Example Assume ideal diodes

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Example (Assume ideal diodes)
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Example

• Determine the output voltage waveform (Assume
  non-ideal diodes)

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Solution

• When the voltage at point A reaches 7.7 V, diode
  D1 conducts and limits the waveform to 7.7 V
  Diode D2 does not conduct until the voltage
  reaches -5.7 V Therefore, positive voltages above
  7.7 V and negative voltages below -5.7 V are
  clipped off.

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Voltage-Divider Bias

• In practice, the bias voltage sources that have
  been used to illustrate the basic operation of
  diode limiters can be replaced by a resistive
  voltage divider that derives the desired bias
  voltage from the dc supply voltage The bias
  resistors must be small compared to R1

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Example

• Describe the output voltage waveform for the
  diode limiter in figure

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Solution
The positive part of the output voltage waveform
is limited to VBIAS 0.7 V
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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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ZENER DIODE CHARACTERISTICS

• 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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ZENER DIODE CHARACTERISTICS

• Voltage after breakdown is also called Zener
  voltage Vz.
• Vz remains nearly a constant, even though current
  Iz varies considerably.

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ZENER DIODE RATINGS

• A Zener data sheet typically provides
• - the maximum power rating PzM
• - the nominal zener voltage Vz at test
  current IzT
• - the maximum DC zener current IzM
• Example
• - 1N752 has a power rating of 500mW, a
  nominal Zener voltage of 5.6V at a test
  current of 20 mA, a maximum Zener current of
  80 mA.

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ZENER DIODE MODEL

• Zener model to be applied

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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 VgtVz, the Zener diode is "on" else the Zener
  diode is "off"

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• Step 2 Substitute the appropriate equivalent
  circuit and solve for the desired unknowns
• If the Zener is on, then VLVz
• IzIR-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.
• (b) Repeat part (a) with RL 3 kO.

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Solution

• (a)
• Since VltVz then the diode is off and the circuit
  becomes

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• (b)
• Now the diode is on and the circuit becomes

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Fixed Vi, variable RL

• The minimum load resistance that will turn the
  Zener diode on
• In this case the load current is maximum
• Once the diode is on we have

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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.

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Example

• (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

• (a)

• (b)

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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

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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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Solution
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Series Zener diodes

• As long as Vi is greater than the sum of VZ1 and
  VZ2, both diodes will be in the "on" state and
  the three reference voltages will be available

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Back-to back-Zener diodes