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Thyristor

Model thyristor using NPN and PNP transistors

Library

Semiconductor Devices

Description

The Thyristor block provides two ways of modeling a thyristor:

• As an equivalent circuit based on NPN and PNP bipolar transistors

• By a lookup table approximation to the on-state I-V (current-voltage) curve

Representation by Equivalent Circuit

The equivalent circuit contains a pair of NPN and PNP bipolar transistors, as shown in the following illustration.

The P-N-P-N structure of a thyristor is matched by the P-N-P and N-P-N structures of the bipolar transistors, the base of each device being connected to the collector of the other device. Ensuring that this circuit behaves like a thyristor is primarily picking suitable parameter values of the NPN and PNP devices, plus external resistors. For example, for the circuit to latch into the on-state, once triggered by a suitable gate current, the total gain of the two transistors must be greater than one. This model structure replicates the behavior of a thyristor in typical application circuits, while at the same time presenting a minimum number of equations to the solver, to improve simulation speed.

 Note:   It is extremely important that you parameterize the thyristor component correctly before using it in your model. To help you do this, there are two test harnesses in the SimElectronics® examples, Thyristor Static Behavior ValidationThyristor Static Behavior Validation and Thyristor Dynamic Behavior ValidationThyristor Dynamic Behavior Validation. Follow the help text for these two examples, plus a datasheet for your device, to re-parameterize the thyristor component so that it replicates the required behavior. You can then copy the parameterized component into your model. Take care to model the gate drive circuitry correctly, including circuit series resistance. Connecting a controlled voltage source directly to the thyristor gate gives nonphysical results because it clamps the gate to the cathode voltage when the gate demand is zero.

The model captures the following thyristor behaviors:

• Off-state currents, IDRM and IRRM. These are typically quoted for the maximum off-state voltages VDRM and VRRM. It is assumed, as is the case for most thyristors, that IDRM = IRRM and VDRM = VRRM.

• The gate trigger voltage is equal to the Gate trigger voltage, v_GT parameter value when the gate current is equal to the Gate trigger current, i_GT parameter value.

• The thyristor latches on when the gate current is equal to the Gate trigger current, i_GT. The thyristor does not latch on until the gate current reaches this value. To ensure this is the case, you must set the Internal shunt resistor, Rs parameter correctly. If the resistance is too high, then the gate triggers before the gate current reaches iGT. If the resistance is too small, then the gate does not trigger.

You can determine the value of the internal shunt resistor Rs by running the simulation. To see how this can be done, refer to the Thyristor Static Behavior ValidationThyristor Static Behavior Validation example. Alternatively, if you are using the thyristor in a circuit where there is an external resistor RGK connected from gate to cathode, then the effect of Rs is usually very small, and it can be set to inf.

• With the thyristor in the on-state, if the gate current is removed, the thyristor stays in the on-state, provided that the load current is higher than the latching current. You do not specify the latching current directly because its value is primarily determined by other block parameters.

However, the latching current can be influenced by the Product of NPN and PNP forward current gains parameter on the Advanced tab. Reducing the gain increases the latching current.

• The on-state voltage is equal to the On-state voltage, V_T parameter value when the load current is equal to the On-state current, I_T parameter value. This is ensured by the R_on resistance value, which takes into account the voltage drop seen across the PNP and NPN devices.

• Triggering by rate of rise of off-state voltage. A rapid change in anode-cathode voltage induces a current in the base-collector capacitance terms. If this current is large enough, it triggers the thyristor into the on-state. The thyristor initialization routine calculates a suitable value for the base-collector capacitance, so that when the rate of change of voltage is equal to the Critical rate of rise of off-state voltage, dV/dt parameter value, the thyristor triggers on. This calculation is based on the approximation that the required current is vGT / RGK where RGK is the gate-cathode resistance value used when quoting the critical dV/dt value.

• A nonzero gate-controlled turn-on time. This is primarily influenced by the NPN transistor forward transit time, TF. You either specify this parameter directly, or calculate an approximate value for TF from the turn-on time.

• A nonzero commuted turn-off time. This is primarily influenced by the PNP transistor forward transit time, TF. You can either specify this parameter directly, or set it to be equal to the forward transit time for the NPN transistor.

Resistors Gmin1 and Gmin2 improve numerical robustness at large forward and reverse voltages. Their values influence the off-state currents by no more than 1% at the maximum off-state forward and reverse voltages.

