Voltage equalizer bridge

A diode equalizing bridge wherein one pair of opposed diodes are forward-biased and a second pair of opposed diodes are reverse-biased so that the two capacitors at the opposed load terminals are charged and discharged by a current source and current sink connected to two opposed power terminals until the capacitors have equal voltages. The current source and sink are gated to activate and deactivate the bridge. For an AC power source, two diode bridges are used, one connected to each terminal of the capacitor to operate during the respective half cycle of the AC power source.

BACKGROUND OF THE INVENTION 
The present invention relates to voltage equalizer circuits for capacitors. 
In analog controls circuits, a variable, which is monitored, may be stored 
as a voltage on a capacitor. In certain applications, it may be desirable 
to balance the voltages stored on at least two or more capacitors. For 
example, in a motor control circuit, having three phases of current, the 
variable stored on the capacitors may be equalized during a portion of the 
control cycle. This equalizing of voltage would provide a common starting 
voltage from which the charge of voltage on the capacitors would be varied 
during the remainder of the monitor or measurement cycle. 
With the advent of the monolithic controllers for induction motors, it is 
desirable that the equalizer include as few as possible elements and 
preferably operate off the alternating current source used to drive the 
motor. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a monolithic equalizer for 
capacitors. 
Another object of the present invention is to provide an equalizer for 
capacitors which can operate with an alternating current source. 
An even further object of the present invention is to provide an equalizer 
for capacitors which may be selectively operable during a given portion of 
the cycle of an alternating current supply. 
These and other objects of the invention are obtained by an equalizer 
including a diode bridge having opposed power terminals and opposed load 
terminals. A first pair of opposed diodes of the bridge being 
forward-biased and a second pair of opposed diodes of the bridge being 
reverse-biased when the voltage of the capacitor of the first load 
terminal is less than the voltage of the capacitor at the second load 
terminal and power is applied to the power teminals. This charges the 
capacitor at the first load terminal and discharges capacitor at the 
second load terminal until the voltages are equal. When the voltage of the 
capacitor of the first load terminal is more than the voltage of the 
capacitor of the second load terminal, the first pair of diodes are 
reverse-biased and the second pair of diodes are forward-biased so as to 
charge the capacitor at the second load terminal and discharge the 
capacitor at the first load terminal until the voltages are equal. 
Switches or switchable current source and sinks are provided between the 
power terminal of the bridge and the power source for permitting 
equalization when activated and for preventing equalization and preventing 
discharging of the capacitors when deactivated. 
The bridge may be a pair of bridges, one connected to each of the terminals 
of the capacitor to provide separate control of the positive and negative 
terminals of the capacitor during the positive and negative portions of an 
AC power source. The control of the current sources and sinks for each 
half of cycle is controlled by phase to voltage converters. In a three 
phase motor, the phase feedback from the motor is used in the input of the 
phase to voltage converters. The voltage on the capacitor is used to 
control the driver/switching circuit for one coil of a three phase motor. 
Other objects, advantages and novel features of the present invention will 
become evident upon review of the detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A voltage equalizer 10 is illustrated in FIG. 1 and includes a diode bridge 
having diodes 12, 14, 16 and 18. The bridge includes a pair of opposed 
power terminals 20 and 22 and a pair of opposed load terminals 28 and 30. 
The anode of diode 12 is connected to terminal 20 and its cathode is 
connected to terminal 28. The anode of diode 14 is connected to terminal 
28 and its cathode is connected to terminal 22. The anode of diode 16 is 
connected to terminal 20 and its cathode connected to terminal 30. The 
anode of diode 18 is connected to terminal 30 and its cathode connected to 
terminal 22. 
A current source 24 is connected to power terminal 20 and a current sink 26 
is connected to power terminal 22. A first capacitor 32 is connected to 
load terminal 28 and a second capacitor 34 is connected to load terminal 
30. The current source 24 includes a control gate 25 and the current sink 
26 includes a control gate 27. As will be explained fully below, this 
allows the current source and sink to be activated and deactivated to 
control the operation of the equalizer 10. 
To fully understand the equalizer of FIG. 1, its operation will now be 
described. With power applied to power terminals 20 and 22 by current 
source 24 and current sink 26 and when the voltage or charge on capacitors 
32 and 34 are connected to load terminals 28 and 30, respectively, are 
equal, all the diodes 12, 14, 16 and 18 are forward-biased. The current 
from source 24 is sunk by current sink 26 and the capacitors 32 and 34 
maintain their equal charge. 
If equalization is not desired, the current source 24 and current sink 26 
are deactivated leaving the power terminals 20 and 22 floating such that 
the diode bridge is inactive and the charge or voltage on the capacitors 
32 and 34 are unaffected by the equalizer 10. Without the control of the 
connection of the power terminals 20 and 22 to the power source, a path 
may be provided to ground through power terminal 22 which would discharge 
the capacitors 32 and 34 or a path to the power terminal 20 which could 
create a charging of the capacitors 32 and 34. Thus, by providing 
switchable current source 24 and current sink 27, the operation of the 
equalizer may be controlled and the capacitors connected to the load 
terminals are unaffected when the equalizer is inactive. It should be 
noted that the capacitors 32 and 34 are storage devices having a voltage 
stored thereon representative of a monitored variable in the circuit to be 
controlled. Thus, the voltage equalizer 10 can be used to equalize the 
voltage on two capacitors at a controlled point in time at any control 
system by the use of the gated sources and sinks 24 and 26. 
