chapter 27: diodes & diode applications · zener diode optimized to run in the voltage...
TRANSCRIPT
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Chapter 27: Diodes & Diode Applications
12/3/2010
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Semiconductors
� Properties are between those of conductors and insulators
� Silicon, germanium, carbon
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Types of Semiconductors
� Intrinsic Semiconductor
� Material in its purest form
� Silicon crystal
� Extrinsic Semiconductor
� Semiconductor with other atoms which are called impurity atoms
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Doping
� Process of adding impurity atoms
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Silicon Properties
� In its pure form silicon has 4 electrons in its outer shell
� Valence electrons
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Silicon Atom Structure
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Silicon Crystalline Structure
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Electron-Hole Pairs
� As silicon is heated some electrons will free themselves and leave vacancies (holes) behind
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Electron-Hole Pairs
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N-Type Semiconductors
� Silicon doped with a pentavalent atom
� 5 electrons in its outer shell
� Materials include antimony, arsenic, and phosphorus
� Produces an extra electron
� Electrons are the majority carriers
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n-Type Semiconductor
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p-Type Semiconductor
� Silicon doped with trivalent atoms
� 3 electrons in the valence shell
� Materials include aluminum, boron, and gallium
� Causes holes in the structure
� Absence of electrons
� Holes are the majority carriers
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P-Type Semiconductor
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Diode
� Created by joining a p- and n- type of semiconductor material
� Allows current to flow in one direction only
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Diode
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Depletion Zone
� At the p-n junction
� Where the p- and n- semiconductors join
� Electrons migrate to equalize the charge
� Create a depletion zone
� Depleted of all charge carriers
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Barrier Potential
� Created by the depletion layer
� Voltage potential between the p and the n layer
� Vb� Germanium - 0.3 volts
� Silicon – 0.7 volts
� Cannot be measured externally
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Barrier Potential
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Bias
� Control voltage or current
� Forward biasing allows current to flow easily through the diode
� Must be greater than Vb to produce current flow
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Forward-Biased Diode
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Reverse-Biasing
� Increases the size of the depletion zone
� Pulls electron and holes away from the junction
� Diode does not conduct any current
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Reverse-Biased Diode
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Leakage Current
� A reverse-biased diode will conduct a small amount of current
� Due to the action of the minority carriers in the p and n layers
� Number of which are determined by the operation temperature
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Volt-Ampere Characteristics
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Breakdown Voltage, VBR
� Occurs when the diode is reverse-biased
� Voltage at which there is a sudden increase in the leakage current (IR)
� Avalanche effect produced by thermally produced free electrons
� Diodes should not be operated in this voltage range
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DC Resistance of a Diode
� RF – DC resistance
� VF – Forward voltage drop
� IF – Forward current
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Diode Approximations
� First Approximation
� Second Approximation
� Third Approximation
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I. First Approximation
� Treats forward-biased diode as a closed switch with a voltage drop of zero volts.
� Treats a reverse-biased diode as an open switch with zero current flow.
� Ideal Diode Approximation
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First Approximation of a Diode
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II. Second Approximation
� Treats a forward-biased diode as like an ideal diode with a battery in series
� The battery voltage is 0.7 volts
� Barrier potential (Vb)
� A reverse-biased diode is treated as an open switch
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Second Approximation
� No current flow until VF > VB
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III. Third Approximation
� Includes bulk resistance (rB)
� Resistance of the p and n material in the diode
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Third Approximation
� VF – Forward-biased voltage
� VB – Barrier voltage
� rB – Bulk resistance
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Calculating rb
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How to Use Approximations
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Diode Ratings
� Breakdown Voltage, VBR
� Average Forward Current Rating, I0
� Maximum Forward-Surge Current Rating, IFSM
� Maximum Reverse Current, IR
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I. Breakdown Voltage, VBR
� Maximum reverse voltage
� Current avalanche occurs here
� Referred as
� Peak Inverse Voltage (PIV)
� Peak Reverse Voltage (PRV)
� Breakdown Voltage Rating (VBR)
� Peak Reverse Voltage Maximum (VRRM)
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II. Average Forward Current
Rating, I0
� Maximum current that a diode can safely handle
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III. Maximum Forward-Surge
Current Rating, IFSM
� Maximum instantaneous pulse current that a diode can safely handle
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IV. Maximum Reverse Current, IR
� Diode in reverse-bias
� Current at a reverse-bias voltage value
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Rectifier Circuits
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Function
� Convert alternating current (AC) to direct current (DC)
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Half-Wave Rectifier
� Forward-biased on the positive AC peaks
� Reverse-biased on the negative AC peaks
� Frequency of the output voltage is the same as the input voltage
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Half-Wave Rectifier Circuit
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Half-Wave Rectifier – Reverse Biased Mode
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Full-Wave Rectifier
� Uses a center-tapped transformer� Center tap is at ground potential
� Has two diode which are alternately forward and reverse biased� Provides output at both the positive and
negative portions of the AC cycle
� Output voltage has twice the frequency of the input voltage
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Full Wave Rectifier
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Full-Wave Bridge Rectifier
� Has the same type of output
� Does not require a center-tapped secondary on the transformer
� Output voltage has twice the frequency of the input voltage
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Full-Wave Bridge Rectifier
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Filtering
� Output from a rectifier is pulsating DC
� Capacitors smooth out the variation in in the output voltage
� Residual variation after filtering is known as ripple voltage
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Capacitors in Power Supplies
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Calculating the Ripple Voltage
� Vripple – Ripple voltage
� Vout(pk) – Peak output voltage (peak value)
� t – Capacitor discharge time
� C – value of the capacitor
� RL – Load resistor value
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Special Diodes
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Light-Emitting Diodes (LED)
� Use of doping material can make diodes emit light
� Gallium, phosphorus, arsenic
� Common colors include red, green, yellow, orange, blue, infrared
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How LED’s Emit Light
� Diode must forward-biased
� Electrons cross from the n region to the p region fall into holes
� Release energy in the form of light and some heat
� Higher barrier voltage (VB) of 1.5 to 2.5 volts
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LED Schematic Symbol
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LED Operating Characteristics
� Higher barrier voltage (VB) of 1.5 to 2.5 volts
� Very low voltage breakdown values
� 3 to 15 volts are common
� Can be easily damaged
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Typical LED Circuit
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LED with Diode Protection
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Zener Diode
� Optimized to run in the voltage breakdown region
� At the breakdown voltage, known as the zener voltage, the voltage remains constant as the current increases up to the maximum value
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Zener Voltage Operation
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Zener Diode Operating Parameters
� Vz – Zener voltage, operating voltage
� IZ – Zener current
� IZT – Zener test current
� IZM – maximum zener current
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Zener Diode Formulas
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Zener Diode Circuit #1
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Zener Diode Circuit #2
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Current for a Loaded Zener Circuit