Types of Power Diodes

1. COLLEGE OF ENGINEERING ROORKEE
Established in 1998
Lecture 3: Types of Power
Diodes
Dr. Aadesh Kumar Arya
Associate Professor & Head

2. Introduction
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The diode totally stops any leakage current never considering any breakage of the junction barrier. But since things are
not ideal in this world, we do face some problems considering non ideal behavior regarding the reverse leakage current
in the diodes. So to adjust this, we associate a reverse recovery time with each diode, which marks the time required for
the diode to recover from the reverse current. But the reverse recovery time acts as a bargain with the diode cost. A diode
requiring lesser reverse recovery time will be more expensive than a simple diode with a larger reverse recovery time.
Power Diode Applications:
As a rectifier Diode
For Voltage Clamping
As a Voltage Multiplier
As a freewheeling Diode

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Diodes are classified according to their reverse
recovery characteristics
The three types of power diodes are as under
1. General Purpose Diodes
2. Fast Recovery Diodes
3. Schottky Diodes

4. General Purpose Diodes
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General-purpose diodes are two-terminal electronic components that allow current to flow in only one direction, from
an anode (+) to a cathode.
These simple semiconductors are PN junctions with a positive or P-region with positive ions and a negative or N-
region with negative electrons
 These diodes have high reverse recovery time. It will be in the range of 25μs.
• .The diode current rating will be from 1A to several thousand amperes.
• The voltage rating will be from 50V to 5kV.

5. Applications
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General-purpose diodes are often used in any application that does not require a specialized function or power
requirements. Some of the applications include:
• Battery charging circuits
• Voltage regulators
• Switching applications
• Clamping circuits
• Power supplies
• DC-DC converters
• AC-DC converters
• DC-AC inverters
• Signal rectifiers
• Oscillators
• Signal modulators / demodulators
• Cellular phones
• TV chassis
• Camcorders
• Motherboards
• electrical traction

6. Fast Recovery Diodes
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Fast Recovery Diode is a semiconductor device, which possesses short reverse recovery time for rectification purpose
at high frequency. A quick recovery time is crucial for rectification of high-frequency AC signal. Diodes are mostly
used in rectifiers because they possess ultra-high switching speed.
The major problem with the conventional diode is that they possess quite high recovery time. Due to which
the rectification of high frequency is not possible with a conventional diode.
These are used in high frequency circuits in combination with controllable switches where a small reverse recovery
time is needed. At power levels of several hundred volts and several hundred amperes, these diodes have trr ratings of
less than a few microsecond.

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• As the name indicates, these diodes have very low reverse recovery time. It will be less than 5μs.
• They are mainly used in switching circuits like choppers, commutation circuits, switching mode power
supplies etc.
• The current rating of the diode will be from 1A to several thousand amperes
• The voltage rating will be from 50V to 3kV.
• In this type of diode,
(1) For low voltage ratings ( below 400V), the epitaxial process is used for the diode fabrication.
(2) For diodes having voltage rating above 400V, diffusion technique is used for the fabrication of
diodes.

8. Construction of Fast Recovery Diode
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The fast recovery diode is constructed in the similar manner by which ordinary diode is constructed.
The major difference in construction between these diodes and conventional diodes is the presence of recombination centres.
In fast recovery diodes, Gold (Au) is added to the semiconductor material. This leads to augmentation in the numerical value
of recombination centres due to which the lifetime (?) of charge carriers decreases.
The semiconductor material used in these diodes is Gallium Arsenide (GaAs).
And with the addition of Gold (Au) in the semiconductor material, the recovery time becomes less (about 0.1ns).
On the other hand, the value of recovery time in case of silicon is 1-5 ns. Thus, it is evident that addition of materials like
gold decreases the recovery time.

9. Applications of Fast Recovery Diode
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1. Rectifier: These diodes are used in rectifier especially for high-frequency rectification.
2. Industrial and commercial areas: They are used in electronics circuits in various industries and automobile sector.
3. Radio signal detector: They are used in radio signal detectors to detect high-frequency RF waves
4. Analog and Digital communication Circuits: In analogue and digital communication circuit these diodes are
extensively used for rectification and modulation purpose

10. Schottky Diodes
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Schottky diode is a metal-semiconductor junction, which does not store charge carriers at the junction because it has no
depletion layer. It finds its application where fast switching is needed.
Significance of Schottky diode
When a P-N junction diode is forward biased, it starts conducting, but when it is reverse biased, it stops conduction. However,
this transition from conduction to insulation is not instant. Diode takes some time to reach a steady state of no conduction
when it is reverse biased.
This happens because during forward biasing charge carriers move across the junction and when it is suddenly
reverse biased, some of the charge carriers are still at the junction, but they have not recombined yet.

