Safe torque-off design example

Motor-drive stages are essential for many industrial processes, and they are used in many environments where safety is of utmost importance. Many robots use motors as they work side by side with humans. In a motor-drive application, it is absolutely essential to take appropriate actions to shut down the system in the event of a potentially dangerous state. Circumstances such as those listed in Table 1 can lead to sudden dangerous motor operation.

One important aspect of safe motor drives is implementing a safe torque-off circuit. Every motor drive has a power stage that consists of gate drivers and potentially isolation in addition to a power-stage circuit. Using a voltage supervisor IC to monitor the power-supply rails for the gate drivers and power stage is extremely beneficial to determining the functional safety rating of your system. Figure 5 is an example of a SIL 2- or PL d-rated safe torque-off system block diagram.

In Figure 5, there are a few instances of voltage monitoring: the 24V power supply and the various isolated inputs used in the power stage for an MCU or field-programmable gate array. These voltage monitoring schemes have a hardware fault tolerance of zero. But because of the high diagnostic coverage offered by the window voltage monitoring of the TPS3762 and its BIST feature, you can still obtain a SIL 2- or PL d-rated system.