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Introduction to Hot Swap

ABOUT THE AUTHOR

Jonathan M. Bearfield has
worked in the field of electronics for 15 years, the last five
supporting Texas Instruments' hot swap and power distribution
product lines.

Hot swap, hot plug, and hot dock are terms
used interchangeably to refer to hot insertion and removal. Hot
swap lets you insert and remove cards, PC boards, cables, and/or
modules from a host system without removing power. Because of the
need for High Availability (HA) systems, hot swap has quickly
become part of every designer's vocabulary. To increase system
availability, hot swap is used to reduce down time, simplify system
repair, and allow for system upgrade. Because of these advantages,
hot-swap solutions are finding their way into a wide variety of
applications.

Applications using hot swap designs include Telecom Systems,
Servers, RAID, Hot Plug PCI, CompactPCI, USB, and Device Bay.
Employing hot swap techniques allows each of these applications to
be upgraded, expanded, and repaired without affecting the rest of
the system. If implemented properly, a Hot Swap IC solution creates
a highly reliable, HA system.

In an HA system, hot-swap power management is used to maintain
100% up time. In some systems, power is not applied to the card
socket until after the card is plugged in. Once power is applied to
the card, the system polls the power requirements and then applies
power. You need to address every application's power-management
needs up-front to design the correct product. These applications
have key considerations to address early in the design.

The Hot Swap Insertion / Removal Event

In order to understand what happens to a system during the hot
swap event, it is important to understand the status of the system
before the event. At the outset, the host system's backplane is
fully powered (live). In a live system, all bulk and bypass
capacitors are fully charged. Inserting an uncharged card into the
live system will quickly charge the card and discharge the live
system. Uncontrolled card charging demands a large inrush of
current and uncontrolled system discharging significantly reduces
the backplane voltage.

Hot-swap solutions control the power up of uncharged cards and
manage system response. Cards mating into a live system connector
will connect and disconnect power (bounce power on and off) as the
card is rocked into the connector. It can take several milliseconds
for the card to mate properly. As the card is inserted, the
capacitors on the card start to charge and draw current from the
live system. As the capacitors initially charge, the card appears
as a short and instantaneously draws a large amount of current.
This inrush current produces a large demand on the system and can
cause the system capacitors to discharge and the system voltage to
droop.

Hot Swap Control Solutions

There are four basic ways of implementing an effective hot-swap
design. The simplest and lowest cost utilizes discrete components.
An improvement over the simple discrete solution requires the
addition of a discrete MOSFET. The most reliable solutions combine
a MOSFET with a hot-swap IC, or comprise hot-swap switch ICs and
hot-swap power manager (HSPM) ICs. There are advantages and
disadvantages to each solution and the trade-offs are total
solution cost to application availability requirements.

Simple Discrete Solutions
The basic discrete or mechanical solution combines staggered pins
with resistors and capacitors or implements Positive Temperature
Coefficient Resistors (PTC)/Negative Temperature Coefficient
Resistors (NTC) to manage the power event.

In the staggered-pin solution, the connector has a combination
of long and short pins. The longer pins mate first and start
charging the board capacitance through a series of resistors. When
the board is fully seated, the shorter pins mate, bypassing the
resistors connected to the longer pins and creating a low-impedance
path for powering the inserted card. One flaw with this solution is
that it requires a specialized connector, which can be expensive.
Another flaw is that the card capacitance charge rate is impossible
to control because the rate of card insertion is impossible to
control. The card capacitance charge is variable and unknown prior
to connecting the low impedance path to the live system. The simple
discrete solution exhibits large inrush currents.

Two solutions that are popular in older systems include the use
of PTCs or NTCs. A standard connector is used in the PTC or NTC
simple discrete solution. The PTC or NTC limitations are the
additional impedance in the power path, lack of fault management
features, and that the power management is dependent on
temperature. Although PTCs have been used in hot-swap designs, they
are not suited for high availability applications. The basic
function of a PTC is to change from a low-impedance to a
high-impedance in high current situations, effectively turning off
the card. However, PTC reacts to temperature, not current, and
therefore takes a long time to respond to a change in load. PTC
does not have the ability to respond rapidly to suppress the large
inrush currents seen during a hot-swap insert event. Also PTC
cannot sense over-current or respond to fault conditions.

