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

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AN-3008 APPLICATION NOTE Examples of Snubber Applications REV In Figure 35, TRIACs switch a 3 phase motor on and off and reverse its rotation. Each TRIAC pair functions as a SPDT switch. The turn-on of one TRIAC applies the differential voltage between line phases across the blocking device without the benefit of the motor impedance to constrain the rate dV of voltage rise. The inductors are added to prevent static ------ dt firing and a line-to-line short. 115 91 0.1 91 R S T2323D FWD 0.1 C S 46 V/µs MAX L S 3.75 330V 500µH 5.6 MOTOR 1/70 HP 0.26A Figure 36 shows a split phase capacitor-run motor with reversing accomplished by switching the capacitor in series with one or the other winding. The forward and reverse TRIACs function as a SPDT switch. Reversing the motor applies the voltage on the capacitor abruptly across the blocking thyristor. Again, the inductor L is added to prevent ⎛dV ------ ⎞ ⎝ firing of the blocking TRIAC. If turn-on occurs, dt ⎠s the forward and reverse TRIACs short the capacitors (C S ) resulting in damage to them. It is wise to add the resister R S to limit the discharge current. Figure 36. Split Phase Reversing Motor Figure 37 shows a “tap changer.” This circuit allows the operation of switching power supplies from a 120 or 240 vac line. When the TRIAC is on, the circuit functions as a conventional voltage doubler with diodes D 1 and D 2 conducting on alternate half-cycles. In this mode of operation, dI inrush current and ---- are hazards to TRIAC reliability. dt Series impedance is necessary to prevent damage to the TRIAC. SNUBBER φ1 100µH 300 4 MOC 3081 FWD 2 1 6 G 91 22Ω 2W WIREWOUND 0.15 µF SNUBBER 300 4 MOC 3081 REV 2 1 6 G SNUBBER 91 SNUBBER ALL MOV’s ARE 275 V RMS ALL TRIACS ARE MAC218-10 1/3 HP 208V 3 PHASE 91 φ2 100µH 300 4 MOC 3081 FWD 2 1 6 G SNUBBER 91 6 MOC 3081 4 43 G 1 2 SNUBBER 2 1 φ3 300 4 MOC 3081 REV 6 G 91 N Figure 35. 3 Phase Reversing Motor 16 REV. 4.01 6/24/02
APPLICATION NOTE The TRIAC is off when the circuit is not doubling. In this state, the TRIAC sees the difference between the line voltage and the voltage at the intersection of Cl and C2. Transients on the line cause ⎛dV ------ ⎞ ⎝ firing of the TRIAC. High inrush dt ⎠s dI current, ---- and overvoltage damage to the filter capacitor are dt possibilities. Prevention requres the addition of a RC snubber across the TRIAC and an inductor in series with the line. Figure 37. Tap Changer For Dual Voltage Switching Power Supply Thyristor Types Sensitive gate thyristors are easy to turn-on because of their low trigger current requirements. However, they have less dV ------ capability than similar non-sensitive devices. A nonsensitive thyristor should be used for high ------ . dt dV dt TRIAC commutating static dV ------ dt SUBBER INDUCTOR 120 VAC OR 240 VAC ratings. D D 1 2 + C 1 – D 3 D 4 240V 0 + C 120V 2 – dV ------ dt R S C S G R L ratings are 5 to 20 times less than Appendix A Testing Snubber Discharge dI dt AN-3008 The equations in Appendix D do not consider the thyristor’s turn-on time or on-state resistance, thus, they predict high dI values of ---- . dt Figure 38 shows the circuit used to test snubber discharge dI ---- . A MBS4991 supplies the trigger pulse while the dt quadrants of operation are switch selectable. The snubber was mounted as close to the TRIAC under test as possible to reduce inductance, and the current transformer remained in the circuit to allow results to be compared with the measured dI ---- value. dt What should the peak capacitor voltage be? A conservative approach is to test at maximum rated V DRM , or the clamp voltage of the MOV. What is the largest capacitor that can be used without limiting resistance? Figure 39 is a photo showing the the current pulse resulting from a 0.001 µF capacitor charged to 800 V. dI The 1200 A/µs ---- destroyed the TRIAC. dt Is it possible for MOV self-capacitance to damage the TRIAC? A large 40 Joule, 2200 A peak current rated MOV was tested. The MOV measured 440 pF and had an 878 volt breakover voltage. Its peak discharge current (12 A) was half that of a 470 pF capacitor. This condition was safe. dV Phase controllable optocouplers have lower ------ ratings than dt zero crossing optocouplers and power TRIACS. These should be used when a dc voltage component is present, or to prevent turn-on delay. dV Zero crossing optocouplers have more ------ capability than dt power thyristors; and they should be used in place of phase controllable devices in static switching applications. REV. 4.01 6/24/02 17
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