Application Note

VISHAY GENERAL SEMICONDUCTOR
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Rectifiers
Application Note
Physical Explanation
GENERAL TERMINOLOGY
Semiconductor diodes are used as rectifiers, switches,
varactors and voltage stabilizers (see Zener data book).
Semiconductor diodes are two-terminal solid-state devices
having asymmetrical voltage-current characteristics. Unless
otherwise stated, this means a device has single pn-junction
corresponding to the characteristics shown in figure 1.
Breakdown voltage, VBR
Reverse voltage at which a small increase in voltage results
in a sharp rise of reverse current. It is given in the technical
data sheet for a specified current.
Forward voltage, VF
The voltage across the diode terminals which results from
the flow of current in the forward direction.
Forward current, IF
The current flowing through the diode in the direction of
lower resistance.
I
Forward resistance, rF
The quotient of DC forward voltage across the diode and the
corresponding DC forward current.
V
Fig. 1
Parallel resistance, rP
Diode resistance resulting from HF rectification which acts
as a damping resistance to the pre-tuned demodulation
circuit.
Differential resistance
See forward resistance, differential
Diode capacitance, CD
Total capacitance between the diode terminals due to case,
junction and parasitic capacitances.
Revision: 16-Aug-11
Case capacitance, Ccase
Capacitance of a case without a semiconductor crystal.
Integration time, tav
With certain limitations, absolute maximum ratings given in
technical data sheets may be exceeded for a short time. The
mean value of current or voltage is decisive over a specified
time interval termed integration time. These mean values
over time interval, tav, should not exceed the absolute
maximum ratings.
Average rectified output current, IFAV
The average value of the forward current when using the
diode as a rectifier. The maximum allowable average
rectified output current depends on the peak value of the
applied reverse voltage during the time interval at which no
current is flowing. In the absolute maximum ratings, one or
both of the following are given:
• The maximum permissible average rectified output
current for zero diode voltage (reverse).
• The maximum permissible average rectified output
current for the maximum value of VRRM during the time
interval at which no current is flowing.
Note
• IFAV decreases with an increasing value of the reverse voltage
during the interval of no current flow.
Document Number: 84064
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APPLICATION NOTE
An application of the voltage current curve is given by
V
I = I S  exp ------ – 1
VT
where
I S = saturation current
kT
V T = ------------ = temperature potential
q
If the diode is forward-biased (anode positive with respect
to cathode), its forward current (I = IF) increases rapidly with
increasing voltage. That is, its resistance becomes very low.
If the diode is reverse-biased (anode negative with respect
to cathode), its reverse current (-I = IR) is extremely low. This
is only valid until the breakdown voltage VBR has been
reached. When the reverse voltage is slightly higher than the
breakdown voltage, a sharp rise in reverse current results.
Bulk resistance
Resistance of the bulk material between junction and the
diode terminals.
Forward resistance, differential rf
The differential resistance measured between the terminals
of a diode under specified conditions of measurement, i.e.,
for small-signal AC voltages or currents at a point of forward
direction V-I characteristic.
Application Note
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Vishay General Semiconductor
Physical Explanation
Rectification efficiency, r
The ratio of the DC load voltage to the peak input voltage of
an RF rectifier.
Series resistance, rs
The total value of resistance representing the bulk, contact
and lead resistance of a diode given in the equivalent circuit
diagram of variable capacitance diodes.
Junction capacitance, CJ
Capacitance due to a pn junction of a diode which
decreases with increasing reverse voltage.
Reverse voltage, VR
The voltage drop which results from the flow of reverse
current (through the semiconductor diode).
Reverse current, IR (leakage current)
The current which flows when reverse bias is applied to a
semiconductor junction.
Reverse resistance, RR
The quotient of the DC reverse voltage across a diode and
the corresponding DC reverse current.
Switching on Characteristic
Forward recovery time, tfr
The time required for the voltage to reach a specified value
(normally 110 % of the steady state forward voltage drop),
after instantaneous switching from zero or a specified
reverse voltage to a specified forward biased condition
(forward current).
This recovery time is especially noticeable when higher
currents are to be switched within a short time. The reason
is that the forward resistance during the turn-on time could
be higher than the DC current (inductive behavior). This can
result in the destruction of a diode because of high
instantaneous power loss if constant current control is used.