The basic function of a NTC is to decrease from a high-impedance
to a low-impedance during start-up when current is charging the
card. However, NTC adds a voltage drop to the system that changes
with changes in the load. The change in resistance from near open
to a few tenths of an Ohm is driven by a change in temperature and
takes a long time. NTC does not respond rapidly enough to suppress
the large currents of a fault event.

Each of the simple discrete solutions requires some type of fuse
or other rapid fault protection device. The fuse adds a voltage
drop to the power path and service costs for board repair.

Discrete MOSFET
The feature set of a discrete MOSFET makes it popular in power
management solutions. MOSFETs provide a low drain-to-source
on-resistance (RDS(on) ), and the MOSFET functionality is
similar to that of an ideal switch. As a voltage controlled device,
MOSFETs require a small additional current from the system to
operate. MOSFETs can be turned on and off almost instantaneously,
several orders of magnitude faster than fuses, PTCs, or NTCs.

A MOSFET is not sufficient to meet all of the sensing and
control circuitry necessary for a HA hot swap solution. A discrete
MOSFET circuit requires additional resistors and capacitors to
control rise time and sense over-current or fault conditions. One
negative feature of discrete MOSFETs is that most of them have a
parasitic diode connected from the drain to the source. If there is
more voltage on the output of the device than on the input, current
will be conducted back across the device. Preventing the back-flow
of current is of critical importance in most HA systems.

Hot-Swap Switch
The next step beyond the discrete MOSFET is a solution using a
hot-swap switch IC. These are ICs with n-channel or p-channel
MOSFETs that have drive, sense, and reporting circuitry designed
in. Hot-swap switches are readily available in the industry today.
They incorporate the drive circuitry necessary to control the rise
time and/or the current ramp rate of the MOSFET. In addition to
sensing faults and protecting against over-current conditions,
these ICs also have the ability to report these conditions to a
system controller. Hot-swap switch solutions with current limit,
shut down, and thermal protection features may remove the need for
fuses.

Hot-swap switch solutions are easy to design. A solution can be
as simple as a timing capacitor and a hot-swap switch IC. Hot-swap
switch ICs reduce design times and lower engineering costs that are
required when trying to implement a discrete MOSFET solution.

Hot-Swap Power Manager and Controller
Some hot swap applications require more than the previous solutions
can deliver. In HA applications like CompactPCI and Hot-Plug PCI,
there are many control and interface requirements that disqualify
simpler solutions. Most HA applications have very specific di/dt,
sequencing, fault management, and reporting requirements that
prohibit discrete solutions.

Two types of HSPMs exist. In lower current applications the
MOSFET can be integrated into the hot swap manager similar to a
hot-swap switch. For higher current applications the HSPM works
with a discrete external MOSFET. The external MOSFET solution has
the advantage of tailoring the IC and MOSFET to the solution needs.
Almost all HSPMs have rise-time turn-on control and circuit breaker
fault protection features. Other HSPM features include di/dt
turn-on ramp, sequencing, healthy signal, and I/O command.

Selecting A Solution
The decision of what hot swap solution to implement should be made
early in the design. Every hot swap application requires a unique
power management feature set. The power supply has to manage the
transients generated by the hot swap insertion and removal events.
There needs to be enough bulk and bypass capacitance to maintain
stable system power during card insertion. Inrush current needs to
be limited, including current ramping, and the connector selection
will depend on the need to pre-charge the inserted card.

You can design hot swap solutions on the card or the system
backplane. In server applications, the hot swap solution is
designed on the backplane. For CompactPCI applications, the hot
swap solution is designed on the card. Standards exist to define
and specify hot swap requirements. CompactPCI, Hot-Plug PCI, and
USB hot swap solutions are specified and mandatory.

For low-availability systems where boards are rarely removed or
inserted, a simple discrete implementation may suffice. For HA
computer and telecom systems, hot-swap switch or HSPM ICs are the
most appropriate solution.

