The example shown in Figure 2 uses a full bridge on the primary (HV) side of the transformer and a full-bridge synchronous rectifier on the secondary (LV) side. The choice of switches on the HV side will be based on cost versus efficiency; typically, IGBTs would be used, but newer APMs are likely to use Silicon Carbide (SiC) MOSFETs to achieve maximum efficiency.
Regardless of the switch technology, isolated gate drivers play a critical role. Digitally isolated gate drivers leverage CMOS technology to create both the device itself and the isolation barrier. Figure 3 shows a block diagram of a single channel in the Si8239x isolated gate driver, which uses an RF carrier to pass information across the isolation barrier. This digital isolation technology provides a robust, isolated data path which is easy to integrate with other CMOS technologies, like gate drivers.
Figure 3: A single state of the automotive qualified Si8239x isolated gate driver family from Silicon Labs (Source: Silicon Labs)
Extending digital isolation
The circuit shown in Figure 3 is managed by an APM controller, which generates the PWM signals to control the power switches' gate drivers. To achieve maximum efficiency, the controller needs to monitor the voltages being generated, a process that also requires an isolated solution such as a galvanically isolated analog amplifier. Isolation is also required for the system bus that connects the APM to the wider automotive control system. Many designs use a CAN bus and the APM includes digital isolators for the CAN bus signals. A multichannel digital isolator with 5 kVrms isolation, such as the Si86xx from Silicon Labs, is optimized for this application. Just like with isolated gate drivers, the CMOS isolation barrier allows the integration of high performance analog and digital I/O functions.
The move to EVs involves significant design challenges for OEMs and Tier 1s. Maintaining, at least for now, 12 V power rail for auxiliary power simplifies the task by supporting legacy systems. However, the removal of the main source 12 V battery energy source – the alternator driven by the engine – increases the complexity of the auxiliary power module. The advances in integration, brought about by CMOS isolation technology, simplify the APM’s design while providing safe, reliable operation for the lifetime of the vehicle.
About the author:
Charlie Ice is Senior Product Manager, Power over Ethernet, at Silicon Labs.