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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Dr. Kilovolt wrote ...
Harry, I have driven FETs and IGBTs from the NE555N directly without problems (scoped the waveform too).
It's better to supply the 555 (and any gate drivers for that matter) with 15 rather than 12 volts.
I always feel that the gate input capacitance of MOSFETS is so high, that I can't imagine the unaided efforts of a 555 being able to keep up those right-angles we all know and love. In any case, I think it good practice to have a buffer amplifier between a low powered oscillator and a high power device to reduce the chance of any unwholesome interaction between the two.
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
I am going to use my already build 555 circuit based on this:
Should there be a resistor between the 555 and the gate of the IGBT or not? Anything I need to look out for? If this doesn't work, I'll try a buffer circuit :)
EDIT: I've been told that underdriving IGBT's can cause them to run into linearity, causing them to overheat and burn out?!?!
Also, apparently a IGBT rated for 1200V won't actually start acting like a IGBT untill around 50V, so I guess my main problem is getting >50VDC ...
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
The 68 ohm resistor limits the 555 output current to ~200mA in the event that the mosfet fails. The drive frequency is only 40kHz at the most, so waveforms will remain crispy even with a relatively large gate resistor. If you have a scope you can see for yourself what the gate waveform looks like, and adjust it to your taste. If your IGBT doesn't have an anti-parallel diode or is avalanche-rated you'll need some extra diodes to protect it.
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
Uzzors wrote ...
The 68 ohm resistor limits the 555 output current to ~200mA in the event that the mosfet fails. The drive frequency is only 40kHz at the most, so waveforms will remain crispy even with a relatively large gate resistor. If you have a scope you can see for yourself what the gate waveform looks like, and adjust it to your taste. If your IGBT doesn't have an anti-parallel diode or is avalanche-rated you'll need some extra diodes to protect it.
How many turns do you think would be optimal on the flyback? I have 18 currently, though my input voltage is only 16V max, so would more turns be beneficial ?
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Best circuit I can make ...
