UL certified IEEE 1394 standard single/dual port FireWire protection circuit

introduction

After extensive testing, the MAX5943A single-port and MAX5944 dual-port FireWire current limiters and low-dropout OR logic switching controllers have passed UL certification under Underwriters Laboratories, Inc.®'s component qualification process, meaning that this circuit ( not a component in the circuit) is UL certified. In order to help designers implement this UL certified circuit, this article not only provides the schematic diagram of each circuit, but also provides a bill of materials (BOM) for each circuit, and details the requirements for components for UL certification. Any changes to the basic circuit or the current-carrying components in the circuit (current-sense resistors or MOSFET switches) require additional UL testing to achieve UL certification. However, if you simply increase the sense resistor value to lower the current limit, no other safety checks are required.

In order to meet the safety specifications of UL, these circuits need a fuse in series, so that when any component between the three MOSFET or BJT terminals fails (open circuit or short circuit), safe operation can still be guaranteed. These circuits use two MOSFETs in series, and either MOSFET can withstand drain-to-source short-circuit current without compromising the safety of the entire circuit to meet UL's safety requirements. An isolation network consisting of 4 resistors and 2 PN transistors is placed between the MOSFET gate and the controller. The gate control signal from the controller (each channel of the dual-port circuit) drives the two MOSFETs through the isolation network. , UL testing uses 12V power supply.