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AN437 Datasheet(PDF) 3 Page - STMicroelectronics |
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AN437 Datasheet(HTML) 3 Page - STMicroelectronics |
3 / 18 page AN437 Snubber circuit functions and drawback 3/18 1.1.2 Snubber circuit benefit at TRIAC turn-off The TRIAC turn-off behavior is characterized by the datasheet curve between the critical rate of decrease of commutating on-state current ((dI/dt)c) and the critical rate of rise of commutation off-state voltage ((dV/dt)c) (refer to Figure 3). These parameters are specified for the maximum operating junction temperature (worst case). In practice, the current waveform, and thus the slope of the decreasing current, is imposed by the load. The user can then only limit the slope of the reapplied voltage. Indeed, by adding an snubber circuit across the TRIAC, the circuit time response is increased and thus, dV/dtOFF is decreased (refer to Figure 3). Figure 3. (dI/dt)c versus (dV/dt)c curve for Z01 standard TRIACs and snubber circuit impact An RC snubber circuit must be used when there is a risk of TRIAC spurious triggering, i.e. when the dI/dtOFF -dV/dtOFF couple, measured in the application, is higher than the TRIAC datasheet values, (dI/dt)c at a given (dV/dt)c. Figure 4 shows the turn-off behavior of a Z0103 standard TRIAC which controls a 26 W drain pump. Without snubber circuit and for the maximum junction temperature (110° C), a spurious triggering appears at turn-off. Indeed, the measured (dI/dt)OFF and (dV/dt)OFF values, equal respectively to 0.13 A/ms and 10 V/µs, are higher than the guarantee (dI/dt)c - (dV/dt)c point (only 7 V/µs @ 0.13 A/ms, see Figure 3). Thanks to an RC snubber circuit (10 nF and 2.7 k Ω), the slope of the reapplied voltage can be limited to 1.5 V/µs and thus spurious triggering at turn-off can be avoided (see Figure 3 and Figure 4). V T I T Load V Mains R C Area of spurious firing at commutation Safe area Operating point with RC snubber Operating point |
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