Power factor and harmonic correction circuit including ac startup circuit

A circuit for correcting power factor and for reducing ac line current harmonic components in conjunction with bridge-connected capacitive-filtered loads and for providing a source of ac startup current for associated circuitry, for example a switch mode power supply which may form part of the load. The circuit comprises a rectifier bridge, a dual-tuned circuit having a tuned filter in series between an ac line input and the bridge, and a shunt capacitor across the input to the bridge. The tuned filter comprises a parallel connected inductor and capacitor, the shunt capacitor being tuned with the inductor to form a trap filter to reduce third-order and higher harmonic components in the ac line current. The circuit also includes a current source operatively connected to the shunt capacitor for providing a source of ac current immediately upon application of ac power to the ac line input.

FIELD OF THE INVENTION 
The present invention relates to power supply circuits, and in particular 
relates to a passive circuit which corrects line current harmonics and 
power factor and provides startup power for associated circuitry, such as 
control circuitry for a switch-mode power supply. 
BACKGROUND OF THE INVENTION 
Conversion of ac power (such as from ac lines from utilities or generators 
and alternators) to dc power has typically been done by using a diode 
bridge rectifier and a filter capacitor connected across the rectifier 
output in parallel with a load. A typical single-phase ac-to-dc diode 
bridge rectifier circuit is shown in FIG. 1. In FIG. 1, the rectifier 10 
is fed by a single-phase ac line input 12 which is connected across the ac 
input terminals of a diode bridge 14. The rectified dc output from diode 
bridge 14 is fed to a load, modeled in FIG. 1 as a resistor R.sub.L. A 
filter capacitor C.sub.o is connected across the load to smooth the dc 
current to the load. Parasitic line inductance associated with this type 
of circuit is shown as a lumped inductor L.sub.i. 
The circuit shown in FIG. 1 has the disadvantage of generating pulsed ac 
line currents drawn from the ac lines. The non-ideal character of the 
input currents from the ac lines creates several problems for the power 
distribution network and for other apparatus in the vicinity of the 
rectifier. Among the problems are high input-current harmonic components, 
low rectifier efficiency because of the large rms value of the input 
current, input ac line voltage distortion because of the associated peak 
currents, and a maximum input power factor much, much less than unity 
(e.g., 0.5 to 0.75), due primarily to the presence of a third harmonic 
component of the ac frequency of considerable amplitude. 
A modified rectifier circuit to increase power factor and reduce harmonics 
has been proposed as shown in FIG. 2. In FIG. 2, rectifier circuit 16 is 
fed by a single-phase ac line input 18 which is connected across the ac 
input terminals of a diode bridge 20. The rectified dc output from diode 
bridge 20 is fed to a load, modeled in FIG. 2 as a resistor R.sub.L. A 
filter capacitor C.sub.o is connected across the load to smooth the dc 
current to the load. An input filter circuit 22 is located in series with 
one leg of the ac line input 18. Input filter circuit 22 consists of an 
inductor L.sub.r and a capacitor C.sub.r connected in parallel. 
The circuit shown in FIG. 2 is stated to have advantages over the circuit 
of FIG. 1. Input peak current is said to be lower, reducing input voltage 
distortion. Power factor is said to be increased somewhat, to 0.887. 
Efficiency is said to be increased because of the low rms values of the 
input current. However, the circuit of FIG. 2 is admittedly more complex 
in operation than the circuit of FIG. 1. 
Other proposed solutions to improve power factor and reduce line current 
harmonics have taken the approach of using passive circuits to correct 
line current harmonics and separate passive circuits to increase power 
factor, but this approach has not yielded acceptable performance levels. 
Still other proposed solutions have taken the form of active circuits. 
These active circuits do work well, but require a large number of 
components, and are not very rugged in terms of line transient 
susceptibility and damage from overload or short circuit. 