Representation by Lookup Table

If using the lookup table representation, you provide tabulated values for anode-cathode current as a function of anode-cathode voltage when in the on state. The main advantages of using this option are simulation speed and ease of parameterization. To further simplify the underlying model, this representation does not model:

• Device triggering due to rate of rise of off-state voltage

• Commuted turn-off time

The turn-on delay is represented by an input gate-cathode capacitor, the value of which is calculated so that the delay between gate voltage rising and the device starting to turn on is equal to the value specified by the Turn-on delay time parameter. The turn-on rise time for the load current is implemented by ramping nonlinearly between zero and the current determined by on-state current-voltage profile over a time period specified by the value of the Turn-on rise time parameter. Note that the resulting turn-on current profile is an approximation to an actual device.

Thermal Port

The block has an optional thermal port, hidden by default. To expose the thermal port, right-click the block in your model, and then from the context menu select Simscape block choices > Show thermal port. This action displays the thermal port H on the block icon, and adds the Thermal port tab to the block dialog box.

Use the thermal port to simulate the effects of generated heat and device temperature. For more information on using thermal ports and on the Thermal port tab parameters, see Simulating Thermal Effects in Semiconductors.

Basic Assumptions and Limitations

The Thyristor block has the following limitations:

• This block does not model temperature-dependent effects. SimElectronics simulates the block at the temperature at which the component behavior was measured, as specified by the Measurement temperature parameter value. All parameters must be quoted for this temperature.

• If you use the equivalent circuit representation:

• In sensitive gate circuits (that is, where there is no external gate-cathode resistor RGK), you must set the value of the Internal shunt resistor, Rs parameter to ensure correct triggering. If the internal shunt resistance is too high, then the thyristor triggers for currents less than iGT. If the internal shunt resistance is too low, the thyristor does not trigger for an input current of iGT. For details on using simulation to determine the acceptable internal shunt resistance value, see the Thyristor Static Behavior ValidationThyristor Static Behavior Validation example.

• Triggering by exceeding the break-over voltage is not modeled.

• Numerically the thyristor can be demanding to simulate, given the very small gate currents in comparison to the load current, and also the very steep current gradients during switching. However, for most typical thyristor-based circuits, you can use the default simulation parameters. In some cases you may need to tighten the Absolute Tolerance and Relative Tolerance parameters on the Solver tab of the Configuration Parameters dialog box, to ensure convergence. In such cases, changing the default value of Absolute Tolerance from auto to 1e-4 or 1e-5 is usually sufficient, because it prevents adaptive changing of this parameter during simulation.

• The leakage currents are approximated by the diodes D1 and D2, as shown in the equivalent circuit. This approach assumes that the leakage via the two transistors is small in comparison. This assumption is not valid for values of vGT that are significantly smaller than the typical forward voltage drop of 0.6 V.

• If you use the lookup table representation:

• Triggering by exceeding the break-over voltage or by rate of change of off-state voltage is not modeled.

• Commutated turn-off time is not modeled. Check that your circuit does not violate the stated commutated turn-off time for the thyristor.

• When you specify a turn-on rise time, the resulting current-time profile is an approximation.

Dialog Box and Parameters

Main Tab

I-V characteristics defined by

Select the thyristor representation:

• Fundamental nonlinear equations — Use an equivalent circuit based on NPN and PNP bipolar transistors. This is the default.

• Lookup table — Use a lookup table approximation to the on-state I-V curve.

On-state voltage, V_T

The anode-cathode static voltage drop when in the on-state, and the current flowing is equal to the on-state current IT. The default value is 1.2 V. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

On-state current, I_T

Static load (or equivalently anode) current that flows when the anode-cathode voltage is equal to the on-state voltage VT. The default value is 1 A. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Vector of on-state voltages, V_T

The vector of on-state voltages, to be used for table lookup. The vector values must be strictly increasing, and the first value must be greater than zero. The values can be nonuniformly spaced. The default values, in V, are [0.75 1 1.25 1.5 1.75 2 2.25]. This parameter is visible only when you select Lookup table for the I-V characteristics defined by parameter.

Vector of corresponding currents, I_T

The vector of currents corresponding to the on-state voltages vector values, to be used for 1D table lookup. The two vectors must be of the same size. The default values, in A, are [0.015 0.22 0.75 1.4 2 2.75 3.45]. This parameter is visible only when you select Lookup table for the I-V characteristics defined by parameter.

Off-state current, I_DRM

The off-state anode current IDRM that flows when the anode-cathode voltage is equal to the off-state voltage VDRM. The default value is 0.01 mA.

Corresponding off-state voltage, V_DRM

Corresponding off-state voltage, VDRM. The anode-cathode voltage VDRM applied with the thyristor in the off-state when quoting the off-state current IDRM. The default value is 400 V.

Holding current

Minimum current at which the thyristor stays in the off state when gate voltage is not commanding the on state. The default value is 1 mA. This parameter is visible only when you select Lookup table for the I-V characteristics defined by parameter.

