Briefly describe the six basic principles that should be followed in automotive power supply design

　　The following are six basic principles that need to be followed when designing automotive power architecture.

　　1. Input voltage VIN range: The transient range of the 12V battery voltage determines the input voltage range of the power conversion IC

　　The typical automotive battery voltage range is 9V to 16V. When the engine is turned off, the nominal voltage of the automotive battery is 12V; when the engine is working, the battery voltage is around 14.4V. However, under different conditions, transient voltages may also reach ±100V. The ISO7637-1 industry standard defines the voltage fluctuation range of automotive batteries. The waveforms shown in Figures 1 and 2 are part of the waveforms given by the ISO7637 standard, which show the critical conditions that high-voltage automotive power converters need to meet. In addition to ISO7637-1, there are also some battery operating ranges and environments defined for gas engines. Most new specifications are proposed by different OEM manufacturers and do not necessarily follow industry standards. However, any new standard requires the system to have overvoltage and undervoltage protection.

　　Figure 1 Cold start voltage waveform

　　Figure 2 Load dump voltage waveform

　　2. Heat dissipation considerations: Heat dissipation needs to be designed based on the minimum efficiency of the DC-DC converter

　　In applications with poor or no airflow, if the ambient temperature is high (> 30°C) and there is a heat source (> 1W) in the enclosure, the device will quickly heat up (> 85°C). For example, most audio amplifiers need to be mounted on a heat sink and have good airflow to dissipate the heat. In addition, the PCB material and a certain amount of copper area help to improve the heat transfer efficiency to achieve the best heat dissipation conditions. Without a heat sink, the exposed pad on the package is limited to 2W to 3W of heat dissipation at 85°C. As the ambient temperature increases, the heat dissipation capacity will decrease significantly.

　　When converting battery voltage to low voltage (e.g. 3.3V) output, linear regulator will lose 75% of input power, which is extremely inefficient. In order to provide 1W output power, 3W of power will be consumed as heat. Limited by ambient temperature and tube/junction thermal resistance, the 1W maximum output power will be significantly reduced. For most high-voltage DC-DC converters, LDO can provide a higher cost-effectiveness when the output current is in the range of 150mA to 200mA.

　　To convert the battery voltage into a low voltage (e.g. 3.3V) and reach a power of 3W, a high-end switching converter is required, which can provide an output power of more than 30W. This is why automotive power supply manufacturers usually choose switching power supply solutions and reject traditional LDO-based architectures.

　　High power design (> 20W) has strict requirements for thermal management and needs to adopt synchronous rectification architecture. In order to obtain heat dissipation capability higher than that of a single package and avoid package "heating", an external MOSFET driver can be considered.

　　3. Static operating current (IQ) and shutdown current (ISD)

　　As the number of electronic control units (ECUs) in cars grows rapidly, the total current drawn from the car battery is also growing. Even when the engine is turned off and the battery is depleted, some ECUs remain operational. To keep the quiescent operating current IQ within a controllable range, most OEMs have begun to limit the IQ of each ECU. For example, the EU requirement is 100µA/ECU. Most EU automotive standards specify that the typical IQ of an ECU is less than 100µA. Current consumption of devices that always remain operational, such as CAN transceivers, real-time clocks, and microcontrollers, is a major consideration for ECU IQ, and the power supply design needs to consider the minimum IQ budget.

　　4. Cost control: The trade-off between cost and specification by OEMs is an important factor affecting the bill of materials for power supplies

　　For mass-produced products, cost is an important factor to consider in the design. The PCB type, heat dissipation capability, the package selection allowed, and other design constraints are actually limited by the budget of a specific project. For example, the heat dissipation capability of the PCB will be very different when using a 4-layer FR4 board and a single-layer CM3 board.

　　Project budgets also lead to another constraint: users can accept higher-cost ECUs but will not spend time and money to transform traditional power supply designs. For some new development platforms with high costs, designers simply make some simple modifications to the traditional power supply designs that have not been optimized.

　　5. Position/Layout: PCB and component layout in power supply design will limit the overall performance of the power supply

　　Structural design, circuit board layout, noise sensitivity, interconnection issues of multilayer boards, and other layout restrictions will restrict the design of high chip integrated power supplies. Using point-of-load power supplies to generate all necessary power will also lead to high costs, and it is not ideal to integrate many components into a single chip. Power designers need to balance overall system performance, mechanical limitations, and cost based on specific project requirements.

　　6. Electromagnetic radiation

　　The time-varying electric field will generate electromagnetic radiation. The radiation intensity depends on the frequency and amplitude of the field. The electromagnetic interference generated by one working circuit will directly affect another circuit. For example, interference in the radio channel may cause the airbag to malfunction. In order to avoid these negative effects, OEM manufacturers have set maximum electromagnetic radiation limits for ECU units.

　　To keep electromagnetic radiation (EMI) under control, the type of DC-DC converter , topology, peripheral component selection, circuit board layout and shielding are all very important. After years of accumulation, power IC designers have developed various technologies to limit EMI. External clock synchronization, operating frequency higher than the AM modulation band, built-in MOSFET, soft switching technology, spread spectrum technology, etc. are all EMI suppression solutions introduced in recent years.