Download Choke-Input Filter

NEWMAR 115 – 12 – 20AU
Full-Wave Rectifier with Choke-Input Filter
Choke-Input Filter
L
100mH
C
25000uF
Rload
5 Ohm
DC Operation
Rinductor
+
Vdc
-
0.1 Ohm
+
V’
dc
-
V’dc = Vdc(Rinductor) / (Rinductor + Rload)
Rload
5 Ohm
AC Operation (approximate)
Rinductor
jXinductor
100mH
0.1 Ohm
jXcapacitor
25000uF
Normally, make Xcapacitor << Rload
Rload
5 Ohm
Approximate the Circuit
jXinductor
100mH
+
+
Vr
V’r
-
-
jXcapacitor
25000uF
Usually, Xinductor >> Xcapacitor
V’[rms] = (Xcapacitor / Xinductor)(Vr[rms])
Full-Wave Rectifier Output
Output frequency = twice the line frequency
2Vm
4Vm
v

cos 2t  ....

3
4Vm
 0.425Vm
Second Harmonic
3
(first ac component)
'
r [ rms ]
V

X capacitor
X inductor
Vr [ rms ]
1
'
Vr [ rms ]  2C [0.707][0.425Vm ]
2 L
7
(0.707)(0.425Vm ) 5.29 10
'
Vr [ rms ] 

Vm
2
4 LC
LC
0.529
'
Vr [ rms ] 
Vm
LC
2π(120)
L   Henrys
C    farads
ripplefactor  r
r
Vr'[ rms ]
'
dc
V
0.529
Vm
0.83
LC


2
LC
Vm

“Critical” Inductance, Lc
Must keep the load current flowing for the
complete cycle of the input voltage
The dc load current must be > the peak value of
the ac component of the load current
2Vm
4Vm

 Rload 3 X inductor
2Vm
4Vm

 Rload 3 Lc
2 Rload
Rload
Lc 

3(2 )(120) 1130
Rload
Minimum Inductance
Lc 
Lc
1000
“Bleeder” Resistor
• The bleeder resistor provides 2 functions
– keeps a minimum current flowing through the
inductor
– discharges the filter capacitor after turn-off
• Rbleeder = R1 = the largest value of Rload
– sets the maximum value of inductor L
Range of values for the Filter Choke
• NEWMAR 115 – 12 – 20AU Power Supply
• Lmax > R1/1000 = 5/1000 = 5mH  50mH
• Lc >= R1/1000 = (12 V/20 A)/1000 = 0.6mH