Thyristor
Thyristor using NPN and PNP transistors.
Description
The Thyristor block allows you to model a teryristor in two ways:
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As an equivalent circuit based on NPN and PNP bipolar transistors.
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Approximation of the I-V (current-voltage) curve in the switched-on state using a matching table.
Representation using equivalent circuit
An equivalent circuit contains a pair of NPN and PNP bipolar transistors as shown in the following figure.
The P-N-P-N thyristor structure corresponds to the P-N-P and N-P-N bipolar transistor structures, with the base of each connected to the collector of the other device. To make this circuit behave like a thyristor, it is necessary to select suitable values for the parameters of the NPN and PNP transistors, as well as the external resistors. For example, in order for the circuit to lock into a conducting state after being triggered by an appropriate control current, the total gain of the two transistors must be greater than unity. This model structure reproduces the behaviour of the thyristor in typical application circuits and at the same time presents the solver with a minimum number of equations, which increases the speed of simulation.
| It is very important to parameterise the thyristor component correctly before using it in your model. According to your device datasheet, modify the parameters of the thyristor component so that it models the desired behaviour. You can then copy the parameterised component into your model. Take care to correctly model the control electrode connection circuit, including the series resistance of the circuit. Connecting a controlled voltage source directly to the control electrode of a thyristor gives unphysical results, because the voltage at the control electrode is pressed against the voltage at the cathode when the voltage at the control electrode is zero. |
The model reflects the following thyristor characteristics:
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Closed-state currents, and . These are usually specified for maximum closed state voltages, and . It is assumed, as for most thyristors, that and .
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The opening voltage at the control electrode is equal to the value of Corresponding gate voltage, V_GT when the current in the control electrode is equal to the thyristor turn-on current, the value of Gate trigger current, I_GT.
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The thyristor switches on when the value of the control current reaches the value of the thyristor trigger current, Gate trigger current, I_GT. The thyristor will not switch on until the control current reaches this value. To ensure this, the Internal shunt resistor, Rs must be set correctly. If the resistance is too high, the thyristor will switch on before the control current reaches . If the resistance is too low, the thyristor will not switch on.
You can determine the value of Internal shunt resistor, Rs by running the simulation. If you use the thyristor in a circuit where there is an external resistor , connected from the control electrode to the cathode, the effect of the resistance determined by the value of Internal shunt resistor, Rs is usually very small and can be set to
Inf. -
If the thyristor is on, the thyristor remains on when no control signal is applied, provided that the load current is greater than the holding current. The holding current is not set directly, as its value is mainly determined by other block parameters. However, the holding current can be influenced by the parameter Product of NPN and PNP forward current gains. Decreasing the gain increases the holding current.
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The turn-on voltage is equal to the voltage drop across the thyristor, the value of On-state voltage, V_T, when the load current is equal to the turn-on current, the value of On-state current, I_T. This is ensured by the resistance value , which takes into account the voltage drop on PNP and NPN devices.
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Triggering by the rate of voltage rise in the closed state. The rapid change in anode-cathode voltage induces a current in the base-collector circuit. If this current is large enough, it switches the thyristor to the open state. A suitable value of the base-collector capacitance is calculated so that at a rate of voltage change equal to the maximum allowable rate of rise of off-state voltage, Critical rate of rise of off-state voltage, dV/dt, the thyristor will trip. This calculation is based on the approximation that the required current is equal to , where is the value of the resistance between the control electrode and the cathode used in the calculation of the maximum allowable Critical rate of rise of off-state voltage, dV/dt.
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The turn-on delay time depends mainly on the value of the NPN device forward transit time, TF parameter. You either specify this parameter directly or calculate an approximate value of from the switch-on time.
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The switch-off delay time, which is primarily affected by the value of the PNP device forward transit time, TF parameter. You can set this parameter directly or set it equal to the forward transit time for the NPN transistor.
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The pnp_resistor and npn_resistor increase the stability of the numerical solution at large forward and reverse voltages. Their values affect the turn-off currents by no more than 1% at maximum forward and reverse closed state voltages.
| Since this implementation of the block includes a charge model, the total resistance of the circuit driving the gate must be modelled in order to obtain clear on/off dynamics of the thyristor. Therefore, if you simplify the electrode control circuitry by representing it as a controlled voltage source, you must include a suitable resistor in series between the voltage source and the control electrode. |
parameterization using a matching table
When you parameterise a thyristor using a matching table, the anode-to-cathode current value is a function of the voltage between the anode and cathode in the open state. The main advantages of using this option are speed of modelling and ease of parameterization. To further simplify the basic model, this representation does not simulate:
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Switching the device on due to the rate of voltage rise in the off state.
