Power Electronics Europe Issue 4 - November 2022

20 AUTOMOTIVE POWER SUPPLY DESIGN www.MonolithicPower.com Issue 4 2022 Power Electronics Europe www.power-mag.com modulator is added to achieve low EMI in the overall system. Design challenges and proposed solutions Designers face two key design challenges due to the increasing power demands in the automotive sector. First, the power supplies for automotive applications must meet standardized EMC requirements, such as CISPR 25 Class 5. This means that the PCB layout should incorporate all EMC design recommendations. Furthermore, to ensure that this 240 W system can stay within the regulated EMC limits, certain complementary solutions are utilized (including an interleaved topology, EMC filter, and FSS modulator). Designs must also manage board thermals. It is recommended to choose the appropriate circuit components to achieve high efficiency levels. By increasing efficiency, designers can reduce power loss, which minimizes temperature rise. In particular, designers should carefully select the MOSFETs and inductor(s) in their system. Figure 2 shows the efficiency of the original 240 W power stage at four different input voltages: 24 V, 36 V, 48 V, and 60 V. There are other ways to improve the automotive power management system thermals beyond selecting optimal components. For example, the MPQ2908A-AEC1 allows the designer to select the converter’s switching frequency (fSW). In general, f SW should be as low as possible to reduce switching losses. A lower frequency increases efficiency while reducing thermal overheating for the board. For this example, with fSW set to 225 kHz, the higher EMI peak is placed at 450 kHz (2 x f SW ) to reduce the switching losses without impacting EMC. Apart from the thermal and EMC constraints, interleaved topologies typically require excellent thermal distribution to equalize MOSFET degradation and prevent parts of the board from overheating. To overcome this thermal constraint, it is vital to select an appropriate PCB layout and optimize current sharing between the two controllers. With an optimal current-sharing scheme, the load current is equally distributed between all of the converters in the system. Therefore, all of the MOSFETs have the same thermal rise. Consider a system without thermal balancing. For a system in steady state with a 20 A load current, there is a 1 A difference between the averaged current of each phase (denoted as the light blue and green traces in Figure 3). This results in an unbalanced temperature for both phases. If there is suboptimal thermal distribution between the phases (phase temperatures denoted as the dark blue and pink traces in Figure 3), then the hotter phase may experience faster degradation. Thermal-balancing system For this article, we designed a simple and easy circuit that equalizes the temperature for both phases via accurate temperature-sensing. This circuit is incorporated into the original 240 W system, then it senses and compares the temperatures of the two phases. As a result, the load current supplied by each converter can be changed accordingly (see Figure 4). For example, if T 1 > T 2 , the thermal- balancing system modifies phase 2’s compensation signal to increase its output voltage (V OUT2 ). Since the total output current is fixed by the load, the phase 2 current (I PHASE2 ) increases while the phase 1 current (I PHASE1 ) decreases. Thus, the power dissipation and temperature of phase 1 decreases until T 1 is equal to T 2 . Furthermore, this circuit reduces the BOM cost and minimizes the MOSFET and inductor size. If the current is shared unequally between the two phases, then the designer must use physically larger circuit components, such as the MOSFETs and inductor(s), to withstand the larger currents and power caused by current measurement tolerances. When current is Figure 2: System efficiency Figure 3: Current sharing without a thermal-balancing system