Automotive Power Supply Design Considerations

 

Embedded boards are designed for several domains. Power supply design is critical to any embedded design. Whether it be functional or passing any compliance, power design plays a critical role. While the commercial designs do make the designers life easy, it is domains like Automotive or Medical makes the designers life hell especially in the power supply. There are several stringent requirements especially from the safety perspective, environmental perspective which leads to a long list of requirements:

• The high energy transient pulses must not damage the electronics and even though this is not part of actual power supply design, the front end of the power supply must have this protection. This is called load dump protection and is a must requirement
• The sudden burst current requirements that come from various loads must be handled by the Power supply,. This is where the transient response, line and load regulation are vital
• The power supply must be able to cover the entire temperature range of the automotive domain.
• The input supply could be 12V or 24V and the need is that the same power architecture must support both the typical voltages, including tolerances. This requirement is the most critical one and is the point where most designs fail. Covering this huge range is a big challenge. While operating at typical voltage is as per the specification, handling the under voltage condition is another scenario. Cold crank or Warm Crank are scenarios Automotive engineers have to deal with.
• Thermal calculations are very important when selecting the supply ICs and heat sink requirements must be estimated. If the design requires a heat sink then, vibration requirements need to be considered for installing the heat sink
• While complying to the above requirements, it is important that cost is within the limit. Any increase in $ of the product cost due to supply could be a big concern
• The important point to remember while designing power supply is that the power drop in the IC must be less. Considering this scenario, it is understood that from the Input a lower voltage must be generated by using switcher and then further rails must be generated from this output. For example, 12V to 5V converter must be used and then rails like 1.8V, 1.2V, 3.3V, 1.35V must be generated from 5V. After all, this is another battery application and efficient power supply design is the key.
• The switching frequency of the main switcher is typical to the EMI performance of the board. The modern day power supplies do have various techniques to help control EMI.
• Eliminate using LDO in the supply chain unless the load current is less and output to input voltage differential is less