AN2777 Datasheet, PDF (9/13 Pages) STMicroelectronics – A new high-temperature TRIAC family

AN2777
Vacuum cleaner requirements
3.2
Inrush current
Inrush current also causes significant stress on TRIACs, especially if the motor is turned on
in full wave mode. Today, with the electromagnetic standard applied to limit light flickering
due to appliance inrush currents (IEC 61000-3-3 standard), most vacuum cleaners feature a
microcontroller which implements a soft-start function. A full-cycle start-up thus occurs only
with wrong triggering by the microcontroller.
Figure 9 gives the measured inrush current of a 2000 W motor started in full-cycle mode,
with a 264 V rms line voltage (worst case for a 220-240 V line). The inrush current can reach
up to 70 A. This level is well below the maximum peak current allowed for the T1235H-6
device (I_TSM = 120 A for a 20 ms pulse).
Figure 9 also gives the calculated junction temperature for this device in a TO220AB
insulated package. The initial device temperature is 60 °C, as it could occur in the
application if the motor has already operated before a new start-up. Dissipated power is
calculated with max V_to and R_d parameters given in our datasheet [see References 4.] The
thermal impedance taken into account is given in our datasheet (R_th(j-c) = 3.3 °C/W). It can
be seen that the junction temperature remains below 150 °C during this start-up. The
operation is then totally safe for the device.
Figure 9. Junction temperature and current at start-up (2000 W, 230 V motor,
T1235H-6I TRIAC)
Current (A) and Temperature (°C)
200
IT
Tj
150
100
50
0
-50
-100
-0.05
0
time (s)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
3.3
Turn-off requirement
As explained above, the dI/dt_OFF constraint is one of the main points to check, especially for
TRIACs working at high temperatures. Furthermore, universal motors impose high dI/dt_OFF
rates due to the brush commutations. Figure 10, for example, shows that the dI/dt_OFF rate
(➀) can be approximately 50% higher than the value due to the 50 Hz wave shape (dI/dt_OFF
➁, as defined in Equation 1).
Doc ID 14748 Rev 1
9/13