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555 to drive mosfet/igbt

K

kell

Jan 1, 1970
0
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?
 
J

jsmith

Jan 1, 1970
0
Possibly you may not need the electrolytic capacitor since your B+ is a
battery??
 
M

Michael Redmann

Jan 1, 1970
0
kell said:
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?

Perhaps the 555 does not really switch off. What's the voltage at pin 3
when load should be off? The 555 can sink about 100 mA so the gate
should be grounded very well. What else do you have connected to the
555? (pins 2,5,6,7). Maybe something is ringing or even oszillating
which prevents the 555 from switching off correctly.

Regards
 
R

Robert Monsen

Jan 1, 1970
0
kell said:
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?

The 555 should really be able to shut them off. Test the MOSFETs that
fail to turn off by disconnecting them from the 555, and grounding the
gate. Does it turn off?

The gate threshold can be as high as 5V. It may take 10V Vgs to turn it
on completely.

Some mosfets may take even more voltage to turn completely on. The
IRF740 has a slightly lower Vgs(th) than the other.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.
 
M

me

Jan 1, 1970
0
Anyway, when I tried the IRF740 and a IGBT designed for car ignitions
Could be the gate capacitance, try a low value resistor (maybe 200 ohm)
from gate to ground in the circuit...
 
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