Diode current specs - Page 1
Forum section: electronics / beginners

author: PepeChip
author_rank: Contributor
timestamp: September 21, 2017, 01:10:04 pm
content: Hi,I shouldn't actually be a beginner but... here I am, asking this.Many times when I see a diode datasheet, I get confused.I do not know (and I have not been able to find any explanation to) the difference between "Forward Continuous Current" and "Average Rectified Output Current".As an example, one can take the diode BAS16W from Diodes.Can somebody explain to me the difference between these two ratings and say to me if a diode like BAS16W (I_forward = 300mA; I_av = 150mA) could be used if the continuous required current is 250mA?Many thanks,regars

author: orolo
author_rank: Frequent Contributor
timestamp: September 21, 2017, 02:53:56 pm
content: The average rectified forward current rating is the the maximum average current the diode can handle working as a half-wave rectifier when fed a sinewave at mains frequency (seethis).Note that the rectifier current peaks at Pi times the average, and then is zero for half a cycle. This is a diferent figure than the maximum DC current the diode can handle. For example, in theRohm diode FAQ,under "Most of the parameters specify average rectified current. How much DC current can be supplied?", they provide graphs where you can see that the maximum square wave current (duty cycle = 0.5) is a tiny bit higher than the average rectified forward current, while the maximum DC current is about 50% higher than that. Of course, this might change from diode to diode. Read your datasheet carefully.Anyway, applying a +50% rule of thumb, if your max average rectified current is 150mA, increasing that by 50% gives 225mA. So using 250mA of DC current might be pushing the diode a bit.

author: PepeChip
author_rank: Contributor
timestamp: September 21, 2017, 03:04:46 pm
content: Hi Orolo,Thanks for replying but I do not understand your answer well.Where can you see that the rectified current peaks at pi times the average?In the Rohm diode FAQ that you refer to there is no distinction between the two currents I am asking about. It is always average rectified current at different duty cycles, that is all.What is the difference between "Forward Continuous Current" and "Average Rectified Output Current"? (see datasheet for BAS16W)ThanksRegards

author: orolo
author_rank: Frequent Contributor
timestamp: September 21, 2017, 03:10:49 pm
content: Quote from: PepeChip on September 21, 2017, 03:04:46 pm\$\displaystyle I_{av} \ = \ \frac{1}{2\pi}\int_0^{\pi}I_{max}\sin t\, \mathrm{d}t \ = \ \frac{2I_{max}}{2\pi} \ = \ \frac{I_{max}}{\pi}\$QuoteThere are three curves, one for sine (only 180 degrees, half a cycle), other for a square wave with 1/2 duty cycle, and other for DC. For DC the average current equals the instantaneous current.QuoteCan you provide a link? I made a quick search when answering the first time, and I got several datasheets for the part.

author: PepeChip
author_rank: Contributor
timestamp: September 21, 2017, 03:16:12 pm
content: https://www.diodes.com/assets/Datasheets/ds30094.pdf

author: orolo
author_rank: Frequent Contributor
timestamp: September 21, 2017, 03:31:52 pm
content: I think it's clear that the forward continuous current is the absolute maximum forward DC current for the diode.However, I think that figure is probably too optimistic. Note that figure 2 forinstantaneousforward voltage ends at 300mA. At 25ºC and 300mA, following that figure, the forward voltage is about 1.15V, and that means a power dissipation of 345mW, quite higher than the 200mW maximum. I don't think that is good for continous operation.Even at 250mA the forward voltage is over 1V, and then the power dissipation is over 200mW. I don't think I would operate the diode continuosly at that power. For a thermal resistance of 625ºC/W that would mean going well over 125ºC.I think the best you can do is to test the diode working and see if it gets too hot to be safe.