VDR Silicon Carbide Varistors

Thermal Design Parameters

Energy Absorption

The specified energy absorption values apply to pulse lengths within a range of milliseconds. The strong dependence of the permissible energy absorption on the pulse length and pulse count which applies to metal oxide varistors does not arise with silicon carbide varistors. For this reason they are particularly suitable in cases of high pulse frequency.

Temperature Range

Temperature Range for lacquered varistors (SDL Series):
- At Full Load - 40°C to + 55°C
- At Zero Load - 40°C to + 130°C
Temperature Range for unlacquered varistors (S and SD Series):
- At Full Load - 40°C to + 75°C
- At Zero Load - 40°C to + 150°C

Power disspation or operating voltage as specified in derating curves must not be exceeded if the ambient temperature is above 55°C or 75°C respectively.

VDR Silicon Carbide Varistors

Derating Curve for Power Dissipation

 

VDR Silicon Carbide Varistors

Derating Curve for Operating Voltage

 

Temperature Coefficient

Silicon carbide varistors have a temperature coefficient which must be observed in certain fields of application. Since temperature increases , voltage drops with constant current, or current rises if voltage is constant. The temperature coefficient of voltage with constant current TKU is differentiated from that of current with constant voltage TKI:

TKIJ = - 0.12 to - 0.18 % / K

TKI = 0.5 to 0.8 % / K

The exponent n is practically temperature independent.

Reaction Time

The reaction time of silicon carbide varistors is mainly determined by connection inductance. In order to attain short reaction times (<10ns) low inductive installation is essential. Measurements have shown that silicon carbide varistors follow current changes of 5*1010 A/s without measureable delay.