The following is one difficult design problem some former colleagues of mine came across for which they haven’t been able to come up with a good solution. Of course, some semiconductor company may have solved the problem already, but they haven’t seen anything along the lines of what they need. Perhaps you, as savvy electronics gurus on Planet Analog, can help:
- Both sensored and sensorless 3-phase brushless DC motors are in wide use for various medical devices and their uses in portable medical devices are increasing. These types of motors often use between 10 W to 100 W of power and are driven by a DC voltage in the 12 V to 30 V range. The common configuration for the motors is a Y arrangement of three power leads connected to either a positive voltage or ground via six FETs, three on the high side and three on the low side.
As is common for medical design, it is desirable to have a backup motor controller as well as backup FETs to drive the motor. This requires detecting that the primary motor controller is functional and that none of the six FETs have failed in a shorted or open state. In addition, it would be ideal to check that the backup FETs are also functional as well as the backup motor controller while the motor is operating properly.
This is a challenging design problem because the failure of a single component, either primary or backup, must not cause the motor to stop for any significant length of time. The amount of time allowed varies depending on the application, but many applications require it to be less than one or two seconds. In addition, the backup circuitry must be able to operate the motor even if a primary FET is shorted, so there must be some way to isolate that component. The good news is that the design has to accommodate only one failed component.
Finally, portable products require small and lightweight motor drivers. So, the design should not increase the overall size too much. Ideally, the design would not be much larger than duplicating the basic motor drive design twice.
Please contribute your ideas and comments. I believe that this is a real design challenge to be met head-on by our Planet Analog readers. I look forward to your solutions.