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Galvanic Isolation of Logic Signals at Extreme Temperatures
Keywords: Isolation, Transformer, Pulse
Many extreme temperature applications such as power converters and motor drivers operate from power rails that are floating, noisy and have extremely high voltages. Typically the logic signal is only a few volts but is must be transmitted in an environment with hundreds or thousands of volts of common mode voltage. In some cases isolation is required for safety reasons to prevent leakage currents that could present an electrical shock hazard. For example the ground reference for a high side driver of an all N channel half bridge is referenced to the switch pin and switches between ground and the power rail within a few nanoseconds. At normal operating temperatures there are many specialized components such as opto-isolators, capacitive coupled devices and devices with integrated high frequency transformers that transmit logic signals across the isolation barrier. These specialized integrated circuits that provide isolation may not function reliably at elevated temperatures; those that do tend to be expensive. With special consideration of the system partitioning, it may be possible to avoid the need for galvanic isolation at extreme temperatures. It may be possible to provide the isolation in a remote more benign environment and send the isolated signal down a cable with appropriate impedance matching. When galvanic isolation at extreme temperatures is necessary, there must be temperature considerations for the components used. For example, when designing pulse transformers, the temperature effects on the magnetic core material properties such as permeability and frequency response must be considered as well as the Curie temperature. Modulation of the logic signals can help to reduce transformer size. Since the isolated driver functions typically require power, a novel method of winding the transformer is discussed; this method allows the drive signal to be transferred across the isolation barrier in the same transformer core that is used to provide isolated power to the driver. This paper addresses these topics and provides several alternative solutions to the problem of galvanic signal isolation at extreme temperatures.
Bruce Ferguson, Chief Engineer
Microsemi Inc
Also Viejo, CA

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