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Designing Military-Grade, Rapid-Start Power Applications

Author: David Berry, Principal Applications Engineer

Design of a mission-critical, rapid-start, isolated power system needs to be flawless. These systems need to start in milliseconds and start every time. The purpose of the power supply in an electronic system is to provide a regulated voltage and / or current to a load. Attention is given to the performance of the power system in its response to rapidly-changing loads and to rapidly-changing voltages demanded by the load. The input voltage and start-up time of the power system is given much less consideration because it is generally assumed that
the power source to the system is always present.
However, there are systems where the power source isn’t present and the system must be active within milliseconds of application of input power. These systems typically require isolation from the power source to keep grounds isolated or to meet military specifications such as Military Standard 704. For example, in many missile launch systems, the target information isn’t programmed into the missile until moments before the time of launch. There are several systems that need to be active within the missile before target information is loaded and the power system is only one of these systems and it needs to be the first system that is active to drive all others. So it is essential to get it right.

Designing mission critical power systems
There are a few critical areas of consideration when designing an isolated power system that must start within 10ms from application of input voltage. The input dV/dt should not exceed the ratings of the power components, the input capacitance cannot cause the sourcing components to exceed their ratings and the output capacitance value should be selected such that the system does not go into
current limit or exhibit instability.
The input section of many DC-DC converters includes an LC filter. If this LC filter is hit with a step voltage, i.e., application of DC to the power system, the LC filter can ring up to a voltage level that can damage the internal circuitry of the power component. A common specification for maximum input dV/dt is 10V/ms. A mechanical switch or FET that closes too quickly can easily exceed a 10V/ms rise time. Using the Power Component Design Methodology, an input filter with current limiting would keepthe input to the DC-DC converter within its input dV/dt specifications.

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