Active circuit filter for reducing conducted radiation from a load back to its power supply

An embodiment of the present invention comprises a series pass transistor with its emitter arranged to receive a source of DC power, its collector arranged to supply a DC load, and its base connected to the junction of a capacitor to ground and an inductor to the collector. An output capacitor from the collector to ground is alternatively included. In a second embodiment of the present invention, an operational amplifier is inserted in the base circuit to increase the effective circuit gain.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to power supplies and more specifically to 
active circuits used as filters for controlling conducted radiation, such 
as the coupling of noise generated by switching power supplies back into 
their sources of power. 
2. Description of the Prior Art 
A common technique for distributing electrical power around a vehicle, 
vessel and even a building involves stringing an unregulated power supply 
bus around to all the points that will consume power and then to place a 
voltage regulator at each of the points of consumption. Each individual 
voltage regulator can then supply an appropriate voltage to its respective 
load. 
Switching power supplies have become popular because they offer high 
efficiency and can produce voltages above or opposite to the incoming 
power source voltage. However, switching power supplies can generate noise 
that is both radiated and directly coupled to its inputs and outputs. With 
higher power levels, the generated noise can be very energetic and 
difficult to control. The radiated noise can typically be controlled with 
proper shielding and the conducted noise is conventionally filtered out. 
Without adequate filtering, an unregulated common power source can couple 
noise between equipment loads and cause erratic operation. Highly 
sophisticated gear tends to be the most susceptible, in particular to 
their highly sensitive radio receivers, such as used in navigation 
instrumentation. 
Capacitors and inductors, and even capacitor and resistor combinations have 
long been used in pi-network and T-network configurations to construct 
filters. However, capacitors and inductors large enough to control noise 
in distributed-point regulation configurations can be bulky and expensive. 
Conventional filters have been designed to filter out AC to DC 
rectification chop to provide a smoothed voltage to a load downstream of 
the power supply. An unusual situation is encountered in distributed-point 
regulation configurations because the smoothing is required upstream of 
the load, in particular, upstream of the switching power supply and its 
associated load. This requirement is opposite to what has been the 
conventional circuit design problem. 
SUMMARY OF THE PRESENT INVENTION 
It is therefore an object of the present invention to provide a filter for 
the control of conducted noise from a switching power supply back to its 
source of power. 
Briefly, an embodiment of the present invention comprises a series pass 
transistor with its emitter arranged to receive a source of DC power, its 
collector arranged to supply a DC load, and its base connected to the 
junction of a capacitor to ground and an inductor to the collector. An 
output capacitor from the collector to ground is alternatively included. 
In a second embodiment of the present invention, an operational amplifier 
is inserted in the base circuit to increase the overall circuit gain. 
An advantage of the present invention is that a filter is provided that is 
effective in controlling noise. 
Another advantage of the present invention is that a filter is provided 
that is economical to manufacture. 
A further advantage of the present invention is that a filter is provided 
that is small in bulk. 
A still further advantage of the present invention is that a circuit for a 
filter is provided that can be easily integrated into a semiconductor chip 
that includes a switching regulator integrated circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a DC power supply filter embodiment of the present 
invention, referred to herein by the general reference numeral 10. Power 
supply filter 10 comprises a series pass PNP type bipolar transistor 12 
with an emitter 14 arranged to receive a positive connection of a DC power 
source 16, a collector 18 arranged to supply a DC load 20, and a base 22 
connected to a junction 24 of a capacitor 26 connected to ground and an 
inductor 28 connected to the collector 18. An output capacitor 29 from the 
collector to ground is alternatively included. DC power source 16 will 
typically deliver an unregulated voltage in the range of 9-36 volts and 
will additionally supply other loads that need to be protected by power 
supply filter 10 from noise generated in load 20. Good results have been 
obtained when capacitor 26 was selected to be a 4.7 microfarad (MFD), 
fifty volt electrolytic, and inductor 28 was chosen to be 1000 
microhenries (.mu.H). Transistor 12 may be a bipolar junction type 2N2907. 
The current through transistor 12 ranged between 50-200 milliamperes (mA). 
The DC power source may be reversed, as in FIG. 2, to provide a negative 
supply terminal input. In such a case, an NPN type bipolar transistor 12' 
would be substituted for the transistor 12 of FIG. 1. 
In operation, DC bias supplied to base 22 will normally keep transistor 12 
turned on and in its active region. Inductor 28 will present a high 
impedance to high frequency noise coming from load 20 and will tend to 
resist turning transistor 12 on harder for positive swings and to resist 
lessening current through transistor 12 for negative swings of the noise. 
Capacitor 26 acts in concert with inductor 28 by presenting a low 
impedance to ground for high frequency noise at the base 22 and a high 
impedance for DC. 
Noise coming in from DC power supply 16 will couple through from emitter 14 
to collector 18 because transistor 12 presents itself in a common-base 
configuration and therefore has little or no effect in filtering out noise 
from upstream. This is the opposite of a conventional active filter which 
would reverse the collector and emitter connections and use an NPN type 
transistor for positive power supply inputs. 
FIG. 3 illustrates a cascode filter embodiment of the present invention, 
referred to by the general reference numeral 30. Filter 30 is similar to 
filter 10 and further includes a transistor 32 with an emitter 33, a 
collector 34 and a base 35. A diode 36 provides bias for transistor 12. 
In another embodiment of the present invention, as illustrated in FIG. 4, a 
DC power supply 40 includes a pair of inductors 42 and 44 in series, a 
resistor 45, a series of input capacitors 46, 48 and 50 connected to 
ground, a transistor 52, an operational amplifier (op-amp) 54 inserted in 
the base circuit of transistor 52 to increase the overall circuit gain, an 
inductor 56 in series connection with a capacitor 58, a pair of output 
capacitors 60 and 62, an output inductor 64 and a diode 66. Inductors 42 
and 44, and capacitors 46, 48 and 50 provide input filtering. Op-amp 54 
improves the effective gain of transistor 52 and can improve the response 
to noise coming from a load 68. Since op-amps have limited high frequency 
performance compared to a single transistor, a high-frequency response 
type op-amp may be appropriate for use as op-amp 54 in particular 
applications. Good results have been obtained when capacitor 50 was 
comprised of two 4.7 MFD capacitors rated for fifty volts, capacitor 58 
was a single 4.7 MFD capacitor also rated at fifty volts, and transistor 
52 as a type 2N2907. Op-amp 54 may be omitted in certain high frequency 
applications or those not needing the higher gain. 
Transistor 52 may also be a field effect transistor (FET) type, shown as 
transistor 52' in FIG. 5, with appropriate connections for the source, 
drain and gate that respectively match connections for emitter, collector 
and base of transistor 52 in FIG. 4. Inductor 56 may be substituted by a 
resistor 70, as shown in FIG. 5. 
In general, any active device having a control input may be connected 
between a power supply input terminal and output terminal such that a load 
current passes through the active device and can be regulated by it. A 
passive filter network, such as a capacitor-inductor combination or a 
capacitor-resistor combination, is connected to the control input and 
between the input and output terminals. In operation, the relatively 
higher frequencies of load current will be attenuated more than relatively 
lower frequencies. Therefore, the conducted radiation from the load to the 
power supply will be reduced or completely eliminated. 
Although the present invention has been described in terms of the presently 
preferred embodiment, it is to be understood that the disclosure is not to 
be interpreted as limiting. Various alterations and modifications will no 
doubt become apparent to those skilled in the art after having read the 
above disclosure. Accordingly, it is intended that the appended claims be 
interpreted as covering all alterations and modifications as fall within 
the true spirit and scope of the invention.