Doc ID 15772 Rev 2

5/14

AN2991

Single-phase induction motor drive topologies

14

When dealing with the current rating for AC semiconductor switches, the rate of decrease of

the current must also be checked. This constraint will depend on the chosen circuit topology.

The worst case of turn-off stress appears with a compressor without any start capacitor. In

this case, the rise in voltage will not be slowed by the motor capacitor. The higher stress

occurs for the “START” winding (where the impedance is lower than the “RUN” winding one)

and when the rotor is stalled. These two conditions yield a higher current and therefore, a

higher rate of decrease for the ACST current.

Then, for a stalled 150 W compressor, supplied with a 264 V rms voltage, the dI/dtc and

dV/dtc equal respectively 2.4 A/ms and 9.6 V/µs through the start ACST (see Figure 5,

measured with THERM01EVAL board). This is far below the maximum ratings for ACST610

devices, which is 3.5 A/ms with a 15 V/µs rate.

Figure 5. Turn-off constraint for the worst case scenario

1.3

Protective inductor

With the two Triac topology, a spurious discharge of the start capacitor can occur when the

start Triac is accidentally turned on. To reduce the dI/dt stress through the silicon switch, a

small protective inductor can be added in series with this Triac.

In order to optimize the solution cost, this inductor can be implemented on the printed circuit

board (PCB). For example, an inductor with 12 turns of 0.51 mm width track (see Figure 6),

made on a 35 µm FR4 PCB, produces a 5 µH inductor and a 1.6

 resistor.

dI/dt = 2.4 A/ms

dV/dt = 9.6 V/µs

dI/dt = 2.4 A/ms

dV/dt = 9.6 V/µs

dI/dt = 2.4 A/ms

dV/dt = 9.6 V/µs

dI/dt = 2.4 A/ms

dV/dt = 9.6 V/µs

dI/dt = 2.4 A/ms

dV/dt = 9.6 V/µs