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EETE JAN 2014

ANALOG & MIXED SIGNAL DESIGN Detecting and selecting the right power supplies for portable electronics By Pinkesh Sachdev The bom in portable electronics has made it commonplace to find devices capable of powering from multiple sources. For example, an industrial handheld instrument or portable medical diagnostic equipment runs mostly on battery, but when plugged into a wall adapter or USB port draws power from that port, both for charging the battery and powering the system. On the other end of the mobility spectrum, big high-availability servers have chassis that take in at least two power supplies to maintain operation through the failure of any one of them. Storage servers employ super-capacitor backups to facilitate a clean error-free shutdown when main power turns off, while some servers have a high-current main and a low-current auxiliary supply. All of these systems face the non-trivial task of dynamically selecting one among the various available supplies to power the system load. Pitfalls of supply multiplexing While the task of selecting the appropriate supply for the given circumstance sounds easy, an improper implementation can cause system malfunction and damaged supplies. Switching between supplies operating in parallel runs the risk of pushing current back in to a supply when a higher voltage is imposed at its output. Some supplies malfunction when energy is pumped back into them, disrupting their control loops to cause an overvoltage at their input terminals, potentially blowing up capacitors and other devices. The other risk during switchover is that all supplies may disconnect from the output for too long, drooping Fig. 1: Two-supply diode-OR powering a load. Fig. 2: LTC4352 ideal diode with UV/OV & ideal diode I/V. the output voltage to reset or corrupt system operation. A third issue occurs when supply voltages are close together. Some comparator-based control methods enter an oscillation mode, switching continuously between supplies. Thus, the need for careful switching around of supplies is required. Identical supplies Let’s start with the simplest scenario – a system powered by two identical supplies. Here, identical implies an equal nominal voltage with variation over the supply tolerance range, usually a few percent. This occurs in high reliability servers, which are equipped with two or more redundant supplies to achieve continuous operation through any supply failure. In such a system, a simple method is to select the supply with the highest voltage to power the system. Two diodes with the supply at their anodes and cathodes connected together, a so-called diode-OR, implements this highest voltage function - see figure 1. This works properly with just one supply plugged in. With both supplies present, the one with the highest voltage has its diode forward biased with the other diode reverse biased. In a modern server populated by multiple cards, power levels easily reach over a kilowatt, causing a 12V DC supply to source currents in the 50A to 100A range. Employing plain old diodes, even of the lower-drop Schottky kind, to diode-OR two such 12V supplies is a daunting if not impossible thermal management task as the diodes drop 1V at high currents, dissipating 50W at 50A. An ideal diode with a much lower drop is needed. As with so many other circuit problems, MOSFETs once again come to the rescue. MOSFETs with a sensing circuit around them are made to behave like an ideal diode, turning on with a very low drop in the forward bias condition (input greater than output) and turning off when reverse biased (input below output). A 10x reduction in the voltage drop brings the power dissipation of the ideal diode to a manageable 5W. This is easily accomplished by a single or paralleled N-channel MOSFETs with an RDS(ON) of 2mΩ. Such a circuit is shown in figure 2 with the associated I-V curves. Linear Technology’s LTC4352 controls an Nchannel MOSFET to implement an ideal diode function. Two of these circuits connected in parallel form an ideal diode OR for redundant supply systems. A linear servo of the MOSFET voltage drop ensures smooth supply switchover without oscillations, while 0.5μs fast turn-on and turn-off minimizes output droop and reverse currents. Ideal diodes possess capabilities that a passive diode only dreams of. The LTC4352 enables the ideal diode only when the input is within a valid range set by undervoltage (UV) and overvoltage (OV) thresholds. STATUS# pin signals the downstream circuitry about the on or off condition of the MOSFET. FAULT# indicates whether the MOSFET is off for a UV/OV condition or has excessive drop across it due to a resistive or open MOSFET, the latter warning of an impending failure before it occurs. Let’s share the load A diode-OR is a winner-take-all system in that the supply with the highest voltage sources the entire load current. Supply lifetime is extended when both supplies source the load equally, Pinkesh Sachdev is Product Marketing Engineer for Mixed Signal Products at Linear Technology – www.linear.com 24 Electronic Engineering Times Europe January 2014 www.electronics-eetimes.com


EETE JAN 2014
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