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1
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- Diode Theory and Application
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2
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- Ideal model is a switch
- Forward-biased ideal model
- Short Circuit
- VD = 0
- RD = 0
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3
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- Reverse-biased ideal model
- Open Circuit
- VD = Supply Voltage
- RD = ∞
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4
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- Characteristic curve shows Current vs Voltage
- 1st approximation
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5
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- Circuit model – Ideal Diode
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6
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- 2nd approximation
- Ideal Switch
- Barrier potential, VB
- Si ≈ 0.7 volts
- Ge ≈ 0.3 volts
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7
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8
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- 3rd approximation
- Barrier potential, VB
- VB
- Internal Resistance
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9
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10
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- Regions of a real diode curve
- Forward Region
- Conduction
- Knee
- High resistance
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11
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- Reverse Region
- High resistance blocking
- Diode breakdown
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12
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13
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- Forward region
- Conduction region
- Dynamic resistance is
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14
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- Reverse region
- Small reverse voltages yield very small currents (uA)
- Reverse breakdown voltage, VR(BR)
- Peak Inverse Voltage (PIV), Peak Reverse Voltage (PRV) or VR(max)
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15
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- Manufacturer specifications (specs)
- Describe product electrical characteristics
- Recommended operating conditions
- Maximum ratings (PIV, power = I2R)
- ac
- dc
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16
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- Part number with 1N prefix
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17
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- Two sections
- Maximum ratings: limits that must not be exceeded
- Electrical Characteristics: typical and max values during operation
- Forward voltage drop
- Reverse voltage
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18
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- Always need a safety margin
- At least 20% more I or V than your circuit
- e.g., if PIV expected in your circuit is 150 V, choose diode with PIV
of >180 V
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19
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- Many manufacturers
- Many diode types (e.g. bridge, high-speed switching, small signal,
Varactor, Zener)
- Data sheets on Internet
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20
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21
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- Reverse Voltage
- Forward Current
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22
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- Maximum Instantaneous Forward Voltage Drop, vF
- Maximum Full-Cycle Average Voltage Drop, VF(AV)
- Temperature Derating
- I2R in diode generates heat
- Derating curve
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23
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- Parameter Shifts
- Temperature increase Forward Region
- Generates more e- - hole pairs
- Changes Barrier Potential, VB
- VB decreases ≈ 2.5mV per 1° C increase
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24
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- Temperature increase Reverse Region
- More minority carriers
- Reverse current, IS ≈ doubles per 10° C increase
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25
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- Reverse Recovery Time
- Switching time
- From On to Off state
- trr
- Nanoseconds for switching diodes
- Microseconds for rectifier diodes
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26
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- Special purpose diode
- Operates in reverse-bias region
- Breakover voltage called Zener Voltage, VZ
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27
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- VZ is close to constant
- From knee current, IZK
- To maximum rated current, IZM
- VZ is set by amount of doping used
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28
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- Symbol:
- Available with ~2.4 V < VZ < ~200 V
- Forward direction – like a standard diode
- Reverse direction – sharp breakdown region
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29
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30
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- Zener Specification Sheet
- Zener test current, IZT
- Nominal Zener Voltage, VZ (measured at IZT)
- Maximum Zener current, IZM
- Knee current, IZK
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31
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- Zener impedance, ZZ @ IZT
- Dynamic Z =
- 2 – 45 Ω
- Almost constant in operating region
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32
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- Power Rating
- Maximum dc power dissipation, PDmax
- PDmax = VZ * IZM watts
- .25 W < PDmax <
50 W
- Power Derating
- Factor such as 6.67 mW per °C
- Graph
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33
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- Modeling
- Ideal
- 2nd approximation
- Open circuit if IZ < IZT
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34
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- Applications
- Use ideal model
- Commercial Tolerance, ±5% to 10% for VZ
- Design
- Determine limits imposed by IZK and IZM
- Design circuit well within these limits
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35
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36
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- Current divider between Zener and Load
- IZK < IZ < IZM
- Input regulation
- Limits input voltage: Emin < Ein < Emax
- Load regulation
- Limits load resistance: RLmin < RL < RLmax
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37
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- Clippers
- Limit amplitude of input ac waveform
- Clampers
- If Vin ≥ VZ then Vout = VZ
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38
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- Transient suppression
- Greater power capability
- Use Back-to-Back Zeners
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39
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- Also called varicap, epicap, or tuning diode
- Voltage variable capacitor
- Symbols:
- or
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40
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- Nonlinear V vs C curve
- Increase reverse Voltage decrease C
- Reverse biased
- Increase voltage decreases diode junction
- Capacitance inversely proportional to distance between plates
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41
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- Normal diode operation when forward biased
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42
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- Specs
- Nominal capacitance, CT (given at a specific voltage)
- Reverse breakdown voltage
- Temperature coefficient
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43
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- Specs
- Figure of Merit, Q
- Capacitance ratio (tuning ratio)
- CR e.g. if 5 pF < C < 30 pF, CR = 6 (30
pF/5 pF = 6)
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44
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- Half-Wave rectification
- Sine Wave input with no dc component
- Single diode
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45
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- Half-Wave output
- Upper ½ of sine wave
- Diode in forward direction
- Lower ½ of sine wave
- Diode in reverse direction
- dc value = .318 Vm (not counting VB)
- PIV ≈ 2 * Em
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46
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47
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- Full-Wave rectification
- Sine Wave input with no dc component
- Center-tap transformer with two diodes
- Full-Wave Bridge with four diodes
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48
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- Full-Wave output
- Upper ½ of sine wave and inverted lower half of sine wave
- dc value = .637 Vm (not counting VB)
- PIV ≈ Em
- Bridge rectifier package (4 matched diodes)
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49
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- Parallel RC circuit with half-wave rectified input
- Capacitor charges during first ¼ cycle
- Capacitor holds during rest of cycle
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50
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- Output
- Less ripple
- Closer to steady dc
- Larger capacitor yields less ripple
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51
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- Parallel RC circuit with full-wave input
- T = Period of Sinusoid
- RL = Load resistance
- C = Filter capacitance
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52
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- Ripple
- Expressed in rms volts
- Ripple factor
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53
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- Diode Forward Current
- Repetitive surge currents
- Maximum listed on many rectifier data sheets
- Unregulated power supplies
- Output dc voltage varies with input voltage
- Regulated power supplies
- Simplest regulator is a Zener diode
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Notes