Power supply for electrostatic apparatus

A power supply for electrostatic apparatus provides a high voltage output. The high voltage output is provided by the half wave rectification and filtering of a pulse signal from a secondary winding of a high voltage pulse transformer. The primary winding side of the high voltage pulse transformer is connected in a series loop circuit with a capacitor and a switching device. The capacitor is charged through an input choke connected to a DC supply source. The switching device is triggered after the capacitor is charged such that the capacitor is discharged through the primary winding of the high voltage transformer. The inductive collapse of the high voltage output transformer provides for the turnoff of the switching device and also serves to partially recharge the capacitor. The half wave rectified high voltage output of the power supply is obtained from the recovery pulse of the high voltage pulse transformer. The efficiency of the power supply is improved since ringing in the transformer is dampened due to the extraction of energy from the recovery pulse. The recovery pulse is much higher in amplitude than the firing pulse during which the switching device discharges the capacitor through the primary winding of the pulse transformer. During each repetitive period, the capacitor is charged, the switching device discharges the capacitor, and a short durational recovery pulse is generated. Several independent outputs of the power supply are obtained by providing respective, independent series combinations of transformers and capacitors with the series combinations being connected in parallel across the switching device.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The present invention relates generally to the field of power supplies for 
electrostatic apparatus and more particularly to a power supply that 
operates on a repetitive pulse basis. 
B. Description of the Prior Art 
Apparatus utilizing electrostatically charged components conventionally 
utilize a standard core and coil type high voltage transformer, and a 
solid state voltage doubler and filter. The transformer is large, heavy 
and rather expensive and the voltage doubler circuit with required 
components is also large and relatively costly. Further, the operation of 
this type of power supply circuit is usually of relatively low efficiency 
and of poor power factor correction. 
Although the electrostatic power supplies of the prior art are generally 
suitable for their intended use, it is always desirable to provide more 
cost effective and energy efficient power supplies. 
SUMMARY OF THE INVENTION 
Accordingly it is a principal object of the present invention to provide a 
power supply for electrostatic apparatus which is more efficient and more 
cost effective than power supplies of the prior art. 
It is another object of the present invention to provide a power supply for 
electrostatic apparatus that operates on a pulse repetitive basis having a 
relatively short durational pulse output relative to the period of the 
repetitive pulse, the repetitive pulse frequency being substantially 
higher than the operating frequency of conventional supplies. 
It is yet another object of the present invention to provide a power supply 
for electrostatic apparatus wherein a capacitor is discharged through the 
primary winding of a high voltage pulse transformer wherein the inductive 
collapse of the output transformer results in a recovery pulse that is 
half wave rectified and filtered to provide the output of the power supply 
at a secondary winding of the high voltage transformer. 
Briefly these and other objects of the present invention are achieved by 
providing a power supply for electrostatic apparatus that generates a high 
voltage output. The high voltage output is provided by the half wave 
rectification and filtering of a pulse signal from a secondary winding of 
a high voltage pulse transformer. The filtering for most electrostatic 
applications is primarily accomplished by the capacitance of the 
electrostatic apparatus. The primary winding side of the high voltage 
pulse transformer is connected in a series loop circuit with a capacitor 
and a switching device. The capacitor is charged through an input choke 
connected to a DC supply source. The DC supply source is commonly achieved 
by full wave rectification and filtering from a 120 or 240 volt AC power 
source. The switching device is triggered after the capacitor is charged 
such that the capacitor is discharged during a firing pulse through the 
primary winding of the high voltage transformer. The inductive collapse of 
the high voltage output transformer provides for the turnoff of the 
switching device and also serves to partially recharge the capacitor. The 
half wave rectified high voltage output of the power supply is obtained 
from the recovery pulse of the high voltage pulse transformer. The 
efficiency of the power supply is improved since ringing in the 
transformer is dampened due to the extraction of energy from the recovery 
pulse. The secondary recovery pulse is much higher in amplitude than the 
firing pulse. The power supply in one arrangement operates to generate 
repetitive pulses at a frequency of 1-3 KHz. During each repetitive 
period, the capacitor is charged, the switching device discharges the 
capacitor, and a short durational recovery pulse is generated, and the 
capacitor is partially recharged. Several independent outputs with 
independent output energy of the power supply are obtained by providing 
respective, independent series combinations of transformers and capacitors 
with the series combinations being connected in parallel across the 
switching device. Various outputs may be individually switched without 
affecting the output of the other independent outputs. The electrostatic 
apparatus operable by the power supply includes electrostatic air cleaners 
or filters and electrostatic charging apparatus in office copying 
equipment. 
