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Power Semiconductor Diodes

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This barrier simulates the behavior of a pn-junction. Recovery charge of this diode is much less than the equivalent pn-junction diode. ... – PowerPoint PPT presentation

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Title: Power Semiconductor Diodes


1
Power Semiconductor Diodes
  • Introduction

2
Function of power semiconductor diodes in power
electronic circuits
  • Switches in rectifiers
  • Freewheeling in switching regulators
  • Charge reversal of capacitor energy transfer
    between components
  • Voltage isolation
  • Energy feedback from load to power source
  • Trapped energy recovery

3
Diode Characteristics
  • A power diode is a two terminal pn-junction
    device.

4
Forward Biased Diode
  • A diode is said to be forward biased or
    conducting when the anode potential is positive
    with respect to the cathode.
  • In this state, the diode has a small forward
    voltage drop across it.

5
The magnitude of this voltage
drop depending on
  • the manufacturing process and
  • the junction temperature

6
Reversed Biased Diode
  • When the cathode potential is positive with
    respect to the anode, the diode is said to be
    reversed biased.
  • Under reversed biased conditions, a small reverse
    or leakage current flows.
  • The magnitude of the reverse current increases in
    magnitude with reverse voltage until the
    avalanche or zener voltage is reached.

7
Diode Characteristics
  • The practical characteristics shown in figure
    2.2a can be expressed by the schockley diode
    equation.

8
Examination of the diode characteristics reveal
three distinct regions
  • Forward-biased region
  • Reverse-biased region
  • Breakdown region

9
Reverse Recovery Characteristics
  • The current in a forward-biased junction diode is
    made up of two components.
  • A forward conducting diode whose forward current
    has been reduced to zero, continues to conduct
    minority carriers from two different regions of
    the diode.

10
Reverse Recovery Characteristics(continued)
  • These minority carriers requires some finite time
    to recombine with opposite charges in order to be
    neutralized. This time is called the reverse
    recovery time.
  • Two reverse recovery characteristics exist.

11
Reverse Recovery Time
  • The reverse recovery time is measured from the
    initial zero crossing from forward conduction to
    reverse blocking condition of the diode current
    to 25 of the maximum reverse current.

12
Reverse Recovery Charge
  • This is the amount of charge carriers that flow
    across the diode in the reverse direction due to
    changeover from forward conduction to reverse
    blocking condition. Its value is determined from
    the area enclosed by the path of the reverse
    recovery current.

13
Power Diode Types
  • Standard or general-purpose diodes
  • Fast-recovery diodes
  • Schottky diodes

14
General Purpose Diodes
  • High reverse recovery time typically around 25
    microseconds
  • Used in low speed applications, e.g., rectifiers
    and converters, with frequencies up to 1kHz

15
Fast-recovery Diodes
  • low recovery time, typically around 5
    microseconds
  • Used in ac-dc and dc-ac converters where speed of
    recovery is of critical importance

16
Schottky Diodes
  • The barrier potential is accomplished with a
    contact between a metal and a semiconductor. This
    barrier simulates the behavior of a pn-junction.
  • Recovery charge of this diode is much less than
    the equivalent pn-junction diode. It is due only
    to junction capacitance and is independent to
    reverse di/dt.

17
Schottky Diodes(continued)
  • It has a relatively low forward voltage drop.
  • The leakage current is higher than that of a
    pn-junction diode.
  • They are mainly used in high current low voltage
    power supplies.

18
Effects of Forward and Reverse Recovery Time
  • The switch is turned on at time0 and remains on
    long enough for a steady load current to flow.
  • The freewheeling diode is reversed-biased under
    these conditions.
  • The diode conducts load current through it due to
    the energy stored in the inductor.

19
Effects of Forward and Reverse Recovery
Time(continued)
  • The diode would act as a short circuit to supply
    the voltage.
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