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Minimising diode losses in redundant systems

Posted: 15 Jun 2015     Print Version  Bookmark and Share

Keywords:power management  Schottky diodes  muxing  FET  MOSFET 

Many power management applications use Schottky diodes for the parallel operation of multiple power sources. This type of power redundancy is often found in systems with solid-state drives (SSDs), hard disc drives (HDDs), programmable logic controllers (PLCs), peripheral component interconnect express (PCIe) cards, network and graphic cards, and some others used in automotive, industrial, personal electronics and telecommunications infrastructure applications. The diodes do a great job of isolating redundant power sources to keep the system operational in the event that any one power source fails, while also preventing current flow from one supply to the other.

The diode power-muxing configuration gives a seamless transition from one voltage rail to the other (figure 1). However, system and circuit designers need to find methods to reduce circuit losses associated with these diodes. The diode also reduces the available supply voltage at the system input. This becomes critical for the lower side of the input operating voltage range. A 0.5 V drop across a diode represents four per cent of the power consumption in a 12 V system.

Figure 1: Multiple power sources muxing with diodes.

A second consideration is over-current protection to prevent bus droops during overload events and short circuits. Maintaining load voltage above the under-voltage level prevents system interrupts while reducing downtime and increasing customer satisfaction.

For a number of years there has been a trend moving away from MUXing with power diodes and moving towards MUXing with ideal diodes. An Ideal diode is a circuit that "makes a FET act like a diode." Although somewhat more complex than a simple diode, the ideal diode can significantly improve system efficiency and consume less power supply margin. There are many controllers in the market today that can make a FET behave like a diode. There are also integrated devices that have a FET and a controller in a single package. Typically these are available for lower voltages and currents. The devices in the spotlight of this article contain an ideal diode as part of a total, integrated solution.

To determine power loss in a diode simply multiply current by VF, the forward voltage drop of the diode. VF is temperature- and current-dependent, and typically ranges from 0.3 to 0.7 V.

To calculate power loss in a FET, simply multiply RDS(ON) by the square of the load current. (I2 x RDS(ON)). Modern MOSFETs have very low on-state resistance, RDS(ON), which results in a low-voltage drop even under load. In turn, this results in much lower power losses than the equivalent system using diodes. This means greater system efficiency, more available power supply margin, and fewer thermal issues during design.

Effective and reliable active ORing is not as simple as it may appear and comes with a few trade-offs. When the MOSFET is turned ON by its associated controller, the current can flow in either direction through its channel. Should the input power source fail due to a short circuit or voltage drop at the input, this will not prevent a reverse-current flow. A longer period of reverse current will discharge the output bus voltage, causing system-level damage. These conditions mandate that the active ORing control be capable of detecting the reverse current accurately, and turn OFF the MOSFET immediately.

An example of an intelligent ORing control that provides seamless transition between two power sources is shown in figure 2. This solution gives a distinctive feature set of true-reverse current blocking, auto-forward conduction, and fast switchover.

Figure 2: Example schematic of an auto-ORing implementation.

Auto-power multiplexing
In addition to the best possible diode implementation, the schematic in figure 2 limits inrush current and protects each rail from potential overload, short circuit, and over/under-voltage faults. Now let's take a look at the operation and experimental results of this implementation.

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