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Electronics => Beginners => Topic started by: king.oslo on May 18, 2013, 12:31:36 pm

Title: Estimate silicon rectifier forward current as a function of time
Post by: king.oslo on May 18, 2013, 12:31:36 pm
Hello there,

I am interested in the HVP12 HV diode. Across the diode I need 0V which ramps up to 325V. I need to determine the desired rise time of that voltage. In order to do this, I guess I must know what kind of (http://latex.codecogs.com/gif.latex?\frac{\partial\,I}{\partial\,t}) for the diode.

What is the equation to calculate or estimate silicon rectifier forward current as a function of time (and VF, perhaps)?

Thanks.M
Title: Re: Estimate silicon rectifier forward current as a function of time
Post by: Paul Price on May 18, 2013, 12:52:43 pm
The forward current though any diode is almost instantaneous and current starts to flow in a sub-nanosecond time after voltage is applied. You don't need to know Di/Dt unless the diode of interest is an DIAC or the anode and cathode of a TRIAC or SCR. Look at the data sheet(alldatasheet.com) for your diode to see switching voltage waveforms and forward voltages v. current.  Most ordinary silicon diodes do not start to conduct current(microamps) until after approx .65V of voltage has been applied(room temperature) and the voltage rises up to 1.2V to 1.5V approx when the current approaches the maximum allowable for the diode. At higher temperatures, the threshold forward voltage of conduction will decrease with temperature at a rate of approx. -2.2mv/deg C of temperature difference from room temp.

What is often more important is the type of diode. Schotkey diodes will conduct with faster turn on and faster turn off and have a forward threshold voltage of .12V to .18V with a milli-amp of current noticed, depending on the diode.
Title: Re: Estimate silicon rectifier forward current as a function of time
Post by: king.oslo on May 18, 2013, 01:17:51 pm
Quote from: Paul Price on May 18, 2013, 12:52:43 pm
The forward current though any diode is almost instantaneous and current starts to flow in a sub-nanosecond time after voltage is applied.

Are you saying silicon (not schottky)HVP12 has tRISE < 1ns to its peak current?M
Title: Re: Estimate silicon rectifier forward current as a function of time
Post by: Paul Price on May 18, 2013, 02:05:50 pm
No, a slow diode rectifier like (!N4003) will be much slower than this <nS, one can only to notice a forward current in the high uA to 1 mA range after a few nS.
Title: Re: Estimate silicon rectifier forward current as a function of time
Post by: king.oslo on May 18, 2013, 02:46:12 pm
The datasheet for the HVP12 says nothing about this (as far as I can tell). Here is a link:

http://www.mouser.com/catalog/specsheets/hvp.pdf (http://www.mouser.com/catalog/specsheets/hvp.pdf)

It is critical to the efficiency of the application to know time it takes to reach about 60% of peak surge current 50A. How can I estimate this time?

Thank you for your time.

Kind regards,
Marius
Title: Re: Estimate silicon rectifier forward current as a function of time
Post by: Paul Price on May 18, 2013, 03:10:09 pm
See the attached files...learn to goooooooogle!