Measurement temperature

The device simulation temperature. You must specify all block parameter values for this temperature. The default value is 25 °C. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Gate Triggering Tab

Gate trigger current, I_GT

Critical gate current iGT required to turn the transistor on, resulting in a gate voltage equal to the gate trigger voltage vGT. You must set the value of the Internal shunt resistor, Rs parameter on the Advanced tab to ensure that the gate triggers at iGT, and not for currents less that iGT. The default value is 3 μA.

Corresponding gate voltage, V_GT

Gate-cathode voltage vGT when the gate current is equal to the gate trigger current iGT. The default value is 0.6 V.

Test voltage, V_D

Supply voltage used when quoting values for vGT and iGT. The default value is 12 V. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Test load resistor

Load resistor used when quoting values for vGT and iGT. The default value is 120 Ω. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

dV/dt Triggering Tab

Critical rate of rise of off-state voltage, dV/dt

Rate at which the off-state anode-cathode voltage must be increased for the thyristor to turn on. The default value is 150 V/μs. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Test gate-cathode resistor, R_GK

Gate-cathode resistor used when quoting the critical rate of rise off off-state voltage. The default value is 1 KΩ. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Time Constants Tab

NPN device forward transit time parameterization

Select one of the following options:

• Derive approximate value from gate-controlled turn-on time — The block calculates the NPN device forward transit time based on the values for the gate-controlled turn-on time and corresponding gate current that you specify.

• Specify directly — Provide the value directly by using the NPN device forward transit time parameter.

This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Gate-controlled turn-on time

Time for the gate to turn from the off to the on state when a gate current is applied. The default value is 2 μs. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter and Derive approximate value from gate-controlled turn-on time for NPN device forward transit time parameterization.

Corresponding gate current

The gate current used when quoting the gate-controlled turn-on time. The gate current and turn-on time are used to calculate an approximate value for the NPN transistor forward transit time on the assumption that all of the input charge is used to raise the gate voltage to the gate trigger voltage vGT. The default value is 10 mA. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter and Derive approximate value from gate-controlled turn-on time for NPN device forward transit time parameterization.

NPN device forward transit time

Represents the mean time for the minority carriers to cross the base region from the emitter to the collector of the NPN device [1]. The default value is 0.3 μs. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter and Specify directly for NPN device forward transit time parameterization.

PNP device forward transit time parameterization

Select one of the following options:

• Set equal to NPN device forward transit time — The block uses the NPN device forward transit time value.

• Specify directly — Provide the value directly by using the PNP device forward transit time parameter.

This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

PNP device forward transit time

Represents the mean time for the minority carriers to cross the base region from the emitter to the collector of the PNP device [1]. The default value is 0.3 μs. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter and Specify directly for PNP device forward transit time parameterization.

Turn-on delay time

Time delay before the device starts to turn on following a step in current on the gate from zero to the value specified by the Gate current for turn-on delay time parameter. The default value is 0 s. This parameter is visible only when you select Lookup table for the I-V characteristics defined by parameter.

Gate current for turn-on delay time

The gate current used when measuring the turn-on delay time. The default value is 1 mA. This parameter is visible only when you select Lookup table for the I-V characteristics defined by parameter.

Turn-on rise time

Time it takes for the thyristor to turn on fully following the turn-on delay event. The default value is 0 s. This parameter is visible only when you select Lookup table for the I-V characteristics defined by parameter.

Advanced Tab

Internal shunt resistor, Rs

Represents the gate-cathode shunt resistance. It is important to set this parameter value to ensure that the gate triggers at iGT, and not for currents less that iGT. For details, see the Thyristor Static Behavior ValidationThyristor Static Behavior Validation example. If you are using the thyristor in a circuit where there is an external gate-cathode resistor RGK, then usually the effect of Rs is small, and it can be set to inf. The default value is 87 kΩ. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Internal series gate resistor, Rg

Represents the resistance associated with the gate connection. A typical value is of the order of a few ohms, and its impact on static and dynamic characteristics is small. Therefore, its precise value is not important, but its presence helps avoid numerical simulation issues if the gate is driven directly by a voltage source. You can specify any positive value. The default value is 10 Ω.

Product of NPN and PNP forward current gains

This is the product of the NPN forward gain BFNPN and the PNP forward gain BFPNP. The value must be greater than one for latching to occur. The smaller the value, the larger the latching current becomes. However, latching current is primarily set by other block parameters, and the total gain has only a small effect. The default value is 10. This parameter is visible only when you select Fundamental nonlinear equations for the I-V characteristics defined by parameter.

Ports

The block has the following ports:

G

Electrical conserving port associated with the gate

A

Electrical conserving port associated with the anode

K

Electrical conserving port associated with the cathode

References

[1] G. Massobrio and P. Antognetti. Semiconductor Device Modeling with SPICE. 2nd Edition, McGraw-Hill, 1993.

See Also

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