Download surge-currrent-filters

Surge Current in the Capacitor filter
• Initially the filter capacitor is uncharged.
• At the instant the switch is closed, voltage is connected
to the bridge and the capacitor appears as a short circuit.
• This produces an initial surge of current Isurge through
the two forward-biased diodes. The worst-case situation
occurs when the switch is closed at a peak of the
secondary voltage and a maximum surge current, is
produced, as illustrated in the figure.
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• It is possible that the surge current could destroy the
diodes, hence a surge-limiting resistor is sometimes
connected,
The value of this resistor must be small compared to
RL. Also, the diodes must have a maximum forward
surge current rating such that they can withstand the
momentary surge of current. This rating is specified on
diode data sheets as IFSM. The minimum surge resistor
value can be calculated as follows :
V
 1.4
Rsurge 
p (sec)
I FSM
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The LC Fitter
• When an inductor is added to the filter additional
reduction in the ripple voltage is achieved. The inductor
has a high reactance at the 120 Hz ripple frequency, and
the capacitive reactance is low compared to both XL and
RL. The two reactances form an ac voltage divider that
tends to significantly reduce the ripple voltage from that
of a straight capacitor filter
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Vr ( out)

X
C


 X L  XC
VDC (OUT )

Vr ( in)


 RL 
VDC ( IN )
 
 Rw  RL 
• Where RW is the
winding resistance of
the inductor
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Example
• A 120 Hz full-wave rectified voltage with a peak
value of 162 V is applied to the LC filter shown.
Determine the filter output in terms of its dc
value and the rms ripple voltage given that the
rms ripple of an unfiltered full-wave rectified
signal is calculated as Vr=0.308 Vp
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Diode Clipper Circuits
• These circuits clip off portions of signal voltages
above or below certain limits, i.e. the circuits limit
the range of the output signal.
• Such a circuit may be used to protect the input
of a CMOS logic gate against static.
• Many examples can be found in transmitters and
receivers in TV or radar or equipment control
and protection circuits
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• There are two general categories of clippers: series
and parallel. The series configuration is defined as
one where the diode is in series with the load, while
parallel variety has the diode in a branch parallel to
the load
• Series clippers:
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Series clipper with bias voltage
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Example
Determine the output waveform for the network
shown
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Solution
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Parallel Diode Clipping circuit
Vout
 RL 
Vin
 
 R1  RL 
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Example
• What would you expect to see displayed
on an oscilloscope connected across RL
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Solution
V p ( out)
 RL 
V p (in)  9.09V
 
 R1  RL 
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Biased Limiters
• The positive limiter can be modified to limit
the output voltage to the portion of the
input voltage waveform above VBIAS - 0.7
V.
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• Similarly, the negative limiter can be
modified to limit the output voltage to the
portion of the input voltage waveform
below - VBIAS + 0.7 V
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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
VBIAS
 R3 
VSUPPLY  8.25 V
 
 R2  R3 
The positive part of the output voltage waveform is limited to VBIAS + 0.7 V
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