Hot swapping is a way to manage power that turns the once catastrophic event of plugging something into a powered piece of equipment into a controlled environment that allows the host unit to be repaired, upgraded or expanded during operation. What hot swap brings to the portable product environment is similar to what the DSP brings to the signal-processing environment: flexibility. Whether using a cabled interface like USB or 1394, or a card or module interface like CardBus or PCI, hot-insertion capability extends the usefulness and the useful life span of portable equipment.
Nowhere is that more visible than in the world of the personal digital assistant, the increasingly ubiquitous pocket-sized computer or PDA. But how do you fit a desktop computer in your pocket? You start by removing all the features that are not essential to basic operational requirements and turn them into hot-swappable peripheral modules. (In reality, it is not quite that simple, but the basic notion still holds true.)
Although it is a paradigm that has been changing, people tend to equate size with capability as a general measure of what a computer can actually do. The perception is that desktops are more powerful than notebooks, which are in-turn more powerful than palmtops and so on. There is an ever-growing demand to get desktop performance in a notebook computer and notebook computer performance in a PDA. Even though every year notebook PC and PDA designers display a new level of ingenuity, they still have to contend with the fact that they are working with a relatively small form factor that still must support all the features and performance that the industry demands. So they create hot-swap peripherals.
Basically, hot swapping (or hot docking, hot plugging, hot insertion) is the act of inserting or removing cards or modules in a power system. It is a rudimentary means by which to add functionality to a piece of equipment without having to support the cost or space requirements of the feature in the host equipment. It increases the runtime of the basic model and allows end users to define their own platform on the fly, without having to power down. A hot socket can be a card, cable or module connection that receives power from the main power supply without user intervention. This basically means that whatever gets plugged in will automatically receive power immediately or soon.
From a very basic perspective, there are two hot-swap implementation types. The first is true hot swap. This is where the connector or socket into which the card, cable or module is inserted is powered at the time of insertion. The second is a pseudo-hot swap. This is where, although the main unit is powered, there is no power applied to the connector or socket until after the card, cable or module is inserted and recognized.
It is important to realize the existence of those two types and the difference between them. In a true hot-swap implementation, power is always available at the point of interconnection. This type of application carries with it key power-management concerns for the host, such as output capacitance, system IR losses and di/dt sourcing capabilities. From the hot-plugged device's perspective the concerns are startup voltage threshold, input capacitance, di/dt sinking requirements and voltage sequencing. With pseudo-hot plug a device is inserted into the system and polled to determine its voltage requirements, then the appropriate voltages are applied. The concerns in this implementation are similar, but their impact on the system is not as significant. Since additional control circuitry would be added to the voltage bus, this implementation may have higher system voltage drops than a true hot-plug implementation.
Applications using 1394, USB, PC Cards, Smart Cards, Compact Flash, hot-Plug PCI, Compact PCI or other cable-, card- or module-based expandability concepts may implement either of the hot-plug implementation methods. The power requirements of these applications vary widely, from amps of current at 32 V for 1394, down to milliamps at 3.3 V for a Smart Card implementation. However, they all share concerns based on the hot-swap environment. In many of the industry specifications that control these implementations, the values for system capacitance, di/dt, tolerance and connector ratings are all defined. Unfortunately, not all systems and devices are compatible from a power-management standpoint because of the flexibility of the specifications.
If not properly managed, a hot-insertion event will generate large voltage and current transients in the system. That's because the device that is about to be plugged in is completely discharged-basically it is empty. At the moment that it is plugged into the host, the host tries to instantaneously charge all of the capacitance that exists on card, cable or module. This inrush of current will cause a voltage droop in the host until the power supply can compensate for it, which in some cases causes over-regulation of the supply once the transient has diminished.
As can be seen from the hot-plug systems sources graph, the power supply is often the last power source in the system to respond to a current transient. This means that the rest of the power is delivered from components outside the power supply. Placing additional capacitance close to the hot-swap connections will greatly reduce these transients.
The inrush current seen in an unprotected system can cause damage to the connectors as well as to components in both the host and the device being inserted. Even if it does not cause damage, the inrush current can cause a significant enough voltage droop in the host system to corrupt data or even cause a complete shutdown by resetting the main power supply or damaging the batteries.
There are many ways to manage and control power for hot-insertion events. Defining the specific application's power requirements during the event helps narrow the selection.
Mechanical solutions could be as simple as a staggered-pin configuration to precharge on-board capacitance. The ground and precharge pins mate first, followed by the primary power pins. The precharge pins connect the load capacitance through a series resistance to the main supply. This resistance reduces the di/dt of the circuit based on the RC time constant. When the primary power pins mate this resistor is bypassed.
Electrical solutions include everything from PTCs and NTCs, discrete and intelligent MOSFETs to hot-swap power managers. The choice depends on overall system power, control and cost requirements and limitations. Discrete solutions are fairly limited in performance. For any type of high-availability systems, or systems that see hot insertion events on a more regular basis, something more along the lines of an intelligent MOSFET or hot swap power manager is required.
Maximizing power control
Where intelligent MOSFETs integrate the switches and are typically targeted at applications below 5 A, hot-swap power managers typically integrate only the driver for the MOSFETs but include a more advanced feature set, along with the ability to drive larger loads. A hot-swap power manager must have adjustable circuit breaker fault protection, controlled voltage or current ramps or both, and current, voltage or power monitoring. These features help minimize inrush currents and maximize power control. Depending on the implementation, it may be necessary to fit devices that can also control the sequencing of the power rails or filter out the transients.
Imagine being able to swap batteries on your cell phone without dropping a call, pop a GPS module into your PDA while you are reading directions, or even hot swap your disk drive or CD-ROM while you are running an application on your notebook computer. You could even have a module that functions as a flashlight and pops into your cell phone or PDA when you are headed to your car at night.
The major drivers of hot-insertion interfaces in the portable world are the desire to become more portable combined with the desire for increased run-time. To meet these goals, certain features need to be defined as plug-in options. This allows the user to determine just how long they want the batteries to last and define exactly what they want the equipment to do.
And hot swap is not just for telecom, servers, workstations and other high-availability systems any more. Portable products like PDAs and games have been integrated into our way of life and the personalization of those devices is an absolute requirement. Where would Game Boy or Playstation be today if the game cartridges could not be hot swapped?
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