AN1048/D RC Snubber Networks For Thyristor Power Control and Littelfuse.com Transient Suppression By George Templeton APPLICATION NOTE Thyristor Applications Engineer INTRODUCTION Edited and Updated RC networks are used to control voltage transients that ǒdVǓ DEVICE PHYSICS could falsely turn-on a thyristor. These networks are called dt s snubbers. Static dV turn-on is a consequence of the Miller effect The simple snubber consists of a series resistor and dt capacitor placed around the thyristor. These components and regeneration (Figure 1). A change in voltage across the along with the load inductance form a series CRL circuit. junction capacitance induces a current through it. This cur- Snubber theory follows from the solution of the circuit’s rent is proportional to the rate of voltage change ǒdVǓ . It differential equation. dt Many RC combinations are capable of providing accept- triggers the device on when it becomes large enough to able performance. However, improperly used snubbers can raise the sum of the NPN and PNP transistor alphas to unity. cause unreliable circuit operation and damage to the semi- conductor device. A Both turn-on and turn-off protection may be necessary A for reliability. Sometimes the thyristor must function with a I IA BP PE range of load values. The type of thyristors used, circuit V PNP configuration, and load characteristics are influential. I CJ I I 1 P J CP NB CJ Snubber design involves compromises. They include N C C I J CN I2 dv I G P cost, voltage rate, peak voltage, and turn-on stress. Practi- J dt B cal solutions depend on device and circuit physics. NPN G t IB N NE IK dV CJ dV K + dt STATIC IA * a ) a dt 1 ( N p) K TWO TRANSISTOR MODEL CJ OF C + INTEGRATED dV EFF 1*(aN)ap) WHAT IS STATIC ? SCR STRUCTURE dt Static dV is a measure of the ability of a thyristor to dt retain a blocking state under the influence of a voltage dV Figure 6.1. ǒ Ǔ Model transient. dt s Specifications subject to change without notice. © 2016 Littelfuse, Inc. 1 Publication Order Number: September 19, 2016 − Rev. 3 AN1048/D AN1048/D 170 CONDITIONS INFLUENCING ǒdVǓ dt s 150 Transients occurring at line crossing or when there is no MAC 228A10 130 initial voltage across the thyristor are worst case. The col- VPK = 800 V lector junction capacitance is greatest then because the μ 110 depletion layer widens at higher voltage. dt dV 90 Small transients are incapable of charging the self- capacitance of the gate layer to its forward biased threshold 70 STATIC (V/ s) STATIC voltage (Figure 2). Capacitance voltage divider action 50 between the collector and gate-cathode junctions and built- in resistors that shunt current away from the cathode emit- 30 ter are responsible for this effect. 10 25 85705540 100 115 130 145 TJ, JUNCTION TEMPERATURE (°C) 180 dV Figure 6.3. Exponential ǒ Ǔ versus Temperature 160 dt s MAC 228A10 TRIAC 140 TJ = 110°C ǒdVǓ FAILURE MODE μ 120 dt s 100 Occasional unwanted turn-on by a transient may be dt dV acceptable in a heater circuit but isn’t in a fire prevention 80 sprinkler system or for the control of a large motor. Turn-on STATIC (V/ s) STATIC 60 is destructive when the follow-on current amplitude or rate 40 is excessive. If the thyristor shorts the power line or a charged capacitor, it will be damaged. 20 1000 600500400300200 700 800 Static dV turn-on is non-destructive when series imped- dt PEAK MAIN TERMINAL VOLTAGE (VOLTS) ance limits the surge. The thyristor turns off after a half- dV dV Figure 6.2. Exponential ǒ Ǔ versus Peak Voltage cycle of conduction. High aids current spreading in the dt s dt thyristor, improving its ability to withstand dI. Breakdown dt turn-on does not have this benefit and should be prevented. Static dV does not depend strongly on voltage for opera- dt tion below the maximum voltage and temperature rating. 140 Avalanche multiplication will increase leakage current and 120 reduce dV capability if a transient is within roughly 50 volts MAC 228A10 dt 100 800 V 110°C of the actual device breakover voltage. μ A higher rated voltage device guarantees increased dV at 80 dt dt dV lower voltage. This is a consequence of the exponential rat- 60 ing method where a 400 V device rated at 50 V/μs has a STATIC (V/ s) STATIC 40 RINTERNAL = 600 Ω higher dV to 200 V than a 200 V device with an identical dt rating. However, the same diffusion recipe usually applies 20 for all voltages. So actual capabilities of the product are not 0 much different. 