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Scr Gate Firing


spark_student

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Hello all,

 

I have read SCR datasheets and have noticed a graph showing gate trigger current and gate trigger voltage. It shows an area that the devices could be in depending on the temperature of the device. I have done a load line analysis of a project I am working on and noticed that beyond my load line is a possibility given my temperature operation range. I am picket fence firing my gate in an soft starter application for a three phase induction motor.

 

Does this mean the gate drive won't be sufficient to turn on the SCR or will it simply require a greater forward voltage and current from the circuit I have connected it up to? Is the consequence just greater power dissipation in my device than a perfect hard turn on. How much is too much? Does it damage the SCR?

 

How can you compare this graph to the typical SCR graph showing anode current and voltage with latching and holding points? These usually show with zero gate current it will reach breakover at the device limit. When the gate current is increased the amount of forward voltage required to activate the SCR lowers.

 

Is the other graph with area for a particular case, say at max di/dt for the device and a particular load?

 

I would assume you would do another load line analysis taking into account the anode cathode circuit max voltage and maximum current depending on the impedance in the load connected to the SCR. The intercepts between where the external circuit load line intercepts your given characteristic at a certain gate current would determine where you were operating and if the SCR would latch. But this load line would change as you start a motor.

 

Another problem/advantage is that the load is a motor, inductive, so would it mean that even weak firing would probably turn it on as the di/dt is low, so weak picket fence is okay? The anode current and voltage will help it turn on. What about partial firing, localised heating in the SCR as a result?

 

The obvious solution is to increase gate power, but I want to optimise the design for the absolute minimum amount of gate power.

 

Sorry about the numerous questions, but I would appreciate any help people can offer.

Thanks in advance.

 

 

 

 

 

 

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Hello spark_student

 

Welcome to the forum.

 

The shaded area in the gate trigger graph covers a number of parameters such as the temperature and the device characteristics spread.

If the gate energy is too low, the SCR will partially turn ON and the charge will slowly spread through out the gate region. If the di/dt is high, the power dissipation can be concentrated in a portion of the die and cause a failure due to localized heating.

An important characteristic is the rate of rise of current in the gate trigger waveform leading edge. It is common to provide a high initial pulse magnitude which drops to a lower steady state. This helps to ensure a fast turn ON.

 

Picket fence firing is used as a means to reduce the total gate power dissipation and to minimize the size of the firing or pulse transformers used.

The pulse width of the initial pulse must be wide enough to allow the current to rise above the latching current of the SCR. If the initial pulse width is too narrow, the SCR will not latch and will turn ON for the duration of the pulse only.

 

If you control a load with a high di/dt, you need a higher initial gate energy with a very steep leading edge. If the load has a lower di/dt, then a softer gate trigger pulse is acceptable. There is no disadvantage in applying trigger pulses designed for a high rate di/dt and there are definite advantages under fault conditions.

 

Best regards,

 

 

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