Power Electronics Europe Issue 4 - November 2022

www.MonolithicPower.com AUTOMOTIVE POWER SUPPLY DESIGN 21 www.power-mag.com Issue 4 2022 Power Electronics Europe shared equally between phases, the design can be optimized for physically smaller MOSFETs and inductor(s), thereby reducing BOM cost. In this circuit, the temperatures of the two phases are sensed via two negative temperature coefficient (NTC) thermistors. Then the temperature difference is fed to a proportional-integral (PI) control circuit that outputs a signal to phase 2’s compensation (COMP) pin. If T 2 < T 1 , then the voltage on phase 2’s COMP pin increases along with the current (or vice versa if T 2 > T 1 ). Phase 1 is not connected to the thermal-balancing circuit. The output current (I LOAD ) is the combination of both phase currents (I PHASE1 + I PHASE2 ) and is set by the load, independent of the current distribution in the phases, as demonstrated with Equation (1): I LOAD = I PHASE1 + I PHASE2 (1) Because of this, if I PHASE2 decreases due to thermal balancing control, then automatically increases (and vice versa). As a result, phase 1 is influenced by the thermal-balancing circuit despite not being directly connected to it. Figure 5 shows the thermal balancing schematic design. A resistor (R8) placed between phase 1 and phase 2’s COMP pins ensures that that the current difference between the two phases does not reach critical or dangerous levels. The experimental results identified 270 k Ω as the optimal value for R8. The simplified circuit only requires correctly sizing the PI circuit components to achieve temperature control. The PI’s circuit transfer function (H(s)) can be calculated using Equation (2), where C1, R4, and R7 are the components of the PI compensation loop. The proportional gain of the PI circuit (KP) can be calculated using Equation (3): The integral gain of the PI circuit (KI) can be calculated using Equation (4): Results By using a thermal-balancing system, drastic improvements in current sharing and temperature equalization can be observed. Specifically, the temperature difference shown in Figure 3 (about 2°C) drops to less than 0.5°C (denoted as the dark blue and pink traces in Figure 6). Conclusion New automotive designs are adopting 48 V power management systems to reduce weight and power loss in the vehicle’s Figure 4: 240W Power stage with thermal-balancing system Figure 5: Thermal balancing schematic design