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

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AN-3008 APPLICATION NOTE TRIAC Design Procedure dV ( dt ) c dV 1. Refer to Figure 18 and select a particular damping factor (ρ)giving a suitable trade-off between V PK and ------ . dt dV Determine the normalized ------ corresponding to the dt chosen damping factor. The volage E depends on the load phase angle: E 2 V RMS Sin (φ) where φ = tan – 1 X = ⎛------ L ⎞ where ⎝R L ⎠ φ = measured phase angle between line V and load I R L = measured dc resistance of the load. dV ( Safe Area Curve dt ) c dV Figure 26 shows a MAC16 TRIAC turn-off safe operating area curve. Turn-off occurs without problem under the curve. The region is bounded by static ------ at low values of ⎛dI ----⎞ dt ⎝dt⎠c and delay time at high currents. Reduction of the peak current permits operation at higher line frequency. This TRIAC operated at f = 400 Hz, T J = 125 °C, and I TM = 6.0 amperes using a 30 ohm and 0.068 µF snubber. Low damping factors extend operation to higher ⎛dI ----⎞ ⎝ , but capacitor sizes dt⎠c increase. The addition of a small, saturable commutation inductor extends the allowed current rate by introducing recovery delay time. Then V RMS 2 2 Z -------------- R I L + X L X L Z 2 2 = = – R L and RMS X L = --------------------- L 2 π f Line If only the load current is known, assume a pure inductance. This gives a conservative design. Then: V RMS L = --------------------------------------- where E = 2 V 2 π f LINE I RMS RMS For Example: 120 E = 2 120 = 170 V; L = ----------------------------------- = 39.8 mH ( 8 A) ( 377 rps) (V/µs) 100 10 c dV dt 1 0.1 10 dl dt -ITM = 15A c = 6 f ITM x 10-3 A/ms WITH COMMUTATION L 14 18 22 26 30 34 38 42 46 50 dV AMPERES/MILLISECOND dt c MAC 16-8, COMMUTATIONAL L = 33 TURNS #18, 52000-1A TAPE WOUND CORE 3/4 INCH OD Read from the graph at ρ = 0.6, V PK = (1.25) 170 = 213V. dV Use 400V TRIAC. Read ------ = 1.0 dt ( ρ = 0.6) 2. Apply the resonance criterion: ω ⎛ 0 spec dV ------ ⎞ dV = ⁄ ⎛------ E⎞ ⎝ dt ⎠ ⎝ dt ( P) ⎠ 510 × 6 V/S ω 0 = ---------------------------- = ( 1) ( 170 V) 29.4 × 10 3 rps 1 C = ------------- = 0.029 µF 2 ω 0 L 3. Apply the damping criterion: R S 2ρ L 39.8 × 10 – 3 = --- = 2( 0.6) C ------------------------------ 0.029 × 10 – 6 = 1400 ohms Static dl ( ) c ( ) c dV Figure 26. dt versus dt T J = 125 °C dV dt Design There is usually some inductance in the ac main and power wiring. The inductance may be more than 100 µH if there is a transformer in the circuit or nearly zero when a shunt power factor correction capacitor is present. Usually the line inductance is roughly several µH. The minimum inductance must be known or defined by adding a series inductor to insure reliable operation (Figure 27). One hundred µH is a suggested value for starting the design. Plug the assumed inductance into the equation for C. Larger values of inductance result in higher snubber resistance and dI reduced ---- . For example: dt 12 REV. 4.01 6/24/02
APPLICATION NOTE AN-3008 100 µH 20A LS1 10 0.33 µF < 50 V/µs MAC 218-6 8A LOAD R 68Ω 0.033 µF L 120V 60Hz 340 V 12Ω HEATER dV dt s = 100 V/µs dV = 5 V/µs dt c Figure 27. Snubbing for a Resistive Load Given E = 240 2 = 340V Pick ρ = 0.3 Then from Figure 18, V PK = 1.42 (340) = 483 V. Thus, it will be necessary to use a 600 V device. Using the previously stated formulas for ω 0 , C and R we find: 5 0× 10 6 V/S ω 0 = ---------------------------------- = 201450 rps ( 0.73) ( 340 V) 1 C = -------------------------------------------------------- ( 201450) 2 ( 100 × 10 – 6 = 0.2464µF ) 100 × 10 – 6 R = 2( 0.3) --------------------------------- 0.2464 × 10 – 6 = 12ohms Variable Loads The snubber should be designed for the smallest load dV inductance because ------ will then be highest because of its dt dependance on ω 0 . This requires a higher voltage device for operation with the largest inductance because of the corresponding low damping factor. dV Figure 28 describes ------ for an 8.0 ampere load at various dt power factors. The minimum inductance is a component dV added to prevent static ------ firing with a resistive load. dt Figure 28. Snubbing For a Variable Load Examples of Snubber Designs Table 2 describes snubber RC values for ⎛dV ------ ⎞ ⎝ . Figures 31 dt ⎠s and 32 show possible R and C values for a 5.0 V/µs ⎛dV ------ ⎞ ⎝ dt ⎠c assuming a pure inductive load. dV Table 2. Static Designs dt (E = 340 V, V peak = 500 V, ρ = 0.3) L µH dV R L V step V PK ρ dt Ω MHY V V V/µs 0.75 15 0.1 170 191 86 0.03 0 39.8 170 325 4.0 0.04 10.6 28.1 120 225 3.3 0.06 13.5 17.3 74 136 2.6 C µF 5.0V/µs 50V/µs 100V/µs R Ohm C µF R Ohm C µF R Ohm 47 0.15 10 100 0.33 10 0.1 20 220 0.15 22 0.03 47 3 500 0.06 51 0.01 110 8 5 100 3.0 11 0.03 100 0 3 REV. 4.01 6/24/02 13
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