In many electrostatic applications, the transformer does not require a 
magnetic core. 
The invention both as to its organization and method of operation together 
with further objects and advantages thereof will best be understood with 
reference to the following specification taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, the power supply 10 of the present invention is 
connected to an AC input source generally referred to at 12. The AC power 
source connection includes supply lines L1 and L2 and a neutral line N 
connected to ground potential at 14. The L1 supply line is connected 
through a series resistor 16 to one input 18 of a full wave rectification 
bridge 20. The L2 supply line is connected to a second input 22 of the 
full wave rectification bridge 20. The L2 supply line is also connected to 
the ground potential 14 through a filter capacitor 24. The bridge input 18 
is connected to the reference potential 14 through a filter capacitor 26. 
The positive DC output terminal 28 of the bridge 20 is connected through 
the series combination of an input choke 30, a primary winding 32 of a 
high voltage pulse transformer 34, and a capacitor 36 to the low DC 
reference output 38 of the bridge 20. A filter capacitor 40 is connected 
across the bridge outputs 28 and 38. 
An SCR 44 is connected across the series combination of the primary winding 
32 and the capacitor 36. A diode 46 is connected in parallel across and 
oppositely poled to the SCR 44. Thus the anode of the SCR 44 and the 
cathode of the diode 46 are connected to the junction 48 of the input 
choke 30 and the primary winding 32. Further the cathode of the SCR 44 and 
the anode of the diode 46 are connected to the low reference output 38. 
The gate lead 49 of the SCR 44 is connected to an output 50 of a 
triggering network generally referred to at 52. 
The triggering network 52 includes the series combination of a fixed 
resistor 54, a variable resistor 56 and a capacitor 58 connected between 
the circuit nodes 48 and 38. The triggering circuit includes a triggering 
device 60, a DIAC in one specific embodiment, that is connected between 
the output 50 of the triggering circuit and the junction of the resistor 
56 and the capacitor 58. Further a resistor 62 is connected between the 
output 50 of the triggering arrangement and the reference point 38. 
The high voltage pulse transformer 34 includes a secondary winding 64 that 
can be air coupled to the primary winding 32; the transformer 34 not 
requiring a ferromagnetic core in many low power applications. The 
polarity dots of FIG. 1 indicate the coil ends of the primary winding 32 
and the secondary winding 64 that are simultaneously at common potential. 
Thus when a positive voltage is present at the coil end 66 of the primary 
winding 32, a positive voltage is also present at the lower winding end 68 
of the secondary winding 64. 
The upper winding end 70 of the secondary winding 64 is connected through a 
diode 72 poled anode to cathode to an output 74 of the power supply 10. 
The coil winding end 68 of the secondary winding 64 is connected to the 
ground reference potential 14 and forms a ground reference output 76 of 
the power supply 10. A filter capacitor 78 is connected across the output 
terminals 74 and 76. 
It should be understood that in various other specific embodiments and 
applications, the output of the secondary winding 64 is utilized directly 
or through appropriate rectification to provide appropriate positive 
and/or negative output voltages. 
Considering an application of the power supply circuit 10 for an 
electrostatic air cleaner or filter, the output 74 is connected through a 
resistor 80 to the positive plate connections 82 of the electrostatic 
filter apparatus (not shown). The output 74 is also connected through a 
resistor 84 to the ion wires connection 86. The output 76 is connected to 
the ground plate connections of the electrostatic air cleaner. 
