Author Topic: Flyback Diode  (Read 7411 times)

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Offline Chet T16

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Flyback Diode
« on: April 30, 2011, 11:54:08 am »
I'm building a quick and simple fuel injector pulser for a friend that will drive up to 4 low impedance injectors at once but i'm not sure what diode i need to put in parallel with the output.

From what i have read the induced voltage is around 90V. The max output current 28A @12V

So, what rating diode do i need? (or more importantly, teach me what i need to know!)
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Re: Flyback Diode
« Reply #1 on: April 30, 2011, 12:37:58 pm »
on the whole a diode rated for about the same current as the coils consumption is ok giving emphasis on the peak/instantaneous current capability if you have a choice of parts
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Offline tecman

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Re: Flyback Diode
« Reply #2 on: April 30, 2011, 01:20:41 pm »
The diode will only see the supply voltage applied (28 vdc), and the current will be the coil current, max.  Generally diodes are available in 50 or 100 volts as a minimum.  The only real factor is how fast you want the diode.  A standard diode is likely OKay, and perhaps a fast recovery type may be better.  Depends on how fast you will cycle the coil.

paul
 

Offline Chet T16

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Re: Flyback Diode
« Reply #3 on: April 30, 2011, 02:57:19 pm »
So the 90v is irrelevant? Once i have a diode rated >12v (i assume you meant 12) and >28A i'm fine?

I'm not sure how fast its possible to cycle under i get some testing done and see how fast they open. No more than 130Hz anyway
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Re: Flyback Diode
« Reply #4 on: April 30, 2011, 03:00:53 pm »
your diode must have a reverse voltage of at least the supply (2x recomended) 130 Hz is not too fast so no need to worry too much about speed but you require decent recovery time as it is being pulsed and is not a once now and then pulse
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Offline Chet T16

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Re: Flyback Diode
« Reply #5 on: April 30, 2011, 03:46:50 pm »
If i use an IRFZ34N to drive the injectors then i wouldn't need to worry about an external diode, would i? :-\
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Re: Flyback Diode
« Reply #6 on: April 30, 2011, 04:01:36 pm »
not recomended ! you will be putting more stress on the mosfet and the mosfet diode voltage drop is generally not as low as a descrete diode so your making more heat than need be. all mosfets have those diodes in them but they are for protection and last resort not as an active circuit part
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Offline jahonen

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Re: Flyback Diode
« Reply #7 on: April 30, 2011, 04:39:20 pm »
Internal diode in the MOSFET (in low-side switch configuration) can't be substituted for a flyback diode as it does not form a loop where current can flow when MOSFET turns off. Remember that current wants to flow just in the same direction which it flowed just before the switch-off.

Also, one possible problem (while it works ok) with the conventional flyback diode across the injector coil is that it limits the voltage to about 0.7-1 volts when conducting so turn-off might be too slow (as current decay rate in an inductor is proportional to the voltage across the inductor). Ideally, one would want the voltage to be clamped to high, but not too high value, just below breakdown voltage of the switching transistor (or limited by coil insulation).

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Offline Chet T16

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Re: Flyback Diode
« Reply #8 on: April 30, 2011, 05:10:18 pm »
This doesn't need to be either efficient or ideal, as my friend said "Something better than connecting them direct to a battery" :)
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Re: Flyback Diode
« Reply #9 on: April 30, 2011, 05:32:25 pm »
yes but you are using the internal diode for something it is not meant for, I did the same on an 8 amp fan speed PWM control and the mosfet got significantly hotter: it is not designed for that. You also have to think safety and failure. it is going on a car, you cannot afford to take a stupid shortcut on something like that. What will happen if the mosfet fails and goes open or closed circuit permanently ? it could cause an accident and at the least leave the driver stranded.
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Offline Chet T16

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Re: Flyback Diode
« Reply #10 on: April 30, 2011, 05:45:07 pm »
My last post was in reference to jahonens reply. I've taken on board what you've said and don't intend on taking shortcuts. I know the level of understanding is far above me on this forum but i'm trying to learn

