ECE 255 Fall 2019 Purdue University ECE 255
- Slides: 20
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 Lundstrom: Fall 2019 1
Goals 1) To practice using diode models 2) To discuss “model parameters” 3) To discuss two points about real diodes Lundstrom: Fall 2019 2
But first: A practice problem Assume ideal diodes and find V and all diode currents Lundstrom: Fall 2019 3
Practice problem Guess: D 1 (FB) D 2 (FB) D 3 (FB) Show why this does not work Lundstrom: Fall 2019 4
Practice problem Guess: D 1 (FB) D 2 (FB) D 3 (FB) Show why this does not work Find: IR 1 = (10 -5)/8. 2 K=0. 61 m. A IR 2 = (5 -0)/12 K =0. 42 m. A Implies ID 2 < 0 D 2 is NOT FB Lundstrom: Fall 2019 5
Practice problem Guess: D 1 (FB) D 2 (RB) D 3 (FB) Show that this does work. Lundstrom: Fall 2019 6
Quiz problem Guess: D 1 (FB) D 2 (RB) D 3 (FB) Show that this does work. Find: IR 1 = (10 20. 2 / (0 –K = 0. 495 m. A V = 10 – 0. 495 x 8. 2 = 5. 94 VD 2 < 0 RB IR 3 = (0 -(-5))/10 = 0. 5 m. A ID 3 + IR 2 = IR 3 ID 3 = IR 3 –IR 2 = 0. 005 m. A 7
Outline 1) Practice using diode models 2) “model parameters” 3) Two points about real diodes Lundstrom: Fall 2019 8
Three diode models Exponential model Ideal diode Constant-voltage-drop model (silicon) Big idea: Complicated devices can be “simply” modeled. 9
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 10
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 11
Determining the model parameter ✓ Lundstrom: Fall 2019 12
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 13
Determine the model parameter ✓ But this would not be considered good engineering judgment. Why? How could we do better? Lundstrom: Fall 2019 14
Outline 1) Practice using diode models 2) “model parameters” 3) Two points about real diodes Lundstrom: Fall 2019 15
A question How much do we need to increase VD to increase the current by 10 X? How much do we need to decrease VD to decrease the current by 10 X? Lundstrom: Fall 2019 16
The answer Lundstrom: Fall 2019 17
Temperature sensitivity 301 K 300 K How much does the voltage need to change to keep the current constant if T increases by 1 deg C? Answer: ~ -2 m. V/deg. C Lundstrom: Fall 2019 18
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 19
Modeling diodes 1) Practice using diode models 2) “model parameters” 3) Two points about real diodes Lundstrom: Fall 2019 20
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