10100 1000 10,000 Heat increases current gain and leakage, lowering GATE‐MT1 RESISTANCE (OHMS) ǒdVǓ , the gate trigger voltage and noise immunity ǒdVǓ dt s Figure 6.4. Exponential dt s versus (Figure 3). Gate to MT1 Resistance Specifications subject to change without notice. © 2016 Littelfuse, Inc. 2 Publication Order Number: September 19, 2016 − Rev. 3 AN1048/D AN1048/D ǒdVǓ 10 IMPROVING MEG MCR22‐006 dt s TA = 65°C dV Static can be improved by adding an external resistor A dt 10 from the gate to MT1 (Figure 4). The resistor provides a 1 V G K path for leakage and dV induced currents that originate in MEG dt the drive circuit or the thyristor itself. Non-sensitive devices (Figure 5) have internal shorting resistors dispersed throughout the chip’s cathode area. This 100 design feature improves noise immunity and high tempera- K ture blocking stability at the expense of increased trigger and holding current. External resistors are optional for non- (OHMS) RESISTANCE GATE‐CATHODE sensitive SCRs and TRIACs. They should be comparable in 10K size to the internal shorting resistance of the device (20 to 0.001 0.010.1 1 10 100 100 ohms) to provide maximum improvement. The internal STATIC dV (Vńms) dt resistance of the thyristor should be measured with an ohm- ǒdVǓ meter that does not forward bias a diode junction. Figure 6.6. Exponential versus dt s Gate-Cathode Resistance A gate-cathode capacitor (Figure 7) provides a shunt 2200 path for transient currents in the same manner as the resis- tor. It also filters noise currents from the drive circuit and 2000 enhances the built-in gate-cathode capacitance voltage MAC 15‐8 1800 divider effect. The gate drive circuit needs to be able to VPK = 600 V μ 1600 charge the capacitor without excessive delay, but it does not need to supply continuous current as it would for a dt dV 1400 resistor that increases dV the same amount. However, the 1200 dt capacitor does not enhance static thermal stability. STATIC (V/ s) STATIC 1000 130 800 120 600 MAC 228A10 9080706050 100 110 130120 110 800 V 110°C TJ, JUNCTION TEMPERATURE (°C) μ ǒdVǓ 100 Figure 6.5. Exponential versus dt dt s dV Junction Temperature 90 STATIC (V/ s) STATIC 80 70 Sensitive gate TRIACs run 100 to 1000 ohms. With an 60 0.001 0.01 0.1 1 dV external resistor, their capability remains inferior to μ dt GATE TO MT1 CAPACITANCE ( F) non-sensitive devices because lateral resistance within the ǒdVǓ Figure 6.7. Exponential versus Gate gate layer reduces its benefit. dt s Sensitive gate SCRs (IGT t 200 μA) have no built-in to MT1 Capacitance resistor. They should be used with an external resistor. The The maximum ǒdVǓ improvement occurs with a short. recommended value of the resistor is 1000 ohms. Higher dt s Actual improvement stops before this because of spreading values reduce maximum operating temperature and ǒdVǓ dt s resistance in the thyristor. An external capacitor of about (Figure 6). The capability of these parts varies by more than 0.1 μF allows the maximum enhancement at a higher value 100 to 1 depending on gate-cathode termination. of RGK. Specifications subject to change without notice. © 2016 Littelfuse, Inc. 3 Publication Order Number: September 19, 2016 − Rev. 3 AN1048/D AN1048/D for sinusoidal currents is given by the slope of the secant One should keep the thyristor cool for the highest ǒdVǓ . dt s line between the 50% and 0% levels as: Also devices should be tested in the application circuit at 6fITM the highest possible temperature using thyristors with the ǒdIǓ + Ańms dt 1000 lowest measured trigger current. c where f = line frequency and ITM = maximum on-state cur- TRIAC COMMUTATING dV rent in the TRIAC. dt Turn-off depends on both the Miller effect displacement dV WHAT IS COMMUTATING dV ? current generated by across the collector capacitance dt dt and the currents resulting from internal charge storage The commutating dV rating applies when a TRIAC has dt within the volume of the device (Figure 10). If the reverse been conducting and attempts to turn-off with an inductive recovery current resulting from both these components is load. The current and voltage are out of phase (Figure 8). high, the lateral IR drop within the TRIAC base layer will The TRIAC attempts to turn-off as the current drops below forward bias the emitter and turn the TRIAC on. Commu- the holding value. Now the line voltage is high and in the tating dV capability is lower when turning off from the pos- opposite polarity to the direction of conduction. Successful dt itive direction of current conduction because of device turn-off requires the voltage across the TRIAC to rise to the geometry.