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CHAPTER 17 DIODES AND APPLICATIONS Typical Diode package Diode symbol Diode Approximations The ideal diode model การป้ อนไฟให้ Diode Forward bias: ให้ ศักย์ ไฟฟ้ าด้ าน Anode ( P ) สูงกว่ า Cathode ( N ) VF > VB (VB (Si ) = 0.7 V; VB (Ge ) = 0.3 V ) ป้ อนไฟให้ กระแสไหลในทิศตามลูกศร ของสัญลักษณ์ ไดโอด Diode จะทาตัวเหมือน switch ปิ ด ยอมให้ กระแสผ่ านได้ Diode Approximations The complex diode model DIODE CHARACTERISTICS Diode Characteristic Curve Reverse bias As the voltage (VR) increases to the left, the current remains near zero until the breakdown voltage (VBR) is reached When breakdown occurs, there is a large reverse current that can destroy the diode การป้ อนไฟให้ Diode Reverse bias: ให้ ศักย์ ไฟฟ้ าด้ าน Anode ( P ) ต่ากว่ า Kathode ( N ) ป้ อนไฟให้ กระแสไหลในทิศย้ อนลูกศร ของสัญลักษณ์ ไดโอด Diode จะทาตัวเหมือน switch เปิ ด ทาให้ กระแสผ่ านไม่ ได้ Breakdown Voltage (VBR) Reverse bias สูงมาก จนกระทั่ง Depletion region ขยายจนเต็มพืน้ ที่ N และ P Avalanche Effect Free e- ฝั่ ง P มีพลังงานสูงมากจนกระทั่ง ชน valence e- ใน P กลายเป็ น Free eทาให้ จานวน Free e- เกิดเป็ นจานวนเท่ าทวีคูณ ( rapid multiplication ) ทาให้ เกิด Free e- ในฝั่ ง P จานวนมาก ไหลผ่ านรอยต่ อได้ กลายเป็ นกระแสไหลในทิศย้ อนกลับ Diode Testing Working diode Forward bias: Voltmeter: 2.5 – 3.5 v (internal meter voltage) Open diode 0.7 v Reverse bias: Voltmeter: Forward & Reverse bias Voltmeter: 2.5-3.5 v Ohm meter: High resistance Shorted diode Forward & Reverse bias Voltmenter: Ohm meter: 0v Low resistance การใช้ งาน diode กาหนดทิศการไหลของกระแสในวงจร ตัวอย่ างวงจรที่ใช้ งาน diode วงจร rectifier ใน DC Power Supply กระแสสลับ (AC) 2 ขัว้ (+/-) ให้ เป็ น กระแสตรง ( DC) (ขัว้ เดียว) วงจร Diode Limiter (จากัดความต่ างศักย์ ของสัญญาณ AC) วงจร Diode Clamper (ยกระดับความต่ างศักย์ ของสัญญาณ AC) Basic DC power supply The dc power supply converts the standard 220 V,50 Hz into a constant dc voltage They consist of three parts : Rectifier, Filter, and Regulator The dc voltage produced by a power supply is used to power all types of electronic circuits, such as television receivers, stereo systems, VCRs, CD player Power supply Figure 17-1 The Half-Wave Rectifier Figure 17-2 Average Value of the Half-wave output voltage VAVG=Vp (out) / Effect of Diode Barrier Potential on Half-Wave Rectifier Output Voltage During the positive half-cycle, the input voltage must overcome the barrier potential before the diode becomes forward-biased Vp (out) =Vp (in) - 0.7 V Figure 17-5 Peak Inverse Voltage (PIV) The maximum value of reverse voltage, sometimes designated as PIV, occurs at the peak of each negative alternation of the input cycle when the diode is reverse-biased Figure 17-7 FULL-WAVE RECTIFIERS The full-wave rectifier is the most commonly used type in dc power supplies allowing unidirectional current to the load during the entire input cycle differ from the half-wave rectifier that allows only during one-half of the cycle Figure 17-9 Average Value of the Full-wave rectified output voltage VAVG= 2Vp (out) / Center-Tapped Full-Wave Rectifier Using two diode connected to the secondary of a center-tapped transformer At the positive half-cycle Forward-biases the upper diode D1 Reverse-biases the upper diode D2 At the negative half-cycle Reverse-biases the upper diode D1 Forward-biases the upper diode D2 Center-Tapped Full-Wave Rectifier Effect of the Turn Ratio on Full-Wave Output Rectifier If the turn ratio of the transformer is 1:1, the output of the rectifier is equal to ½ of the input voltage Vp Owing to the voltage input is approximately equal to the output voltage, we must use the step-up transformer Peak Inverse Voltage (full-wave rectifier) VD 2 Vp (sec) 2 Vp ( out) Vp (sec) 2 Vp (sec) Vp (sec) 2 Vp (sec) 2 2Vp (sec) PIV Vp (sec) 2Vp ( out) Full-Wave Bridge Rectifier Bridge Output Voltage PIV Vp (sec) Vp ( out) POWER SUPPLY FILTER AND REGULATORS After passed the rectifier, the output of the power supply is filtered for reduce the ripple, on the other hand, for make an