Currently, sensing temperature across an isolation boundary can be done with a non-isolated temperature sensor, such as an NTC. Isolation devices that can normally bridge an isolation boundary in a high voltage environment must meet a minimum clearance threshold depending on the high voltage level. If using a non-isolated device, it must be placed a minimum distance from the isolation boundary specified by the clearance. Clearance is the shortest distance in air between the high voltage signal pin and a low voltage signal pin. Creepage is similar to clearance, it describes the shortest distance between the high voltage and low voltage sides of a device along surfaces, such as the package or the PCB. Creepage can never be less than clearance. The minimum clearance required is determined by multiple factors, although it is primarily determined by the working voltage of the isolated device.

In the case of ISOTMP35, which is a basic isolation device, the minimum clearance is 4mm (the width of the package body). If not using an isolated temp sensor, a user must place their non-isolated sensor across the high voltage boundary at least as far away as the minimum clearance. The main drawback with this method is that while it is easy to implement, placing the sensor in the low voltage region several millimeters away from the high voltage heat source means the heat must pass across FR4 (standard PCB dielectric) which has a relatively poor thermal conductivity. This means temperature response time is reduced and the final achieved temperature is much lower than what it can be with a direct connection.

FR4 has relatively poor thermal conductivity, which means that the NTC can respond slowly to the change in temperature in the high voltage region and hence has both slow temperature response time and does not closely approach the final true temperature value. Both response time and final temperature value can be improved by using a non-conductive thermal epoxy to thermally couple the NTC with the high voltage heat source. While this can improve the thermal performance of the NTC, the performance is still not as good as direct metal-to-metal contact with the high voltage heat source.

Figure 1-2 Low Voltage NTC Design