ENSC2003 - Lecture 8

← Back to Home

ENSC2003 - Lecture 7
Diodes and Models - Slides

Diodes and Models

• Diode $-$ made of two semiconducting materials:

  • p-type + n-type -> they form a junction

  

• Two terminals of a diode -> anode $(+)$ and cathode $(-)$

  • A silver line on a power diode indicates the cathode end

  

• Diodes are passive elements, so current enters the $+$ve end

Exponential Mode of Diodes

• Diodes are non-linear elements

  • Do not use superposition when a diode is present
  • Do not include diode in Thevenin and Norton simplification

  

• Direction of diode is important as the current vs voltage behaviour depends of its orientation in circuit

• ! The current-voltage characteristic of a diode is exponential

Our graph is split into three regions:

1. Forward-bias region $-$ exponential increases in current
   $i_D=I_S(e^{\frac{V_D}{V_T}}-1)$
2. Reverse-bias region $-$ small negative current flow
   • Essentially minimises current flowing in other direction
     $i_D\approx -I_S$
3. Breakdown region $-$ significant current in reverse direction
   • Diodes cannot handle this and will usually break
   • This region is not covered in this unit

• $I_S$ is the reverse saturation current -> strongly temperature dependent
  • Approximately doubles for every $10°\text{C}$ rise in temperature
• $V_T$ is the thermal voltage -> about 25mV at room temperature ($20°\text{C}$)

PWL Model

• Graphical analysis is complicated and impractical for complex circuits

  • Graph model not used in this unit

• Thus, the piecewise linear model (PWL) is used instead

  • It describes the gross behaviour of the device in the presence of relatively large voltages and currents

  

• We treat an OFF diode as an open circuit

Only two states:

• Reverse bias -> current ($i_D$) $=0$
  • $V_D<V_{D,\text{on}}$
• Forward bias -> current ($i_D$) $>0$
  • $V_D=V_{D,\text{on}}$

Ideal silicon diodes have $V_{D,\text{on}}=0.6\text{V}$, which is always fixed

• In calculations involving diodes, we cannot predict whether the diode is in ON or OFF mode beforehand, so we must make an assumption

• You must include a statement such as:

  • Assume diode is forward-biased (on-mode).
    Thus $V_D=V_{D,\text{on}}$ for all positive $i_D$
  • Then you must either prove or disprove your equations

• Replace the diode in diagram with the relevant equivalent circuit:

  

• You can see an example in slides

Light Emitting Diodes

• Light emitting diodes dissipates power as light emission

  

  • LED behaves electrically like a diode

• $V_{D,\text{on}}$ for an LED depends on emission colour

  • Red-end of spectrum -> lower $V_{D,\text{on}}$
  • Blue end of spectrum -> higher $V_{D,\text{on}}$

• Light intensity depends on current:
  $i_D=I_S(e^{40V_D}-1)$

---

ENSC2003 - Lecture 9