LINEAR dn363f Data Handbook

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Replace ORing Diodes with MOSFETs to Reduce Heat and Save Space – Design Note 363 James Herr and Mitchell Lee Introduction The LTC4354 implements two ideal diodes, simulta-

High availability telecom systems employ redundant power neously controlling two external N-channel MOSFETs supplies or battery feeds to enhance system reliability. with the source pins tied together, as shown in Figure 2. Discrete diodes are commonly used to combine these This common source node is connected to the VSS pin, the power sources at the point of load. The disadvantage of negative supply of the device. Its positive supply is this approach is the significant forward voltage drop and derived from –48V_RTN through an external current resulting power dissipation, even with Schottky diodes. limiting resistor. The LTC4354 includes an internal shunt This drop also reduces the available supply voltage, which to regulate the VCC pin at 11V.

is sometimes critical at the low end of the input operating , LTC and LT are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

range. A circuit with “ideal” diode behavior overcomes the

dissipation and voltage loss problems by eliminating the PO ER IS SI PA TI ON (W

forward drop. The LTC®4354 negative voltage diode-OR controller real-

DIODE (MBR10100)

izes near-ideal diode behavior. External N-channel MOSFETs, actively driven by the LTC4354, act as pass

POWER

transistors to replace the diodes. The device maintains a SAVED small 30m V forward voltage drop across the MOSFETs at light load; under heavy load the voltage drop becomes a function of RDS(ON). For example, an 18mΩ MOSFET and

FET (IRFR3710Z)

5A load current produce a drop of 90m V, representing a more than five fold improvement in drop and power

CURRENT (A)

dissipation over a Schottky diode, which exhibits 500m V

DN363 F01

drop under the same operating conditions. Lower power

dissipation conserves board space and saves the cost of Figure 1

heat sinks. At the same time, 410m V of input operating range is added—a critical factor when the system is

–48V_RTN

running on hold-up capacitors with only a few volts of

headroom.

Ideal –48V ORing Diode

Figure 1 shows a comparison of power dissipation for a LTC4354 FAULT LOAD

diode and a MOSFET driven by the LTC4354. At 10A, the

DB GADA GB VSS

voltage drop across a 100V Schottky diode (MBR10100)

is around 620m V; a heat sink is required to handle LED1µF

resulting 6.2W of power dissipation. Using an LTC4354 VA = –48V

DN363 F02

IRFR3710Z

driving a 100V N-channel MOSFET (IRFR3710Z), the

VB = –48V

dissipation is only 1.8W due to the low 18mΩ (max) IRFR3710Z RDS(ON) of the MOSFET. This is easily dissipated in the

circuit board with no additional heat sinking. Figure 2