diodes

# diodes - Module 1 Semiconductor (pn-Junction) Diodes What...

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Module 1 Semiconductor ( pn -Junction) Diodes What are they and what is their purpose? Diodes are electronic "check valves" which allow current to flow freely in one direction, but block current flow in the opposite direction. How do they work? (Module 2) Essentially, they serve the same function in circuits, as the valves in your heart and veins do in your body. Open Closed 2 Semiconductor ( pn -Junction) Diodes Physically, diodes are formed by the interface between two regions of oppositely doped semiconductor ( i.e. , pn junction) and are thus, structurally, the simplest semiconductor devices used in electronics. Circuit Symbol: Note: From the above, it follows that zero power dissipation occurs in an ideal diode! “open-circuit” “short-circuit” Ideal Diode An ideal diode is a two-terminal device defined by the following non-linear (current- voltage) iv -characteristic:

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3 Forward Biased Regime (v > 0) : Zero voltage drop occurs across a forward-biased ideal diode ( i.e. , the diode behaves like an ideal short circuit). Reverse Bias Regime (v ± 0) : Zero current flows in a reverse-biased ideal diode ( i.e. , the diode behaves like an open circuit). Determine the polarity of the voltage across the diode, then “replace” the diode with its equivalent circuit (either open-cct. or short cct.) and proceed with linear circuit analysis. DC Behavior: e.g. The diodes in each of the following circuits can be assumed to be ideal. D Fwd. biased ± V = 0 I = + 10 V ² 0 V 1 k ³ = 10 mA D Rev. biased ± I = 0 V = + 10 V ² (0 A )(1 k ³ ) = 10 V ± 4 e.g. The diodes in each of the following circuits can be assumed to be ideal, and the voltages v A and v B can be either +5 V or 0 V. 5 5 0 5 5 0 0 0 v Y v B v A 0 0 0 5 v Y = v A v B AND Gate 5 5 0 5 5 0 0 0 v Y v B v A 0 5 5 5 v Y = v A + v B OR Gate v Y v Y Alternatively, v Y = v A v B v C v Y = v A + v B + v C v Y v Y
5 AC Behavior: e.g. The diode in the following circuit can be assumed to be ideal. 6 AC Behavior: e.g. The diode in the following circuit can be assumed to be ideal.

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7 AC Behavior: e.g. The diode in the following circuit can be assumed to be ideal. 8 Conduction Angle: Consider the current flowing in the following circuit where the diode is ideal and v S = 24 sin ( ± t ) V. The diode begins to conduct when v S = 12 V. t The voltage across the resistor has the same waveform as the current (i.e., remaining zero until v S = 12 V). The amplitude (i.e., maximum) load voltage is thus V R = V S ± 12 V = 24 V ± 12 V = 12 V. t
9 Conduction Angle: Consider the current flowing in the following circuit where the diode is ideal and v S = 24 sin ( ± t ) V. 24 sin 2 ² ³ ´ µ · ¸ ¹ =12 V or 24 cos ( )=12 V º 2 =2 cos -1 12 V 24 V ´ µ · ¸ ¹ ¹ =120 ° The diode begins to conduct when v S = 12 V, which occurs at a phase angle defined by 24 sin ( t )=12 V. Or, in terms of the conduction 2 ² as defined in the figure above, The conduction angle characterizes the fraction of a period that the diode is “on”.

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## This note was uploaded on 02/03/2010 for the course EECE EECE 254 taught by Professor Robinturner,purangabomasumi during the Fall '09 term at UBC.

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diodes - Module 1 Semiconductor (pn-Junction) Diodes What...

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