zener diode.ppt

Clamper examples

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ZENER DIODES

• The simplest of all voltage regulators is the Zener diode voltage regulator.

• A Zener diode is a special diode that is optimized for operation in the breakdown region.

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ZENER DIODE CHARACTERISTICS

• In the forward region, the Zener diode acts like a regular silicon diode, with a 0.7 volt drop when it conducts.

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• In the reverse bias region, a reverse leakage current flows until the breakdown voltage is reached.

• At this point, the reverse current, called Zener current Iz, increases sharply.

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• Voltage after breakdown is also called Zener voltage Vz.

• Vz remains nearly a constant, even though current Iz varies considerably.

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Basic Zener Regulator• Fixed Vi and RL

• Step 1: Determine the state of the Zener diode by removing it from the network and calculating the voltage across the resulting open circuit

• If V>Vz, the Zener diode is "on" else the Zener diode is "off"

L

iLL RR

VRVV

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• Step 2: Substitute the appropriate equivalent circuit and solve for the desired unknowns

• If the Zener is on, then VL=Vz• Iz=IR-IL• The power dissipated by the

Zener diode is Pz= Vz Iz which must be less than the PZM specified for the device.

• Note that when the system is turned on the Zener diode will turn "on" as soon as the voltage across the Zener diode is Vz volts. It will then "lock in" at this level and never reach the higher level of V volts,

• Zener diode is used as a simple regulator designed to maintain a fixed voltage across the load RL.

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Example

• (a) For the Zener diode network shown, determine VL, VR, Iz, and Pz.

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Solution

• (a)

• Since V<Vz then the diode is off and the circuit becomes

VRR

VRV

L

iL 73.8

WIVP

AI

VVVV

VVV

ZZZ

Z

LiR

L

0

0

27.7

73.8

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Repeat part (a) with RL = 3 kΩ.

• (b)

• Now the diode is on and the circuit becomes

VRR

VRV

L

iL 12

WIVP

mAIII

VVVV

VVV

ZZZ

LRZ

LiR

ZL

7.26

67.2

6

10

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Fixed Vi, variable RL

• Once the diode is on we have

Zi

ZL VV

RVR

min

minmax

L

Z

L

LL R

V

R

VI

LRZ

RR

ZiR

IIIR

VI

VVV

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• Since Iz is limited to lZM as provided on the data sheet, it does affect the range of RL and therefore IL. min

max

min

L

ZL

ZMRL

I

VR

III

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• (a) For the network shown, determine the range of RL and IL that will result in Vz being maintained at 10 V

• (b) Determine the maximum wattage rating of the diode

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Solution

250minZi

ZL VV

RVR

mAR

VI

VVVV

RR

ZiR

40

40

kI

VR

mAIII

L

ZL

ZMRL

25.1

8

minmax

min

mWIVP ZMZ 320max

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Fixed RL , variable Vi

• For fixed value of RL, Vi must be sufficiently large to turn the Zener diode on

• Vi is also limited to a maximum value limited by IZM

L

ZLi R

VRRV

)(min

ZRi

LZMR

VRIV

III

maxmax

max

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Example

• Determine the range of values of Vi that will maintain the

Zener diode in the "on" state

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SolutionV

R

VRRV

L

ZLi 67.23

)(min

VVRIV

mAIII

ZRi

LZMR

87.36

67.76

maxmax

max

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Voltage-Multiplier CircuitsVoltage-Multiplier Circuits

• Voltage Doubler

• Voltage Tripler

• Voltage Quadrupler

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Voltage multiplier circuits use a combination of diodes and capacitors to step up the output voltage of rectifier circuits.

Voltage DoublerVoltage Doubler

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This half-wave voltage doubler’s output can be calculated by:

Vout = VC2 = 2Vm

where Vm = peak secondary voltage of the transformer

Voltage DoublerVoltage Doubler

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• Positive Half-Cycleo D1 conductso D2 is switched offo Capacitor C1 charges to Vm

• Negative Half-Cycleo D1 is switched offo D2 conductso Capacitor C2 charges to Vm

Vout = VC2 = 2Vm

Voltage Multipliers - Triplers

The voltage tripler arrangement adds another diode/capacitor set. + half-cycle: C1 charges to Vp through D1,

- half-cycle: C2 charges to 2Vp through C2, Next + half-cycle: C3 charges to 2Vp through C2. Output is across C1 & C3.

Voltage Multipliers - Quadruplers

The voltage tripler arrangement adds another diode/capacitor set. + half-cycle: C1 charges to Vp through D1,

- half-cycle: C2 charges to 2Vp through C2, Next + half-cycle: C3 charges to 2Vp through C3. Next - half-cycle: C4 charges to 2Vp through C4 Quadruple Output is across C2 & C4.

Voltage Tripler and QuadruplerVoltage Tripler and Quadrupler

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Practical ApplicationsPractical Applications

• Rectifier Circuits– Conversions of AC to DC for DC operated circuits– Battery Charging Circuits

• Simple Diode Circuits– clippers and clamper circuits in order to generate

different output voltage waveforms

• Zener Circuits– Overvoltage Protection– Setting Reference Voltages

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Voltage Doubler

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+ve half cycle -ve half cycle

zener diode.ppt

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