Tài liệu ZENER DIODES ppt

3.1 ZENER DIODES VOLTAGE REGULATION aA voltage regulator circuit automatically maintains the output voltage of a power supply constant, regardless of -a change in the load - a change in the source voltage ZENER DIODES aThe simplest of all voltage regulators is the zener diode voltage regulator. aA zener diode is a s pecial diode that p is optimized for operation in the breakdown re gion.g ZENER DIODE CIRCUITZENER DIODE CIRCUIT a The zener diode is typically connected a The zener diode is typically connected reverse biased, in parallel with the load. a Resistor Rs limits current to zenera Resistor Rs limits current to zener. ZENER DIODE CHARACTERISTICS aIn the forward region, the zener diode acts like a regular siliconregular silicon diode, with a 0.7 volt drop when itvolt drop when it conducts. ZENER DIODE CHARACTERISTICS aIn the reverse bias region a reverseregion, a reverse leakage current flows until theflows until the breakdown voltage is reached. aAt this point, the reverse current, called zener current Iz, increases sharplyincreases sharply. ZENER DIODE CHARACTERISTICS aVoltage after bkd ilbreakdown is also called zener voltage Vzvoltage Vz. aVz remains nearly a constant evena constant, even though current Iz varies considerably. ZENER DIODE RATINGS aA zener spec sheet typically provides - the maximum power rating Pz - the nominal zener voltage Vz at test current Iz T - the maximum DC zener current Iz M Example: 1N752 h ti f 500 W- 1N752 has a power rating of 500mW, a nominal zener voltage of 5.6V at a test current of 20 mA a maximum zenercurrent of 20 mA, a maximum zener current of 80 mA. ZENER DIODE CIRCUIT EXAMPLE aIn a zener diode voltage regulator circuit, V 20V V 9V R 82 R 100V DC(IN) = 20V, Vz = 9V, Rs = 82Ω, R L = 100Ω Q: Calculate I L Q: Calculate I L . A: I L = V L /R L = V Z /R L = 9V/100 Ω = 90 mA 82Ω 20V 9V 100Ω 9V ZENER DIODE EXAMPLE (CONT’D) Q: Calculate I T . A: I T = V Rs /R s = [V DC(IN) - Vz]/R s = [20 - 9]/82 Ω = 134 mA 82Ω 20V 100Ω 9V [...].. .ZENER DIODE EXAMPLE (CONT’D) Q: Calculate Iz A: Iz = IT - IL = 134mA - 90mA = 44mA 82Ω 20V 9V 100Ω ZENER DIODE EXAMPLE (CONT’D) Q: If zener Pz = 500mW, is Iz safe ? A: IzM = Pz / Vz = 500 mW / 9V = 55 mA Since circuit Iz = 44 mA (< 55mA), zener is safe ZENER DIODE EXAMPLE (CONT’D) Q Q: Suppose load changes from 100 Ω to 200 Ω... 9V/200 Ω = 45 mA 20V 9V 200Ω ZENER DIODE EXAMPLE (CONT’D) A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs = [20 - 9]/82 Ω = 134 mA (no change) Iz = IT - IL = 134mA - 45mA = 89mA Since IzM = 55mA, zener is not safe 20V 9V 200Ω ZENER DIODE EXAMPLE ( (CONT’D) ) Q: Suppose load changes from 100 Ω to 80 Ω Find new IL, Iz, IT Is Iz safe ? A: IL = VL/RL = VZ/RL = 9V/80 Ω = 112 5 mA 112.5 20V 9V 80Ω ZENER DIODE EXAMPLE (CONT... change) Iz = IT - IL = 134mA - 112 5mA = 21.5 mA 112.5mA 21 5 Since IzM = 55 mA, zener is safe 20V 9V 80Ω ZENER DIODE EXAMPLE (CONT’D) Q: Suppose zener regulator circuit has no load connected, is Iz safe ? A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs = [20 - 9]/82 Ω = 134 mA (no change) Iz = IT = 134 mA this is very Unsafe 20V 9V ZENER DIODE EXAMPLE ( (CONT’D) ) Q: In the previous example, suppose VDC(IN) changes... ) Q: In the previous example, suppose VDC(IN) changes from 20V to 30V Is Iz safe ? A: IL = 9V/100 Ω = 90 mA IT = [30-9]/82 Ω = 256 mA Iz = IT - IL = 256 mA - 90 mA = 166 mA Very Unsafe 82Ω 30V 9V 100Ω ZENER DIODE EXAMPLE ( (CONT’D) ) Q Q: Suppose VDC(IN) changes from 20V to 18V pp g ( ) Is Iz safe ? A: IL = 9V/100 Ω = 90 mA IT = [18-9]/82 Ω = 110 mA Iz = IT - IL = 110 mA - 90 mA = 20 mA Safe 82Ω 18V . change in the source voltage ZENER DIODES aThe simplest of all voltage regulators is the zener diode voltage regulator. aA zener diode is a s pecial diode. operation in the breakdown re gion.g ZENER DIODE CIRCUITZENER DIODE CIRCUIT a The zener diode is typically connected a The zener diode is typically connected