# Question about the term "Vf*Ron*Goff" in the series voltage in diode block from Simscape Electrical.

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Marcelo on 30 Nov 2021
Commented: David John on 4 Jan 2022
Hi, everyone,
The diode block has the following explanation in its Help file:
So, according this explanation, the series voltage source is:
V = Vf * (1 − Ron*Goff)
V = Vf − Vf*Ron*Goff
V = Vf − (Vf*Goff)*Ron
V = Vf − (Ioff)*Ron
I don't remember seeing this second voltage component (Vf*Ron*Goff) anywhere else.
And, altough it is said that this second term is to ensure zero current when zero voltage is applied across the diode, I could not understand and draw an equivalent electrical circuit that shows this afirmative in a clear and easy way.
Can anyone help me understand this (Vf*Ron*Goff) voltage component?
Thank you.

David John on 16 Dec 2021
In the off-state, the diode equation is:
v = i/Goff
In the on-state, the equation is:
v = i*Ron + Vsource
where Vsource is like a series voltage source. The transition between the two states occurs at v=Vf. In order for the current to be continuous at Vf, we must have Vf*Goff=(Vf-Vsource)/Ron. This implies that Vsource=Vf*(1-Ron*Goff).
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David John on 4 Jan 2022
Thanks for this. I'll feed this back to the library team to see about updating the documentation to make it more clear.

R2019b

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