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flyback transformer back EMF?

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alf

Sat Jun 04 2011, 07:35PM

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

hi,

does a flyback transformer produce any back voltage?
and if so what voltage is it?
could i use a flyback diode to suppress it?

because i keep blowing transistors i use in my 555 flyback driver, but the current and voltage seem to be within range, and it doesnt get *too* hot, i have found that irf mosfets work very well, and the best one so far was the irfp460 that i had lying around. but alas it failed too, and i dont want to buy a new one till i find out why they keep failing,
and finally, could i put two irfp460s in parallel to increase the current rating? and the timer can easily drive 2 of them, so i think itll be ok.

thx. Alf.

Ash Small

Sat Jun 04 2011, 09:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yes, there is a back EMF when the field collapses.

A suitable diode and capacitor in parallel with the transistor should protect it. I'll try and find a link to an old thread on the subject.

The problem with paralleling transistors is if they don't both switch off at exactly the same time (due to manufacturing tolerances), you don't get a fast switch off, which is the main priority with a flyback driver.

Antonio

Sat Jun 04 2011, 10:40PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Flyback circuits not only produce a large voltage pulse over the primary coil when the primary current is interrupted, but require this pulse to operate. It's necessary to add something to limit the pulse amplitude to a value that does not destroy the switching transistor. You can use a varistor, an avalanche diode (that most mosfets already have), or several types of snubbers using diodes, capacitors, and resistors.

Pinky's Brain

Sat Jun 04 2011, 11:33PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Some applications (ignition coils mostly) make use of the primary leakage to give an extra push together with forward transformer action. It is not necessary though. If you could get the coupling factor to 1, the flyback circuit would not give a higher voltage on the primary than Usec/N during the flyback phase ... and it would operate fine. The flyback effect does not rely on the extra pulse created by leakage induction.

Varistor/MOVs are not very reliable long term devices, also not very fast, if you are going to use a thermal dump to get rid of the energy in the primary leakage use a TVS diode.

Sulaiman

Sun Jun 05 2011, 01:00AM

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

A diode across the primary would 'kill' any eht output

The IRFP460 is 500v, 20A, repetitive avalanche rated
so the most likely causes of death are
- over-current
... if the transistor is turned on for too long the primary current will exceed 20a
(if your power supply can supply >20A)
... if the off time is not long enough then the energy stored in the inductor
will not be returned to zero each cycle,
the current will build up little by little each cycle until something gives.

- over-temperature
... did you use a suitable heatsink? did it get very hot?

There are fake IRFP460's on the market, did you buy yours from a 'reputable' source?

alf

Sun Jun 05 2011, 09:42AM

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

Sulaiman wrote ...

A diode across the primary would 'kill' any eht output

The IRFP460 is 500v, 20A, repetitive avalanche rated
so the most likely causes of death are
- over-current
... if the transistor is turned on for too long the primary current will exceed 20a
(if your power supply can supply >20A)
... if the off time is not long enough then the energy stored in the inductor
will not be returned to zero each cycle,
the current will build up little by little each cycle until something gives.

- over-temperature
... did you use a suitable heatsink? did it get very hot?

There are fake IRFP460's on the market, did you buy yours from a 'reputable' source?

thanks!, i did assume that since the mosfet has a built in diode, it would be ok.
i used a heatsink that is pretty big, its about 8x5x1 inches, and it did get hot,
well i dont know where it was from, it was just lying around so i think it would be OK,
could it be that the 555 timer was only supplying 6v and the transistor needs 10v to saturate it?

so it wouldnt be such a good idea to but the cheapest ones from china on ebay?

thx again. Alf.

Ash Small

Sun Jun 05 2011, 11:18AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

alf wrote ...

.could it be that the 555 timer was only supplying 6v and the transistor needs 10v to saturate it?

The MOSFET will get hot if it is not fully 'on'.

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