The voltage sensor provides the accurate, real?time voltage information that underpins every function of the insulation monitoring system:

• High sensitivity: A precise voltage sensor can resolve millivolt?level changes caused by a few megohm insulation faults, enabling early detection of degradation before dangerous current leakage develops.
• Fast response: A fast?response voltage sensor tracks transient voltage variations, allowing the IMD algorithm to filter out noise and maintain measurement accuracy.

The voltage sensor circuit consists of a high?voltage resistor divider and an operational amplifier. The resistor divider is the critical element of the IMD circuit that interfaces with the SSR. The high?voltage resistors not only set the measurement scale but also limit the avalanche current to satisfy the system?accuracy requirements. A standard operational amplifier, such as the LM2904B?Q1, is appropriate for this purpose. When the insulation monitoring function is implemented on the power?factor?correction (PFC) microcontroller, an isolated amplifier with an individual high?voltage supply must be used to maintain proper isolation between the HV side and the control side.

Figure 3-3 is the block diagram of the isolated amplifier. The AC voltage is measured on the high?voltage side with the PFC?side MCU; whereas, the IMD circuit is referred to the vehicle chassis. Using the AMC3330-Q1 eliminates the need for a separate isolated power supply on the high?voltage side, simplifying the design while maintaining the required isolation.