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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Here is a chip I'm looking at on page 2 (the 8 pin)
From what I have read it is better to hit the gate with a negative pulse for a rapid turn-off. It seems to me that the best one will get with the bootstrap is zero volts for turnoff when connecting to the Vs pin. How is this acceptable?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
If your switch mode control Ic doesn’t allow the duty cycle to fall to 0 and 100%, then a bootstrap type of circuit can be used, Though if you are talking about a normal buck convertor you won’t have a lower mosfet to enable it to work anyway.
As just been said, it can be done with a separate supply, plus opto isolator driving a gate drive ic, probably the most difficult part is finding a suitable isolated supply.
How good would be one of those double insulated moulded pcb mounted mains transformers be I wonder? Or what about a decent gate drive ic like a TC4422 run from a continuous square wave oscillator feeding a ferrite toriod transformer like commonly used for gate drive but secondary rectified to supply the high side iso supply. If there are other power transformers in your circuit that have space to add another winding of insulated wire, and you can get enough turns on for the needed voltage, not very professional I admit.
Swinging the gate voltage negative isn’t strictly required but is certainly useful for faster turnoff when using higher switching frequencies. Also some opto’s can be slow if your using the ones with built in amp’s I guess should be ok but best check the data sheet for them.
Most opto's are rather slow :) I'm using 6N137 and TC4420 and it seems to work fine. I'm also using a NOT gate between the optocoupler and the 4420, so if the PWM signal is lost the driving IC will turn off, not on. For a isolated power supply I'm using a 2x9v toroid transformer (that's about 14V DC rectified).
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
This type of ic is great for low power stuff but if you're considering sstc etc then in my opinion the isolated supply for each upper transistor approach is better.
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gate drive bootstrap driver