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Switch-off delay time.
The turn-on delay is represented by an input capacitor between the control electrode and the cathode, the value of which is calculated so that the delay between the voltage rise at the control electrode and the start of device turn-on is equal to the value specified by the Turn-on delay time parameter. The turn-on load current rise time is realised by the non-linear rise of the current from zero to the current defined by the current-voltage profile in the open state in the time specified by the Turn-on rise time parameter value. Note that the resulting turn-on current profile is an approximation to a real device.
Assumptions and limitations
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Temperature dependent effects are not modelled in this block. This block is modelled at the temperature specified by the Measurement temperature parameter value. All parameters must be specified for this temperature.
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If you are using an equivalent circuit model:
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In circuits with a sensitive control electrode (that is, where there is no external resistor for the control electrode-cathode), you must set the Internal shunt resistor, Rs parameter value to ensure proper triggering. If the internal shunt resistance is too high, the thyristor will trip at a current lower than . If the internal resistance of the shunt is too low, the thyristor will not trip at an input current .
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Tripping when the breakdown voltage is exceeded is not modelled.
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Numerical modelling of the thyristor can be difficult, given the very small control currents compared to the load current, and the jumps in current during switching. However, for most typical thyristor-based circuits, default modelling parameters can be used. In some cases, it may be necessary to tighten the Absolute Tolerance and Relative Tolerance parameters in the Solver Configuration block to ensure convergence. In such cases it is usually sufficient to change the Absolute Tolerance by default from
autoto1e-4or1e-5, as this prevents this parameter from being adaptively changed during the simulation. -
The leakage currents are approximated by i-leakage diodes as shown in the equivalent circuit. This approach assumes that the leakage through the two transistors is small in comparison. This assumption is not valid for values of , which are much smaller than the typical forward voltage drop of
0.6V.
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If you use a compliance table view:
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Triggering by excess breakdown voltage or by the rate of change of voltage in the off state is not modelled.
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Off delay time is not modelled. Check to see if your circuit violates the specified off delay time.
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When you specify the rise time of the turn-on current, the resulting current-time dependence is an approximation.
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Parameters
Main
I-V characteristics defined by - thyristor parameterization
Fundamental nonlinear equations (by default) | Lookup table
Use either an equivalent circuit based on NPN and PNP bipolar transistors (Fundamental nonlinear equations) or an approximation of the I-V curve in the on state using the Lookup table to model the thyristor.
On-state voltage, V_T - voltage drop across the thyristor in the on state
1.2 V (by default).
Static anode-cathode voltage drop in on-state, with current flowing equal to the switch-on current .
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
On-state current, I_T - static load current in switched-on state
1 A (By default).
Static load current (anode current) that flows when the anode-to-cathode voltage is equal to the voltage in the on state.
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
Vector of on-state voltages, V_T - vector of on-state voltage values
[0.75 1 1.25 1.5 1.75 2 2.25] V (by default).
Vector of open state voltage values to be used for table lookup. The values of the vector must be strictly increasing and the first value must be greater than zero. The values may not be uniformly distributed.
Dependencies
To use this parameter, set Lookup table for I-V characteristics defined by.
Vector of corresponding currents, I_T - vector of values of corresponding currents
[0.015 0.22 0.75 1.4 2 2.75 3.45] A (by default).
A vector of currents values corresponding to the values of the vector of inclusion voltages to be used for searching the 1D table. The two vectors must be of the same size.
Dependencies
To use this parameter, set Lookup table for I-V characteristics defined by.
Off-state current, I_DRM - anode current in off state
0.01 mA (by default).
Off-state anode current , which flows when the anode-to-cathode voltage is equal to the off-state voltage .
Corresponding off-state voltage, V_DRM - anode-cathode voltage in the off state
`400 V (by default).
Anode-to-cathode voltage , applied to the thyristor in the off-state when the off-state current is reached .
Measurement temperature - device modelling temperature
25 °C (By default)
Device modelling temperature. All values of the unit parameters must be specified for this temperature.
Dependencies
To use this parameter, set Fundamental nonlinear equations for the parameter I-V characteristics defined by.
Gate Triggering
Gate trigger current, I_GT - gate triggering current.
3 µA (by default)
Threshold value of current through the control electrode , necessary to switch on the transistor, as a result of which the voltage on the gate becomes equal to the corresponding voltage on the control electrode . You must set Internal shunt resistor, Rs to ensure that the gate is triggered at , not at currents less than .
Corresponding gate voltage, V_GT is the voltage between the control electrode and the cathode
0.6 V (by default)
Voltage between the control electrode and the cathode , when the current at the control electrode is equal to the turn-on current .