The present invention overcomes the drawbacks of the prior solutions, and 
uses a passive circuit which reduces line current harmonics and increases 
power factor using only three passive components. The circuit can be tuned 
to reduce total line current harmonics or total harmonic distortion (THD) 
to below 5%, while increasing power factor to greater than 0.95. The 
present invention also provides start up power for other circuitry, such 
as a switch-mode power supply which may form part of the load R.sub.L, 
without the need for additional active or passive components. The present 
invention significantly increases power factor, reduces THD, is rugged and 
reliable, and is simple and inexpensive to construct. 
Other objects and advantages of the invention will appear hereinafter. 
SUMMARY OF THE INVENTION 
The present invention is a circuit for correcting power factor and reducing 
ac line current harmonics in conjunction with bridge-connected 
capacitive-filtered loads and for providing a source of ac startup current 
for associated circuitry, for example a switch-mode power supply which may 
form part of the load. The circuit comprises a rectifier bridge, a 
dual-tuned circuit having a tuned filter means in series between an ac 
line input and the bridge, and a shunt capacitor across the input to the 
bridge. The tuned filter comprises a parallel-connected inductor and 
capacitor, the shunt capacitor being tuned with the inductor to form a 
trap filter means to reduce third-order and higher harmonics in the ac 
line current. The circuit also includes current source means operatively 
connected to the shunt capacitor for providing a source of ac startup 
current immediately upon application of ac power to the ac line input.

DESCRIPTION OF THE INVENTION 
Referring now to the drawings, wherein like numerals indicate like 
elements, there is shown in FIG. 3 a circuit 24 in accordance with the 
present invention. Circuit 24 comprises a pair of ac line inputs 26 for 
connecting circuit 24 to a source of single-phase ac power, such as a 
generator or alternator, or such as ac mains from a distribution system. 
Circuit 24 comprises a conventional diode bridge 28 to rectify ac input 
current to dc output current. The dc output of bridge 28 supplies a load 
30, which for the present may be considered to be any circuit or device 
that consumes power, and may be resistive, reactive or both. A filter 
capacitor C.sub.o is connected across the dc output of bridge 28 in 
parallel with load 30 to smooth out ripples in the dc output current from 
the bridge, in known manner. 
In one leg of the ac line inputs 26 there is located a tuned filter means 
32. Tuned filter means 32 comprises a capacitor C.sub.1 connected in 
parallel with an inductor L.sub.1. Inductor L.sub.1 provides power factor 
correction for the bridge-coupled capacitive load comprising both filter 
capacitor C.sub.o and load 30. Preferably, but not necessarily, the 
impedance of the inductor is approximately 0.125 times the impedance of 
the bridge-coupled capacitive load at the frequency of operation, i.e., at 
the ac line frequency. 
Capacitor C.sub.1 and inductor L.sub.1 are tuned to form a trap filter to 
reduce third harmonic components of the ac line frequency. Preferably, but 
not necessarily, C.sub.1 and L.sub.1 are tuned to approximately 1.2 times 
the third harmonic of the line frequency. At this trap frequency, 
sufficient damping of the third harmonic is achieved and higher order 
harmonics are more effectively damped. 
A second capacitor C.sub.2 is connected across the ac input terminals of 
bridge 28. Shunt capacitor C.sub.2 and inductor L.sub.1 together form a 
second trap filter for reducing higher order (i.e., greater than third 
order) harmonics. By tuning capacitor C.sub.2 with inductor L.sub.1, 
higher order harmonics can be significantly reduced. 
Those skilled in the art will appreciate that, by tuning components 
L.sub.1, C.sub.1 and C.sub.2, power factor and harmonics can be 
controlled. By properly tuning these components, total line current 
harmonics (THD) can be reduced to below 5%, while power factor can be 
increased to greater than 0.95. Thus, it will be appreciated that the 
present invention offers significant improvements over prior rectifier 
circuits which exhibit much lower power factors and greater THD. 