Hot Swap Sequencing

In modules with multiple supply voltages, both the on-board
electronics and the host system may impose sequencing requirements
on the power interface. System configuration also plays a role in
determining the hot swap and sequencing implementation. For
example, a high-end computer system may have a plug-in power bus of
+3.3V, +5V, and +12V supplies. Each plug-in slot sits directly on
this power bus; thus, each slot is always live. In this case, each
plug-in module will have to control the hot insertion event and
supply sequencing.

Sequencing refers to the control of the relative levels and
timing between two or more supply voltages during both power-up and
power-down transitions. This requirement may be specified as a
minimum time delay between a primary supply achieving its
steady-state level before a secondary supply is turned on. Or, it
may be stated as the fixed threshold of one supply at which the
second is ramped up. Other systems or processors may have the
requirement that two supplies are ramped together, such that the
voltage differential between them never exceeds a specified
maximum. This is often referred to as supply tracking.

As a second example, consider a server system with ±5V and
±12V routed to each plug-in connector. The proper hot swap of
a mating card assembly may require specific di/dt control on each
supply line, and a time delay for power-on reset (POR) of a slave
controller before the higher voltage supplies are ramped up to
drive the Rx/Tx circuitry. In addition, an over-current or
under-voltage on any one of the supplies will need to be signaled
to the host, for either external (host) or automatic (module)
shut-down of the remaining voltages. The hardware configuration at
the mating interface or under host software control set the
sequencing order.

Dual-supply devices, such as DSPs and ASICs often impose supply
sequencing requirements. Many of the devices now on the market use
a +3.3V, 2.5V, or even 1.8V supply that powers all the core logic,
including the CPU, clock-generation circuitry, on-chip memory, and
hardware peripherals. A second, isolated supply drives the I/O
circuitry to interface to other 5V or 3.3V devices. Two issues
arise from this configuration:

  1. Current flow within internal isolation or ESD structures
    between supply rails
  2. Bus contention between the I/O pins of the processor or ASIC
    and other drivers on the bus.

Excessive current flow within the device can occur if the
voltage differential between the two supplies exceeds a particular
threshold. Nominal operating voltages for the two supplies are such
that one supply is at a higher potential. But during power-up and
power-down transients, differences in load characteristics can
affect voltage ramp rates such that the relative level of the
supplies overstresses the device.

The caveats regarding voltage differentials also apply during
power-down events. Differences in the load current drawn from each
supply during power-down, as well as the capacitance associated
with each rail, affect the rate of voltage decay. If these factors
can vary based on board configuration (number of ports or channels
and processor speed), or the level of peripheral activity (active,
standby, or sleep mode), then predicting the ramp-down rates of the
different supplies becomes increasingly difficult. When multiple
supply nodes are present at the hot swap interface, the system
design should address the orderly turn-on and subsequent status of
these supplies.

Ideal Hot-Swap Power Manager Feature
Set

Along with sequencing considerations, you may also use
multiple-supply boards in hot swap applications. In these
situations, there are a number of additional voltage and current
control issues. Hot swap implementations can range from simple
discrete solutions with current limiting resistors and
staggered-pin connectors, to full-featured, highly integrated hot
swap controllers. However, for high reliability/HA systems, the
solutions will tend toward the more complex end of the spectrum.
This is typically preferable because of the number of monitoring,
control, and reporting functions needed to achieve the demanding
performance.

The following list provides insight into the functions of a
well-designed hot swap controller. Obviously, different systems may
require more or less functionality, and the designer should
evaluate the needs of his design against the requirements of
reliability and up-time specification, industry standards, and the
anticipated frequency of live insertions and removals. The list is
generally organized in order of importance, with the basic
components shown first, progressing towards functions that become
more peripheral to the hot swap event:

  • Current limiting
  • Controlled di/dt or dv/dt (soft-start)
    v(t) = L * (di/dt)
  • Circuit breaker function
  • Over-current time-out
  • Tight fault threshold tolerance
  • Programmable / adjustable
  • Sequencing control
  • Power good reporting.

All of this means that a good hot-swap power management solution
will have a high level of integration, otherwise it will be very
cumbersome and difficult to use.

