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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
555 is an oscillator chip not a gate drive chip. A gate drive chip is essentially a high current output signal buffer. Usually a 10 ohm is placed in series with the gate because it may be too fast without it and cause ringing.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
Correct me if I'm wrong, but really high speed driver chips use push-pull configurations to rapidly charge and discharge MOSFET/IGBT gates (typically using a mini-fullbridge to run a gate-drive transformer.
It really depends on what frequency you are aiming for, once you know what frequency you want to drive your ignition coil at you can calculate your astable 555 resistor/cap values.
So, short answer: typically a 555 can maybe drive in the KHz range reliably, but I highly doubt you could (or would want to) go to Mhz range. Try substituting the resistor with a potentiometer so you can play with the pulse frequency and find the point where you get the most power.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
A 555 has a totem pole output good (if the data sheet is to be believed) to 200mA, so it should be perfectly adequate for anything up to a few 10s of kHz.
The "slow-on, fast off" network between the 555 and the FET is a little disturbing. It looks like it has been borrowed from another very low speed application where it might have been used in the gate drives to two FETs, where it was important to make sure that one was off, switched off fast by the 68R, before the other was on, switched on very slowly by the 3.3k. 10v into 3.3k gives only 3mA.
Switching on this slowly gives a large energy loss into the FET. Not too much of a problem for very slow switching, but wastes a lot of power at 1kHz, and is unusable at 10kHz.
There's nothing wrong with a 555 in this application, just replace those three components with the 10R that Avi suggested.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Actually it was borrowed from some other ignition coil driver. I don't remember much benefit from having it in place to tell the truth, but I didn't notice any extra losses either. So I trusted that it had some positive effect and left it in place. The risetime isn't effected much at 400Hz or whatever the timer is running at, and with the large amount of primary inductance presented by the ingnition coil I don't think it matters much anyway.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The 555 will probably output over an amp of peak current. I have verified it works well for driving moderately large gate capacitances up to several tens of kHz.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Relatively slow turn-on is probably acceptable since the primary inductance of the ignition coil will limit the rate at which drain current rises at turn-on.
Provided the ignition coil is run in discontinuous current mode, then the slow MOSFET turn-on probably won't cause particularly high turn-on losses. I'd be more worried about the lack of an over-voltage clamp across the ignition coil or turn-off snubber across the MOSFET!!!
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IRFP450 High Speed Driver?
