AN2777 Datasheet, PDF (11/13 Pages) STMicroelectronics – A new high-temperature TRIAC family

AN2777
Vacuum cleaner requirements
3.4
Jammed nozzle operation
For vacuum cleaners, the worst operating condition occurs when the tube is blocked. This
operation does not lead to a higher current. On the contrary, as there is no air flow anymore,
the motor torque is lower and the motor rms current can decrease down to 8 A.
In fact, the stress comes from the fact that the heatsink thermal impedance drastically
increases as there is no cooling air flow anymore. The case temperature can then reach up
to 120 or 140 °C. The TRIAC (dI/dt)c capability is then highly reduced. This can cause failed
turn-off. The motor suddenly goes from low speed to high speed with a half-cycle full
conduction mode. Such operation causes noise variation and vacuum cleaner vibration.
Appliance manufacturers try to reduce this kind of behavior as much as possible, since it
may give a poor quality image of their equipment to the end-user.
This is the reason why some closed-box tests are usually performed by vacuum cleaner
designers to check the TRIAC’s ability to withstand such a stressful operation. We have
performed such a test with the following conditions:
● TRIAC enclosed in a 10.5 x 8 x 5 cm cardboard box
● Plastic foam around the box to thermally insulate it
● 1000 W, 110 V motor to reach 17 A/ms dI/dt_OFF rate
● Motor rms current: 5.5 A (medium speed)
● Line voltage: 120 V, 60 Hz
● No heatsink
Figure 12. TRIAC performance comparison for closed-box test
Tcase (°C)
180
160
140
120
100
80
60
40
20
0
0
50
T1235H - ST
Device A
Device B
Device C
time (s)
100
150
200
250
The case temperature is measured from motor start-up to spurious TRIAC turn-off.
Figure 12 gives the results with the T1235H-6I and other 12 A, 600 V, 35 mA, 150 °C TRIAC
devices in insulated packages. Device A case temperature increases faster than all other
devices. This means that its power losses are higher than the other devices. This could be
certainly due to a smaller die size. Device B heating time is the slowest. But this device is
only able to withstand the dI/dt_OFF rate up to a 116 °C case temperature, whereas the
T1235H works up to 156 °C. Device C presents the same power losses as the T1235H but
works well only up to 144 °C.
Using a T1235H device thus helps to withstand the closed-box test by more than 1 mn
beyond the other devices.
Doc ID 14748 Rev 1
11/13