Comprehensive power supply system designs for harsh automotive environments : Page 4 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.

High peak, narrow pulses that appear on input of automotive electronic systems usually come from two sources:

  • The disconnection of input power supply when there is inductive load in series or parallel.
  • The switching processes of a load influencing the distributed capacitance and inductance of a wire harness.

Figure 7. Thermal performance comparison.

Some of these pulses could have high voltage peaks. For example, pulse 3a defined in ISO 7632-2 is a negative spike whose peak voltage exceeds −220 V, while pulse 3b defines a pulse with maximum peak voltage of 150 V, on top of the battery’s initial voltage. Although they feature a large internal impedance and very narrow duration time, downstream electronics could be easily damaged if they see these pulses.

Two properly sized TVSs are installed in the front end to suppress such spikes. In fact, some of the low energy pulses could be absorbed directly by filter effect of input capacitor and parasitic wire inductance.

Figure 8. Severe cold crank for the 12 V system defined in LV 124.

Figure 9. Cold crank event.

Multiple Rail Regulator Rides Through Cold Crank Events

The LT8602 provides compact solutions for up to four regulated rails (for example, 5 V, 3.3 V, 1.8 V, 1.2 V) with an input voltage range from 5 V to 42 V, suitable for functions that do not necessarily need to be on during a cold crank. Otherwise, for functions that must operate even during cold crank— such as the spark plug controller or alarm—solutions like the LT8603 work down to 3 V (or lower) inputs.

LV 124 has defined the worst case of cold crank, shown in Figure 8. It indicates that the lowest battery voltage can go down to 3.2 V and last for 19 ms at car start-up. This specification challenges applications to keep running as low as

2.5 V when faced with the extra diode voltage drop from battery reverse protection in a traditional (nonideal diode) solution. In a passive diode protection scheme, buck-boost regulators may be required instead of less complex and more efficient buck regulators to provide a stable 3 V supply often required by many microcontrollers.

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