Analysis of Diode Circuits

Analysis of Diode Circuits

[TOC]

Summary of Diode Models

• Diode Model : exp, constant-v, ideal
  • All three models are nonlinear.
  • All three models distinguish between positive and negative voltages.
    ("Rectification"(整流): keep all the positive voltages, and throw all the negative voltages)
  • The ideal model "passes" positive voltages; the constant-v model "passes" voltages greater than V_{D-on}; the exp model "passes" voltages significantly if V_D > 700 - 800 mV.

Application Examples

• Charger/Adapter（充电器/电源适配器）
  1. 通过变压器降低电压幅值。
  2. 通过整流电路（内含二极管），使电流方向恒为正。
  3. 通过低通滤波器，保留电流的直流部分。

Practice with Diode Circuits

Example (Using ideal model of diode)

• (a)

  
  
  

  Analysis of Diode Circuits 1

• (b)

  
  
  

  Analysis of Diode Circuits 2

• (c)
  Find the relationship between V_x and I_x ; V_x and V_{out}
  

  

  Analysis of Diode Circuits 3
  
  
  

  Analysis of Diode Circuits 4

• (d)

  
  
  

  Analysis of Diode Circuits 5

Application Example 2

Analysis of Diode Circuits 6

每个二极管上有约为800mV的压降，两个串联，就能形成1.6V的压降。

• The voltage across each diode is a weak function of its current.
• The diodes operate as a simple voltage regulator.

Principles of Diode Circuit Analysis

1. Begin by assuming certain states of all diodes, check the final result against these assumptions.
2. If a diode is about to turn on or off, it must sustain a voltage of V_{D-on}, but its current is small
3. If a diode is on and carries a current, the current must flow from the anode to the cathode.

Example 1

Using constant-voltage model of diode

Analysis of Diode Circuits 7

Raise V_x so much that V_N reaches (V_{D-on} + V_B) \Rightarrow D1 begins to turn on
Since the current thru D1 is still small \Rightarrow Voltage drop across R1 is small. (which can be neglected)

Quiz 1

Plot "V_N vs V_x"

Analysis of Diode Circuits 8

Example 2

Using constant-voltage model of diode

Swap D1 with R1 , the relationship between V_x and I_x doesn't changed.

Analysis of Diode Circuits 9

Quiz 2

Plot "V_N vs V_x"

Other Examples of Diode Circuits

Example

Analysis of Diode Circuits 10

Quiz

Plot V_N vs V_x

Analysis of Diode Circuits 11