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cooling flyback driver transistor?

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alf

Fri Jun 24 2011, 07:41PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

hey,
i have recently discovered that the mosfet i had thought had failed, and tested manyyyy times,

is actually working perfectly, but it gets too hot in about 10 seconds of use, the heatsink is pretty big (around 6x4"),
i have a cpu cooler heatsink with attached fan, would this work better?
or maybe a peltier device, but then that has to be heatsinked too.... hmm

i know the flyback driver is (very) inefficient but it is simple, and it makes a good plasma speaker and big hot sparks lol.

i cant use the zvs driver, as it cannot be modulated for use as a plasma speaker.
the circuit used about 10 amps at 24v, or 15 amps at 24v if used as a plasma speaker.

i am gonna rewind a MOT and make several tappings, so i can operate it at the highest voltage only when necessary, to reduce power and avoid damaging the transformer with overvoltage. as my previous FBT failed recently, and this is an almost new one (taken from a recently repaired TV which then failed shortly after.)

thanks for your help. Alf.

Inducktion

Fri Jun 24 2011, 08:15PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Well, the easiest way to fix that is to use a different circuit design. Single transistor flyback drivers tend to generate a lot of heat due to the way they're designed, so if you want less heat, use a different design, simply put. A 555 timer plasma speaker using two complementary mosfet drivers to a halfbridge is probably going to be your best bet for simplicity and heat generation.

Sulaiman

Sat Jun 25 2011, 11:31AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

15A @ 24V = 360 Watts,
what flyback transformer are you using?

A good cpu cooler is an excellent transistor heatsink
(provided that the reliability, power consumption and noise are not an issue)

Daedronus

Sat Jun 25 2011, 07:09PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

how do you driver your mosfet?
what model is the mosfet you use?

alf

Sat Jun 25 2011, 07:48PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Sulaiman wrote ...

15A @ 24V = 360 Watts,
what flyback transformer are you using?

A good cpu cooler is an excellent transistor heatsink
(provided that the reliability, power consumption and noise are not an issue)

hey,

haha yeah it is
im using a one from a sony tv, well actually it failed today so im using another, which also failed, but then i found a different winding on it which produces a 2cm HOT spark,

any idea why they keep failing? im driving them with 24v, and the spark is a maximum of about 2 inches, is this too much for it to produce constantly?

and cool ill try the cpu cooler tomorrow maybe.

thanks. Alf.

alf

Sun Jun 26 2011, 06:55PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Daedronus wrote ...

how do you driver your mosfet?
what model is the mosfet you use?

hey,

its driven with a 555 timer, and the model is irfp630 i think, but ill check later to make sure.

thx. Alf.

Patrick

Sun Jun 26 2011, 07:28PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

they die when you push too much power through them, and 360 watts is quite a lot, i pulse my flybacks for 200 watt use 10 seconds on, 60 seconds off, for 3 minutes, and i have them under oil.

A question:
Are you using a mica insulator and thermal grease between your heatsink and transistor? Is your heatsink floating?

Under Canola oil.

Before completion.

Daedronus

Mon Jun 27 2011, 09:15AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

a 555 is a bad mosfet driver.....
It's possible your mosfets heat up because they turn on/off too slowly
irfp630 is also small, use a mosfet that is rated for 30-40A.
With a bigger mosfet you usually get higher gate capacitance, so a proper gate driver is even more important.

alf

Mon Jun 27 2011, 11:11AM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Patrick wrote ...

they die when you push too much power through them, and 360 watts is quite a lot, i pulse my flybacks for 200 watt use 10 seconds on, 60 seconds off, for 3 minutes, and i have them under oil.

A question:
Are you using a mica insulator and thermal grease between your heatsink and transistor? Is your heatsink floating?

Under Canola oil.

Before completion.

hey,

right, it is a irfp460, my mistake lol
right, could i put the transistor in oil to cool it?

im just using thermal grease, no insulator, the heat sink is at whatever voltage the drain is i guess, because i connect 1 side of the flyback to the heatsink, instead of to the drain pin.

btw does the secondary coil heat up too? or just the primary.

thanks. Alf.

Patrick

Mon Jun 27 2011, 06:30PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

alf wrote ...

right, could i put the transistor in oil to cool it?

Dont bother. Waste of time, difficult to replace when thay blow.

alf wrote ...

im just using thermal grease, no insulator, the heat sink is at whatever voltage the drain is i guess, because i connect 1 side of the flyback to the heatsink, instead of to the drain pin.

Yes, this is the right way to go.

alf wrote ...

btw does the secondary coil heat up too? or just the primary.

Yes, both heat up, as does the ferrite.

What voltage is your 555 at?
Most are only good for 16 or 18 volts absolute max.

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