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ECE 255: Fall 2019 Purdue University

ECE 255: Modeling Diodes ( Sedra and Smith, 4.1-4.2) Mark Lundstrom School of ECE Purdue University West Lafayette, IN USA. ECE 255: Fall 2019 Purdue University. Goals. To practice using diode models To discuss “ model parameters ” To discuss two points about real diodes.

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ECE 255: Fall 2019 Purdue University

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  1. ECE 255:Modeling Diodes(Sedra and Smith, 4.1-4.2)Mark LundstromSchool of ECEPurdue UniversityWest Lafayette, IN USA ECE 255: Fall 2019 Purdue University Lundstrom: Fall 2019

  2. Goals To practice using diode models To discuss “model parameters” To discuss two points about real diodes Lundstrom: Fall 2019

  3. But first: A practice problem Assume ideal diodes and find V and all diode currents Lundstrom: Fall 2019

  4. Practice problem Guess: D1 (FB) D2 (FB) D3 (FB) Show why this does not work Lundstrom: Fall 2019

  5. Practice problem Guess: D1 (FB) D2 (FB) D3 (FB) Find: IR1 = (10-5)/8.2K=0.61 mA IR2 = (5-0)/12K =0.42 mA Implies ID2 < 0 D2 is NOT FB Show why this does not work Lundstrom: Fall 2019

  6. Practice problem Guess: D1 (FB) D2 (RB) D3 (FB) Show that this does work. Lundstrom: Fall 2019

  7. Quiz problem Guess: D1 (FB) D2 (RB) D3 (FB) Find: IR1 = (10 – 0) / 20.2 K = 0.495 mA V = 10 – 0.495x8.2 = 5.94 VD2 < 0 RB IR3 = (0-(-5))/10 = 0.5 mA ID3 + IR2 = IR3 ID3 = IR3 – IR2 = 0.005 mA Show that this does work.

  8. Outline Practice using diode models “model parameters” Two points about real diodes Lundstrom: Fall 2019

  9. Three diode models Exponential model Ideal diode Constant-voltage-drop model (silicon) Big idea: Complicated devices can be “simply” modeled.

  10. Model parameters 1) Ideal diode model No model parameters 2) Constant-voltage-drop model One model parameter 3) Exponential model One model parameter Lundstrom: Fall 2019

  11. 1) Constant voltage drop model Now suppose that we want to use this diode in a circuit for which What VD should we use? Lundstrom: Fall 2019

  12. Determining the model parameter ✓ Lundstrom: Fall 2019

  13. 2) Exponential model Question: Given a diode that we want to model with the exponential equation, how do we determine IS? Answer: Do a measurement at 300 K: e.g. What is IS? Lundstrom: Fall 2019

  14. Determine the model parameter ✓ But this would not be considered good engineering judgment. Why? How could we do better? Lundstrom: Fall 2019

  15. Outline Practice using diode models “model parameters” Two points about real diodes Lundstrom: Fall 2019

  16. A question How much do we need to increase VD to increase the current by 10X? How much do we need to decrease VD to decrease the current by 10X? Lundstrom: Fall 2019

  17. The answer Lundstrom: Fall 2019

  18. Temperature sensitivity 300 K 301 K How much does the voltage need to change to keep the current constant if T increases by 1 deg C? Answer: ~ -2 mV/degC Lundstrom: Fall 2019

  19. Summary We discussed three types of diode models: i) ideal, ii) constant-voltage-drop, and iii) mathematical (exponential). Models have model parameters. To get good results, we need a good model and accurate model parameters. Lundstrom: Fall 2019

  20. Modeling diodes Practice using diode models “model parameters” Two points about real diodes Lundstrom: Fall 2019

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