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Design Of Emergency Start Power Supply

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    ١٣ أغسطس، ٢٠٢٠ ١٢:٣٤:٥٢ ص PDT

     

    The power architecture of most cars must follow the most basic principles when designing, but not every designer has a thorough understanding of these principles. The following are the 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 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 around 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 Figure 1 and Figure 2 are part of the waveforms given by the ISO7637 standard. The figure 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.

     

    2. Heat dissipation considerations: heat dissipation needs to be designed according to the lowest efficiency of the DC-DC converter

    For applications with poor air circulation or even no air circulation, 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 installed on heat sinks and need to provide good air circulation conditions to dissipate heat. In addition, the PCB material and a certain copper-clad area help to improve the heat transfer efficiency, so as to achieve the best heat dissipation conditions. If a heat sink is not 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 case/junction thermal resistance, the 1W maximum output power 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 higher cost performance.
    To convert the battery voltage to low voltage (for example: 3.3V), when the power reaches 3W, a high-end switching converter needs to be selected, which can provide an output power of more than 30W. This is exactly the reason why automotive power supply manufacturers usually choose switching power supply solutions and reject traditional LDO-based architectures.

    3. Quiescent current (IQ) and shutdown current (ISD)

    With the rapid increase in the number of electronic control units (ECUs) in automobiles, the total current consumed from the car's battery is also increasing. Even when the engine is turned off and the battery is exhausted, some ECU units still 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 EU requirement is: 100μA/ECU. Most EU automotive standards stipulate that the typical value of ECU IQ is less than 100μA. Devices that always keep working, such as CAN transceivers, real-time clocks, and current consumption of microcontrollers are the main considerations for ECU IQ. The power supply design needs to consider the minimum IQ budget.

    4. Cost control: OEM manufacturers’ compromise between cost and specifications is an important factor affecting the power supply bill of materials

    For mass-produced products, cost is an important factor to be considered in the design. PCB type, heat dissipation capability, package options 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 PCB will be very different.

    The project budget will also lead to another constraint. Users can accept higher cost ECUs, but will not spend time and money on transforming traditional power supply designs. For some high-cost new development platforms, designers simply make some simple modifications to the unoptimized traditional power supply design.

    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, 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 to generate all necessary power will also lead to high costs, and it is not ideal to integrate many components on a single chip. Power supply designers need to balance overall system performance, mechanical constraints, and cost according to specific project requirements.


    6. Electromagnetic radiation

    The electric field that changes with time will produce electromagnetic radiation. The intensity of radiation depends on the frequency and amplitude of the field. Electromagnetic interference generated by one working circuit will directly affect another circuit. For example, the interference of radio channels may cause the airbag to malfunction. In order to avoid these negative effects, OEM manufacturers have established maximum electromagnetic radiation limits for ECU units.

    In order to keep 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 all very important. After years of accumulation, power IC designers have developed various techniques to limit EMI. External clock synchronization, operating frequency higher than AM modulation frequency band, built-in MOSFET, soft switching technology, spread spectrum technology, etc. are all EMI suppression solutions introduced in recent years.

     

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