Most automotive power supply architectures must follow the most basic principles when designing, but not every designer has a thorough understanding of these principles. The following are six basic principles to be followed in the design of automotive power supply architecture.
- Input voltage VIN range: The transient range of the 12V battery voltage determines the input voltage range of the power conversion IC
The typical car battery voltage range is 9V to 16V. When the engine is off, the nominal voltage of the car battery is 12V; when the engine is working, the battery voltage is about 14.4V. However, under different conditions, the transient voltage 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 shows the critical conditions that high-voltage automotive power converters need to meet. In addition to ISO7637-1, there are some battery operating ranges and environments defined for gas engines. Most of the 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.
- Heat dissipation considerations: heat dissipation needs to be designed according to the minimum efficiency of the DC-DC converter
For applications with poor air circulation or even no airflow, if the ambient temperature is high ( 30°C), the housing has a heat source ( 1W), the device will quickly heat up ( 85°C). For example, most audio amplifiers need to be installed on a heat sink, and need to provide good air circulation conditions to dissipate heat. In addition, the PCB material and certain copper areas help to improve the heat transfer efficiency, so as to achieve the best heat dissipation conditions. If no heat sink is used, the heat dissipation capability of the exposed pad on the package is limited to 2W to 3W (85°C). As the ambient temperature increases, the heat dissipation capacity will decrease significantly.
When the battery voltage is converted into a low-voltage (for example: 3.3V) output, the linear regulator will consume 75% of the input power and the efficiency is extremely low. In order to provide 1W of output power, 3W of power will be consumed as heat. Limited by the ambient temperature and the thermal resistance of the case/junction, the maximum output power of 1W will be significantly reduced. For most high-voltage DC-DC converters, when the output current is in the range of 150mA to 200mA, LDO can provide a high cost performance.
Convert the battery voltage to low voltage (for example: 3.3V), when the power reaches 3W, you need to choose a high-end switching converter, this converter can provide more than 30W output power. This is why automotive power supply manufacturers usually choose switching power supply solutions, and reject the traditional architecture based on LDO.
3. Static working current (IQ) and shutdown current (ISD)
With the rapid growth of the number of electronic control units (ECUs) in automobiles, the total current consumed from automobile batteries is also increasing. Even when the engine is turned off and the battery is exhausted, some ECU units keep working. In order to ensure that the static operating current IQ is within the controllable range, most OEM manufacturers begin to limit the IQ of each ECU. For example, the requirements proposed by the European Union are: 100μA/ECU. The vast majority of EU automotive standards stipulate that the ECU IQ typical value is less than 100μA. Devices that keep working all the time, such as CAN transceivers, real-time clocks, and microcontroller current consumption are the main considerations for ECU IQ, and power supply design needs to consider a minimum IQ budget.
4. Cost control: OEM manufacturers' compromise between cost and specifications is an important factor that affects the bill of materials for power supplies
For mass-produced products, cost is an important factor to consider in design. PCB types, heat dissipation capabilities, allowable package selection and other design constraints are actually limited by the budget of a particular project. For example, using 4-layer board FR4 and single-layer board CM3, the heat dissipation capacity of the PCB will be very different.
The project budget will also lead to another constraint, users can accept higher cost ECU, but will not spend time and money to transform traditional power supply design. For some high-cost new development platforms, designers simply make some simple modifications to the unoptimized traditional power supply design.
- Location/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, multi-layer board interconnection issues, and other layout restrictions will restrict the design of high-chip integrated power supplies. The use of point-of-load power supplies to generate all necessary power supplies also results in high costs, and it is not ideal to integrate many components into a single chip. Power supply designers need to balance overall system performance, mechanical limitations, and costs based on specific project requirements.
6. Electromagnetic radiation
The electric field that changes with time generates electromagnetic radiation, and the intensity of radiation depends on the frequency and amplitude of the field. The electromagnetic interference generated by one working circuit directly affects the other circuit. For example, radio channel interference may cause airbag malfunctions. To avoid these negative effects, OEM manufacturers have set maximum electromagnetic radiation limits for ECU units.
In order to keep the electromagnetic radiation (EMI) within the controlled range, the type, topology, selection of peripheral components, circuit board layout and shielding of the DC-DC converter are very important. After years of accumulation, power IC designers have developed various techniques 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.
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