Comprehensive power supply system designs for harsh automotive environments : Page 3 of 7

October 07, 2019 //By Bin Wu and Zhongming Ye, Analog Devices
Comprehensive power supply system designs for harsh automotive environments
Advances in automotive technology have significantly increased the electronic content of modern automobiles to enhance safety, improve the driving experience, enrich entertainment functions, and diversify the power and energy sources. We continue to commit engineering resources to improving power management solutions for the automotive market. Many of the technologies from that effort have resulted in significant advances in power supply efficiency, compactness, robustness, and EMI performance.

The LT8672 is an ideal diode replacement to the passive diode to protect the downstream systems from the negative voltages, as shown in Figure 3.

Under normal conditions, the LT8672 controls an external N-channel MOSFET to form an ideal diode. The GATE amplifier senses across DRAIN and SOURCE and drives the gate of the MOSFET to regulate the forward voltage to 20 mV. D1 protects SOURCE in the positive direction during load steps and overvoltage conditions. When a negative voltage appears in the input side, GATE is pulled to SOURCE when SOURCE goes negative, turning off the MOSFET and isolating DRAIN from the negative input. With its fast pull-down (FPD) capability, LT8672 can quickly turn off the external MOSFET.


Figure 4. Waveform of LT8672 response to reverse polarity.

 

Superimposed Alternating Voltage

A common disturbance on the battery rail is a superimposed ac voltage. This ac component can be an artifact of the rectified alternator output or a result of frequent switching of high current loads, such as motors, bulbs, or PWM controlled loads. According to automotive specifications ISO 16750 and LV 124, an ECU may be subjected to an ac ripple superimposed on its supply, with frequencies up to 30 kHz and amplitudes of up to 6 V p–p. In Figure 5, a high frequency ac ripple is superimposed on the battery line voltage. Typical ideal diode controllers are too slow to react, but the LT8672 generates high frequency gate pulses up to 100 kHz to control external FETs as needed to reject these ac ripples.


Figure 5. Waveform of LT8672 response to superimposed alternating voltage.

The unique ability of the LT8672 to reject common ac components on a power rail are a function of its fast pull-up (FPU) and FPD control strategy and its strong gate driving capability, where the gate driver is powered by an integrated boost regulator. Compared with a charge pump gate power solution, this boost regulator enables the LT8672 to maintain a regulated 11 V voltage to keep the external FET on, while providing strong gate souring current to reduce switching loss for high frequency ac ripple rectification. Its 50 mA source current capability enables super-fast turn-on of the FET, minimizing power dissipation; its 300 mA sinking current capacity realizes fast turn-off, minimizing the reverse current conduction. In addition, this significantly reduces the ripple current in the output capacitor. Typical rectification waveforms for a superimposed alternating voltage are shown in Figure 6.


Figure 6. Waveform of LT8672 response to superimposed alternating voltage.

 

In addition, the LT8672 effectively reduces conduction losses when compared with a traditional Schottky diode solution under the same load conditions. As seen in the thermal images of Figure 7, the solution using the LT8672 is almost 60°C cooler than a traditional diode-based solution. It not only improves the efficiency, but also eliminates the need for a bulky heat sink.

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