How to estimate the diode forward voltage drop by forward current?

Thread Starter

shenhome

Joined Oct 10, 2009
3
I got the diode forward voltage drop in the datasheet, which is 3.5V@If=30A. I didn't get any profile about the current vs voltage drop in the datasheet. Now I would like to know how to estimate the drop voltage if If=10A is used in my design.
 

SgtWookie

Joined Jul 17, 2007
22,221
3.5v @ 30A is a pretty hefty power dissipation; 105 Watts.

It would help a great deal knowing first the part number of the rectifier.
It would also help to know the voltage range you're dealing with. You may be better off using a different rectifier.
 

rjenkins

Joined Nov 6, 2005
1,013
You can very roughly estimate it as being linear, starting at 0.7V for low current.

3.5 - 0.7 = 2.8V
/3 (for 10A) = approx 0.9
add back the 0.7 = 1.6V


So roughly 1.6V @ 10A for about 16W dissipation.
 

bountyhunter

Joined Sep 7, 2009
2,512
I got the diode forward voltage drop in the datasheet, which is 3.5V@If=30A. I didn't get any profile about the current vs voltage drop in the datasheet. Now I would like to know how to estimate the drop voltage if If=10A is used in my design.
The specs can either be "typical" or "maximum". The data sheet usually has a typical curve showing VF versus current.

The formula for calculating the change in diode drop at two currents is:

delta VBE = .026 x natural log (I1 / I2)

In your case, going from 30A down to 10A only changes the VBE by about 30 mV. Not much. In reality, it may be more due to other effects.
 

bountyhunter

Joined Sep 7, 2009
2,512
You can very roughly estimate it as being linear, starting at 0.7V for low current.

3.5 - 0.7 = 2.8V
/3 (for 10A) = approx 0.9
add back the 0.7 = 1.6V


So roughly 1.6V @ 10A for about 16W dissipation.
I don't think this method will yield accurate values. Here is a VF curve for a high current silicon rectifier:

http://pdf1.alldatasheet.com/datasheet-pdf/view/81693/DIOTEC/1N3671.html

The VF @ 30A is 1.1V and at 10A it's about 0.92V. The curve is not close to being linear.

Do you have a data sheet for the part in question?
 

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SgtWookie

Joined Jul 17, 2007
22,221
Yes, it could very well be that the OP meant 3.5v @ 30mA instead of Amperes; and they want to try running it at 10mA.

That would make a lot more sense than a Vf of 3.5v for a rectifier diode.
 

Thread Starter

shenhome

Joined Oct 10, 2009
3
I am talking about TVS diode from Onsemi, whose part number is P6SMB47AT3G. Maybe you would ask why I need Vf for TVS, but I do need it for some reason. I am asking data from supplier, but right now I don't get it. I would like to know if there is any method to estimate the Vf vs If.

Thanks a lot for your great help!
 
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