Test voltage, V_D - test supply voltage
`12 V (by default)
Supply voltage used when specifying values and .
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
Test load resistor - test load resistor
`120 ohms (by default)'.
Load resistor used in calculating the values of and .
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
dV/dt Triggering
Critical rate of rise of off-state voltage, dV/dt - maximum allowable rate of rise of off-state voltage
150 V/sec (by default)
If the anode and cathode voltages rise faster than this rate, the thyristor will experience parasitic switching due to capacitive effects.
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
Test gate-cathode resistor, R_GK - test resistor between the control electrode and the cathode
`1000 ohms (by default)'.
Resistor between the control electrode and the cathode used in calculating the maximum allowable voltage rise rate in the off state.
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
Time Constants
NPN device forward transit time parameterization - NPN device forward transit time parameterization
Derive approximate value from gate-controlled turn-on time (by default) | `Specify directly `
Select one of the following options:
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Derive approximate value from gate-controlled turn-on time- the block calculates the NPN device turn-on time based on the turn-on time and the corresponding control current you specify. -
`Specify directly' - specify the value directly using the NPN device forward transit time parameter.
Dependencies
To use this parameter, set Fundamental nonlinear equations for the I-V characteristics defined by parameter.
Gate-controlled turn-on time - time of delay of thyristor turn-on after the control pulse is applied
2 ms (By default).
Time of thyristor switching from off to on when control current is applied.
Corresponding gate current - test current of the control electrode
`10 mA (by default).
Control electrode current used in determining the on-time delay of the thyristor after a control pulse is applied. The gate current and turn-on time are used to calculate an approximate value for the forward run time of the NPN device, assuming that all input charge is used to raise the gate voltage to the corresponding gate voltage . The value by default is 10 mA.
Dependencies
To use this parameter, set Fundamental nonlinear equations for the I-V characteristics defined by parameter and Derive approximate value from gate-controlled turn-on time for the NPN device forward transit time parameterization parameter.
NPN device forward transit time - average forward transit time
0.3 ms (By default).
Average transit time of non-basic charge carriers through the base region from emitter to collector of NPN device [1].
Dependencies
To use this parameter, set Fundamental nonlinear equations for the I-V characteristics defined by parameter, and Derive approximate value from gate-controlled turn-on time for the NPN device forward transit time parameterization parameter.
PNP device forward transit time parameterization - PNP device forward transit time parameterization
Set equal l to NPN device forward transit time (by default) | Specify directly.
Select one of the following options:
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Set equal to NPN device forward transit time- the block uses the NPN device forward transit time value. -
Specify directly- specify the value directly using the *PNP device forward transit time parameter.
Dependencies
To use this parameter, set Fundamental nonlinear equations for the parameter I-V characteristics defined by.
Turn-on delay time - thyristor turn-on delay time
`0 s (by default)
Turn-on delay time after the current on the control electrode changes from zero to the value set by the Gate current for turn-on delay time parameter.
Dependencies
To use this parameter, set Lookup table for I-V characteristics defined by.
Gate current for turn-on delay time - current at the control electrode for turn-on delay time
1 mA (by default).
The current at the control electrode used for turn-on delay time measurement.
Dependencies
To use this parameter, set Lookup table for I-V characteristics defined by.
Turn-on rise time - rise time of the turn-on signal
0 s (By default).
The time required for the thyristor to fully switch on after the turn-on delay time has passed.
Dependencies
To use this parameter, set Lookup table for I-V characteristics defined by.
Advanced
Internal shunt resistor, Rs - shunt resistance between the control electrode and the cathode.
`87 kOhm (by default).
The shunt resistor between the control electrode and the cathode. It is important to set the value of this parameter so that the gate trips at , not at currents less than . If you are using a thyristor in a circuit where there is an external resistor , the effect of is usually small and can be set to Inf.
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.
Internal series gate resistor, Rg - resistance associated with the control electrode connection
`10 ohms (by default)'.
Resistance associated with the control electrode connection. A typical value is of the order of a few ohms, and its effect on static and dynamic characteristics is small. Its exact value is therefore not important, but its presence helps to avoid problems in numerical modelling if the gate is controlled directly by a voltage source. You can specify any positive value.
Product of NPN and PNP forward current gains - product of NPN and PNP forward current gains
10 (By default)
This is the product of NPN forward current gain and PNP forward current gain . The value must be greater than unity for latching to occur. The smaller the value, the larger the latching current. However, the latching current is mainly set by other block parameters, and the overall gain has only a minor effect.
Dependencies
To use this parameter, set Fundamental nonlinear equations for I-V characteristics defined by.