# Two Diodes In Parallel

Discussion in 'Homework Help' started by ElectronicReaper, Oct 14, 2012.

1. ### ElectronicReaper Thread Starter New Member

Oct 14, 2012
2
0
Hello, all

I am given the following circuit

The problem states: $D_1$ and $D_2$ have different cross sectional areas but othewise identical

(1) Find the current following through the diodes.

(2) If $I_{in}=8mA$ and $D_2$ has 2 times the larger cross sectional area than $D_1$. Calculate current following through each diode

No values for the components are given.

My complications: I am on part one.

I am sure as what diode model to use. If I use ideal diode model then the diodes would be forward biased and would essentially act as a short circuit thus, shorting out the other diode. Correct?

Or I could use the constant voltage drop model with $V_D=0.7V$
and do KVL for both loops but that doesn't workout either. Likewise, I could use the battery plus model and solve for some general equation for the currents through each diode but it doesn't seem to make much sense.

I, also, know that for the exponential model for a diode that the saturation current of the diode ($I_s$) is proportional to the junction area of the diode. So I know at least for part two that diode $D_2$ should exhibit a large current.

2. ### WBahn Moderator

Mar 31, 2012
23,862
7,384
So I know at least for part two that diode $D_2$ should exhibit a large current.

How does I_s depend on the junction area?

Write things in terms of the saturation current for one of the diodes (you pick one, it doesn't matter, though choosing the smaller one will probably make the math a bit easier and less error prone). Call this I_so.

Look at the equations and how they behave when they are in parallel. How will the currrent always split between the two (assuming that all of the other parameters, including the temperature, can be kept the same). Come up with a model for single diode that is equivalent to the two diodes in parallel. Use the single diode to find the total current in the circuit to answer the first part and your knowledge of how the current will split to answer the second part.

If you can assume that the voltage source is significantly greater than the diode knee voltage, then you can assume a constant voltage across both diodes to answer the first and use your analysis of the parallel situation to answer the second. Even if you don't do this, you should use it as a simple sanity check on your answer.

3. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
510
In IC design we set up bias chains and ratioed current based on emitter area. I recall we could always assume the current was proportional to the area, so I believe you can assume the current will ratio itself as 2:1 through the two diodes and solve using that. YMMV, but that's what I remember (from 1980)