Author Topic: Using SCR's as diode's - reverse blocking voltage  (Read 1157 times)

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Offline adamcordTopic starter

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Using SCR's as diode's - reverse blocking voltage
« on: February 12, 2020, 04:13:07 pm »
In a project I'm working on, I'm thinking of using some beefy SCR's as diodes by tying the anode to the gate (a practice which is usually kosher). I want to use these SCR's due to their high blocking voltage and high current ratings. But since the Gate would be shorted to the Anode, I'd eliminate one of the P-N junctions that might otherwise be there to block the reverse voltage, leaving only one P-N junction, right?

So my question is: Does shorting gate to anode on an SCR lower it's reverse blocking capability? (I guess similarly, what if my gate driver conducts between gate-anode during the reverse voltage condition? Again, would this pose a risk of reverse breakdown?)
 

Offline schmitt trigger

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #1 on: February 12, 2020, 04:38:56 pm »
I randomly checked the very first power SCR that I found a Vishay VS-111RKI120PBF. This is a 110 amp device.

Its data sheet indicates a VDRM/VRRM of 1200 volts, yet its maximum peak gate voltage is only rated between 20 volt and -10 volts.
 

Online Zero999

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #2 on: February 12, 2020, 04:39:59 pm »
Dumb question: why not use a diode?

To answer the original question. SCRs can normally block the same reverse voltage as their off voltage rating. The only issue is the gate typically has a maximum reverse voltage rating of around 5V. Connect a small diode with a high enough reverse voltage rating in series with the gate and a high value resistor (100k to 1M) between the gate and cathode.

You should use a diode, if you don't need a switch. It's much cheaper and easier.
 

Offline T3sl4co1l

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #3 on: February 12, 2020, 04:40:53 pm »
G-K has a low reverse breakdown voltage, like BJT B-E (and for the same reasons).

IIRC, there are some SCRs made that way, with high breakdown on both terminals; I forget what special application they serve though.  Or maybe it's what a TRIAC is internally, of course pasted together with its complement so the device switches AC without much concern over drive magnitude or sign.

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Online wraper

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #4 on: February 12, 2020, 04:55:20 pm »
You should first have tried to measure G-K resistance with multimeter and find that it conducts in both directions with tens to hundreds of Ohm resistance.
 

Online wraper

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #5 on: February 12, 2020, 04:56:27 pm »
G-K has a low reverse breakdown voltage
Around 0V to be exact, it's simply conductive both ways.
 

Online Zero999

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #6 on: February 12, 2020, 06:16:35 pm »
You should first have tried to measure G-K resistance with multimeter and find that it conducts in both directions with tens to hundreds of Ohm resistance.
It depends on the SCR. K-G is a diode connection. In small, sensitive gate SCRs it should measure the same as a diode, using a multimeter. Larger non-sensitive gate SCRs have a gate-cathode resistor built-in.
 

Online wraper

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #7 on: February 12, 2020, 08:12:28 pm »
Larger non-sensitive gate SCRs have a gate-cathode resistor built-in.
It's not a built-in resistor, it's an intrinsic feature of SCR. Small SCR may have this resistance on higher side but it's still there.  And this resistance may easily vary 5+ times times within single part number (and so does gate sensitivity).

« Last Edit: February 12, 2020, 10:18:51 pm by wraper »
 
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Offline NiHaoMike

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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #8 on: February 13, 2020, 12:16:31 am »
Dumb question: why not use a diode?

To answer the original question. SCRs can normally block the same reverse voltage as their off voltage rating. The only issue is the gate typically has a maximum reverse voltage rating of around 5V. Connect a small diode with a high enough reverse voltage rating in series with the gate and a high value resistor (100k to 1M) between the gate and cathode.

You should use a diode, if you don't need a switch. It's much cheaper and easier.
I'm assuming he has the SCR lying around but not any large diodes and that it's for a temporary application. In that case, add a series diode and resistor combination from anode to gate to trigger it.
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Re: Using SCR's as diode's - reverse blocking voltage
« Reply #9 on: February 13, 2020, 12:33:59 am »
I'm assuming he has the SCR lying around but not any large diodes and that it's for a temporary application. In that case, add a series diode and resistor combination from anode to gate to trigger it.
That resistor in series with the diode should be something like 10-50 ohms . The value is not critical, just that this lower value makes sure the SCR turns on before the forward voltage gets too high, and the on-stage voltage drop does not give too high a continuous gate current like it would if there was no resistor at all.
 
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