Hot Swap Applications

There are several applications that require the protection and
versatility of a hot swappable interface. They are typically either
HA systems such as servers or telecom systems, or they are systems
looking for rapid expansion capability such as notebook computers
using USB.

USB
Perhaps the most common hot swappable interface in the industry
today, USB is a simple four-wire interface that contains both data
and power. The USB interface is very similar to the 1394 interface
in that they are both hot pluggable interfaces. However, USB
transmits data at 1.5Mbps and 12Mbps (USB 1.0) or 480Mbps (USB
2.0), and distributes power at approximately 5V in increments of
500 mA and 100 mA, well below the 1394 capabilities and
requirements. The power management requirements of USB are also
much more explicitly defined than those of the 1394 interface,
although there are still three tiers in the USB peripheral
platform: Host/Self-power hub, Bus-powered hub, and USB
function.

USB data is a 3.3V level signal, but power is distributed at 5V
to allow for voltage drops in cases where power is distributed
through more than one hub. Each function must provide its own
regulated 3.3V from the 5V input or its own internal power
supply.

Designing the simplest power management solution for port power
control may protect your power supply, but will probably not ensure
the continuous operation of the system. Taking just a few items
into consideration can turn this simple solution into an effective
means of both managing power and ensuring reliable operation over a
wide range of conditions. Selecting capacitors with an appropriate
ESR, the position and type of ferrite bead, and using individual
current limit devices or power switches maximize the performance of
the USB power interface without adding much complexity to the
design.

Infiniband
Truly an emerging application, Infiniband (IB) is a switched-fabric
architecture in which daughter cards are hot-inserted into a
server-type system. IB is a scalable, modular, channel-based,
switched-fabric architecture with a performance range from 500Mbps
to 6Gbps. Supporting a wide range of applications, IB is defined
for the connection of servers with other servers, as well as with
storage and networking devices. The application requires the
management of two separate supplies during insertion and removal.
The main supply is a 12V rail capable of delivering approximately
2.5A, and the auxiliary supply is only 5V at 250mA. Due to the high
availability requirements of the system, card insertion and removal
must take place without generating any adverse effects in the
system. On the card side of the hot-swap power management device,
power is regulated up or down based on what the card requires. The
IB specification stipulates the di/dt requirements of the hot swap
event.

There are two power connections available for each IB module.
Bulk power is intended for the major functions of the module. This
12V, ±2V supply will have to be regulated down to a more
useful voltage level, such as 3.3V. The maximum load current is
2.5A dc. Auxiliary power is intended for management and enumeration
functions of the module, even when the bulk power is not available.
Auxiliary power is supplied to the module as 5V and may be
regulated within the module as required. The maximum load current
is 0.26A dc.

A single IB port is capable of providing up to 50W, but
provisions are made to allow the chassis backplane the option to
supply 25W and indicate that only 25W is available. Concurrently,
the IB module indicates that up to either 25W or 50W is required.
This ensures that an appropriate match of power required versus
power delivered is in place while allowing for both flexible
systems and module designs.

Device Bay Power Requirements
Device Bay is intended to support a wide variety of applications,
including mass storage, communications, and security. Typical
devices include CD-ROM drives, DVD-ROM drives, hard disk drives,
and smart card readers. Device Bay is a new form factor for PCs,
and PC peripherals. Just as PC Cards allow the expansion of the
notebook PC platform, Device Bay adds this flexibility to
everything from the desktop PC to the monitor. One intention of the
Device Bay specification is to enable compatibility between any
Device Bay device and any bay. Device Bay provides a simple path
for upgrade and/or system expansion by allowing peripherals to be
changed without opening the chassis. These applications allow the
user to remove a hard drive and take their entire operating system
on the road with them, as well as provide a rather instantaneous
means for upgrading a system.

The Device Bay communication protocol uses the 1394 and USB
interfaces. This provides a broader range of bandwidths with almost
unlimited scaleable performance. Supporting both interfaces in the
host also allows the device designer to select the interface that
is best suited for the functional requirements of the device
application.

