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The back-to-back thyristor model consists of two similar thyristors connected in anti-parallel. This device helps in switching the current in either direction by applying a small current of either polarity between the gate and main terminal. 

The model considers various factors like forward bias threshold, forward and reverse break over voltage, blocking current, snubber elements, holding current, on/off resistance. Depending on the user's requirement, there is also an option to select the precision functionality, which allows the user to compensate for errors due to digital implementation of the model during the state switching as a function of time step. The turnoff time Tq is the recovery time between the instant the anode current has decreased to 0 and the instant when the thyristor is capable of withstanding positive voltage without turning on again.

The model also has an option to be controlled from external signals and an option to enter the precision delay D.

titleTable of Contents

Table of Contents

Mask and Parameters


NameDescriptionUnitVariable = {Possible Values}
VminForward voltage dropV


RopenOpen state resistanceOhm{(0,1e12]}
RcloseClosed state resistanceOhm{(0,1e12]}
RsnubberSnubber resistanceOhm{(0,1e12]}
CsnubberSnubber capacitanceF{[0,1e12]}
Control type

External Input pins: Schematic control signal; the received integer is converted to a binary string where each digit corresponds to a phase

External Input sensors: Target digital inputs


NameDescriptionUnitVariable = {Possible Values}
Fail resetFail reset

{Enable, Disable}

Precision valve

Enable: Enables recalculation of the state switch command using exact time between steps (as percentage of time step)

Disable: does not use precision valve

{Enable, Disable}
IminHolding currentA{[0,1e12]}
TqThyristor turn off times{[0,1e12]}
FbovForward break over voltageV{[0,1e12]}
RbovReverse break over voltageV{[0,1e12]}

Ports, Inputs, Outputs and Signals Available for Monitoring


Net_1Network connection
Net_2Network connection 


DDelay used for precision valves
PBinary coded firing pulses




DDelay used for precision valve, measured from changing point to current time steps
PBinary coded firing pulses
FailSigFail reset signal
IDiode currentA
IswSwitch currentA
DelayDelay due to forward voltage drops
Delay12Delay in forward Thyristor modes
Delay21Delay in anti parallel Thyristor modes
DelayExt12Delay for external triggers
DelayExt21Delay for external triggers
Cmd12Forward Signal state command
Cmd21Anti parallel Signal state command
CmdExt12Signal state external command
CmdExt21Signal state external command
State12Forward Thyristor state
State21Anti-parallel Thyristor state

Note that in the previous table, the suffixes "12" and "21" denote the forward (positive to negative) and anti-parallel (negative to positive) thyristor states respectively. One can identify the positive terminal of the device as the one closest to the input pin. So, in the default (not flipped or rotated) thyristor back-to-back icon, the positive terminal is the top one.