When the voltage on capacitor 32 is greater than the voltage on capacitor 
34, diodes 12 and 18 become reverse-biased. Thus, the current source 24 
drives or charges the capacitor 34 through diodes 16 and the current sink 
26 sinks charge or voltage from capacitor 32 through diode 14. The diodes 
12 and 18 remain reverse-biased until the voltage on capacitors 32 and 34 
become equal. At this point, diodes 12 and 18 again become forward-biased 
and the current from current source 24 is sunk totally by the current sink 
26. 
When the voltage on capacitor 32 is less than the voltage on capacitor 34, 
diodes 14 and 16 become reverse-biased. The current source 24 charges the 
capacitor 32 through diode 12 and the current sink 26 discharges the 
voltage on capacitor 34 through diode 18. When the charge or voltage on 
capacitor 32 equals the voltage or charge on capacitor 34, diodes 14 and 
16 are again forward-biased and the total current from current source 24 
is sunk by current sink 26. 
The use of a diode equalizing bridge to equalize the voltage on three 
capacitors, each of which represents a variable monitored in a three phase 
motor control, is illustrated in FIG. 2. A pair of bridges 10 and 110 are 
shown. It should be noted that the elements in bridge 110 which have the 
same function as those in bridge 10 have the same unit and tens digit. The 
three capacitors which are to be balanced are illustrated as capacitor 32 
connected to load terminal 28 of bridge 10, capacitor 34 connected to load 
terminal 30 of equalizing bridge 10 and load terminal 128 of equalizing 
bridge 110 of equalizing bridge 110, and capacitor 136 connected to the 
load terminal 130 of equalizing bridge 110. The bridge, as illustrated, 
will simultaneously balance the three capacitors. Alternatively, by using 
the gates 25, 125 on current sources 24, 124 and the gates 27, 127 on 
current sinks 26, 126, the equalizing bridges 100, 110 may be operated 
separately. 
By way of example, assume that capacitor 32 has a smaller voltage than 
capacitor 34 and capacitor 35 has a smaller voltage than capacitor 36. For 
this assumed condition, diodes 14 and 16 of bridge 10 and diodes 114 and 
116 of bridge 110 are reverse-biased. Thus, capacitor 32 is charged by 
current source 24 via diode 12 and capacitor 136 is discharged by current 
sink 125 via diode 118. Since diode 18 of bridge 10 is conducting and 
diode 112 of bridge 110 are conducting, the current from current source 
124 is sunk by current sink 26. Thus, the charge on capacitor 34 is not 
initially affected. 
If the difference between capacitors 32 and 34 and between 34 and 36 are 
equal, the charge on capacitor 34 is not affected at all. If the 
difference between the voltage of capacitors 32 and 34 is greater than the 
difference between capacitors 34 and 36, then the discharge on capacitor 
36 will cause capacitor 36 to have a voltage equal to capacitor 34 before 
the charging of capacitor 32 would reach the voltage on capacitor 34. At 
this point, when the charge on capacitors 34 and 36 are equal, diodes 114 
and 116 on bridge 110 are conducting and the current source 124 is being 
sunk by the current sink 126. This will cause the charge on capacitor 34 
to be sunk by current sink 26 via diode 18. This decrease in voltage on 
capacitor 34 will again reverse-bias diodes 114 and 116 causing the charge 
to be further decreased on capacitor 36. This process will continue until 
the charge on capacitors 32, 34, and 36 are equal. Thus, it can be seen 
that only two bridges are needed to perfectly match the voltage on three 
capacitors. 
The application of the equalizer of the present invention using an 
alternating source of voltage is illustrated in FIG. 3. Although the 
circuit of FIG. 3 is specifically for a motor control, it should be noted 
it can be used for equalizing capacitors to be used in any sort of control 
or measurement circuit. The capacitor 232 has its positive electrode and 
its negative electrode connected to a motor control by lines 244 and 246 
respectively. The equalizer circuit includes two separate diode bridges 
210 and 211 connected respectively to the positive and negative terminals 
of capacitor 232. The positive terminal of the capacitor 232 is connected 
to a load terminal of bridge 210 and the negative terminal of capacitor 
232 is connected to a load terminal of bridge 211. 
Each of the diode bridges includes a current source 224 and 224' and 
current current sinks 226 and 226'. The current sink 226 and current 
source 224' connect one power terminal of their respective bridges to a 
common AC return or ground. The current sink 224 connects the other power 
terminal of bridge 210 to the positive AC voltage +VAC while the current 
sink 226' connects the other power terminal of bridge 211 to the negative 
AC voltage -VAC. Current source 224 and current sink 226, illustrated as 
transitors, are gated on and off by the positive phase to voltage 
converter 238 and power source 224' and current sink 226' are gated on and 
off by the negative phase to voltage converter 240. Line 242 provides the 
phase feedback from the motor as an input to the positive and negative 
phase to voltage converters 238 and 240 respectively. 
