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

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AN-3008 APPLICATION NOTE inductor and limits the rate of inrush current if the device dV does turn on. Resistance in the load lowers ------ and V dt PK (Figure 16). / (E W0) dV dt dV dt NORMALIZED 1.4 2.2 E dV 2.1 dt 1.2 2 1.9 1 1.8 M = 1 M = 0.75 1.7 V PK 0.8 0.6 0.4 0.2 M = R S / (R L + R S) 0 0 0.2 0.4 0.6 0.8 1 DAMPING FACTOR R M = RESISTIVE DIVISION RATIO = S R L + R S I RRM = 0 Figure 16. 0 to 63% M = 0.5 M = 0.25 M = 0 dV dt Characteristics Voltage Waves NORMALIZED PEAK VOLTAGE VPK/E Damping factor and reverse recovery current determine the shape of the voltage wave. It is not exponential when the snubber damping factor is less than 0.5 (Figure 17) or when significant recovery currents are present. VMT2-1 (VOLTS) 500 400 ρ = 0 ρ = 0.1 300 0.1 200 0.3 1 ρ = 0.3 ρ = 1 100 0 0 0 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 7 TIME (µs) dV 0-63% = 100 V/µs, E = 250 V, dt s R L = 0, I RRM = 0 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 NORMALIZED PEAK VOLTAGE AND dV dt 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Figure 18. Trade-Off Between V PK and dV A variety of wave parameters (Figure 18) describe ------ . dt Some are easy to solve for and assist understanding. These dV dV include the initial ------ , the maximum instantaneous ------ , and dt dt dV the average ------ to the peak reapplied voltage. The 0 to 63% dt ⎛dV ------ ⎞ and 10 to 63% ⎛dV ------ ⎞ ⎝ definitions on device data sheets dt ⎠s ⎝ dt ⎠c are easy to measure but difficult to compute. Non-Ideal Behaviors Core Losses 10-63% dV dt 0 0 0.2 0.4 0.6 0.8 1.0 2.2 1.4 1.6 1.8 2.0 DAMPING FACTOR ( ρ ) (R L = 0, M = 1, I RRM = 0) NORMALIZED dV = dt dV dt dV dt E dV/dt E ω 0 MAX o 0-63% dV dt V PK 10-63% NORMALIZED V PK = VPK E dV dt The magnetic core materials in typical 60 Hz loads introduce losses at the snubber natural frequency. They appear as a resistance in series with the load inductance and winding dc dV resistance (Figure 19). This causes actual ------ to be less than dt the theoretical value. Figure 17. Voltage Waves for Different Damping Factors 8 REV. 4.01 6/24/02
APPLICATION NOTE AN-3008 L C R Core remanence and saturation cause surge currents. They depend on trigger angle, line impedance, core characteristics, and direction of the residual magnetization. For example, a 2.8 kVA 120 V 1:1 transformer with a 1.0 ampere load produced 160 ampere currents at start-up. Soft starting the circuit at a small conduction angle reduces this current. L DEPENDS ON CURRENT AMPLITUDE, CORE SATURATION R INCLUDES CORE LOSS, WINDING R. INCREASES WITH FREQUENCY C WINDING CAPACITANCE. DEPENDS ON INSULATION, WIRE SIZE, GEOMETRY Complex Loads Figure 19. Inductor Model Many real-world inductances are non-linear. Their core materials are not gapped causing inductance to vary with current amplitude. Small signal measurements poorly characterize them. For modeling purposes, it is best to measure them in the actual application. Complex load circuits should be checked for transient voltages and currents at turn-on and turn-off. With a capacitive load, turn-on at peak input voltage causes the maximum surge current. Motor starting current runs 4 to 6 times the steady state value. Generator action can boost voltages above the line value. Incandescent lamps have cold start currents 10 to 20 times the steady state value. Transformers generate voltage spikes when they are energized. Power factor correction circuits and switching devices create complex loads. In most cases, the simple CRL model allows an approximate snubber design. However, there is no substitute for testing and measuring the worst case load conditions. Surge Currents in Inductive Circuits Inductive loads with long L/R time constants cause asymmetric multi-cycle surges at start up (Figure 20). Triggering at zero voltage crossing is the worst case condition. The surge can be eliminated by triggering at the zero current crossing angle. i (AMPERES) 90 0 240 VAC 40 20 MHY i 0.1 Ω ZERO VOLTAGE TRIGGERING, I RMS = 30 A 80 120 160 200 TIME (MILLISECONDS) Figure 20. Start-Up Surge For Inductive Circuit Transformer cores are usually not gapped and saturate easily. A small asymmetry in the conduction angle causes magnetic saturation and multi-cycle current surges. Steps to achieve reliable operation include: 1. Supply sufficient trigger current amplitude. TRIACS have different trigger currents depending on their quadrant of operation. Marginal gate current or optocoupler LED current causes halfwave operation. 2. Supply sufficient gate current duration to achieve latching. Inductive loads slow down the main terminal current rise. The gate current must remain above the specified I GT until the main terminal current exceeds the latching value. Both a resistive bleeder around the load and the snubber discharge current help latching. 3. Use a snubber to prevent TRIAC ⎛dV ------ ⎞ ⎝ failure. dt ⎠c 4. Minimize designed-in trigger asymmetry. Triggering must be correct every half-cycle including the first. Use a storage scope to investigate circuit behavior during the first few cycles of turn-on. Alternatively, get the gate circuit up and running before energizing the load. 5. Derive the trigger synchronization from the line instead of the TRIAC main terminal voltage. This avoids regenerative interaction between the core hysteresis and the triggering angle preventing trigger runaway, halfwave operation, and core saturation. 6. Avoid high surge currents at start-up. Use a current probe to determine surge amplitude. Use a soft start circuit to reduce inrush current. Distributed Winding Capacitance There are small capacitances between the turns and layers of a coil. Lumped together, they model as a single shunt capacitance. The load inductor behaves like a capacitor at frequencies above its self-resonance. It becomes ineffective in dV controlling ------ and V PK when a fast transient such as that dt resulting from the closing of a switch occurs. This problem can be solved by adding a small snubber across the line. Self-Capacitance dV A thyristor has self-capacitance which limits ------ when the dt load inductance is large. Large load inductances, high power factors, and low voltages may allow snubberless operation. REV. 4.01 6/24/02 9
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AN-3008 RC Snubber Networks for Thyristor Power Control and ...

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