Conventionally, electrostatic air cleaners or filters include a number of 
ion wires at the air flow entrance of the air cleaner that are arranged 
transverse to the direction of air flow through the filter. The filter 
includes positively charged plates alternated with ground reference plates 
with the plates being arranged generally parallel to the direction of the 
air flow and disposed downstream of the ion wires. Thus air cleaners of 
this general type ionize particulate matter in the air flow with the 
plates causing the ionized particulate matter to be deposited on the 
plates of the air cleaner. Electrostatic air cleaners of this type exhibit 
across their grid input supply terminals a large capacitance at relatively 
high frequencies. A typical example of an electrostatic air cleaner 
includes a grid current of approximately 0.4 milliamps at 5000 volts to 
the ion wires and a plate current of approximately 3-4 microamps. 
In operation, the resistor 16 functions as a line fuse and can also 
function as a dropping resistor for 220 volt operation. The input choke 30 
is preferably approximately 200 millihenrys with any suitable resistance 
value. The triggering network of resistors 54, 56 and the capacitor 58 
determine the time constant for the trigger pulse for the SCR 44. In a 
suitable specific embodiment, circuit values of the triggering network 52 
have been found suitable to result in repetitive triggering of the SCR 44 
at a rate in the range of 1 KHz to 3 KHz. Resistor 62 functions to improve 
turnoff characteristics of the SCR 44. The relationship between the 
component values of the choke 30, the capacitor 36 and the primary winding 
32 as well as the coupling between the primary winding 32 and the 
secondary winding 64 are interrelated to provide appropriate operation of 
the present invention. Further, the component values of the circuit 10 
avoid the necessity of elaborate turnoff circuits for the SCR 44. 
With the SCR 44 in the off nonconducting state, the capacitor 36 is charged 
through the primary winding 32. When the SCR 44 is triggered by the 
triggering circuit 52, the capacitor 36 is discharged through the primary 
winding 32 and through the SCR 44. Referring now to FIG. 2, the waveform 
89 represents the signal versus time across the secondary winding 64 
referenced to winding end 68. The curve portion 90 represents the voltage 
waveform across the secondary winding 64 during the time that the 
capacitor 36 discharges through the transformer 32 and while the SCR 44 is 
conductive. The inductive collapse of the transformer 34 turns off the SCR 
44 at a time shortly after the curve point 92. Further, during the 
recovery pulse 94, energy from the transformer 34 is diverted through the 
fast recovery diode 46 to partially recharge the capacitor 36. The 
collapse recovery pulse 94 of the transformer 34 and the recharging of the 
capacitor 36 through the primary winding 32 and the input choke 30 
improves the efficiency of the circuit 10. The diode 46 also functions to 
protect the SCR 44 from excess reverse voltages allowing stable and safe 
operation under adverse conditions such as an open or shorted secondary 
winding 64. 
The recovery pulse 94 provides a large positive voltage pulse across the 
secondary winding 64 at winding end 70 with respect to winding end 68. The 
diode 72 half wave rectifies the output of the secondary winding 64 and 
charges the capacitor 78 and the grid network of the electrostatic air 
cleaner apparatus connected to at 82, 86. The DC output at 74 remains 
relatively constant between the repetitive pulses of relatively short 
duration due to the relatively high frequency of operation, the capacitor 
78, and the capacitance of the electrostatic apparatus. 
It should be understood that in various specific embodiments and 
applications, outputs from the secondary winding 64 can be provided 
including positive and/or negative voltages derived from the discharge 
pulse 90 and/or the recovery pulse 94. 
The resistor 84 functions to current limit the output at 74 under short 
circuit conditions such as are encountered when a large conductive 
particle or mass lodges between the plates of the electrostatic air 
cleaner apparatus. The resistor 80 functions to current limit the output 
86 and also to provide a voltage divider for the plate network connected 
at 82. In many forms of electrostatic air cleaner apparatus, the plate 
network connected across points 82 and 76 is operated at a lower potential 
than the ion wires at 86. In some forms of electrostatic air cleaners, the 
plate network may be directly connected to the ion output at the same 
potential. 
