Saturday, September 24, 2011

NIC Coil Update

Due to lack of planning on layout I have encountered leakage inductance problems in both the bridge and gate driver section. There are also overheating problems. This is a small set-back to finishing the NIC coils for the beginning of the program this year.

I have rebuilt the bridge circuit from scratch, it is much lower inductance layout and now 2 mosfets share 2 heatsinks rather than all 4 on one heatsink and all the blocking and recovery diodes on another. This has reduced the heating problem. The new bridge circuit was tested and the resonant frequency of the coils appears to be 240kHz with the current small top-load toriod lawn-mower wheel things. I will also include more capacitors in the voltage doulber supply and more across the MOSFETs power bus, this will likely mean explosive MOSFETs but will reduce the rate at which they explode ;). So far with the new layout testing at low power the MOSFET's now seem "indestructible" to upset driving conditions below resonant frequency and no problems ever above, likely due to the barrier diodes in series and recovery diodes that bypass the MOSFET body diode. In old designs with audio modulation this was always needed as I pulled the driver below resonant frequency during the off-state for playing notes. This load was far to capacitive for the slow MOSFET body diodes and always killed them. I may change to tuning above resonant frequency for audio modulation but this could stress the gate IC's.

It was found that the GDT had too many turns as I expected a lower resonant frequency around 100 - 200kHz, it seems the lowest I go with the current setup (coupling of primary and secondary, topload and secondary wire length) is from 220kHz to 260kHz , with 240kHz being about the average when placed over its own enclosure left standing in the middle of the room.
I plan to mount the GDT directly onto a plastic spacer over the heat-sink to make servicing the parts easier. I have much better driver IC's and I'm getting pretty close to perfect gate waveforms under load. I have remade the GDT to have only 10 turns made of computer networking cable with the 4 twisted wire-pairs.

I will be making bigger and lighter weight top-loads to replace the heavy foil covered rubber and metal wheels. This is to reduce the resonant frequency a bit more to reduce switching losses in the bridge and more so decrease heating in the gate driver IC's. The new IC's have arrived to replace the latchup problem with the 3710's I also have ordered many types of schottky diodes to help remove some dissipation from freewheeling current inside the driver IC's.

The coils enclosures are being rebuilt for neater and lower inductance layout and cooling needs. This will also mean a repaint to cover up scratchs and a new user interface / control panel.
Sorry no pictures I dont have time, plus no one really reads this blog :P

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