LTspice simulation solid state contactor
Solid state relays (SSR) and solid state contactors (SSC) are widely used in AC networks where silent switching, high switching frequencies, and virtually
unlimited lifetime are required. The robust power semiconductors can be used to switch ohmic, inductive and capacitive loads (zero-point switching or
moment-switching). The load output usually consists of two anti-parallel connected thyristors. The resulting power loss of the thyristors must be dissipated
via a properly sized heat sink.
The following example shows a thermal simulation with LTspice* for a solid state contactor based on forward measurement and Zth-measurement.
Data solid state contactor:Reverse voltage=800V
Load current=20A / 13,2A
Air heat sink=80mm x 76mm x 22mmZero-point switchingMounting for DIN rail
The picture shows the layout with the spatial position of the two thyristors on a DCB in the solid state contactor.Forward voltage measurement:
Thyristor characteristic at Tj=25°C and 125°C, measured at the main terminals.
The picture shows the cooling curves of the two thyristors after switching off the load current of thyristor 1. The cooling curves were created from the extracted
R/C elements of the measurement curve.
Cooling: free convection, heating/cooling time: 7000s each.
Since the module is rotationally symmetrical, the same thermal network is used for both thyristors.Simulation results:
The graph shows the simulated junction temperatures tj_thyr1/2 (°C), the power dissipations pv_thyr1/2 (W), the forward voltages ud_thyr1/2 (V) and theload currents V1/2 (A) of the two thyristors in the time range between 6999,95s and 7000s.Start temperature (Tamb)=25°C, middle junction temperature (Tj)=77,32°C.*LTspice is a free software from the former semiconductor manufacturer Linear Technology (since 2017: Analog Devices) for circuit simulation. It is developedand maintained by Mike Engelhardt.