Author Topic: confuse by diode specs  (Read 2094 times)

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Online Simon

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confuse by diode specs
« on: September 20, 2014, 08:12:40 pm »
I keep seeing on datasheets for diodes:

Non?Repetitive Peak Surge Current
(Surge Applied at Rated Load Conditions Halfwave, 60 Hz)

What does this translate to in practice ? does it mean that the peak can only occur at a rate of 60Hz having just said "Non?Repetitive Peak Surge Current" it seems to be a bit of a contradictory statement to me. How do I translate this to a 1KHz switching rate, ie do i derate the current or can't I ever exceed average current more often than at a rate of 60Hz. I'm trying to select a back emf diode for 15A running current.
 

Offline kxenos

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Re: confuse by diode specs
« Reply #1 on: September 20, 2014, 08:20:55 pm »
It means that this amount of current was applied one time (non repetitive) for a duration half a period of 60Hz (1/120 sec)
 

Offline SeanB

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Re: confuse by diode specs
« Reply #2 on: September 20, 2014, 08:24:35 pm »
Max current that can safely be handled during turn on initially. Afterwards the rating drops to max repetitive current, and the average current over a cycle must be less than the max current.

Basically this is the peak of the turn on surge. Then you integrate ( that video of Dave again ) to get the average current per cycle from the peak current per cycle. Then you use the instantaneous forward voltage multiplied by the instantaneous forward current and integrate that to get the per cycle power dissipation. From that you can see if the diode is capable of handling the current. You can have a circuit where the average current is 3A, but a 3A diode will fail as the power dissipated is too high, or the peak current is too high, or the surge current when you turn on the power is more than the diode can handle. Big toroid and large ultra low ESR capacitor is the ideal demonstrator for this.
 

Online Simon

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Re: confuse by diode specs
« Reply #3 on: September 20, 2014, 08:39:46 pm »
So the non repetitive peak is for 8.3..ms

Now I'm looking at this datasheet as an example: http://www.farnell.com/datasheets/1708329.pdf so my average is 6A for both halves. How would they describe the peak current that I can consider part of the average. I mean 12A for 500ms with a 50% duty is 6A on average or say 24A for 250ms with a 25% duty, at what point do i hit a limit ?
 

Online Simon

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Re: confuse by diode specs
« Reply #4 on: September 20, 2014, 08:56:22 pm »
I think i spotted it, under average current rating it has a comment about 20KHz so presumably the maximum duration of a pulse that produces an average of 6A is 50us so at 50% (25us) that would be peaks of 12A and at 25% (12.5us) 24A, now how long does a back emf pulse last i wonder ? probably depends on the motor and speed etc....
 

Offline kxenos

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Re: confuse by diode specs
« Reply #5 on: September 20, 2014, 10:19:37 pm »
The maximum forward current of this diode is 3A. If your signal is square wave (50% duty cycle [or less]) you can have maximum repetitive current of 6A. The duty cycle is the important factor and not the frequency. If Ton=0.5 then 6A * 0,5 = 3A average current. You shouldn't pass more than 6A no matter the frequency and the average (or DC) current can not exceed 3A.
But anyway, in this diode you have practically another limitation: You have Rjc = 9C and for a DPAK package assuming a PCB land of 0,5in^2 you have also about 60C so if you mount this device in a PCB and have a zone of 0.5in^2 it will heat about 70C above ambient temp for every Watt it dissipates. For an ambient temp. of 30C that means about 100C for 1 Watt. If forward voltage is 1Volt that means that with 1A of average current you will have the device working in 100C which is unacceptable for a design value under normal circumstances because it's going to reduce the life of the part.
 

Online Simon

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Re: confuse by diode specs
« Reply #6 on: September 21, 2014, 07:22:58 am »
Yes power dissipation will be the limiting factor and I'm expecting that as Vf increases with current the short and larger the spikes the more average wattage is dissipated with the same average current. In my case it's hard to predict the amount of average current because I am going to use it as a back emf diode.
 


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