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This application report discusses traditional methods using schottky diodes or P-Channel MOSFETs to provide front-end input protection such as reverse battery protection, reverse current blocking, and protection during input micro-shorts. Next, the report discusses ORing power supplies to provide supply redundancy and increase power capacity. The report discusses in detail the drawbacks of existing methods and the benefits of using TI's Ideal Diode Controllers for input protection and ORing applications.
In front-end power system designs, modules, or subsystems that directly run from battery power require protection from reverse battery connection or dynamic reverse polarity conditions during a inductive load disconnect from the battery. During maintenance of car battery or jump start of the vehicle, the battery can be connected in reverse polarity during reinstallation and can cause damage to the connected subsystems, circuits, and components. Figure 2-1 shows a battery that is reverse connected. When this occurs, huge current flows through ESD diode of micro-controllers, DC/DC converters, or other integrated circuits cause severe damage to battery connected subsystems. Polarized components such as electrolytic capacitors can be damaged by reverse connected battery as shown in Figure 2-2.
Passenger cars and commercial vehicles are fitted with 12-V or 24-V battery and the subsystems powered through the 12-V or 24-V battery are subjected to various electrical transients on their power supply lines during the operating life time of the vehicle. Automotive EMC testing standards such as ISO 7637-2 and ISO 16750-2, among others, specify electrical transients, test methods, and classify functional performance for immunity against the specified transients. Reverse battery protection solution is expected to protect the electrical subsystems from the transients and meet the functional performance status required for each subsystem. Traditionally, schottky diodes are used to provide reverse battery protection and prevent damage to battery connected subsystems.
The simplest method of reverse battery protection is to add a series diode at input of the system power path. Figure 2-3 shows a reverse battery protection using a schottky diode. When the battery is installed correctly, load current flows in the forward direction of the diode. If the battery is installed with the wrong polarity, the diode is reverse biased and blocks reverse current, thereby protecting the load from negative voltage.
Figure 2-4 shows the response to a reverse polarity condition at the input. When the 12 V input is quickly reversed to -20 V, the output voltage remains without collapsing immediately or following the negative input as the schottky diode gets reverse biased and isolates the output from negative voltage. A bulk capacitor placed at the output holds the output from falling immediately and can supply the load for a short time before the input supply recovers.
Drawbacks of using schottky diode for reverse battery protection include:
On systems where large holdup capacitors are used, inrush current during startup can be huge and must not exceed the maximum diode current. This needs to be considered when choosing thermal layout or heat sink.
Schottky diodes are traditionally used to OR two or more power supplies to increase system redundancy or increase power capacity in N+1 configuration. Typically more than one power supply units (PSU) are paralleled using schottky diodes in a N+1 redundant configuration. Minimum 'N' supplies are required to power the load and additional supply unit is provided for redundancy in case of a single point failure: one power supply unit fails. Power supply with higher voltage provides most or all of the power required by the load. To share loads almost equally among the power supplies, power supply DC set point is adjusted to match other units closely.
Figure 3-1 shows dual ORing scheme where two PSUs power the load through two schottky diodes. When one of the power supply fails and its input is shorted, schottky diode in its path is reverse biased and isolates the other power supply from the failure. Load remains fully powered from the working power supply until the faulty unit is replaced.
Load Sharing: Load sharing between two power supplies is mainly dependent on the forward voltage difference of the schottky diodes and voltage difference between two power supplies. Power supply with higher voltage and lower forward voltage schottky diode carries most of the current. Forward voltage drop of the schottky diode has a negative temperature co-efficient and it reduces with increasing temperature. This can lead to situation where a single supply carries the entire load current though second supply is still present and results in increased junction temperature TJ. This necessitates a careful heat sink design and thermal management between two diodes.
Power Dissipation and Thermal Management: Apart from the key concerns such as power dissipation and the associated thermal management, reverse leakage current at a higher temperature can result in efficiency loss and lead to thermal run away situations if thermal design is not done properly. Reverse leakage current of high voltage schottky diodes increase drastically with temperature. For example, 60 V rated schottky diode STPS20M60S has a 100 mA reverse leakage current at 150 °C, which amounts to 6 W of power dissipation at -60 V. Consider a case when only one power supply is fully supplying the load current due to forward voltage difference of schottky diodes or offset in power supply DC set point. If this first power supply fails, the second supply takes over and supplies the entire load, but the schottky diode of the first one had a higher TJ before turning off and conducts large reverse leakage current. This can lead to a thermal run-away situation where the schottky continues to conduct increased reverse current and gets damaged. A damaged schottky diode and failed power supply can pull down the entire power system leading to a system failure. Even if thermal run-away is avoided by careful heat sink design, sustained power dissipation in the reverse conduction results in unwanted power loss.