The waveform 89 of FIG. 2 at portion 96 shows the high frequency ringing of 
the circuit and the transformer 34 that occurs after the recovery pulse 
94. The ringing at 96 is dampened and greatly reduced with resultant 
improved efficiencies since the supply circuit 10 extracts energy for the 
output at 74 and the partial charging of the capacitor 36 from the 
recovery pulse curve portion 94. 
The characteristics of the high voltage pulse transformer 34 are important 
to achieve the low cost features and high efficiency of the power supply 
circuit 10. Thus for many applications of low power requirements, the high 
voltage transformer 34 does not utilize a magnetic core but instead 
provides air coupling of the primary winding 32 and the secondary winding 
64 and operates as a coreless pulse transformer. In the preferred 
embodiment, the primary and secondary windings have the same winding 
widths or traverse. Further additional interlayer insulation is provided 
to reduce interlayer capacitance to improve output voltage. Thus the 
transformer 34 of the power supply circuit 10 of the present invention 
with the noted characteristics results in operation of the circuit 10 in 
accordance with the waveform 89 of FIG. 2. 
Considering electrostatic apparatus for use with the power supply circuit 
10 in addition to electrostatic air precipitators, any apparatus that 
requires a high voltage electrostatic charge for operation at relatively 
high voltages and low currents may be suitably operated from the power 
supply circuit 10. For example in office copier machines, electrostatic 
arrangements are utilized for the charging and discharging of paper or 
other copying medium. The electrostatic charging arrangements of such 
office copiers can be appropriately supplied by the power supply circuit 
10 of the present invention. 
Referring now to FIG. 3 and considering an alternate embodiment of the 
power supply circuit of FIG. 1, the capacitor 36, the SCR 44 and the 
primary winding 32 of the high voltage transformer 34 are arranged in a 
series circuit loop as in FIG. 1. However, in FIG. 3 the capacitor 36 is 
connected between the junction 48 and the low DC supply reference 38 of 
the bridge 20. Further the series combination of the primary winding 32 
and the SCR 44 are connected between the junction point 48 and the low DC 
supply reference output 38. Operation of the alternate arrangement of FIG. 
3 is similar to that of FIG. 1 with triggering of the SCR causing the 
discharge of the capacitor 36 through the primary winding 32 of the 
transformer 34. 
Referring now to FIG. 4, the power supply circuit 10 is capable of being 
arranged to provide multiple independent outputs by the provision of 
additional parallel circuit branches including the series combination of a 
capacitor and a transformer. For example, in FIG. 4, the series 
combination of a capacitor 136 and a primary winding 132 of a transformer 
134 is connected in parallel with the SCR 44 and in parallel with the 
series combination of the primary winding 32 and the capacitor 36. The 
transformer 134 is similar in structure to the transformer 34. Thus, the 
additional output 174 of the power supply circuit may be obtained from the 
secondary winding 164 through a diode 172. Further, the outputs at 74 and 
174 may be independently controlled by the provision of a switch 140 in 
the series circuit branch with the capacitor 136 and the primary winding 
132. The operation of the switch controlling the on or off state of the 
output 174 does not have any appreciable effect on the output 74. It 
should be understood that any desired number of multiple outputs may be 
supplied in this manner. Further, it should also be understood that the 
various output branches allow independent selection of component values of 
the capacitors 36, 136 and transformers 34, 134 to provide desired output 
levels at the independent outputs. 
While there has been illustrated and described several embodiments of the 
present invention, it will be apparent that various changes and 
modifications thereof will occur to those skilled in the art. For example, 
by variations of the resistance of component 56, the output voltage may be 
suitably varied as desired. Further, in order to stabilize the output 
voltage for variations in input supply voltages, a zener diode is added 
having a cathode connected to the junction of resistors 54 and 56 and an 
anode connected to the reference 38 to provide a stable reference voltage 
to the triggering arrangement 52. Although the power supply circuit of the 
present invention is generally intended for electrostatic applications, it 
should also be understood that the present invention is also applicable to 
general power supply applications where power requirements are small. 
It is intended in the appended claims to cover all such changes and 
modifications as fall within the true spirit and scope of the present 
invention.