Diode Circuits.

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Diode Circuits

+ + - - Practical Aspects of pn Junction anode cathode Reversed bias
Forward bias - - cathode The left hand diagram shows reverse bias, with positive on the cathode and negative on the anode (via the lamp). No current flows. The other diagram shows forward bias, with positive on the anode and negative on the cathode. A current flows.

Polarization of the pn Junction
(1) (2) Forward bias examples (4) (3)

Polarization of the pn Junction
(1) (2) Reversed bias examples (3) (4)

Diode Ohms Check: Checks preformed on Si diode, by reversing the leads on the Digital Voltage Mutimeter (DMM). + - P N DMM = 0  DMM =   DMM

Diode Voltages To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.

A Diode Puzzle Which lamps are alight? Some may not be full brightness. + + - -

A Diode Puzzle Which lamps are alight? Some may not be full brightness. + + - -

+ + - - Exercise - a Diode Puzzle
Which lamps are alight? Some may not be full brightness. + + - -

+ + - - Exercise - a Diode Puzzle
Which lamps are alight? Some may not be full brightness. + + - -

circuit A reading Rp R D V reading Ev A diode is a nonlinear device and typical linear circuit analysis methods do not apply!

Diode Characteristic for Small-Signal Diodes
VT ~ 26 mV Vknee less than 1mA at 300K When the temperature is increasing the knee voltage Vknee decreases by about 2mV/K

Analysis of Diode Circuits
Nodal analysis Mesh analysis Kirchhoff’s voltage law Thevenin & Norton theorems Vth/RTh Slope=-1/RTh Vth Example 10.1

Analysis of Diode Circuits
Thevenin equivalent + - io Vo vD iD KVL KCL Their characteristics intersect

Analysis of Diode Circuits
Nodal analysis Mesh analysis Kirchhoff’s voltage law Thevenin & Norton theorems Vth/RTh Slope=-1/RTh Vth Example 10.1

Load-Line Analysis Repeat for: Vss=10V and R=10kW
Problem If the circuit shown below has Vss=2V and R=1kW and a diode with ch-tic shown, find the diode voltage and current at the operating point Repeat for: Vss=10V and R=10kW VDQ=0.68V and iDQ=0.93mA

Zener Diode - Voltage Regulator (reverse biased)
A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".

Vss+ RiD+vD=0 Zener Diode - Voltage Regulator (reverse biased)
Problem Find the output voltage for Vss=15V and Vss=20V if R=1kW and a Zener diode has the ch-tic shown below. Load Line analysis Kirchhoff’s voltage law Vss+ RiD+vD=0 Slope of the load is -1/R Reverse bias region

Load Line Analysis of Complex Circuits Thevenin Equivalent

Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage vL and the source current iS if Vss=24V, R=1.2kW and RL=6kW.

Exercise – find Thevenin equivalent
Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage vL and the source current iS if Vss=24V, R=1.2kW and RL=6kW. Exercise – find Thevenin equivalent

VT=Vss*(RL/(R+RL))=20V RT=(RRL)/(R+RL)=1kW
Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage vL and the source current iS if Vss=24V, R=1.2kW and RL=6kW. VT=Vss*(RL/(R+RL))=20V RT=(RRL)/(R+RL)=1kW Thevenin equivalent

Load line equation VT + RTiD + VD = 0 VL=-VD=10V iD=-10mA
Finally iS=(VSS-VL)/R=11.67 mA (from circuit “a”) Exercise 10.4 & 10.5

Ideal diode Model Useful for circuits with more than one diode
Assume a state for each diode, either “on” or “off” -2n combinations (2) Assume a short circuit for diode “on” and an open circuit for diode “off” (3) Check to see if the result is consistent with the assumed state for each diode (current must flow in the forward direction for diode “on” and the voltage across the diodes assumed to be “off” must be positive at the cathode – reverse bias) (4) If the results are consistent with the assumed states, the analysis is finished. Otherwise return to step (1) and choose a different combination of diode states.

Problem Analyze the circuit shown below using the ideal diode model
Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on. 7V -3V Not consistent with the assumption that D2 if off Exercise 10.6 & 10.7 & 10.8

Problem Analyze the circuit shown below using the ideal diode model
Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on. 7V -3V Not consistent with the assumption that D1 is off

Problem Analyze the circuit shown below using the ideal diode model
Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on. 7V -3V This is OK

Piecewise Linear Diode Models
More accurate that the ideal diode model and do not relies on nonlinear equation or graphical techniques. (1) Diode V-I ch-tic approximated by straight line segments (2) We model each section of the diode I-V ch-tic with R in series with a fixed voltage source v = Rai + Va

Problem Find circuit models for the Zener-diode volt-ampere ch-tic shown in figure below using the piecewise-linear diode model. Draw a line Look for intercept (0.6V) & the reciprocal of the slope (1/R) (1.6V-0.6V)/100mA=10W Open circuit approximation Repeat for the reverse bias Exercise 10.7