K
kell
Guest
I was experimenting driving various mosfets and igbts with a NE555 at a
low freq around one per sec turning a small bulb on and off just to
check how well the 555 works before I try to build the actual circuit I
have in mind, which will have an inductor operating at a few KHz in a
topology similar to a boost converter. (The inductor is intended to
reach a peak current of several amps.) I used no gate resistor, just
connected pin 3 directly to the gate.
Anyway, when I tried the IRF740 and a IGBT designed for car ignitions
(HGTP14N...), which both come in the TO-220 case, the bulb turned on
and off cleanly. But when I tried a couple of devices that come in the
big TO-247 case, the bulb didn't turn off cleanly. With the IRG4PC30F
IGBT, the bulb dimmed before it turned off. With the STW18NB40 mosfet,
the bulb didn't even turn off, it just flickered a little.
I used a sealed lead acid battery and connected the Vcc pin of the 555
through a 75 ohm resistor to B+. I had a 100uF electrolytic across the
power pins.
I was powering the 555 through a resistor because that is the way I
intend to use it in the final circuit, as a way of protecting it from
the inductive spiking.
What is it about these big mosfets and IGBTs, that a 555 can't turn
them off?
low freq around one per sec turning a small bulb on and off just to
check how well the 555 works before I try to build the actual circuit I
have in mind, which will have an inductor operating at a few KHz in a
topology similar to a boost converter. (The inductor is intended to
reach a peak current of several amps.) I used no gate resistor, just
connected pin 3 directly to the gate.
Anyway, when I tried the IRF740 and a IGBT designed for car ignitions
(HGTP14N...), which both come in the TO-220 case, the bulb turned on
and off cleanly. But when I tried a couple of devices that come in the
big TO-247 case, the bulb didn't turn off cleanly. With the IRG4PC30F
IGBT, the bulb dimmed before it turned off. With the STW18NB40 mosfet,
the bulb didn't even turn off, it just flickered a little.
I used a sealed lead acid battery and connected the Vcc pin of the 555
through a 75 ohm resistor to B+. I had a 100uF electrolytic across the
power pins.
I was powering the 555 through a resistor because that is the way I
intend to use it in the final circuit, as a way of protecting it from
the inductive spiking.
What is it about these big mosfets and IGBTs, that a 555 can't turn
them off?