Two Diodes In Parallel

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.

In, short how would I go about this problem

ElectronicReaper said:

My complications: I am on part one.

I am sure as what diode model to use. If I use ideal diode model
[\QUOTE]

The fact that you are given non-ideal information about the diodes is a big hint that, perhaps, using an ideal model is not the best choice.

then the diodes would be forward biased and would essentially act as a short circuit thus, shorting out the other diode. Correct?

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If they are both ideal diodes (whether it is modelled as a short or as a constant voltage) says nothing about how the current will share between them. We can make some assumptions about the current density being the same in each, but the ideal model has no mechanism for inforcing that.

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.

Click to expand...

For the reason given above -- there's no mechanism to determine what fraction of the current goes where.

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.

Click to expand...

What's the "battery plus" model? And why doesn't that 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.

Click to expand...

Think about what you have just said. You are given information about the junction areas and you have a diode model in which the junction area is relevant to the operation of the diode. Sounds like a pretty strong indication that perhaps this is the best model to at least start with.

Click to expand...

So I know at least for part two that diode \(D_2\) should exhibit a large current.

In, short how would I go about this problem[/QUOTE]

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.

ElectronicReaper said:

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.

Click to expand...

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)

See if your calculations agree.



From search engine:

• Lecture 5
  
  PN Junction Diodes ... Since the current flowing across a PN junction is
  proportional to its cross-sectional area, two identical PN junctions connected in
  parallel act effectively as a single PN junction with twice the cross-sectional area,
  hence twice ... Injected minority carriers do not recombine in the quasi-neutral
  base region.
  
• 
  
• www-inst.eecs.berkeley.edu/~ee105/fa11/Lectures/Lecture%206.ppt

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