Aumov LV Ultramov Varistor Design Guide Page 7 Varistor Design Guide For DC & Automotive Applications

DC Application Varistor Design Guide 2015 Littelfuse DC Application Varistor Design Guide www.littelfuse.com Terminology Used in Varistor Specifications Terms and Descriptions Symbol Clamping Voltage. Peak voltage across the varistor measured under conditions of a specified peak V C pulse current and specified waveform. NOTE: Peak voltage and peak currents are not necessarily coincidental in time. V C Rated Peak Single-Pulse Transient Currents. Maximum peak current which may be applied for a single 8/20 s impulse, with rated line voltage also applied, without causing device failure. I TM Lifetime Rated Pulse Currents. Derated values of I TM for impulse durations exceeding that of an 8/20 s waveshape, and for multiple pulses which may be applied over device rated lifetime. - Rated RMS Voltage. Maximum continuous sinusoidal RMS voltage which may be applied. V M(AC) Rated DC Voltage. Maximum continuous DC voltage which may be applied. V M(DC) DC Standby Current. Varistor current measured at rated voltage, V M(DC) . I D For certain applications, some of the following terms may be useful. Nominal Varistor Voltage. Voltage across the varistor measured at a specified pulsed DC current, I N(DC) , of specific duration. I N(DC) is specified by the varistor manufacturer. V N(DC) Peak Nominal Varistor Voltage. Voltage across the varistor measured at a specified peak AC current, I N(AC) , of specific duration. I N(AC) is specified by the varistor manufacturer. V N(AC) Rated Recurrent Peak Voltage. Maximum recurrent peak voltage which may be applied for a specified duty cycle and waveform. V PM Rated Single-Pulse Transient Energy. Energy which may be dissipated for a single impulse of maximum rated current at a specified waveshape, with rated RMS voltage or rated DC voltage also applied, without causing device failure. W TM Rated Transient Average Power Dissipation. Maximum average power which may be dissipated due to a group of pulses occurring within a specified isolated time period, without causing device failure. - Varistor Voltage. Voltage across the varistor measured at a given current, I X . V X Voltage Clamping Ratio. A figure of merit measure of the varistor clamping effectiveness as defined by the symbols (V C ) (V M(AC) ), (V C ) (V M(DC) ). V C /V PM Nonlinear Exponent. A measure of varistor nonlinearity between two given operating currents, I 1 and I 2 , as described by I = kV a where k is a device constant, I 1 I I 2 , and a 12 = ( logI 2 / I 1 ) ( logV 2 / V 1 ) a Dynamic Impedance. A measure of small signal impedance at a given operating point as defined by: Z X = ( dV X ) ( dI X ) Z X Resistance (Varistor). Static resistance of the varistor at a given operating point as defined by: R X = ( V X ) ( I X ) R X Capacitance (Varistor). Capacitance between the two terminals of the varistor measured at specified frequency and bias. C X AC Standby Power. Varistor AC power dissipation measured at rated RMS voltage V M(AC) . P D Voltage Overshoot. The excess voltage above the clamping voltage of the device for a given current that occurs when current waves of less than 8 s virtual front duration are applied. This value may be expressed as a % of the clamping voltage (V C ) for an 8/20s current wave. V OS Response Time. The time between the point at which the wave exceeds the clamping voltage level (V C ) and the peak of the voltage overshoot. For the purpose of this definition, clamping voltage as defined with an 8/20 s current waveform of the same peak current amplitude as the waveform used for this response time. - Overshoot Duration (Varistor). The time between the point at voltage level (V C ) and the point at which the voltage overshoot has decayed to 50% of its peak. For the purpose of this definition, clamping voltage is defined with an 8/20 s current waveform of the same peak current amplitude as the waveform used for this overshoot duration. - Terminology Used in Varistor Specifications 6

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