Those skilled in the art will also appreciate that the circuit of the 
present invention is useful in any bridge-coupled capacitively-filtered 
power supply circuit. This includes any type of circuit which requires a 
dc bus derived from an ac power line. 
One type of power supply circuit in which the circuit of the present 
invention is especially useful is a switch-mode power supply. As those 
skilled in the art will understand, a switch-mode power supply converts ac 
power at a line frequency to rectified dc power which in turn is converted 
to ac power, usually at a frequency much higher than the ac line input 
frequency. A switch-mode power supply typically employs a plurality of 
switching devices which are operated by switch control signals generated 
by control circuitry. When used with a switch-mode power supply, capacitor 
C2 provides a source of ac current which can be applied to the control 
circuitry of the switch-mode power supply to start up the power supply. 
All switch-mode power supplies require some sort of startup circuitry to 
get the power supply running when power is first applied from the ac line. 
As shown in FIG. 4, which is based on a diagram on page 222 of the 
Unitrode Applications Handbook 1987-1988, a typical switch-mode power 
supply may consist of a bridge rectifier 102 connected to ac line input 
terminals 120. The dc output of bridge rectifier 102 is connected to 
filter capacitor 104. Integrated circuit oscillator 112 controls the 
switching of transistor switch 116 which is coupled to a primary winding 
of output transformer 118. Integrated circuit 112 can be a Unitrode 
integrated circuit type UC 3842. 
When power is first applied to ac line input terminals 120, storage 
capacitor 108 is charged up through power resistor 106 and provides power 
to integrated circuit 112. After the power supply has started, a voltage 
is developed across auxiliary winding W.sub.aux of output transformer 118 
and thus provides power to integrated circuit 112 via steering diode 110. 
This saves on total power dissipated in the converter because the 
auxiliary winding can be sized exactly for the control circuitry being 
powered. Because most control circuitry operates on relatively low voltage 
(10-40 volts), and because the input voltages are in the range of 100-300 
Vac, a substantial amount of power (1-2 watts) will be dissipated in the 
start up power resistor if it is not disconnected after the circuit begins 
running. To disconnect the startup power resistor, additional circuitry is 
required. From the standpoint of cost, reliability and space (i.e., 
component volume), it would be desirable not to have this added circuitry. 
The present invention makes it possible to eliminate a start up power 
resistor and, consequently, other circuitry for disconnecting the start up 
resistor after the system is running. 
Referring now to FIG. 5, the present invention is shown in somewhat greater 
detail than in FIG. 3, and the start up feature of the invention is more 
clearly illustrated. In FIG. 5, the same reference numerals and 
designations indicate the same parts as in FIG. 3, and therefore those 
parts need not be described again. With the circuit as shown in FIG. 5, as 
soon as power is applied to the circuit 24 of the invention, an ac 
circulating current begins to flow through capacitor C.sub.2. A pair of 
steering diodes CR.sub.1 and CR.sub.2 are connected anti-parallel and are 
located in series with shunt capacitor C.sub.2 to steer this current 
through a capacitor 34 in the switch-mode power supply, which forms part 
of load 30, as described above. This circulating current charges capacitor 
34 up to the low voltage bus defined by Zener diode 36. When capacitor 34 
is charged to the level of the low voltage bus, current will begin to flow 
through resistor 38 which is not a power resistor to the low voltage 
control circuitry of the switch-mode power supply (such as the supply 
terminal of integrated circuit 112 in FIG. 4), and will start up the 
switch-mode supply. 
Since the circulating current used to start up the switch mode power supply 
is derived without the use of resistors, there is almost no power loss 
incurred by harnessing this current. This novel circuit therefore 
eliminates the need for a power resistor and active components needed to 
disconnect it after the circuit begins to run. This, in turn, yields a 
circuit which is smaller, less expensive, more reliable, and more 
efficient. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification, as indicating the scope of the invention.