3 Key Components

High?voltage switches and voltage?sensing circuitry are the two critical elements of an insulation monitoring device (IMD). The switch changes the equivalent network of the IMD, so the microcontroller (MCU) can determine the system impedance. A commonly used switch is a mechanical relay, but relays are bulky, heavy, and tend to lose reliability over long?term operations.

Voltage sensing is usually performed with an operational amplifier or an isolated amplifier; the choice determines the achievable accuracy and isolation level. The series resistors in the measurement path also have a major influence on the precision of detection, so both the nominal values and tolerances must be selected to meet the overall accuracy specification.

Finally, the placement of the MCU is a key design decision. The following discussion assumes the MCU is located on the low?voltage side of the IMD and shares the vehicle chassis ground. This topology simplifies grounding and communication but imposes additional constraints on the high?voltage front?end design.