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Power Dissipation of Diode

FreshmanNewbie

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I have two diode datasheets.

Onsemi

Vishay

In both the datasheets, why is the maximum allowable power dissipation of the diode not provided in the absolute maximum ratings table.
Usually, in electronic components, I have seen the absolute maximum ratings table which will include the maximum allowable power dissipation for transistors and other components. But why in these diode datasheets, the maximum power is missing?
 

KlausST

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Hi,

in my eyes the max power dissipation makes not much sense here.

Let´s take an example:
Let´s imagine the max. continous power dissipation of the OnSemi diode to be 2W.

What do you do with this information? How do you go on?

Klaus
 

BradtheRad

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I've wondered the same thing. I suppose the reason is because it takes less energy to cite just one or two specs about a component. Thus it makes sense for us to cite: (a) the most informative (or most important) spec, and (b) whatever secondary rating is informative and also easy to remember, or (c) easy to measure, or (d) easy to identify.

With ordinary diodes it's easier to measure Amperes rather than Watts. And by looking at the size of the diode we easily gauge its Amp rating, whether 3A or 1A or small signal.

For zener diodes on the other hand it's important to know the zener voltage and also the Watt rating. Notice their safe Ampere rating can vary over a wide range.

Thus it's easier to cite 1N47xx family are 1W rating, because that's informative and easy to remember. Who can cite safe Amp ratings for zeners in the 1N47xx family?
 

Easy peasy

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max power dissipation is a function of available cooling - it can and sometimes is quoted for a device on an infinite heatsink @ 20 deg C

usually an engineer looks at the package and decides the max power that is acceptable for the pcb cooling or otherwise that is available
--- Updated ---

actually a careful read gives a max current of 1.6A @ 110degC where the forward drop is about 0.85V ( from the graphs )

So Pmax = 1.36 watts if you can keep the pads to 110 deg C or less at this power level - again a function of the heatsinking you provide ...
 
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c_mitra

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But why in these diode datasheets, the maximum power is missing?
Good observation. Relevant point.

Instead of the max power dissipation, you will see the max current (both peak and cont) values should be specified. People are looking for these two parameters for a diode.

The datasheet should also give a graph that says the forward drop vs forward current. This graph basically says the power dissipation.

If you also specify the power dissipation, you are over specifying the parameters. It is possible to have a contradiction there.

In reality, the forward voltage drop is not under your control. This is a device characteristic. Hence it is more relevant to say the forward voltage drop vs the forward current.

For large diodes that need a heat sink, you will see somewhere else the details of the heat sinking needed at max current (peak or cont).

I hope I have made the point clearer.
 

Easy peasy

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The datasheet should also give a graph that says the forward drop vs forward current. This graph basically says the power dissipation.
power dissipation taken from that graph has to be qualified by the heatsinking available - no heatsinking means you cannot use all the points on the graph ...
 

FreshmanNewbie

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Good observation. Relevant point.

Instead of the max power dissipation, you will see the max current (both peak and cont) values should be specified. People are looking for these two parameters for a diode.

The datasheet should also give a graph that says the forward drop vs forward current. This graph basically says the power dissipation.

If you also specify the power dissipation, you are over specifying the parameters. It is possible to have a contradiction there.

In reality, the forward voltage drop is not under your control. This is a device characteristic. Hence it is more relevant to say the forward voltage drop vs the forward current.

For large diodes that need a heat sink, you will see somewhere else the details of the heat sinking needed at max current (peak or cont).

I hope I have made the point clearer.
Thank you for your answer. Very much helped
 

c_mitra

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Let´s imagine the max. continous power dissipation of the OnSemi diode to be 2W.
For Zener diodes, this is very much relevant and useful. It tells you the magnitude of current it is expected to handle.

For rectifier diodes, we look for the forward current as the relevant parameter and the power dissipation is calculated from the forward voltage drop.

For zener diodes, we look for the breakdown voltage and the reverse current is determined from the power dissipation of the diode.

Of course it is possible to state the reverse current for zener diodes also.
 

KlausST

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Hi,

@c_mitra.
in a datasheet where only one part is specified it makes sense to give the max current. This - usually - is what I want to know.
In a datasheet where multiple devices are specified (a complete family) it makes sense to specify the max power dissipation.

Example:
* Resistors: for each resistor value you have the same specified power dissipation, but the current is different depending on resisor value.
( ofr sure one can wirte the max current for 10 Ohms, 12 Ohms, 15 Ohms.... it becomes a huge table..)
* for Zeners: for each zener value you have the same specified power dissipation, but the current is different depending on resisor value.
(same as above)

in opposite:
* a diode (as mentioned by the OP): we have a somehow fixed forward voltage. Thus we get a fixed current. And usually this is what we want to know.

Klaus
 

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