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Chapter 2: Diode Applications

Chapter 2: Diode Applications. 1. Load-Line Analysis. The load line plots all possible current (I D ) conditions for all voltages applied to the diode (V D ) in a given circuit. E / R is the maximum I D and E is the maximum V D .

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Chapter 2: Diode Applications

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  1. Chapter 2:Diode Applications 1

  2. Load-Line Analysis The load line plots all possible current (ID) conditions for all voltages applied to the diode (VD) in a given circuit. E / R is the maximum ID and E is the maximum VD. Where the load line and the characteristic curve intersect is the Q-point, which specifies a particular ID and VD for a given circuit. 2

  3. Series Diode Configurations Forward Bias Constants • Silicon Diode: VD = .7V • Germanium Diode: VD = .3V Analysis • VD = .7V (or VD = E if E < .7V) • VR = E – VD • ID = IR = IT = VR / R 3

  4. Series Diode Configurations Reverse Bias Diodes ideally behave as open circuits Analysis • VD = E • VR = 0 V • ID = 0 A 4

  5. Parallel Configurations 5

  6. Half-Wave Rectification The diode only conducts when it is in forward bias, therefore only half of the AC cycle passes through the diode. The DC output voltage is 0.318Vm, where Vm = the peak AC voltage. 6

  7. PIV (PRV) • Because the diode is only forward biased for one-half of the AC cycle, it is also reverse biased for one-half cycle. • It is important that the reverse breakdown voltage rating of the diode be high enough to withstand the peak, reverse-biasing AC voltage. • PIV (or PRV) > Vm • PIV = Peak inverse voltage • PRV = Peak reverse voltage • Vm = Peak AC voltage 7

  8. Full-Wave Rectification The rectification process can be improved by using more diodes in a full-wave rectifier circuit. Full-wave rectification produces a greater DC output: • Half-wave: Vdc = 0.318Vm • Full-wave: Vdc = 0.636Vm 8

  9. Full-Wave Rectification Bridge Rectifier • Four diodes are required • VDC = 0.636 Vm 9

  10. Full-Wave Rectification • Center-Tapped Transformer Rectifier • Requires • Two diodes • Center-tapped transformer • VDC = 0.636(Vm) 10

  11. Summary of Rectifier Circuits Vm = peak of the AC voltage. In the center tapped transformer rectifier circuit, the peak AC voltage is the transformer secondary voltage to the tap. 11

  12. Diode Clippers The diode in a series clipper circuit “clips” any voltage that does not forward bias it: • A reverse-biasing polarity • A forward-biasing polarity less than .7V for a silicon diode 12

  13. Biased Clippers Adding a DC source in series with the clipping diode changes the effective forward bias of the diode. 13

  14. Parallel Clippers The diode in a parallel clipper circuit “clips” any voltage that forward bias it. DC biasing can be added in series with the diode to change the clipping level. 14

  15. Summary of Clipper Circuits more… 15

  16. Clampers A diode and capacitor can be combined to “clamp” an AC signal to a specific DC level. 16

  17. Biased Clamper Circuits The input signal can be any type of waveform such as sine, square, and triangle waves. The DC source lets you adjust the DC camping level. 17

  18. Summary of Clamper Circuits 18

  19. Zener Diodes The Zener is a diode operated in reverse bias at the Zener Voltage (Vz). • When Vi Vz • The Zener is on • Voltage across the Zener is Vz • Zener current: IZ = IR – IRL • The Zener Power: PZ = VZIZ • When Vi < Vz • The Zener is off • The Zener acts as an open circuit 19

  20. Zener Resistor Values If R is too large, the Zener diode cannot conduct because the available amount of current is less than the minimum current rating, IZK. The minimum current is given by: The maximum value of resistance is: If R is too small, the Zener current exceeds the maximum current rating, IZM. The maximum current for the circuit is given by: The minimum value of resistance is: 20

  21. Voltage-Multiplier Circuits Voltage multiplier circuits use a combination of diodes and capacitors to step up the output voltage of rectifier circuits. • Voltage Doubler • Voltage Tripler • Voltage Quadrupler 21

  22. Voltage Doubler • This half-wave voltage doubler’s output can be calculated by: • Vout = VC2 = 2Vm • where Vm = peak secondary voltage of the transformer 22

  23. Voltage Doubler • Positive Half-Cycle • D1 conducts • D2 is switched off • Capacitor C1 charges to Vm • Negative Half-Cycle • D1 is switched off • D2 conducts • Capacitor C2 charges to Vm • Vout = VC2 = 2Vm 23

  24. Voltage Tripler and Quadrupler 24

  25. Practical Applications • Rectifier Circuits • Conversions of AC to DC for DC operated circuits • Battery Charging Circuits • Simple Diode Circuits • Protective Circuits against • Overcurrent • Polarity Reversal • Currents caused by an inductive kick in a relay circuit • Zener Circuits • Overvoltage Protection • Setting Reference Voltages 25

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