Fwiw the intended use is for testing/cleaning injector so it would be used off car and only for a couple of seconds at a time. Of course, thats not to say it couldn't be used for extended periods so that needs to be considered!
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Re: Flyback Diode
« Reply #11 on: April 30, 2011, 05:50:31 pm »
well if it's not for road use you don't have huge safety issues although it is often easier to do it right first time. I've been designed some electrical power management for older cars and am very aware that failure is not an option (most cars don't like 40V or no power at all)
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Offline Chet T16

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Re: Flyback Diode
« Reply #12 on: April 30, 2011, 06:38:42 pm »
Oh yeah, do it once, do it right.

The ironic thing is i have an exam on thursday covering diodes, mosfets and bjts and none of it is any use here!
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Re: Flyback Diode
« Reply #13 on: April 30, 2011, 06:40:08 pm »
you will often find blowing stuff up is how you learn the practical side  ;D
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Offline Chet T16

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Re: Flyback Diode
« Reply #14 on: April 30, 2011, 06:44:40 pm »
Oh i can do that!
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Offline Chet T16

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Re: Flyback Diode
« Reply #15 on: May 01, 2011, 12:10:58 pm »
Right, so this diode should do the job perfectly then?

I never knew there were so many different diodes  :D
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Re: Flyback Diode
« Reply #16 on: May 01, 2011, 03:35:35 pm »
should do although I'd try and get something with 30V reverse voltage but guess it is not a huge issue as if it really fails it won't be while the car is moving
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Offline Chet T16

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Re: Flyback Diode
« Reply #17 on: May 01, 2011, 04:44:28 pm »
Why 30V? Is that so its twice the supply voltage?
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Re: Flyback Diode
« Reply #18 on: May 01, 2011, 05:04:59 pm »
remember there are other things in the car circuit that generate transients. There is a standard "transient spec" for automotive systems that states certain positive voltage spikes for a certain amount of time and anything made for a car must have adequate protection against it, we are talking over 100V for very short periods like mS but still worthy of consideration. You also never use anything to it's recommended limit although I guess that as voltages get higher you leave less percentage margin (say a 100V part for 70V 200V would be overkill).

there are people on here that are quite experienced with automotive stuff, they may be able to offer shrewder advice, in the face of doubt I err on the side of caution. If your designing with IC's I suggest adequate spike protection like MOV's or TVS diodes.
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Offline Chet T16

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Re: Flyback Diode
« Reply #19 on: May 01, 2011, 07:20:45 pm »
Thanks for all your help, its very much appreciated.

I have so much to learn which is equally annoying and exciting!
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Re: Flyback Diode
« Reply #20 on: May 01, 2011, 07:39:36 pm »
like I said, the more you blow up the more you learn  :o
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Offline vk6zgo

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Re: Flyback Diode
« Reply #21 on: May 02, 2011, 06:09:51 am »
I would definitely suggest a " Fast Recovery diode."

Just search on your Farnell site under that title,& you should find one,

or:-

If you have an old dead CRT based TV or monitor,you can steal one from the "Boost HT" circuit.

This circuit uses an overwind on the EHT transformer,with a single diode to rectify the 15kHz from the overwind.

This is then filtered by a low ESR cap,to produce about +150v for use in those parts of the TV/Monitor that require

such a voltage.

If you use an ordinary diode here,it overheats & breaks down,so a diode from this circuit should do your job.



VK6ZGO
 

Offline vl400

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Re: Flyback Diode
« Reply #22 on: May 02, 2011, 06:40:21 am »
Being for an injector cleaner you prob are going to be using the injectors afterwards right? Might want to consider how the manufacturers drive these injectors, they use a peak and hold setup. So you see a large current surge which opens the injector quickly and then drops to a steady lower current keeping it open using a current limiting circuit. You may need to check what damage occurs if you drive a low impedance injector at full current while cleaning, or can just add in a series resistor (of high enough wattage) so you dont damage the injector coils.
 


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