ELE1110D Basic Circuit Theory Tutorial 6

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ELE1110D Basic Circuit TheoryTutorial 6

• Diode Circuits
• By Xu Ceng
• SHB 832

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Diode Circuits Basic Concept

• Diode
• Simply a P-N junction
• Due to diffusion and recombination, Potential
  Barrier formed in Depletion Region.
• Forward Bias (VD gt 0)
• Apply a positive voltage VD across PN junction.
• Decrease the potential barrier? force diffusion
• Current ID can flow in the direction from P to N.
• Reverse Bias (VD lt 0)
• Apply a negative voltage VD across PN junction.
• Increase the potential barrier? further stop
  diffusion
• Current ID cannot flow in the direction from P to
  N.
• Reverse Saturation Current (IS)
• Minority Carriers created by thermal energy.
• A tiny current due to minority carriers flow

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Diode Circuits Current Model

• An equation that approximates the diode current
• vD is the voltage across the diode
• I0 is the reverse saturation current
• k is Boltzmann constant
• T is temperature in unit of Kelvin
• q is charge

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Diode Circuits Current Model

• Example Consider the diode equation with
  I010-9A. If VD is varied between 0.4V to 0.8V,
  what is the corresponding diode current?

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Diode Circuits Simple Circuits

• Simplified model
• In practice, Diode conducts only when the
  potential difference from P to N is greater than
  0.6V.
• We say it is in Forward Bias.
• Voltage drop across diode is fixed at 0.6V.
• Diode is open circuit in Reverse Bias.

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Diode Circuits - Simple Circuits

• Diode conducts only when _________.
• In Forward Bias, voltage across diode is _______.
• Therefore, voltage at B is ________.
• When _______, the diode is in Reverse Bias.
• The diode is open-circuit.
• Therefore, voltage at B is ________.

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Diode Circuits - Simple Circuits

• Diode conducts only when _________.
• When in Reverse Bias, diode is ________________.
• Therefore, VOUT ______________.
• When in Forward Bias, voltage across diode is
  always ________.
• Therefore, VOUT ___________.

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Diode Circuits Simple Circuits

• Diode X conducts only when ________.
• When in Forward Bias, voltage across Diode X is
  ________.
• Therefore VA VOUT ________.
• Diode Y conducts only when ________.
• When in Forward Bias, voltage across Diode Y is
  ________.
• Therefore, VB VOUT ___________.

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Diode Circuits Simple Circuits

• Example (Midterm 2003)
• In the circuit shown below, the input is a
  sinusoid with peak voltage equal to /- 10V.
  Assume that the diode starts to conduct when the
  forward biased voltage is greater than 0.6V,
  sketch the output voltage

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Diode Circuits Application

• Full-wave bridge rectifier