output smoothly Capacitor-Input filter Ripple Voltage The voltage which change due to charging and discharge of the capacitor is called “ripple voltage” Ripple Voltages for half-wave and full-wave Ripple Factor (r) Ripple factor is the ratio of the Vr to VDC, expressed as : Vr r 100% VDC NOTE: the frequency in the full-wave rectifier is twice of the half-wave rectifier Surge Current in the Capacitor-Input Filter IC Regulators An integrated circuit regulator is a device that is connected to the output of a filtered rectifier and maintains a constant output voltage The capacitor-input filter reduces the input ripple to the regulator to an acceptable level and it is combined in IC regulator. The most IC regulators have three terminal Input terminal Output terminal Reference terminal IC Regulators Basic regulated power supply Percent Regulation Line regulation Specifies how much change occurs in the output voltage for a given change in the input voltage Line regulation = ∆VOUT ∆VIN 100% Load regulation Specifies how much change occurs in the output voltage over a certain range of load current value VNL- VFL Load regulation = 100% VFL Diode Limiter Diode Limiters Diode limiters (clipper) cut off above or below specified levels Vout RL Vin RS RL Diode Limiting and Clamping Circuits Diode Limiters Adjustment of the limiting level Diode Clampers Diode clamper known as a dc restorer Add a dc level to an ac signal Diode Clampers ชนิ ดของ Diode Regular diode Zener diode Varactor diode Light-Emitting diode (LED) Photo diode ZENER DIODES The zener diode is a silicon pn junction device and operate in the reverse breakdown region symbol Zener Breakdown (Vz) Two types of reverse breakdown in a diode Avalanche also occurs in the rectifier diode (regular diode) Zener Occurs in a zener diode at low reverse voltages NOTE : Zeners with breakdown voltage of 1.8 to 200 V are commercially available Breakdown Characteristic Regulator: ตัวคงค่าควา I ZK I zener I ZM Zener Equivalent Circuit Regulator: ตัวคงค่าความต่างศ ักย ์ Zener diode impedance The ratio of ∆Vz to ∆Iz is the zener diode impedance Zz = ∆VZ ∆IZ Normally, ZZ is specified at IZT ZZ is approximately constant over the full range of reverse-current values Zener Voltage Regulation Zener diodes can be used for voltage regulation in noncritical low-current applications I ZK I zener I ZM Zener Voltage Regulation As the input voltage varies, the zener diode hole the constant voltage across the output terminals Zener Regulation with a Varying Load The zener diode maintains a constant voltage across RL as long as the zener current is greater than IZK and less than IZM, this process is called load regulation VARACTOR DIODES A varactor is basically a reverse-biased pn junction that utilizes the inherent capacitance of the depletion region The depletion region acts as a capacitor dielectric A C d VARACTOR DIODES A C d VARACTOR DIODES VARACTOR DIODE APPLICATIONS fr 1 2 LC LEDs and PHOTODIODES There are two types of optoelectronic devices The Light Emitting Diode (LED) The photodiode (light detector) The Light Emitting Diode (LED) When the device is forward-biased, electrons across the pn junction from the ntype material and recombine with the holes in the p-type material When recombination takes place, the recombining electrons release energy in the form of heat and light The Light Emitting Diode (LED) The semiconductive materials used in LEDs are gallium arsenide(GaAs), galium arsenide phosphide (GaAsP), and Gallium phosphide (GaP) Silicon and Germanium are not used because they are very poor at producing light GaAs LEDs emit infrared (IR) radiation GaAsP produces either red or yellow visible light GaP emits red or green visible light The Light Emitting Diode (LED) symbol electroluminescence The Light Emitting Diode (LED) The Light Emitting Diode (LED) Applications The Photodiode The photodiode is a pn junction device that operates in reverse bias The Photodiode The Photodiode operation The Photodiode Applications The diode data sheet TROUBLESHOOTING