AN-3008 RC Snubber Networks for Thyristor Power Control and ...

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AN-3008 APPLICATION NOTE dV Static ------ does not depend strongly on voltage for operation dt below the maximum voltage and temperature rating. Avalanche multiplication will increase leakage current and dV reduce ------ capability if a transient is within roughly 50 volts dt of the actual device breakover voltage. dV A higher rated voltage device guarantees increased ------ at dt lower voltage. This is a consequence of the exponential rating method where a 400 V device rated at 50 V/µs has a dV higher ------ to 200 V than a 200 V device with an identical dt rating. However, the same diffusion recipe usually applies for all voltages. So actual capabilities of the product are not much different. Heat increases current gain and leakage, lowering ⎛dV ------ ⎞ ⎝ , dt ⎠s the gate trigger voltage and noise immunity (Figure 3). dV STATIC (V/µs) dt 180 160 140 120 100 80 60 40 Failure Mode MAC 228-10 V PK = 800 V 20 20 100 200 300 400 500 600 700 800 T J, JUNCTION TEMPERATURE (°C) dV Figure 3. Exponential ( dt ) versus Temperature s dV ( ) dt s dV Occasional unwanted turn-on by a transient may be acceptable in a heater circuit but isn’t in a fire prevention sprinkler system or for the control of a large motor. Turn-on is destructive when the follow-on current amplitude or rate is excessive. If the thyristor shorts the power line or charged capacitor, it will be damaged. Static ------ turn-on is non-destructive when series impedance dt limits the surge. The thyristor turns off after a half-cycle of dV conduction. High ------ aids current spreading in the thyristor, dt dI improving its ability to withstand ---- . Breakdown turn-on dt does not have this benefit and should be prevented. Improving dV ( ) dt s dV Static ------ can be improved by adding an external resistor dt from the gate to MT1 (Figure 4). The resistor provides a path dV for leakage and ------ induced currents that originate in the dt drive circuit or the thyristor itself. dV STATIC (V/µs) dt 140 120 100 80 60 40 20 MAC 228-10 800V 110°C R INTERNAL = 600Ω 0 10 100 1000 10000 GATE-MT 1, RESISTANCE (OHMS) ( ) s dV Figure 4. Exponential dt versus Gate to MT 1 Resistance Non-sensitive devices (Figure 5) have internal shorting resistors dispersed throughout the chip’s cathode area. This design feature improves noise immunity and high temperature blocking stability at the expense of increased trigger and holding current. External resistors are optional for non-sensitive SCRs and TRIACs. They should be comparable in size to the internal shorting resistance of the device (20 to 100 ohms) to provide maximum improvement. The internal resistance of the thyristor should be measured with an ohmmeter that does not forward bias a diode junction. dV STATIC (V/µs) dt 2200 2000 1800 1600 1400 1200 1000 800 600 50 60 70 80 90 100 110 120 130 T J, JUNCTION TEMPERATURE (°C) ( ) s dV Figure 5. Exponential dt versus Junction Temperature MAC 16-8 V PK = 600 V 2 REV. 4.01 6/24/02
APPLICATION NOTE AN-3008 Sensitive gate TRIACS run 100 to 1000 ohms. With an gate drive circuit needs to be able to charge the capacitor dV without excessive delay, but it does not need to supply continuous current as it would for a resistor that increases ------ external resistor, their ------ capability remains inferior to dt dV non-sensitive devices because lateral resistance within the dt dV dV Figure 7. Exponential ( versus Device Physics dt ) s ( ) dt c gate layer reduces its benefit. the same amount. However, the capacitor does not enhance static thermal stability. Sensitive gate SCRs (I GT < 200 µA) have no built-in resistor. They should be used with an external resistor. The recommended value of the resistor is 1000 ohms. Higher values The maximum ⎛dV ------ ⎞ ⎝ improvement occurs with a short. dt ⎠s reduce maximum operating temperature ⎛dV Actual improvement stops before this because of spreading ------ ⎞ ⎝ (Figure 6). dt ⎠ resistance in the thyristor. An external capacitor of about 0.1 s The capability of these parts varies by more than 100 to 1 µF allows the maximum enhancement at a higher value of depending on gate-cathode termination. R GK . 10 MEG MCR22-2 One should keep the thyristor cool for the highest ⎛dV ------ ⎞ ⎝ . T A = 65°C dt ⎠s Also devices should be tested in the application circuit at the A 10 highest possible temperature using thyristors with the lowest 1 MEG V G measured trigger current. K dV TRIAC Commutating dt 100K dV What is Commutating ? dt dV The commutating ------ rating applies when a TRIAC has been dt conducting and attempts to turn-off with an inductive load. 10K 0.001 0.01 0.1 1 10 100 The current and voltage are out of phase (Figure 8). The dV TRIAC attempts to turn-off as the current drops below the STATIC (V/µs) dt holding value. Now the line voltage is high and in the opposite polarity to the direction of conduction. Successful turnoff requires the voltage across the TRIAC to rise to the dV Figure 6. Exponential ( versus dt ) s Gate-Cathode Resistance instantaneous line voltage at a rate slow enough to prevent retriggering of the device. 130 R L 120 i 2 V LINE MAC 228-10 G V MT2-1 110 800V 110°C 1 dl 100 PHASE dt c 90 ANGLE φ 80 TIME TIME dV 70 i V LINE dt c 60 dV 0.001 0.01 0.1 1 Figure 8. TRIAC Inductive Load Turn-Off ( dt ) GATE TO MT 1, CAPACITANCE (µF) c Gate to MT 1 Capacitance A TRIAC functions like two SCRs connected in inverseparallel. So, a transient of either polarity turns it on. GATE-CATHODE RESISTANCE (OHMS) dV STATIC (V/µs) dt A gate-cathode capacitor (Figure 7) provides a shunt path for transient currents in the same manner as the resistor. It also filters noise currents from the drive circuit and enhances the built-in gate-cathode capacitance voltage divider effect. The VOLTAGE CURRENT There is charge within the crystal’s volume because of prior conduction (Figure 9). The charge at the boundaries of the VMT2-1 REV. 4.01 6/24/02 3
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