The Current Load: Why Do Two Diodes in Parallel Not Share It?

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Discussion Overview

The discussion revolves around the behavior of two diodes connected in parallel and the reasons they may not share the current load equally. Participants explore the implications of diode characteristics, such as forward voltage and dynamic resistance, and how these factors influence current sharing.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the current sharing between two parallel diodes depends on their forward voltage and dynamic resistance.
  • Others argue that while the current may not be shared equally, it is incorrect to say that one diode carries all the current.
  • It is noted that due to the non-linear I-V characteristics of diodes, one diode may draw more current, leading to thermal effects that further increase its current draw.
  • One participant explains that the saturation current (Is) of a diode is temperature-dependent and varies among individual diodes, contributing to unequal current sharing.
  • A suggestion is made that using small valued series resistors with each diode can help enforce current sharing between them.
  • There is a mention of the similarity in behavior between diodes and transistors due to their non-linear characteristics.

Areas of Agreement / Disagreement

Participants generally agree that diodes do not share current equally and that thermal effects play a significant role in this behavior. However, there is no consensus on the extent of current sharing or the conditions under which it may occur.

Contextual Notes

The discussion highlights the complexity of diode behavior in parallel configurations, including the influence of temperature and individual diode characteristics, but does not resolve the specific conditions under which current sharing may be optimized.

iScience
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why do two diodes in parallel not share the current load?
 
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iScience said:
why do two diodes in parallel not share the current load?
They may, but the current sharing depends on:
  • The forward voltage of the diodes
  • The "dynamic resistance" of the diodes (dVf/dI)
 
They don't? The shares may not be equal but how could they not share? You mean the current goes all through only one of them?
 
nasu said:
They don't? The shares may not be equal but how could they not share? You mean the current goes all through only one of them?
Yes, 1 device tends to hog most of the current. Diodes have a highly non=linear I-V curve, i.e. Id = Is*exp((Vd/Vt) - 1). Important to know is that "Is" is a strong function of temperature, and varies among parts. Due to mismatches in parameters, as soon as 1 diode draws more current, it gets hotter. Then Is goes up with the higher temperature, which increases diode forward current Id further. Then temp goes higher, Is increases, etc. It is a slim chance that 2 paralleled diodes share current 50%/50%. Possible, but very unlikely.

If, however, each diode is connected to a small valued series resistor, then the diode-resistor pairs are connected in parallel, the resistors force current sharing. Does this help at all?

Claude
 
"Sharing" does not imply 50-50. As long as one portion is not zero, they share.
 
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cabraham said:
Yes, 1 device tends to hog most of the current. Diodes have a highly non=linear I-V curve, i.e. Id = Is*exp((Vd/Vt) - 1). Important to know is that "Is" is a strong function of temperature, and varies among parts. Due to mismatches in parameters, as soon as 1 diode draws more current, it gets hotter. Then Is goes up with the higher temperature, which increases diode forward current Id further. Then temp goes higher, Is increases, etc. It is a slim chance that 2 paralleled diodes share current 50%/50%. Possible, but very unlikely.

If, however, each diode is connected to a small valued series resistor, then the diode-resistor pairs are connected in parallel, the resistors force current sharing. Does this help at all?

Claude

Definitely, thanks

and since transistors are also nonlinear this accounts for their observed behavior as well?
 
iScience said:
Definitely, thanks

and since transistors are also nonlinear this accounts for their observed behavior as well?
Yes.

Claude
 

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