In a desktop, the peak power requirements for Device Bay, on a
per-bay basis, is 45W. This is derived from the combination of
required voltages VID , 3.3V, 5V, and 12V at 0.45A,
3.75A, 3A, and 5.25A peak, respectively. The electrical continuous
power requirement drops the total power requirement to 30W. For a
notebook computer, the power requirement is much lower. The peak
power requirement is 8W and is derived from the combination of
required voltages VID , 3.3V, and 5V at 0.45A, 1.6A, and
2.4A peak, respectively. Note that the 12V supply is not required
in the notebook computer.

Hot-Plug PCI Power Requirements
PCI hot-plug is a specification derived from the standard PCI local
bus specification. PCI provides a means of defining the interface
and power requirements of add-in cards for a PC system.
Graphics-oriented systems and applications have created throughput
problems between the main host processor and any of its
peripherals. The PCI bus provides a means of moving the more
demanding, higher bandwidth peripherals closer to the processor on
a communications level. This provides measurable gains to most
applications. The PCI hot-plug addendum provides a means of
hot-plugging these add-in cards, which increases system
functionality and allows system upgrade and repair without powering
down a system, thereby minimizing the effective down-time.

The add-in card has been the standard upgrade path for the PC
since the first PC was created, providing a means of altering the
operating environment and changing the system functionality.
Utilizing the PCI interface and add-in boards as a method of
upgrading a system has a cost in terms of power. All PCI connectors
must support four power rails, regardless of the need for all four
rails anywhere else in the system. The power requirements are 3.3V,
5V, 12V, and -12V at 7.6A, 5A, .5A, and .1A, respectively. In
theory a PCI expansion card could draw more than 25W, and there
are, at a minimum, four PCI slots per host system. This is a worst
case additional power consumption requirement exceeding
100W-per-system.

Although using the PCI add-in cards for upgrades has existed for
several years, it can still weigh heavily on a system in terms of
power requirements. Most designs do not demand the maximum amount
of power available. However, as with the other interfaces, the host
power supply must be designed to handle the worst-case loads.

Telecom
Perhaps one of the highest voltage hot swap applications, Telecom
systems definitely fall into the HA requirement category. Typically
distributing 48V and sometimes 12V or 24V on the backplane, these
systems can not afford to fail. If a single card goes out, only a
handful of users may be affected. However, having to turn off the
system to repair or upgrade the system would take down an entire
city grid.

These systems distribute higher voltages so that they do not
have to distribute high currents. The voltage drops in low-current,
high-voltage systems are negligible, especially since regulators on
the plug-in cards and modules will not be effected by even sizable
changes in the input voltages. This advantage does not outweigh the
criticality of needing to insert and removes cards without
impacting system performance. Strict attention must be paid to
inrush current, voltage, and current transients, as well as voltage
tolerances. The later due to the fact that system voltages often
fluctuate between 30V and 76V in many systems. Not only does the
hot-swap solution need to manage the insertion and removal event,
it must function properly across this entire input voltage
range.

The Need for Hot Swap

The ability to withstand the stresses of live insertion and
removal of plug-in modules is a requirement placed on many of
today's electronics systems. These systems range from small
computers to large-scale servers and telecom systems. The use of
hot swap capability is growing, due to both the growth of
applications such as HA environments, and industry standard
requirements.

Many of the hot swap modules are driven by multiple power
sources. The removable assembly may contain logic blocks operating
from different regulation points, may need to interface to legacy
logic-levels elsewhere in the system, or may contain devices that
operate from split-rail supplies. These systems and modules require
a power interface that provides a sophisticated level of hot swap
control combined with proper supply sequencing, and sequencing
order and timing may be defined at different levels in the system
hierarchy.

For possible device issues, always look to the manufacturer's
specifications to identify any unique considerations. The
combination of hot swap requirements with supply sequencing calls
for a power interface that performs the functions of both.
Obviously, the degree of the interface's complexity can vary from
system to system. However, the feature set previously presented in
this article can serve as a checklist to identify the pertinent
functions of each target application. This, together with the
functional blocks presented, should help the system, or module
designer, develop the appropriate solution. Today, the more complex
solutions are often built around an integrated hot swap controller,
providing the functionality needed at a lower cost, using less
board space. In addition, this approach generally results in a
full-featured solution, providing a robust design that contributes
to the up-time performance and long-term reliability of the overall
system.

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