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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I don't see anything wrong with this, if it doesn't get power it can't turn on...
As I said, it won't turn on, but it's output may be high impedance and mosfet turned on spuriously. But you are probably right that it's not a problem.
My scope is not good with anything that involves HF, HV, EMF or any other kind of interference. It's a 40-year old granddady tube scope so there's no wonder.
Mine is too - you can't see any kind of 'interference' apart from waveform that is causing it, unless something is not right with the circuit.
I don't see why wouldn't it work with a bulb load or no load alone. Streamers from the transformers could cause interference, although I could scope well even in such conditions.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
wrote ...
wrote ... If you need to go above about 1k for whatever reason, it's perfectly acceptable to add gate capacitance,
I don't think so.
why not? I would rather have the extra capacitance to drive from a low impedence driver than add several kohms of gate resistance! Especially in a noisy environment! It surely makes the whole thing more predicatable - and the low drive impedence should help minimise the miller effect once you start switching your load? Struggled to find a decent reference for this other than page 9 of this and the fact that all the motor inverters at work slow down the rise times in this way (extra gate capacitance with a small series resistance to prevent oscillation).
wrote ... I don't know why all these fast gate drivers were invented then.
for switching the large devices in resonant mode supplies I would hope, where the gate capacitance is just a pain in the arse that you want to switch as fast as poss whilst you get the chance (the zero current or zero volt part). Dissipation is something to consider when you start slowing the device down on purpose, it's not always something you just solve by banging on a faster gate drive
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko wrote ...
There are many variables there I don't know si it may help if you bothered to draw at least a quick hand-drawn schematic of current state of the circuit. :p
Here is a schematic
wrote ... Mine is too - you can't see any kind of 'interference' apart from waveform that is causing it, unless something is not right with the circuit.
As i said the scope is to blame, it shows crap even with the probes shorted together not touching anything, just lying near the circuit. (the same crap that is then "added" to the real waveform)
I no longer now use the parallel FETs, I got some more powerful ones and I use 10 ohms gate resistors with them, so far no deaths (there is other little problem but about that another time.)
Registered Member #1125
Joined: Fri Nov 16 2007, 09:13PM
Location: Edmonton, Alberta, Canada
Posts: 62
The problem when parelleling those devices that unless they all turn on and off at the same time the load current will suddenly spike in the one with the least resistance and still on... and in the case of an inductive load the Reverse EMF across it.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Avalanche: don't get me wrong, but we're not building hertz range motor drivers here.
Your recommendation of 1k resistor really isn't quite a ''safe side'' for 50kHz inverter, considering it would give 4,2 microsecond time constant in this case - something in range of ten ohms is like a safe side. I understand if you just wanted to give an example.
But there are a few general rules I guess, like I mentioned turning off an inductive load too quickly - this can create emf that can exceed the voltage rating of your device in some cases - so you can work out your maximum allowable rate of change of current (dI/dt) and make sure that it stays within the voltage range of the mosfet.
Other thing that caught my eye, are you referring to gate drive circuit (with GDT) or the power bridge here?
50kHz is not a too high frequency and we can use relaxed gate drive, and indeed the IR chip alone would probably work well enough. And you really want to have least possible capacitance here so you can make your gate driver smaller and more efficient!
This also allows us to use larger and slower devices if necessary.
Jan: I really don't know what more to say - out of bother, you could reduce the DC blocking cap to 1uF or so and put 1k pulldown resistors onto gates.
Still it's worrying me that you are blowing mosfets *in operation* with lightbulb load, which makes it really look like shoot through is in question.
Regarding your scope, it doesn't work with supšply voltage present even if there is no load, or just lightbulb?
The problem when parelleling those devices that unless they all turn on and off at the same time the load current will suddenly spike in the one with the least resistance and still on...
Yes, but the loss is negligible compared to switching loss. And this is only at turn off.
and in the case of an inductive load the Reverse EMF across it.
If one switch is briefly on others will have no voltage across them.
And in any case VDS can't ever pass supply voltage.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko wrote ...
Jan: I really don't know what more to say - out of bother, you could reduce the DC blocking cap to 1uF or so and put 1k pulldown resistors onto gates.
Still it's worrying me that you are blowing mosfets *in operation* with lightbulb load, which makes it really look like shoot through is in question.
Regarding your scope, it doesn't work with supšply voltage present even if there is no load, or just lightbulb?
The mosfets blew as I tried to draw arc off the xfmrs.. The filter cap probably had enough energy for this, even with the bulb load. Shoot through is out of question as the driver includes 1.2/0.7uS deadtime.
The scope gets fed up anytime I turn on the power part of the driver, I also don't get where it could be getting the interference from
I'll come here again as another FETs blow unexplainably, I'll let it be for now
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The filter cap probably had enough energy for this, even with the bulb load.
The problem is that even if there is no dc block cap and one of your mosfet stacks was permanently on the bulb would light and limit the current to fraction of an amp. You just couldn't have killed mosfets from overcurrent in any case with lightbulb in series.
If something is wrong with your driver, deadtime means nothing - and in this case shoot through for some unexpected reason is only explanation I can think of.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
wrote ... Your recommendation of 1k resistor really isn't quite a ''safe side'' for 50kHz inverter...
ah, nooo, you misunderstood my post (probably because I worded it like crap). Regardless of application, frequency etc I would always want to stay well below 1k on the gate of any power device, but for 'smaller things' I wouldn't be so worried about that limit. In the case of this inverter you're right 1k would be silly.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
have you tried just one set of fets and just the IR chip with a dummy load insted of the tesla to be shure it works like you think. You may have problems with your bootsraping. another thing each fet gate should have its own resister say 7ohms unless all of the fets are mached or have the same bach No. When your testing your circuit use a floting supply on your high side insted of bootstraping
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Too fast gate drive kills FETs
