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Just threw this together... just wondering if there is anything that sticks out as completely messed up.
Basically, a 555 continuous boost converter converts 12v from a lead-acid to 75 volts, then another boost converter uses this to get a max of 450v. A small self-charging circuit was borrowed from Anothercoilgun's site. The 'quasi' half-bridge IGBT's are run from an adjustable pulse from yet another 555... with all the 555/IGBT's being run from a 5v voltage regulator.
Hopefully with an adjustable pulse, a slotted iron projectile, a half-bridge setup, a lubricated thin-walled barrel, and a decently calculated coil, I can achieve a decent level of efficiency.
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I don't understand why you need double conversion, anyway i don't like to see a halfbridge wired up that way, no gate resistors and such hmm, it's gonna blow. Also is the driver ic a halfbridge driver IC, i don't think so? Oh no you cannot drive a halfbridge that way. The high side transistor will float up to 300V above ground, looks like you will kill both fets . Also why use a halfbridge for a single stage? They are most efficient for the latter stages in multi-stage coilguns because of they excellent quench time. Use a simple topology like IGBT-diode or IGBT- quench resistor or even an SCR one if you don't need to turn off the coil. Don't wire it that way or you will kill those expensive IGBTs
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
A quick suggestion for charging. Since its a fairly small capacitor bank you could use a 555 or similar oscillator capable of a few kilohertz and a power transistor to pulse current into the secondary of a 220v-6v stepdown transformer. This can provide upwards of 1kv output from the primary of the transformer at a few watts with just a 9v battery for power. The higher the designed current of the transformer, and the beefier the battery, the more power you can get from the transformer obviously.
I agree with TheMerovingian, its kind of a quick and dirty design. I have some misgivings about how robust that half bridge will be. I'm just starting work on my own half bridge, but I can say you'll need gate resistors for sure. I'm not quite sure about Q1 floating up to 300v. Thats probably just my inexperience though.
Banned Registered Member #110
Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85
Stages/functional blocks. Build and test any schematic by parts then move on to next part once prior parts are good. Build a single booster, make sure it is working good, not generating excessive heat, and not overcharging. Then integrate a pre-boost stage of 75v. If the addition actually produces a higher end Watts than a single booster, then go with it, else just more power is being wasted in another IGBT. One booster will amplify 12v to 450v fine.
I like the use of that second schematic because it is much more easier to follow. In the first, how were you going to turn off the second booster? Pin 4 on second booster is tied to +5v, assuring second booster to be self running. If you turn off the 75v 555 only, the second one will still be boosting because of the direct connection from +12v-inductor1-diode-inductor2-igbt2.
About the booster, inserting a 10 ohm resistor from emitter to ground will not only starve the booster of current (10ohms+CoilZ=huge resistance) whatever little power it does produce will be diverted away from the inductor toward the 10 ohm resistor in the form of a heater and voltage divider. The IGBTs require more than 5v, else little to no power will go to the inductor. Use at least a 9v regulator to power 555s that way it will still function under the singular 12v power source. The IGBTs do need a resistor to gate, if not they will ring and cause the circuit to resest. Even with a fixed voltage and frequency, each type of IGBT needs a different gate resistance. 30-100 ohms at 9v works fine for some but causes huge noise spikes on others. I noticed extra diodes here and there. Most will not aid and the ones connect to the booster IGBT's gate will block a return path to ground guaranteeing a short failure when the IGBTs "latch-ON".
The half bridge was well covered all ready. What level of efficiency are you attempting or try to surpass from previous cgs?
Registered Member #186
Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42
wrote ...
Stages/functional blocks. Build and test any schematic by parts then move on to next part once prior parts are good. Build a single booster, make sure it is working good, not generating excessive heat, and not overcharging. Then integrate a pre-boost stage of 75v. If the addition actually produces a higher end Watts than a single booster, then go with it, else just more power is being wasted in another IGBT. One booster will amplify 12v to 450v fine.
I posted a thread in the HV forum about 12v to 450v from a single booster... it was shot down pretty heavily, even without people answering any real questions. With 12v to 450v, the duty cycle with have to be about 97.3%... a 555 can handle that? Really, the only reason I even wanted to link up two boosters was to get to 450v.
wrote ...
I like the use of that second schematic because it is much more easier to follow. In the first, how were you going to turn off the second booster? Pin 4 on second booster is tied to +5v, assuring second booster to be self running. If you turn off the 75v 555 only, the second one will still be boosting because of the direct connection from +12v-inductor1-diode-inductor2-igbt2.
Ahh yes. I cought that myself last night after I posted. I changed the second booster's reset pin to match the first's... but if I only need one boost circuit, I won't really have to worry about it.
wrote ...
About the booster, inserting a 10 ohm resistor from emitter to ground will not only starve the booster of current (10ohms+CoilZ=huge resistance) whatever little power it does produce will be diverted away from the inductor toward the 10 ohm resistor in the form of a heater and voltage divider. The IGBTs require more than 5v, else little to no power will go to the inductor. Use at least a 9v regulator to power 555s that way it will still function under the singular 12v power source. The IGBTs do need a resistor to gate, if not they will ring and cause the circuit to resest. Even with a fixed voltage and frequency, each type of IGBT needs a different gate resistance. 30-100 ohms at 9v works fine for some but causes huge noise spikes on others. I noticed extra diodes here and there. Most will not aid and the ones connect to the booster IGBT's gate will block a return path to ground guaranteeing a short failure when the IGBTs "latch-ON".
Thanks a lot for the advice. I misread the IGBT's data sheet for required gate-emitter voltages, and since it has a max continuous of 20v, I can use the 12v battery directly (the 555 has a high max voltage as well). I've taken out the boost-IGBT's diodes and emitter resistors, in place of gate resistors, and also added a gate resistor to both firing IGBTs. Oh, and I took out the whole 5v regulator, since it wasn't needed. (Had to take out the 'charged' LED though.)
wrote ...
The half bridge was well covered all ready. What level of efficiency are you attempting or try to surpass from previous cgs?
Well, if I get around 3% efficiency, I'll be happy.... though I want to push 10% from a single stage. (Even if it means absurdely slow velocity)
If everything works out, I'll just tack on another stage, and work from there.
Registered Member #186
Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42
Could I get a bit of clarification?... maybe an example or schematic?
A simple 555 driver wouldn't work at all? Maybe I'm missing something about the concept of the half-bridge design....
It should work like this, right? C1 is at full charge. A trigger pulse (which will be 12v, max 200ma from the 555) turns on both q1 and q2, allowing C1 to discharge through the coil, L1.... when the pulse ends, both q1 and q2 shut off. This causes voltage across L1 to go from say 450v to 0... as the field collapses around l1, it induces back emf, which can freely pass through d1 and d2 in the opposite direction through the coil, re-charging C1 somewhat.
Registered Member #186
Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42
Basic circuitry. Just a few questions now...
What would good base values for r1 and r2 be? 100 Ohms? How would I go about getting optimal values for them?
Mero mentioned that I would need a driver with 'isolated floating outputs'. Why would a single, steady pulse not work (like one from a 555)? halfbridge28sm.jpg Edit: Perhaps I need a power transistor to amplify the 555 pulse current?
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Quick CG schematic sketch, up for criticism.
