylene, the bright white spark still appears, in the same place, and heated the polyethylene up enough to set the inside on fire, but the outside showed no burn marks where it might have arced through! Plus, there should not be more than 170v on that line, and a good cm of air on all sides around it. Bloody odd.
So, anyways, what I have been able to observe with my multimeter, and no, I dont have a scope and cant afford one (yet).
3. The ~12V side of things seems to be just fine. A factory 120->12v, 3A transformer plus a diode bridge scrounged off an ATX PS provides power to this circuit. Tested at various places on the breadboard and the voltage seems right. After the capacitor, my multimeter only shows ~5v though in either DC or AC mode.
4. The GDT itself is wound on a toroid that appeared to be being used a GDT in the ATX PS I recovered it from. I also wound the GDT, as per suggested numerous times on these boards, with wire from some CAT5e line I had kicking around. However, I took the outside insulation off the wires, and I wound them in pairs. I used Orange/White for the primary side, Blue/White for one IBGT and Green/White, wound in the opposite direction, for the other IBGT. At the end of both of the paired wires on the secondary side I twisted then soldered them together (independently), so that on both secondaries white goes to "ground" and colored goes to gate. Testing shows 30v on the colored wires going to the gates, so that sounds about right with a slightly greater than 1:2 ratio on the GDT.
However, the hZ function on my multimeter seems incapable of measuring the frequency of anything but wall current, which is right at 60hZ, as I get odd, changing, and seemingly random readouts.
5. I couldnt find any 750+V capacitors rated at 100uF or more, so I used 2 sets of 3 250V, 210uF capacitors in series for an effective 2 (750V, 70uF) capacitors on the power side. Obviously this now overkill, and I might at some point rearrange them to be parallel instead of series for 2 (250V, 630uF).
6. I mounted both IBGTs and the diode bridge on the same heatsink, however they are insulated with thermal pads and/or 6mil PE sheet with arctic silver thermal paste on both sides of the plastic. None of them have gotten warm yet, but then, I dont think they have actually been running.
7. Well, hell. I just turned the circuit on to do some more tests, this time with the 120v line hooked up to the diode bride. Smelled something burning, killed the power, and found IBGT 1 to be really hot. On closer inspection, it has a hole burned through it right in the dead center, just above the pin. >_< I wondered why I was now finding that 24V AC current everywhere in the power side of the circuit. Damn damn damn.
8. I cant help but wonder if the diode bridge is dead, which exposed the IBGT to AC current until it burned up. Damn, its rated to 1200v though. Ok, lemme test the diode bridge with my multimeter, as I have done before, but I dont know how to interpret the results. I could use some help with this:
From this data, can someone tell me if this thing is fried? My guess is yes, it most certainly is, but I could use some confirmation.
Damn, another mistake I made was trying to build the entire thing so it would fit inside the ATX PS box I got all those parts from. Probably shouldn't have attempted that on my first circuit.
Critiques and helpful suggestions are most welcome, I could use them. :-/
Anyways, here's the pics. Be forewarned, this is a sloppy set up:
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Sorry, but I'm too impatient to read your post throughly- did you use the light dimmer to limit power to the inverter? There's about a thousand things that may be wrong and troubleshooting without actually being there is hard. What you need to do ideally is have a scope and test the circuit without a load to ensure that the gate waveforms are ok. Once ensured then proceed with low voltage on the bridge (50V) and the intended load. If that works then step up to mains. That's the easy way, the hard way is working backwards to find out what died without a scope.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uzzors wrote ...
Just noting that GDT primary clamp diodes are missing in that schematic. Use of the schematic as it is now may lead to MOSFET gate overvoltage damage or gate driver transistor(s) damage.
Registered Member #1506
Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18
Uzzors wrote ...
Sorry, but I'm too impatient to read your post throughly- did you use the light dimmer to limit power to the inverter? There's about a thousand things that may be wrong and troubleshooting without actually being there is hard. What you need to do ideally is have a scope and test the circuit without a load to ensure that the gate waveforms are ok. Once ensured then proceed with low voltage on the bridge (50V) and the intended load. If that works then step up to mains. That's the easy way, the hard way is working backwards to find out what died without a scope.
Hmm, don't have a scope, and cannot afford one right now. I guess I will try the hard way, and if that fails sideline the project until I can afford a scope, probably sometime late next year.
Based on my readings, can you at least tell me if my diode bridge is dead? Pretty please? ^_^
Dr. Kilovolt wrote ...
Just noting that GDT primary clamp diodes are missing in that schematic. Use of the schematic as it is now may lead to MOSFET gate overvoltage damage or gate driver transistor(s) damage.
Recommendations, as far as diodes and placement in the schematic?
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Half-bridge, single 555 inverter questions
