### Author Topic: Diode current in a rectifier circuit  (Read 1595 times)

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#### Meshka7

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##### Diode current in a rectifier circuit
« on: March 01, 2015, 06:48:16 am »
So I'm going through this book and reading about diodes. There are these elaborate equations to calculate the ripple and avg/max current in the diode. In one example, the max current is calculated to be 2.5A. The author concluded that the diode has to be spec'ed for at least 2.5A.

So I simulated circuit, and while LTSpice shows the max current to be only 1.6A, the initial charging current goes all the way to 8.2A. So if I'm using a 2.5A diode, I am out of luck and my rectifier diode will get destroyed by the surge.

Here is the circuit:

https://www.dropbox.com/s/gppvgw7d2ywru9g/Full%20Wave%20Bridge%20Rectifier.asc?dl=0

However I looked up the 1N4001 datasheet and found 2 paramters : Average rectified output current (1A), and peak forward surge current (30A). The first parameter is a little mysterious. Average with respect to what? (given that the diode is on only for a short amount of time). So is it average while the diode is on? That would correspond to the calculated average from the book which is half the max which would be 1.25A (the book derive that iDmax = 2 * iDavg) and we would be out of spec with the 1N4001 (1.25A > 1A).

The 8.2A would be within the surge max 30A parameter and all is good, but that renders the calculated max of 2.5A useless. Is there a rule of thumb to calculate the max surge current out of the max repetitive charging current?

#### JohnnyBerg

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##### Re: Diode current in a rectifier circuit
« Reply #1 on: March 01, 2015, 07:16:15 am »
However I looked up the 1N4001 datasheet and found 2 paramters : Average rectified output current (1A), and peak forward surge current (30A). The first parameter is a little mysterious. Average with respect to what?

They mean that you must take the integral over 1 period.

#### Meshka7

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##### Re: Diode current in a rectifier circuit
« Reply #2 on: March 01, 2015, 07:23:09 am »
The one period average in the same example was calculated to be 66mA. I guess this makes it way within spec. But again, how would I have figured out that the real max is 8.2A??

#### dannyf

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##### Re: Diode current in a rectifier circuit
« Reply #3 on: March 01, 2015, 07:35:45 am »
Quote
The author concluded that the diode has to be spec'ed for at least 2.5A.

The instantaneous current can be extremely high, especially if you have a large capacitance bank with low ESR.

However, most rectifiers are very hardy - they can take repeated peaks 100 - 200x of their average ratings no problem (schottky diodes are not as hardy).

Also, do a FFT on the current and factor in the winding capacitance in the transformer, etc. It is one of those simple circuits that can get very complex quickly.
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#### JohnnyBerg

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##### Re: Diode current in a rectifier circuit
« Reply #4 on: March 01, 2015, 07:40:41 am »
But again, how would I have figured out that the real max is 8.2A??

Thats is not so easy. You have to know al the internal resistances, the moment in the sinewave that power is turned on, and the exact characteristic of the diode. Knowing al that you end up with a first or second order differential equation, that needs solved.
If solved, you get a function for the current in time. To find the peak, you take the derivative and solve it to zero.

#### Meshka7

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##### Re: Diode current in a rectifier circuit
« Reply #5 on: March 01, 2015, 08:57:15 am »
Thanks All.

Smf