The Power Diode
In the previous tutorials we saw that a semiconductor
signal diode will only conduct current in one direction
from its anode to its cathode (forward direction), but not in the
reverse direction acting a bit like an electrical one way valve.
A widely used application of this feature is in the conversion of an
alternating voltage ( AC )
into a continuous voltage ( DC ). In other words, Rectification.
But small signal diodes can also be used as rectifiers in low-power, low current (less than 1-amp)
rectifiers or applications, but were larger forward bias currents or higher reverse bias blocking voltages are involved
the PN junction of a small signal diode would eventually overheat and melt so larger more robust Power Diodes
are used instead.
The power semiconductor diode, known simply as the Power Diode,
has a much larger PN junction
area compared to its smaller signal diode cousin, resulting in a high
forward current capability of up to several hundred amps (KA)
and a reverse blocking voltage of up to several thousand volts (KV).
Since the power diode has a large PN junction, it is
not suitable for high frequency applications above 1MHz, but special and
expensive high frequency, high current diodes are
available. For high frequency rectifier applications Schottky Diodes are generally used because of their short reverse
recovery time and low voltage drop in their forward bias condition.
Power diodes provide uncontrolled rectification of
power and are used in applications such as battery charging
and DC power supplies as well as AC rectifiers and inverters. Due to
their high current and voltage characteristics they can also
be used as freewheeling diodes and snubber networks. Power diodes are
designed to have a forward "ON" resistance of fractions of
an Ohm while their reverse blocking resistance is in the mega-Ohms
range. Some of the larger value power diodes are designed to
be "stud mounted" onto heatsinks reducing their thermal resistance to
between 0.1 to 1oC/Watt.
If an alternating voltage is applied across a power
diode, during the positive half cycle the diode will conduct
passing current and during the negative half cycle the diode will not
conduct blocking the flow of current. Then conduction through
the power diode only occurs during the positive half cycle and is
therefore unidirectional i.e. DC as shown.
Power Diode Rectifier
Power diodes can be used individually as above or
connected together to produce a variety of rectifier
circuits such as "Half-Wave", "Full-Wave" or as "Bridge Rectifiers".
Each type of rectifier circuit can be classed as either
uncontrolled, half-controlled or fully controlled were an uncontrolled
rectifier uses only power diodes, a fully controlled rectifier
uses thyristors (SCRs) and a half controlled rectifier is a mixture of
both diodes and thyristors.
The most commonly used individual power diode for basic electronics applications is the general purpose
1N400x Series Glass Passivated type rectifying diode with standard ratings of continuous forward
rectified current of 1.0 amp and reverse blocking voltage ratings from 50v for the 1N4001 up to 1000v for the 1N4007, with
the small 1N4007GP being the most popular for general purpose mains voltage rectification.
Half Wave Rectification
A rectifier is a circuit which converts the Alternating Current (AC) input power into a Direct Current
(DC) output power. The input power supply may be either a single-phase
or a multi-phase supply with the simplest of all the rectifier
circuits being that of the Half Wave Rectifier. The
power diode in a half wave rectifier circuit passes just one half
of each complete sine wave of the AC supply in order to convert it into a
DC supply. Then this type of circuit is called a "half-wave"
rectifier because it passes only half of the incoming AC power supply as
shown below.
Half Wave Rectifier Circuit
During each "positive" half cycle of the AC sine wave, the diode is forward biased as the anode is positive with respect to the cathode resulting in current flowing through the diode. Since the DC load is resistive (resistor, R), the current flowing in the load resistor is therefore proportional to the voltage (Ohm´s Law), and the voltage across the load resistor will therefore be the same as the supply voltage, Vs (minus Vf), that is the "DC" voltage across the load is sinusoidal for the first half cycle only so Vout = Vs.
During each "negative" half cycle of the AC sinusoidal input waveform, the diode is reverse biased
as the anode is negative with respect to the cathode. Therefore, NO current flows through the diode or circuit. Then in the
negative half cycle of the supply, no current flows in the load resistor as no voltage appears across it so
therefore, Vout = 0.
The current on the DC side of the circuit flows in one direction only making the circuit
Unidirectional. As the load resistor receives from the diode a positive half of the waveform, zero volts, a positive
half of the waveform, zero volts, etc, the value of this irregular voltage would be equal in value to an equivalent DC voltage
of 0.318 x Vmax of the input sinusoidal waveform or 0.45 x Vrms of the input sinusoidal waveform. Then the equivalent DC
voltage, VDC across the load resistor is calculated as follows.
Where Vmax is the maximum or peak voltage value of the AC sinusoidal supply, and VS
is the RMS (Root Mean Squared) value of the supply.
Example No1
Calculate the voltage across, (VDC) and the current, (IDC)
flowing through a 100Ω resistor connected to a 240 Vrms single phase half-wave rectifier as
shown above. Also calculate the DC power consumed by the load.
During the rectification process the resultant output DC voltage and current are therefore both "ON" and "OFF"
during every cycle. As the voltage across the load resistor is only present during the positive half of the cycle (50% of the
input waveform), this results in a low average DC value being supplied to the load. The variation of the rectified output
waveform between this ON and OFF condition produces a waveform which has large amounts of "ripple" which is an undesirable
feature. The resultant DC ripple has a frequency that is equal to that of the AC supply frequency.
Very often when rectifying an alternating voltage we wish to produce a "steady" and continuous DC voltage free
from any voltage variations or ripple. One way of doing this is to connect a large value
Capacitor across the output voltage terminals
in parallel with the load resistor as shown below. This type of capacitor is known commonly as a "Reservoir" or Smoothing Capacitor.
Half-wave Rectifier with Smoothing Capacitor
When rectification is used to provide a direct voltage power supply from an alternating source, the amount
of ripple can be further reduced by using larger value capacitors but there are limits both on cost and size. For a given
capacitor value, a greater load current (smaller load resistor) will discharge the capacitor more quickly
( RC Time Constant ) and so increases
the ripple obtained. Then for single phase, half-wave rectifier circuits it is not very practical to try and reduce the
ripple voltage by capacitor smoothing alone, it is more practical to use "Full-wave Rectification" instead.
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