Linear Design Features LTMag-V18N01-10-LTC4357-Lum Technical Manual

DESIGN IDEAS L

9V to 80V Ideal Diode Reduces Heat Dissipation by Order of Magnitude

over Schottky by Meilissa Lum

Introduction

FDB3632

High availability systems often employ VINA = 48V

parallel-connected power supplies or battery feeds to achieve redun-

IN GATE OUT

dancy and enhance system reliability.

LTC4357 VDD VOUT TO LOAD

Schottky ORing diodes have long been used to connect these supplies at GND the point of load. Unfortunately, the forward voltage drop of these diodes

FDB3632

VINB = 48V

reduces the available supply voltage and dissipates significant power at high currents. Costly heat sinks and IN GATE OUT elaborate layouts are needed to keep

LTC4357 VDD

the Schottky diode cool.

A better solution is to replace the GND

Schottky diode with a MOSFET-based ideal diode. This reduces the voltage drop and power dissipation, thereby Figure 1. Two load-sharing, redundant, 48V/10A power supplies using an ideal diode reducing the complexity, size and cost of the thermal layout and increasing of voltage drop, the MOSFET is driven load current is shared between the two system efficiency. The LTC4357 is an fully on, and the forward drop becomes supplies. Otherwise, the supply with ideal diode controller that drives an equal to RDS(ON) • ILOAD. If the load cur- the highest output voltage provides N-channel MOSFET and operates over rent reverses, as may occur during an the load current. a voltage range of 9V to 80V. input short, the LTC4357 responds by Load sharing is accomplished using

quickly pulling the MOSFET gate low a simple technique known as droop

How It Works in less than 0.5µs. sharing. Load current is first taken The LTC4357’s basic operation is from the highest supply output. As this

straightforward. The external MOSFET Load Sharing output falls or droops with increased source is connected to the input sup- Redundant Supplies loading, the lower supply begins to

ply and acts like the anode of a diode, Figure 1 shows a 48V/10A ideal di- contribute. Regulating the forward while the drain is the cathode. When ode-OR application. An MBR10100 voltage drop to 25m V ensures smooth power is first applied, the load current Schottky diode would dissipate 6W load sharing between outputs without initially flows through the body diode of under these operating conditions. oscillation. The degree of sharing is a the MOSFET. The LTC4357 senses the In contrast, the FDB3632 7.5mΩ function of MOSFET Rds(on), the output voltage drop and drives the MOSFET MOSFET dissipates only 7.5mΩ • impedance of the supplies and their on. The LTC4357’s internal amplifier (10A)2 = 0.75W. The reduced power loss initial output voltages. Backfeeding of and charge pump try to maintain a increases efficiency and saves space one supply into the other is precluded 25m V drop across the MOSFET. If the required for heat sinking. If the power by the diode action of the LTC4357. load current causes more than 25m V supply voltages are nearly equal, the

Solar Power Application

FDB3632

In solar power systems, Schottky di- odes are used to prevent discharge of

the battery during hours of darkness.

IN GATE OUT

12V Unfortunately, the voltage drop and

SOLAR SHUNT LOAD

REGULATOR LTC4357 BATTERYVDD

PANEL

power dissipation of a Schottky diode 0.1µF can be quite large when used with

high wattage solar panels, thus reduc- ing the amount of power available to

Figure 2. Solar panel charging 12V battery through ideal diode to prevent back feeding charge the battery. Figure 2 uses the

Linear Technology Magazine • March 2008