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Thus, this phenomenon is called charge storage. Due to this charge storage, diode takes time while
switching. The time taken by the diode to achieve a steady state of no conduction in reverse biasing is called
reverse recovery time. The effect of reverse recovery time is negligible at frequencies below 10MHz, but at
high frequencies, this effect is significant.
These diodes are used where a low forward voltage drop (usually 0.3V) is needed in low output voltage
circuits.
In high frequency switching circuits like SMPS, the Schottky diodes are used (in those
applications we cannot use the general-purpose diodes).
In this, type of diodes, instead of P-N junction (semiconductor-to-semiconductor) metal
to semiconductor junctions used.
The Schottky diodes have very fast recovery time and low forward voltage drop.
The current flow will be done by only majority carriers.
So the time delay due to reverse recombination is avoided.
The drawback with Schottky diode is they have low voltage ratings (around 100V) and forward current
ratings (upto 300A).

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Advantages of Schottky diode
1. It possesses high switching speed.
2. Due to high switching speed, its reverse recovery time is very less as compared to other bipolar diodes.
3. The value of forward voltage in the case of this diode is also minimal comparatively other bipolar diodes
Disadvantages of Schottky Diode
1. It possesses a significant value of leakage current.
2. The reverse breakdown voltage of these diodes is very small. Thus, even a small amount of reverse voltage can
damage it.
Applications of Schottky Diode
1. It is used as Schottky TTL in digital devices as these devices need fast switching.
2. A Schottky diode is the most significant component for digital computers, as the performance of digital
computers is determined by switching speed of diodes.

13. Difference between Schottky Diode and
PN Junction Diode
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PN Junction Diode Schottky Diode
Bipolar device ( current conduction will happen
due to minority & majority carriers)
Unipolar device ( current conduction will
happen due to majority carriers only)
Semiconductor – semiconductor
junction formation
Semiconductor – Metal junction formation
Relatively large forward voltage drop (typically
0.65V)
Lower forward voltage drop (typically 0.3V)
Available with higher voltage ratings Maximum reverse voltage rating will be 200V
Comparatively On state loss will be more Low On state Losses
Suitable for low-frequency applications (i.e. 50
Hz to 400 Hz)
Suitable for high frequency switching
applications (ie, up to 300MHz)
Slow turn on & turn off process (due to the
presence of stored charge carriers in the
junction. These have the effect of momentarily
allowing current to flow in the reverse direction
when reverse voltage is applied.)
Turn on & turn off faster than pn-junction
diodes (depletion layer is not formed near the
junction because electrons are the majority
carriers on both sides of the junction (ie, in the
N-type semiconductor as well as in the metal).

14. Why Schottky diode switching faster than bipolar
Junction Diode?
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 In this diode, the electrons are the majority carriers on both sides of the junction (i.e, in the N-type
semiconductor as well as in the metal).
 So depletion layer is not formed near the junction. (i.e no stored charge carriers in the junction)
 This scenario will give the following benefits:
With reverse bias condition, there is no significant current from the metal to the semiconductor.
Thus the time delay existing in the junction diodes due to hole – electron recombination is absent in
Schottky diode.
 Hence, the Schottky diode can switch OFF faster than a bipolar diode.

15. How to test Diode?
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We know that fact that resistance of diode in forward biased condition is low and the resistance of diode in reverse
biased condition is high.
Keep the multimeter in the ohmmeter section. If we measure the resistance of a diode using the connections like red
lead to anode and black lead(common) to cathode, a healthy forward biased diode will give low resistance.
 A high resistance reading in both directions indicates an open (defective device) condition, while a very low
resistance reading in both directions will probably indicate a shorted device.

16. Diode Protection
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Snubber circuits are essential for diodes used in switching circuits as they can save a diode from overvoltage
spikes, which may arise during the reverse recovery process. A common snubber circuit consists of a series RC
connected in parallel with the diode.
They have many different purposes, namely the reduction of power dissipation in power electronic switching
networks.

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Series/parallel connections: necessary in high voltage and high current applications. Matching diode in terms of
their reverse recovery properties is important in order to avoid large voltage imbalances between the diodes. A
parallel RC snubber in parallel with each diode overcomes most of these problems