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Occasionally a question arises: Why not use external power transistors with high voltage breakdown to protect standard transceivers against DC over-voltages? The answer is quite simple: A discrete solution consumes more cost, time, and space than an integrated OVP transceiver. For example, assume that the half-duplex OVP transceiver on the left is to be replaced by a discrete design using a standard transceiver plus some external components. Because the over-voltage protection for a standard transceiver must be implemented in the transmit and the receive paths separately, the design requires a full-duplex transceiver. In the receive path, designers must implement a discrete voltage limiter so the bus voltage during an over-voltage event remains transparent. The output stage of the transmit path can then be ruggedized with four discrete transistors. Another option is an integrated H-bridge with control inputs that require the conversion from RS-485 bus signals into TTL or CMOS logic levels. This requires a drive logic circuit between the unprotected driver output and the discrete output stage. The implementation of current limiters is required to limit the power consumption of an active output stage during over-voltage events. Satisfying all of the above requirements makes the design of a discrete over-voltage protected transceiver solution absolutely foolproof.