Turn on transient peak voltage, Vfp
The voltage peak (overshoot) after instantaneous switching
from zero or a specified reverse voltage to a specified
forward biased condition (forward current). The forward
recovery is very important especially when higher forward
currents must be switched on within a very short time
(switching on losses).
Reverse resistance, differential, rr
The differential resistance measured between the terminals
of a diode under specified condition of measurement i.e., for
small-signal (AC) voltage or currents at a point of
reverse-voltage direction V-I characteristic.
Pulse
Rx
VS
Peak forward current, IFRM
The maximum forward current with sine-wave operation,
f  25 Hz, or pulse operation, f  25 Hz, having a duty cycle
tp/T  0.5.
Ri
D.U.T.
APPLICATION NOTE
Peak surge forward current, IFSM
The maximum permissible surge current in a forward
direction having a specified waveform with a short specified
time interval (i.e., 10 ms) unless otherwise specified. It is not
an operating value. During frequent repetitions, there is a
possibility of change in the device’s characteristic.
Peak surge reverse voltage, VRSM
The maximum permissible surge voltage applied in a reverse
direction. It is not an operating value. During frequent
repetitions, there is a possibility of change in the device’s
characteristic.
Power dissipation, PV
An electrical power converted into heat. Unless otherwise
specified, this value is given in the data sheets under
absolute maximum ratings, with TA = 25 °C at a specified
distance from the case (both ends).
Revision: 16-Aug-11
VF
Fig. 2
Peak reverse voltage, VRRM
The maximum reverse voltage having an operating
frequency f  25 Hz for sine-wave as well as pulse operation.
IF
VF
VFP
110 %
100 %
t
tfr
Fig. 3
Switching off Characteristic, Inductive Load
Reverse recovery time, trr
The time required for the current to reach a specified reverse
current, iR (normally 0.25 % of IRM), after switching from a
specified forward current IF to a specified reverse biased
condition (reverse voltage VBatt) with a specified slope
dIF/dt.
Document Number: 84064
2
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ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay General Semiconductor
Physical Explanation
Peak reverse recovery current, IRM
The peak reverse current after switching from a specified
forward current IF to a specified reverse biased condition
(reverse voltage VR) with a specified switching slope dIF/dt.
The reverse recovery is very important especially when
switching from higher currents to high reverse voltage within
a very short time (switching off losses).
V
VBR
t
VS
I
IBR
D.U.T.
Pulse
t
Fig. 7
Fig. 4
Switching off Characteristic, Instantaneous Switching
V, I
Reverse recovery time, trr
The time required for the current to reach a specified reverse
current, iR (normally 0.25 A), after instantaneous switching
from a specified forward current IF (normally 0.5 A) to a
specified reverse current IR (normally 1.0 A).
IF
dIF/dt
trr
VF
iR
VBatt
t
IRM
47 Ω
Pulse
50 Ω
tIRM
Fig. 5
APPLICATION NOTE
Reverse recovery charge, Qrr
The charged stored within the diode when instantaneous
switched from a specified forward current IF (normally 0.5 A)
to a specified reverse current IR (normally 1.0 A).
Reverse avalanche energy, ER
The reverse avalanche energy when using the rectifier as a
freewheeling diode with an indicutive load. When the
inductance is switched off, the current through the
inductance will keep on flowing through the D.U.T. until the
stored energy,
1
2
E R = ---  L  I
2
is dissipated within the rectifier. Under this condition the
diode is in a reverse avalanche mode with a reverse current
at the beginning which is equal to the current that was
flowing through the inductance just before it was switched
off.
The reverse energy capability depends on the reverse
current and the junction temperature prior to the avalanche
mode.
D.U.T.
50 Ω
47 Ω
2Ω
1.5 kΩ
50 Ω
Oscilloscope
47 µF
- VS
Fig. 8
I
IF
trr
t
0
iR
Qrr
VS
IR
Pulse
D.U.T.
Fig. 9
Fig. 6
Revision: 16-Aug-11
Document Number: 84064
3
For technical questions within your region: [email protected], [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
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