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

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AN-3008 APPLICATION NOTE 120 VAC 60Hz SWEEP FOR DESIRED V Ci VTC 5-50 2.5kV 2000 500W 5000 200W X100 V PROBE R S C S OPTIONAL MOV OPTIONAL PEARSON 411 CURRENT TRANSFORMER MT 2 G MT 1 56 50 Ω TRIAC UNDER TEST 91 3000 VMT 2-1 2 1 3 4 QUADRANT MAP VG 12 V Q 1,3 Q 2,4 QUADRANT SWITCH 1 µF MBS4991 Figure 38. Snubber Discharge dI dt Test HORIZONTAL SCALE – 50 ms/DIV. VERTICAL SCALE – 10 A/DIV. C S = 0.001 µF, V Ci = 800 V, R S = 0, L = 250 mH, R TRIAC = 10 OHMS Figure 39. Discharge Current From 0.001 µF Capacitor Appendix B dV Measuring ( dt ) s dV Figure 40 shows a test circuit for measuring the static ------ dt of power thyristors. A 1000 volt FET switch insures that the voltage across the device under test (D.U.T.) rises rapidly from zero. A differential preamp allows the use of a N-channel device while keeping the storage scope chassis at ground for safety purposes. The rate of voltage rise is adjusted by a variable RC time constant. The charging resistance is low to avoid waveform distortion because of the thyristor’s selfcapacitance but is large enough to prevent damage to the dI D.U.T. from turn-on ---- . Mounting the miniature range dt switches, capacitors, and G-K network close to the device under test reduces stray inductance and allows testing at more than 10 kV/µs. 18 REV. 4.01 6/24/02
APPLICATION NOTE AN-3008 27 DIFFERENTIAL PREAMP X100 PROBE X100 PROBE G 2 1 V DRM/V RRM SELECT DUT 20k 2W 2W 0.33 1000V 1000 10 WATT WIREWOUND 0.047 1000V R GK 470pF MOUNT DUT ON TEMPERATURE CONTROLLED Cµ PLATE dV dt VERNIER 100 2W 82 2W 0.001 0.005 0.01 0.047 POWER 1 MEG 2W EACH 1.2 MEG 2W TEST 1N914 0.1 20V f = 10 Hz PW = 100 µs 50 Ω PULSE GENERATOR 56 2W 1000 1/4W 1N967A 18V MTP1N100 0.47 0-1000V 10mA ALL COMPONENTS ARE NON-INDUCTIVE UNLESS SHOWN Figure 40. Circuit For Static dV dt Measurement of Power Thyristors Appendix C Measuring dV ( dt ) c dV A test fixture to measure commutating ------ is shown in dt Figure 41. It is a capacitor discharge circuit with the load series resonant. The single pulse test aids temperature control and allows the use of lower power components. The limited energy in the load capacitor reduces burn and shock hazards. The conventional load and snubber circuit provides recovery and damping behaviors like those in the application. The voltage across the load capacitor triggers the D.U.T. It terminates the gate current when the load capacitor voltage crosses zero and the TRIAC current is at its peak. Each V DRM , I TM combination requires different components. Calculate their values using the equations given in Figure 41. Commercial chokes simplify the construction of the necessary inductors. Their inductance should be adjusted by increasing the air gap in the core. Removal of the magnetic pole piece reduces inductance by 4 to 6 but extends the current without saturation. The load capacitor consists of a parallel bank of 1500 Vdc non-polar units, with individual bleeders mounted at each capacitor for safety purposes. An optional adjustable voltage clamp prevents TRIAC breakdown. To measure ⎛dV ------ ⎞ ⎝ , synchronize the storage scope on the dt ⎠c current waveform and verify the proper current amplitude and period. Increase the initial voltage on the capacitor to compensate for losses within the coil if necessary. Adjust the snubber until the device fails to turn off after the first halfcycle. Inspect the rate of voltage rise at the fastest passing condition. REV. 4.01 6/24/02 19
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AN-3008 RC Snubber Networks for Thyristor Power Control and ...

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