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Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
"Inazuma" Mk. 1 (from the Japanese for "lightning") is going to be a mid sized DRSSTC running with a full bridge of TO-247 IGBTs.
Progress so far:
I've constructed the basic full bridge assembly, with CPU heatsinks, fans and bulk decoupling on a piece of pine shelf
This uses short sections of aluminium U-channel to clamp the IGBTs to the heatsink for reduced thermal impedance
Controller is Steve Conner's PLL controller, essentially unmodified. This is split across two boards connected via ribbon cable and mounted in a diecast aluminium box. Building is still in progress, this is it's current status.
Holes in the top of the lid are to enable me to adjust the pots when the circuit boards are built into the case.
Current transformer is a ferite toroid with 47 turns of Furakawa triple insulated wire and mounted on a bit of copper clad board with hot-melt. The bolts are just for mounting to the controller case top.
I've also built an interrupter based on a couple of 555 timers and housed it in a plastic case.
This uses a TOSLINK / SPDIF fibre optic emitter with a receiver on the coil controller for electrical isolation
The internals are a bit crammed but it all fits and works pretty well.
To do: Complete building the controller circuitry including fan speed controller, LED indicators and test that it all works OK Build into the metal box Complete base assembly Finish winding secondary Decide on primary geometry + tank cap
Onwards!
EDIT: I've also put up a new page on my website which gives more details about the construction.
Registered Member #593
Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50
James,
Very excellent and neat work indeed. I too am working on my first DRSSTC, which will feature a half bridge of TO-264 IGBTs. Great idea for heatsink mounting. I spent some time earlier tonight, going through my box of heatsinks trying to decide how I was going to meet this thermal challenge with those IGBTs. I'm basically still collecting heatsinks and looking.
My coil will probably be a project that I will have to stretch over most of the summer. I hope to have first light sometime by late summer to early fall. I'll probably start a thread when I have more accomplished. Mine will be based around a 12.5" x 4.5" AWG 31 secondary, which I recently wound and coated.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
*** Updates ***
Ran up and tested the controller and it all works OK. It's really crammed in there, I've fallen into the trap of underestimating how big my box needed to be so everything is really crammed in!
In soldering the heavy current connections, one of the GDT wires came unsoldered. I took the opportunity to take a photo of the GDT on the underside. Good labelling of the connections is key!
The bridge is a _lot_ busier now, with mains input for both controller and bridge supplies, mains transformer for the controller being added onto the board.
I've also finished my MMC, two parallel strings of 9 caps to give me 100nF @ 9kV. These were connected using two parallel strips of single sided copper clad board with bits cut out. M4 bolts form the connection points.
Measured the gate drive signal with the deadtime control turned up to maximum and I get nice burts on the gate with very little ringing using a 4R7 resistor
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Have you found balancing resistors for MMC are not required?
I never use them. They function fine without them, but i cant say i recommend handling the MMC afterward (since i have found voltage on the caps individally, yet the bank itself is discharged). This was when using caps of various histories and abuse, so they were all out of capacitance spec. The situation is far less severe when you use all fresh caps and they are all seeing the same use.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Sulaiman wrote ... Looking good and compact. Nice work.
I just have a couple of questions I'd like to ask; Have you found balancing resistors for MMC are not required? Are that many fans required?
Hi Sulaiman, thanks for the comments.
I've got some 9M1 resistors that I was going to add to the MMC when I have time. Even if they aren't strictly necessary they'll serve to discharge the bank when it's not operating.
As to the fans, I've got a PWM control circuit built into the controller so I can vary the voltage between off and way too much. I'm only using TO-247s which have a higher thermal impedance from j-c than ISOTOP bricks. So I'd prefer to run some fans just to keep everything cool and increase reliability. I'll probably need it when it comes to pushing the peak current envelope a little.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
I've completed the primary now, so I should be able to do some initial low voltage tests just using the primary. I still have to clean, make the endcaps, build a winding jig and wind the secondary which is the last major job remaining.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Progress.
Secondary winding is going well, I'm about 60% done and it's only taken a couple of hours so far. I gave up trying to motorise the jig because the gear I had attached to the motor shaft keeps coming unstuck and slipping.
I'm doing some initial testing on the controller and I've got some results. Here is a plot of the primary current waveform in series with the cap, no secondary just the primary circuit. Frequency is 116kHz, pretty much spot on what was predicted.
The current is measured using a second CT + burden (47 turns and a 0.47 ohm resistor to give 10mV/A) and the peak is 228A running from a 22V power supply. This seems high, but then there is no secondary to load the tank so it could be reasonable. Comments?
Looking at the half bridge voltage with a 10:1 probe, it's not pretty (top trace in below plot). You can see that ZCS occurs during the period of deadtime but the actual voltage is all over the shop. Any comments on this?
My tuning LEDs don't appear to light and the PLL doesn't appear to track the frequency. If my phasing was incorrect, the PLL wouldn't track - correct? Also, I wouldn't expect the PLL to track if the input signal was low, or would the diodes in series with the burden resistor help here?
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