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Calculating Gate Current for a SCR

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abbeyromy

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I was plotting characteristic curves for SCR (MCR 228) and observed that gate current is not coming as per the below formula for different Gate Trigger voltage pulses.

IG = (Vtrig-Vg)/Rg


In a book I read that to calculate Ig, the vg is taken as 0.7V ( FB drop of a transistor) but this is not applying when I try to simulate this in pspice.

Then I tried to find the Vg/Ig by plotting and observed that even this ratio is varibale for different Vtrig.

Can anybody tell me how to calculate the gate current for a SCR. What formula or principle can predict the current level?

I am attaching the circuit for your reference. Please help! I am stuck!

-Hemanshu
 

You may want to review the used thyristor model and determine the implemented internal gate resistance...
Who told you, that the SPICE model has to follow your simple formula?

P.S.: Guessed ríght, it's 750 ohm.
 

    abbeyromy

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Hi FvM,

This is the spice model I am using
.subckt MCR22-8 anode gate cathode
* Min and Max parameters
X1 anode gate cathode Scr params:
+ Vdrm=600v Vrrm=600v Ih=2ma Vtm=1.2v Itm=1
+ dVdt=25e6 Igt=0.2ma Vgt=.8v Ton=1.2u Toff=40u
+ Idrm=10u
* 90-5-18 Motorola DL137, Rev 2, 3/89
.ends

How did you calculated the figure of 750 ohms? Please help me understand.
I read this formula in a book and was myself surprised to find that how come gate resistance is not included. I checked the datasheet and found the value RGK = 1000 Ohms mentioned somewhere but then I am not understanding how spice is calculating level of the gate current.
-Hemanshu
 

can somebody please help here
 

The MCR22-8 model instantiates another subcircuit scr, there the inner gate resistance is implemented. It's value is (Vgt-0.65)/Igt.

Actually I don't understand, what you try to achieve.
- Do you want to learn to program SPICE subcircuits? Than you should consult a SPICE reference or text book and figure out,
how the model works.
- Or do you want to understand the empirical behaviour of a thyristor? Then you should be aware of an inner gate resistance in
addition to the well-known diode characteristic. It's not specified in the MCR22-8 datasheet. You can either believe, that the model
provided with PSPICE is representing the device's behaviour correctly or perform measurements on your own.

I checked the datasheet and found the value RGK = 1000 Ohms mentioned
Rgk is an external resistor, not an inner resistance.
 

Hi FvM,

Actually I was trying to understand how gate current calculations are done for SCR.
After reading this equation (IG = (Vtrig-Vg)/Rg ) in a book I tried viewing the same in PSpice but then I was not getting the results.
I will try to understand the gate level resistance from datasheet. Thanks a lot for replying!!
-Hemanshu
 

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