SLUAAY5 December   2024 UCC21551-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Automotive PTC Heater Module Overview
    1. 2.1 Automotive Heating Architectures
      1. 2.1.1 Positive Temperature Coefficient Heaters
      2. 2.1.2 Heat Pumps
    2. 2.2 Automotive Heating Architectures
    3. 2.3 PTC Heater Topologies
  6. 3Design of Automotive PTC Heater Controller
    1. 3.1  Block Diagrams
    2. 3.2  Designing the Power Supplies
    3. 3.3  Picking Low-Dropout Regulators
    4. 3.4  Designing of the Communication Interface
    5. 3.5  Implementation of the Digital Isolator
    6. 3.6  Implementation of the Microcontroller Unit
    7. 3.7  Designing of the Switch Driver Stage
    8. 3.8  Selection of the Power Switches
    9. 3.9  Considerations of the PTC Load
    10. 3.10 Designing the Load Current Monitoring
    11. 3.11 Selection of the Temperature Sensing
  7. 4Summary

3.5 Implementation of the Digital Isolator

The purpose of the digital isolator is to relay information back and forth between the low voltage side and high voltage side of the PTC control module, while providing galvanic isolation between these two sides.

The designer must determine how many signals need to be sent to and from the high voltage side, as well as what signals those will be. The more signals that need to be sent over the isolation barrier, the more channels in the digital isolator are required. The amount and type of signals needed to pass through the isolation barrier are also highly dependent on the topology of the subsystem in regard to the MCUs. Guidance regarding the selected MCU topology can be found in the “Implementation of the microcontroller unit” from Table 3-1.

The EV battery voltage can have an impact on what isolation rating (VISO) you need your digital isolator to have. For up to 400V, basic (3.0kVRMS to 3.7kVRMS) or reinforced (5.0kVRMS to 5.7kVRMS) isolation is suitable. For 800V, however, reinforced isolation tends to be more commonly implemented. Creepage and clearance is also impacted by battery voltage and what standards (IEC, UL and so forth) the design must adhere to. If higher creepage and clearance is needed, there are digital isolators whose package’s creepage could fit said requirement. For example, if a very wide creepage distance is needed, the designer could select the ISO7741-Q1. This device can come in the DWW package (10.30 mm × 14.0 mm). However, PTC heater modules tend to not need this wide of a creepage, so the designer can pick a device like the ISO6741-Q1 with DW package (10.30 mm × 7.50 mm).

When placing the digital isolator block in the PCB layout, the designer can use the Digital Isolator Design Guide for comprehensive guidance. To help reduce the PCB area, a designer can pick a digital isolator with an integrated power supply to eliminate the need for a component powering either the primary or secondary side of the device. This can also reduce total system cost depending on if there would have been a dedicated power supply for powering either side of the digital isolator, and how much that dedicated power supply would have cost (for example low-drop out regulator) A digital isolator that can offer this kind of benefit is the ISOW7741-Q1. This device has an isolated DC/DC converter integrated in it, eliminating the need for a discrete isolated power supply for the secondary side of the isolator.