Chapter 4: Switches, Relay and Power-Control Semiconductors

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Chapter 4Switches, Relay and Power-Control
Semiconductors

• Adapted fromKilian, C. T. (2001), Modern
  Control Technology Components and SystemsDelmar

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Power Transistors

• Used extensively in control circuits as both
  switches and power amplifiers.
• A small current is used control a much larger
  current.

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• Current gain is expressed as
• Power is dissipated anytime there is current
  through it and voltage across it.

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Class A Operation

• Biased in the middle of the operational range.
• Input signal increases or decreases the base
  current.
• Current at the load is inverted.

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Class B Operation

• Transistor is biased just below 0.7V (operational
  bias point).
• Only half a signal will be amplified.

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Class C Operation

• Transistor is either fully saturated or cutoff.
• Acts as a switch.

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Power Dissipation

• Transistors are rated on the amount of power they
  can dissipate.
• The greater the current flow through the
  transistor, the greater power, thus heat, it must
  dissipate.
• Excessive heat will cause damage to the device.

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Heat Sinks

• Heat sinks are used to draw heat from the device.
  Typically metal with fins for air cooling.
• Heat Sink compound is used to increase thermal
  transfer.

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Insulating the Case

• On many power transistors, the case itself is
  connected to the collector and may need to be
  insulated.
• A thin film of mica is typically used.

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Darlington Pair

• Integrated dual transistors that provide a very
  large beta (gt500).
• Small current on base the 1st stage controls a
  large current biasing the 2nd stage.

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Field Effect Transistors

• Performs job similar to a BJT.
• Junctions are Gate, Drain, Source.
• Gate-Source Voltage (VGS) controls the load
  current (IDS).
• JFET (Junction FET) is one version, does not have
  large current control.
• N-Channel use a negative gate voltage, P-Channel
  use a positive.

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• MOSFETs capacitive couple the gate preventing any
  direct path for current flow.
• Can control high currents.
• Gain is called transconductance and measured in
  siemens or mhos. It is the inverse of
  resistance.Gain Change in current
  Change in voltage

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Silicon-Controlled Rectifiers

• 3-terminal power-control device that is a
  thyristor.
• Thyristors are 4-layer semiconductors that act as
  a switch.
• Terminals Anode, Cathode(K), Gate

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• Gate current is used to switch SCR on.
• The gate cannot turn the SCR off.
• An SCR will be off unless the VAK exceeds the
  forward breakover voltage, at which time it will
  be in the forward conduction region.
• The SCR will remain in conduction until the
  forward current drops below the holding current
  (IH).

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• With a gate voltage, the breakover voltage is
  dramatically reduced.
• When gate voltage (thus current) is high enough,
  the SCR starts conducting almost immediately
  (VGT, IGT).
• SCR will remain in conduction even if the gate
  voltage is removed.
• Minimum current required to stay in conduction is
  called Holding Current (IH).

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• Means of turning off the SCR (current lt IH)
• Open the load current.
• Use Forced Commutation to short out the SCR.
• AC Circuit automatically turns it off on the
  opposite alternation.

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• SCRs are often used in AC circuits to control
  power to the load, such as DC Motors for speed
  control.
• By controlling the resistance of the gate, IGT is
  reached based on the instantaneous AC voltage.
• The SCR may be set to fire anywhere within the
  1st 90 degrees of a cycle.

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• The resulting IAK wave form has delay and
  conduction regions.

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• Addition of a capacitor creates a phase shift
  allowing control over a wider range of the
  alternation.

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• Through use of a full-wave rectifier, power may
  be controlled during both alternations for DC
  applications.

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• Other conditions that will cause an SCR to fire
• If the VAK rises too quickly dv/dt. A snubber
  may be added to prevent this effect.
• VAK and IH are not held low long enough.
• Excessive heat can lower the breakover voltage.
• LASCR (Light-Activated SCR) is one which used
  light entering a small window fires the device.

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SCR Ratings

• Current capability 0.5A to 1000A
• VDRM Peak forward blocking voltage
• VRRM Peak reverse blocking voltage
• ITSM Peak surge current
• IGT Gate trigger current
• VGT Gate trigger voltage
• DV/DT Critical voltage rise V/uS

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TRIAC

• Similar to an SCR but can conduct in both
  directions.
• Leads are labeled MT1, MT2 and Gate

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Delay Time Conduction Time

• Delay time (or holdoff) is the time the thyristor
  is shutoff preventing current flow.
• Conduction time is length of time the thyristor
  is in conduction.

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• The time for a half-cycle is0.5 x 1/60Hz
  8.33mS
• Delay Time Conduction Time 8.33mS
• If an SCR had a delay time of 3mS, its
  conduction time would be 5.33mS(8.33mS 3mS)

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Conduction Angle

• Conduction angle is the number of degrees into a
  waveform the thyristor fires.
• With an alternation being 180 degrees, if an SCR
  fires at 30 degrees, it will be in conduction for
  150 degrees.
• Converting between conduction time and conduction
  angle8.33mS/180 46.3uS/deg

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Trigger Devices

• Due to the low tolerances in both the firing
  current of SCRs and Triacs, triggering devices
  are often used to more dependably fire the
  device.

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Unijunction Transistors

• The UJT fires when 2/3 of the voltage between
  Base1 and Base2 is on the emitter, the UJT will
  fire.
• When Vc reaches the trigger voltage (Vp), the UJT
  fires providing gate current to the SCR.

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Programmable Unijunction Transistor

• The PUT behaves like the UJT, but the firing
  voltage is programmable using gate voltage.
• When the Anode voltage reaches the gate voltage,
  the PUT fires gating the SCR.

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DIAC

• A two-terminal bi-directional device which fires
  when the breakover voltage is met in either
  direction.
• Allows for symmetrical firing of TRIACS.

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Solid State Relays (SSR)

• Solid State relays are used for an isolated
  control of high AC or DC loads.
• A small voltage energizes an LED which optically
  fires a triac control circuit.
• Does not control based on AC firing angle, but
  controls the length of time a device is energized.