D. Zhou, Ph.D.

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• AC Surge Protection and Fuse Selection
• Presented by
• Godfrey de la Torre
• EE 136
• Fall 2003
• Professor Zhou
• December 6, 2003

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Introduction

• In todays discussion, we will cover AC Surge
  Protection and Fuse Selection.
• Many companies implement AC Surge Protection
  within their products Belkin, and Newpoint, just
  to name a few.
• The intended market for this application is
  primarily those of consumer electronics devices
  like notebook computers, desktop PCs,
  televisions, and even audio receivers.

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Topic 1 Why bother with AC Surge Protection?

• With many of todays electronics using
  sensitive electronic controls, it becomes
  necessary to protect them from current spikes
  (surges) caused by lightning.
• Lightning causes surges which may destroy
  sensitive circuitry.
• It is not only important to understand why
  surges occur, but rather to understand that they
  will occur, and that it is important to guard
  against them at all costs.

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Some examples of surge voltage waveforms

• The Ring Wave Surge Voltage Waveform
• According to IEEE Standard 587-1980, the most
  common voltage surge waveform is shown below

• This waveform is referred to as the ring wave.
• This wave may vary from 5kHz to 500kHz, but a
  typical residential ring wave varies anywhere
  from 30kHz to 100kHz, and has an amplitude as
  high as 6kV at 200A.

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Unidirectional surge waveform

• This waveform is the called the unidirectional
  wave.
• This waveform most likely occurs near the
  service entrance of a building, and as a result
  carries more energy than the ring wave.
• This wave may carry 6kV and 3kA.

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Likely Rate of Surge Occurrences

• Some surge protection devices have limited
  lives, so it becomes imperative to consider the
  relative rate of surge occurrences.
• Knowing the relative rate of exposure and the
  anticipated surge crest gives the designer an
  idea of the number of surges that may occur
  within a given area.

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Location Categories

• The surge stress to be expected depends upon the
  location of the equipment to be protected.
• For equipment inside a building, the expected
  surge stress depends upon the distance between
  the equipment and the service entrance.
• Category A equipment located furthest from the
  service entrance.
• Category B equipment near the service entrance
• Category C equipment outside the building

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Categories Explained

• Category A the lowest stress category since
  furthest from service entrance applies to all
  outlets(10 to 14 wire) 30 ft. from Category B,
  or to those outlets 60ft. from service entrance
  voltage stress is 6kV with 200 A max
• Category B the highest stress conditions since
  closest to service entrance includes bus panels,
  distribution lines, and lightning systems in
  commercial buildings same stress as Cat. A but
  with 3000 A max
• Category C the location outside the building
  or at the service entrance stress far greater
  than 6kV
• Note Since most power supplies are indoors,
  this report covers only Category A and B designs.

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Categories Explained (continued)
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Available Transient Suppression Devices

• Metal Oxide Varistors at voltages below its
  turnover voltage, these passive elements exhibit
  high resistance at voltages exceeding the
  turnover point, these elements sink excess
  current

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Available Transient Suppression Devices
(continued)

• Transient Suppression Diodes essentially two
  diodes back to back in a shunt configuration
  used for its high clamping action to stop
  transients its resistance is low in conduction

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Available Transient Suppression Devices
(continued)

• Gas-Filled Surge Arrestors handles much larger
  current than previous devices effectively shorts
  to ground when it conducts excess current

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Category A Transient Suppression Filters
This circuit utilizes metal oxide varistors,
transient suppressor diodes, inductors and
capacitors. When L1 (a) and L1 (b) conductive
excessive current, caps C2 and C3 are charged to
a voltage which brings ZD1, ZD2, and ZD3 into
conduction. Once these diodes enter conduction,
they effectively create a short to ground,
thereby protecting the load.
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Category B Transient Suppression Filters
This design is usually reserved for higher-power
applications (when compared to Category A
designs), and as such, utilizes gas-discharge
tubes, spark gaps, fast-acting fuses, as well as
inductors and capacitors. The functionality of
this design is similar to that of Category A
when fast transients enter this circuit,
varistors V1 through V3, L1, L2, and capacitors
C1 through C5 cause conduction in diodes D1
though D3, which short to ground, thereby
protecting the load. This design (as well as the
Category A design) offers full noise protection
as well.
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Category A Transient Suppression Filter Example

• Simplorer Schematic

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Category A results

• Assume varistors are ideal switches that are
  normally open. These switches short for some
  turnover voltage VT.
• For first case, choose VT 90V. Vsource
  120Vac at 60Hz.

