In this article we focus on the US Department of Defense interface standard MIL-STD-1275, which is related to 28V DC military vehicle power supplies, as well as other similar national-level specifications, such as DEFSTAN 61-5 Part 6 in the UK. The aircraft has its own standards, for example, the DO-160 is for civil aircraft and the MIL-STD-704 is for military aircraft. Although the specific pulse characteristics have changed, they are conceptually very similar and therefore apply to the same principle. Voltage spikes are characterized by tens of microseconds and voltages of up to several hundred volts, resulting from inductive coupling of lightning strikes or load steps. The solution currently applied is effective, and this solution typically uses a transient voltage suppressor, supplemented by the required EMI filter circuit and power cable inductance. Voltage surges are typically up to 100V for tens or hundreds of milliseconds and are caused by throwing. When the load circuit or battery is disconnected, the voltage across the alternator rises rapidly in a short period of time, and as a result, other loads using the same power supply encounter the same voltage surge. As we will see later, this can be a challenging and difficult problem to solve. Voltage ripple superimposed on the steady-state voltage rail of the input supply creates further design challenges. Moderate amplitude ripple can be filtered by the input capacitor to the protection circuit, but with larger ripple and higher current, it is more practical and efficient to pass the ripple to the downstream regulator through the protection circuit. Conventional passive overvoltage protection circuits (Fig. 1) require relatively large and bulky components, and such components introduce insertion loss, which may become a problem due to increased power requirements. Diverting a large amount of energy to the ground does not ensure that power is supplied downstream, and that passive components may be damaged due to repeated operations. Figure 1: Passive overvoltage protection circuit A better solution is to use a linear surge suppressor IC that provides better performance, overcurrent protection, and more while reducing the board area required. An example is the LT4363 high voltage surge suppressor (Figure 2). We call this IC a linear surge suppressor because it operates similarly to a linear regulator. Figure 2: LT4363 Surge Suppressor with Current Limit Under normal operating conditions, an external N-channel MOSFET is driven to full pass and acts as a transfer device with very small voltage drops. If the output voltage rises above the regulated value set by the resistor divider on the FB pin, the MOSFET regulates the voltage on the OUT pin, allowing the load circuit to continue to run during transient events. An optional resistor between the SNS and OUT pins is used to control the overcurrent event, and the current limit loop controls the gate voltage on the MOSFET to limit the sense voltage across the resistor to 50mV. Whether an overvoltage or overcurrent event initiates a current source to charge the capacitor connected to the TMR pin. The charge current is related to the input-to-output voltage difference so that the timer period is shortened with increasingly severe faults, ensuring that the MOSFET remains within its safe operating area.
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