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Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
at presant I'm working on a pcb that is simular to yours but I'm using FRED diods RURG80100 that have I belive a 55nS recovery time in the configuration that Steave Conner illistrated. I also included a opto isolator in front of the drivers for galvonic isolation for foalting purpases.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Weee, swinging a small FET gate at 2.2MHz... now to get onto the bigger stuff... where's my signal generator? heheh
Because of my paranoia (and lack of cash), I did isolation by ferrite instead of optoisolation. For the digital signals though, optos are probably the way to go... hehe.. next time! =D
no sparks yet... I just realised I forgot the leakage resistors (1K) on the gatedriver inputs, so I have to add those underneath to the four boards before they'll do anything. There's not enough leakage through IN for a bipolar signal to trigger them. Meh.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Those spikes are a little big, but so long as you aren't exceeding the max voltage of the gates you should be ok... You did put zeners on the gates, right?
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Top waveform is the 5Vp/p signal from the signal generator going to ferrite (isolation) to the isodriver board, the bottom waveform is the voltage across the FET. There are no zeners on the gates, since the gatedrivers are clamped to the rails with diodes, and there is no isolation between the FETs and gatedrivers.
I'm using 10-ohm gate resistors atm, but to square up the waveforms (since the drivers really can provide the current) I'm considering 1-3 ohm.
Here's the circuit driving a bigger FET, (IRFP450) at 2.4MHz
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Nice work BP! Driving a big power FET at 2.4MHz isn't that easy, especially while keeping HV isolation, but you seem to have pulled it off in style. Time to stick some DC bus voltage up it and check for common-mode noise immunity now.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I don't know if this is a good idea but could a DC blocking capacitor (big) in series with FET gate be an additional safety?
When MOSFET dies he usually connects 'all pins together' attempting to fry more parts in the circuit. Then in some occasions mosfet's source pin may blow open and leave DC bus supply voltage on your gate driver resulting in spectacular death of drivers. Cap would just 'charge up' and current will be unable to flow anymore. It would have to be combined with TVS or stronger zenerson UCC's output to survive initial 'shock'. (???)
Those spikes are a little big, but so long as you aren't exceeding the max voltage of the gates you should be ok... You did put zeners on the gates, right?
Spikes are quite normal whebn drving FET directly with gate drivers. Higher current gate drivers usually make bigger spikes.
Top waveform is the 5Vp/p signal from the signal generator going to ferrite (isolation) to the isodriver board, the bottom waveform is the voltage across the FET. There are no zeners on the gates, since the gatedrivers are clamped to the rails with diodes, and there is no isolation between the FETs and gatedrivers.
Yeah, but I would like to see gate waveform! =) From just input oscillatr signal and FET voltage (voltage across MOSFET always looks wierd like that ).
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Aw thanks Steve, I'm chuffed =)
Firkragg, the bottom waveform is the (gate) voltage across the FET. My apologies for the ambuguity =P
On this image: The top waveform is the 5Vp/p signal going into the gatedriver. The bottom waveform is the gate voltage of a random TO-220 FET at 2.2MHz (mind you with a 10-ohm series gate resistor, making the edges very curvy).
On this image: Similarly, the top waveform is a 5Vp/p signal going into the gatedriver (hysteresis is 2V on, 1V off). The bottom waveform is the gate of an IRFP450 FET at 2.4MHz. I need to lower the gate resistor more because the switching times are a little slow.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
This morning i've bolted down all eight packages to a heatsink (precariously), and fired up all the isodrivers at once, with the correct combination of inverting/noninverting chips =P
my power supply is struggling a little (just vibrating a wee bit), but not below the dropout voltage of any of the regulators.
there's sinewavey noise on the signals, regardless of the frequency, which I can see with the timebase on my scope set to fastest and the intensity at brightest... but there's nothing I'm running at that frequency, so must be parasitic ringing from elsewhere.
All the drivers seem to work the same, however I'm curious as to what the little artefact is just prior to the turn-off transition on each pulse... hmmm
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isolated gate drives
Driving a big power FET at 2.4MHz isn't that easy, especially while keeping HV isolation, but you seem to have pulled it off in style. 
Time to stick some DC bus voltage up it and check for common-mode noise immunity now.
