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ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner. www.fairchildsemi.com Application Note AN-3008 RC Snubber Networks for Thyristor Power Control and Transient Suppression Introduction A A RC networks are used to control voltage transients that could IA falsely turn-on a thyristor. These networks are called snub- IBP PE V bers. PNP I CJ I 1 P J ICP NB CJN The simple snubber consists of a series resistor and capacitor CJ C IC I2 dV placed around the thyristor. These components along with N IJ G P dt B the load inductance form a series CRL circuit. Snubber NPN G t I theory follows from the solution of the circuit’s differential BN dV N CJ E IK dt equation. IA = 1– (αN + αp) K Many RC combinations are capable of providing acceptable TWO TRANSISTOR MODEL CJ K CEFF = performance. However, improperly used snubbers can cause OF 1– (αN + αp) INTEGRATED unreliable circuit operation and damage to the semiconduc- SCR STRUCTURE tor device. dV Figure 1. Model Both turn-on and turn-off protection may be necessary for ()dt s reliability. Sometimes the thyristor must function with a dV range of load values. The type of thyristors used, circuit Conditions Influencing () configuration, and load characteristics are influential. dt s Transients occurring at line crossing or when there is no Snubber design involves compromises. They include cost, initial voltage across the thyristor are worst case. The voltage rate, peak voltage, and turn-on stress. Practical collector junction capacitance is greatest then because the solutions depend on device and circuit physics. depletion layer widens at higher voltage. Small transients are incapable of charging the self-capaci- dV Static tance of the gate layer to its forward biased threshold voltage dt (Figure 2). Capacitance voltage divider action between the collector and gate-cathode junctions and built-in resistors dV What is Static ? that shunt current away from the cathode emitter are respon- dt sible for this effect. dV Static ------- is a measure of the ability of a thyristor to retain a dt 180 blocking state under the influence of a voltage transient. 160 MAC 228-10 TRIAC T = 110°C 140 J dV s) ()Device Physics µ dt s 120 dt dV dV 100 Static ------- turn-on is a consequence of the Miller effect and dt regeneration (Figure 1). A change in voltage across the 80 junction capacitance induces current through it. This current STATIC60 (V/ dV is proportional to the rate of voltage change ------- . It triggers 40 dt the device on when it becomes large enough to raise the sum 20 20 100 200 300 400 500 600 700 800 of the NPN and PNP transistor alphas to unity. PEAK MAIN TERMINAL VOLTAGE (VOLTS) dV Figure 2. Exponential versus Peak Voltage ()dt s REV. 4.01 6/24/02 AN-3008 APPLICATION NOTE dV dV Static ------- does not depend strongly on voltage for operation Improving () dt dt s below the maximum voltage and temperature rating. dV Avalanche multiplication will increase leakage current and Static ------- can be improved by adding an external resistor dt dV reduce ------- capability if a transient is within roughly 50 volts from the gate to MT1 (Figure 4). The resistor provides a path dt dV of the actual device breakover voltage. for leakage and ------- induced currents that originate in the dt drive circuit or the thyristor itself. dV A higher rated voltage device guarantees increased ------- at dt 140 lower voltage. This is a consequence of the exponential rating method where a 400 V device rated at 50 V/µs has a 120 dV higher ------- to 200 V than a 200 V device with an identical MAC 228-10 dt 100 800V 110°C s) rating. However, the same diffusion recipe usually applies µ for all voltages. So actual capabilities of the product are not 80 dt much different. dV 60 RINTERNAL = 600Ω dV STATIC40 (V/ Heat increases current gain and leakage, lowering ------- , dt s the gate trigger voltage and noise immunity (Figure 3). 20 0 180 10 100 1000 10000 GATE-MT1, RESISTANCE (OHMS) 160 MAC 228-10 140 VPK = 800 V dV s) Figure 4. Exponential () versus µ dt 120 s Gate to MT1 Resistance dt dV 100 Non-sensitive devices (Figure 5) have internal shorting 80 resistors dispersed throughout the chip’s cathode area. This STATIC (V/ 60 design feature improves noise immunity and high tempera- 40 ture blocking stability at the expense of increased trigger and holding current. External resistors are optional for non-sensi- 20 20 100 200 300 400 500 600 700 800 tive SCRs and TRIACs. They should be comparable in size TJ, JUNCTION TEMPERATURE (°C) to the internal shorting resistance of the device (20 to 100 ohms) to provide maximum improvement.