A switch 266, illustrated as transitor, connects the positive terminal of 
capacitor 232 to the ground reference and is controlled by the negative 
phase to voltage converter 240. A switch 268 illustrated as a transitor, 
connects the negative terminal of capacitor 232 to the ground and is 
controlled by the output of the positive phase to voltage converter 238. 
The switch 266 is turned on during the negative phase to bring the 
positive terminal of the capacitor 232 to ground while the negative 
terminal of the capacitor is being equalized. Similarly, the switch 268 is 
turned on during the positive phase to bring the negative terminal of 
capacitor 232 up to ground while the positive terminal to be equalized. 
The current sources and sinks 224, 244' and 226 and 226' as well as the 
positive and negative grounding switches 266 and 268 of FIG. 3 have not 
been shown in FIG. 4 for sake of clarity and can be considered to be part 
of the equalizer and the phase to voltage converters' circuits 
respectively. 
Line 248 connects the other load terminal of bridge 210 to the positive 
equalization buss and line 250 connects the other load terminal of bridge 
211 to the negative equalization buss. The positive phase to voltage 
converter 238 gates on the power sources and sinks 224 and 226 
respectively during the positive half cycle to allow equalization of the 
positive terminal of capacitor 232 with other capacitors connected to the 
positive equalization buss via line 248. The negative phase to voltage 
converter 240 gates on current sources and sink 224' and 226' respectively 
during the negative half cycle to equalize the negative terminal of 
capacitor 232 with the other capacitors connected to the negative 
equalization buss via line 250. 
The equalizer circuit of FIG. 3 can be applied to a three phase motor as 
illustrated in FIG. 4. If the three capacitors 232, 332 and 432 do not 
have some form of equalization, each capacitor in each phase leg of the 
controller will contain a voltage representing the winding current phase 
and an error voltage resulting from component mismatch and motor phase 
inbalance. If the phase voltage variation among the capacitors remain 
uncorrected, they will cause unsymmetrical firing of the triac resulting 
in unbalanced motor drive. The equalizer circuit as illustrated in FIG. 4 
senses these variations and forces the three capacitors to operate at a 
voltage resulting in symmetrical phasing of three triacs thereby causing 
the motors to operate in a balanced manner. 
A three phase motor is illustrated in FIG. 4 as including coils 264, 364 
and 464 representing phases A, B, and C respectively. Each of the phases 
are connected to a three phase AC power line via a switchable device 
illustrated as triac 262, 362 and 462 respectively. The operation of the 
triacs are controlled by a switch 260, 360 and 460 respectively which are 
illustrated as an optical coupler or isolation switches. The isolation 
switches and couplers 260, 360 and 460 are driven respectively by a 
positive and negative comparators 252, 352, 452 and 254, 354 and 454 
respectively. The positive comparators 252, 352 and 452 receive a voltage 
input from the positive terminal of the capacitors via line 244, 344 and 
444 and have a second input from the positive ramp generators 256, 356 and 
456 respectively. The negative comparators 254, 354 and 454 receive an 
input from the negative terminal of the capacitors via lines 246, 346 and 
446 and have a second input from the negative ramp generators 258, 358 and 
458 respectively. These provide the control signals for the optocoupler or 
isolation switches 260, 360 and 460 respectively. 
The capacitors are connected to the positive and negative equalization buss 
over lines 248, 348, 448 and 250, 350, 450 via positive and negative 
equalizer 210, 310, 410 and 211, 311, 411 respectively. The positive 
equalizer 210, 310 and 410 are controlled by positive phase to voltage 
converters 238, 338 and 438 respectively and the negative phase equalizers 
211, 311 and 411 are controlled by negative phase to voltage converters 
240, 340 and 440 respectively. The positive and negative phase to voltage 
converters are activated and controlled by motor phase feedbacks 242, 342 
and 442 from their respective motor coils 264, 364, 464. 
The three phase circuit of FIG. 4 is basically that of FIG. 3 applied to 
three separate capacitors. The operation of the equalizers or diode 
bridges in FIGS. 3 and 4 is the same as that described for FIGS. 1 and 2 
wherein pairs of opposed diodes are forward-biased to provide or take away 
current from a respective capacitor until the voltage in all the 
capacitors are equal. The capacitors in the configuration of FIG. 4 are 
connected by two diode bridges. 
As is evident from the description of the preferred embodiments, the 
objects of the invention are attained in that a alternating current 
operated voltage equalizer is provided. Although the invention has been 
described and illustrated in detail, it is clearly understood that the 
same is by way of illustration and example only and is not to be taken by 
way of limitation. The spirit and scope of the present invention are to be 
limited only by the terms of the appended claims.