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For Vsource 120Vac at 60Hz, with VT 90V
The top is the graph of the voltage source. The
bottom shows the clamping action of varistors V1
though V3.
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Category A Transient Suppression Filter Example
(continued)

• Next, try the worst case with Vsource 6kV at
  say, 3kHz. Choose VT 600V, which is a common
  turnover voltage for varistors.

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For Vsource 6kVac at 3kHz, with VT 600V
The voltage source waveform is shown on top, and
the resulting clamping action of varistors V1
through V3 is shown at the bottom. Note the
second graph spikes to 600V, then brings the
output to 0 V.
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Category B Transient Suppression Filter Example

• Category B designs are similar to Category A in
  its topology. In fact, the Category A can be
  used as a Category B design if sufficiently large
  devices can be implemented within A. But for
  brevitys sake, Billings uses a gas discharge
  tube, which can be modeled as an ideal switch.
• The switch is normally off, but conducts for some
  terminal voltage. S4 shall be the switch
  modeling the gas discharge tube.

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Category B Transient Suppression Filter Schematic
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Now, same procedure for Category B For Vsource
6kVac at 10kHz, with VT 600V, and S4 set to
4000V (to emulate Gas Tube)
Notice similar results as Category A. The output
spikes to 4000V, then shorts to ground. The left
graph is the input, and the second is the output.
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Topic 2 Fuse Selection

• Types of Fuses
• There are three main kinds of fuses
• Time-Delay Fuses

This fuse is usually the largest among the three,
primarily because this fuse provide large amounts
of currents for some time without rupturing.
These are usually used for devices with large
inrush currents, like motors and solenoids.
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Topic 2 Fuse Selection

• Standard Slow Blow Fuse

The fuse at the top is typical of a standard
slow-blow fuse. These fuses are low-cost, and
are easiest to find among the three classes.
These fuses are for standard applications,
primarily that of short-circuit protection.
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Topic 2 Fuse Selection

• Very Fast-Acting Fuses

These protect semiconductor devices. These fuses
allow the bare minimum energy during overload,
and are usually filled with filler (e.g. sand) to
quench arc voltage in the event of overload.
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Fuse Parameters

• Current Rating this is the maximum current that
  the fuse can handle it must exceed the DC or rms
  current demanded by the circuit
• Voltage Rating this is the fuses ability to
  extinguish the voltage arc that is produced as
  the fuse melts during fault conditions. Failure
  to extinguish the arcing voltage may result in
  explosion.

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Fuse Parameters

• Let-through or I2R rating
• This parameter identifies the amount of energy
  that must be dissipated in the fuse before it
  melts. Having this rating allows categorization
  of fuses into fast-blow or slow-blow fuses.
  Knowing the pre-arcing melting time allows one to
  find the fault current below.

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Fuse Selection

• For line input fuses, the designer must first
  study the turn on of the supply and the action of
  the inrush limiting circuitry at maximum and
  minimum input voltages and current load.
• Choose standard or slow-blow fuse that provides
  sufficient current margin to give reliable
  operation and satisfy the inrush current
  requirements.
• For long fuse life, fuses are chosen to be 150
  of Irms,max.
• The voltage rating must exceed peak supply
  voltage.

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Standard Fuse Example

• Here, we consider a standard fuses
  functionality. The fuse shall be modeled as an
  ideal switch, one that normally conducts, but
  opens when the current is 10A (i.e. the fuses
  current rating in this case is 10A). The source
  is an independent current source with 15Arms at
  60Hz.

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Standard Fuse Example Schematic
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Standard Fuse Example Schematic Results
The input current waveform is on the left and the
output is on the right. Notice how the fuse
brings the current to zero.
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Conclusion

• AC surge protection and fuses play a key role in
  load protection.
• The choice of either Category A or Category B
  design depends on the distance from the service
  entrance.
• Fuses must be selected according to its voltage,
  current, and I2R rating.