Switching Characteristics of SCR
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SCR or Thyristor Turn-on Time:
Thyristor turn-on time may be defined as the time required by the SCR to change its state from forward blocking mode to forward conduction mode when a gate pulse is applied.
The total turn-on time of SCR comprises three different time intervals:
Delay Time:
- It is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value.
Rise Time:
- Rise time is the time taken by the anode current to rise from 10% of its final value to 90% of its final value.
Spread Time:
- It is the time taken by the anode current to rise from 90% of its final value to 100%.
- It is the time required for the forward blocking voltage to fall from 10% of an anode voltage to the on-state voltage drop of the device.
Concept
The current through an inductor is given by:
\(I_L={V_L\over L}\times t\)
\(t={I_L\over V_L}\times L\)
where, t = time period
IL = Latching current
VL = Voltage
L = Inductor
Calculation
Given, IL = 4 mA
VL = 200 V
L = 0.2 H
\(t={4\times 10^{-3}\over 200}\times 0.2\)
t = 4 μs
Key Points Latching Current: It is the
minimum anode current required to maintain the thyristor in the
ON state immediately after a thyristor has been turned on and the gate signal has been removed.
Holding Current: It is the minimum anode current to maintain the thyristor in the on-state.
Latching current is always greater than holding current.
Additional Information The thyristor or SCR is a power semiconductor device which is used in power electronic circuits.
They work like a bistable switch and it operates from nonconducting to conducting.
The designing of thyristors can be done with 3-PN junctions and 4 layers.
It includes three terminals namely anode, gate, and cathode.
Concept:
- Need of series connection of SCR is required when we want to meet the increased voltage requirement by using various SCR's.
- When the required voltage rating exceeds the SCR voltage rating, a number of SCR's are required to be connected in series to share the forward and reverse voltage.
- When the load current exceeds the SCR current rating, SCR are connected in parallel to share the load current.
Application:
According to the question:
Given that
SCR 1 voltage = 350 V; current = 6 A
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SCR 2 voltage = 300 V; current = 9 A
SCR 3 voltage = 250 V; current = 12 A
Let us take the total current to be 'I'
Current through resistor R in shunt with SCR 1 is
I1 = I ' 6
Similarly, current through resistor R is shunt with SCR 2
I2 = I ' 9
And, current through resistor R in shunt with SCR 3
I3 = I ' 12
Now, the string voltage becomes
Vs = I1R + I2R + I3R
Vs = (I - 6) R + (I - 9) R + (I - 12) R
Vs = (I - 6) R + (I - 6) R ' 3 R + (I - 6) R ' 6 R '..(1)
Note that
We should consider the extreme case from the calculation of resistance R for voltage equalization in string of SCR.
In extreme case, the voltage drop across SCR 1 (or the one having highest voltage drop) will be maximum forward blocking voltage.
' Vmax = 350 = (I - 6) R ----(2)
Put (I - 6) R = 350 in equation (1), we get
Vs = 350 + 350 ' 3 R + 350 ' 6 R
(350 + 300 + 250) = ' 9 R
900 = ' 9 R
9 R = ' 900
\(R = \frac{{150}}{9}{\rm{\Omega }}\)
R = 16.66 Ω
Therefore the
value of equalising resistance to be used across each thyrister is 16.66 Ω.
Turn on time: thyristor takes some transition time to go from forward blocking mode to forward conduction mode. This transition time is called turn on time of SCR
Turn off time: time during which a reverse voltage is applied across the thyristor during its commutation process.
Turn off time of a thyristor is greater than turn-on time.
Two thyristors of the same rating and same specifications may have equal or unequal turn-on and turn-off periods.
I2t rating is used to determine the thermal energy absorption of the device. This rating is required in the choice of a fuse or other protective equipment employed for the SCR. This is the measure of the thermal energy that the SCR can absorb for a short period of time before clearing the fault by the fuse.
Important Point:
di/dt rating of thyristor indicates the maximum rate of rising of the anode to cathode current. We use a series reactor to control this limit
dv/dt rating of thyristor indicates the maximum rate of rising of anode voltage that will not trigger the device without any gate signal. We use a snubber circuit to control this limit
For more information, please visit Thyristor Switches.
The only way to be sure that you are testing only the SCR is to take the module you included data on out of the circuit. Set up a simple bench test circuit to check for performance. Since you will be working with out a scope, use a DC supply and just gate the SCR only and see if it fires. You can tell if there is voltage drop across the load resistor you use, or if no voltage there it will all drop across the SCR until it is fired. If the load has voltage across it all the time and you do not have to gate the SCR then the likelihood is that the SCR is shorted. Be sure to place the SCR in the test circuit, not the diode that is in the same package. By the way, you can get some really decent scopes for little money. Especially on Ebay.
mordred said:
testing the biasing is certainly the best method however another quick check is to take a resistive check with your DMM + to anode -to cathode resistance should be a low number. on the reverse bias - to anode + to cathode resistance should be high a shorted junction will show a low resistance in both for/rev bias and high resistance an open junction in both states. it is similar to a diode resistive check in that case
I'm sorry but I must disagree. You can do a DMM check on a Thyristor from gate to cathode and get a low resistance if the device is undamaged, but standard forward and reverse resistance readings (diode test) will be high in both directions because it hasn't been gated. A DMM
WILL tell you if the SCR PN junction is
shorted because it will read low in both directions, it
won't tell you if the device is
opened.
Hi,
Not sure just how familiar you are with thyristors, but what Randy suggests is correct.
But to complete his suggestion:
1. Use a load in the range of 100W (like a plain light bulb) in series with the thyristor anode-cathode and feed with any standard AC mains supply.
2. Use a DC supply (5 to 10 Volts should do the trick) in series with a small resistor like 100Ohms (so you don't damage the thyristor gate) and feed it between the gate and the cathode (don't bother about polarity).
The load current should only flow as long as the thyristor gate current flows.
The diode in the thyristor module can be checked with any diode tester (or DMM).
Lets start with the easiest check of all, one you can do with the thyristor in place and with the load (heating elements) connected. No disassembly required.
Place your temperature control loop in manual and set the output at 0%. You should read near 0 V at the thyristor output. Ramp up the output of your control loop. As the output increases you should read a voltage that is close to an equivalent percentage of your line voltage. IE, if line voltage is 480V, then at 10% you should read about 48V, at 50% you should read about 240V, and at 100% you should read about 480V. If you get a fairly linear reading at the output then the semiconductors and firing circuit are just fine. If you see something like 50% voltage at 0% command, then one of the SCRs is shorted out and you need to replace the module.
Another thing that can cause havoc in a resistive heating system is if one of the heating elements has developed a short circuit to ground somewhere along the element path - something that you may not pick up on right away because it won't trip any breakers for blow a fuse, especially if there is a transformer in the heating circuit as well.
Maybe I'm asking an obvious question, but has anything else changed in the system at all? The problem you describe is not one that would send me looking at the SCRs or triacs right away.
If this is something you do often? If so I recommend that you bite the bullet and purchase an oscilloscope. Its a tool. If you saw the necessity of the DMM and purchased one, then the same thought and justification process should apply to the scope, despite the additonal cost.
Alaric said:
Another thing that can cause havoc in a resistive heating system is if one of the heating elements has developed a short circuit to ground somewhere along the element path - something that you may not pick up on right away because it won't trip any breakers for blow a fuse, especially if there is a transformer in the heating circuit as well.
To expand on this just a bit:
Sometime a problem with a heating element will look like a power control problem because it only shows up when the elements are hot and/or you are at a high power output from the power controller. Elements move around as they heat up. These can be a real bear to track down.