Abstract:
A power controller having a plurality of power output modules which can be added or removed as desired to change the power output rating. The electrical output of the individual power modules are removably attached to an output combining structure. The electrical connections to the output combining structure also provide mechanical support for the power output modules. The servo controller is constructed so that as additional power modules are added, each provides a predetermined proportion of the output power. That is, each module will add in a linear fashion to the total output capability of the amplifier. Load sharing among the modules is provided. An etched copper resistor having a positive resistance versus temperature characteristic is provided to aid load sharing among the output semiconductors. A feedback signal is derived from the power output modules to protect the servo controller. The protection of the power controller is one function of the feedback signal, in that, it limits the maximum current derivable from the controller under normal conditions. The primary function of the current feedback signal is to provide an error signal which when combined with the input command causes the current output to be properly metered to a load. The feedback signal can be obtained from only one of the output modules since load sharing is provided or can be obtained in several or all of the power output modules as desired. Temperature sensing of a representative module is provided for system protection.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is related to a power processing system and more particularly to a power controller wherein the power output capacity can be varied by adding or removing individual power output modules. 
     2. Description of the Prior Art 
     Prior art servo controllers, power amplifiers, and power regulators are normally designed to provide a given output capacity. Once constructed it is difficult, if not impossible, to increase the output rating of prior art devices without major disassembly or redesign. 
     In some prior art power controllers, it is necessary to have a feedback signal related to current to provide adequate protection and control of the device. Some prior art devices have output sections configured as a bridge in which the current feedback signal is derived from a shunt disposed in series with the load at the bridge output. This generally provides a signal having poor form factor with little usable information because of the presence of high common-mode voltages at the shunt. It has been discovered and recognized that by placing two sampling resistors, one in each leg of the bridge circuit at the common connection point, a superior current feedback signal can be derived using a differential amplifier. 
     SUMMARY OF THE INVENTION 
     A power controller is provided wherein the power output capacity of the controller can be varied, in increments, by adding modules as required to meet the load. The disclosed controller utilizes identical power modules which can be easily attached or removed. Each module contains power semiconductors for supplying controlled amounts of output current at a rated voltage. Additional power modules can be attached to a common mechanical and electrical structure so that the addition of each module will add in a linear fashion to the total output capability of the power processor. That is, each module supplies a proportional amount of the total output current. Each module is constructed to assure that the sharing of the load is within certain limits, such as 10%. When the term servo controller, power controller, power regulator, power amplifier or power processor is used herein, it is intended to be inclusive of the other designated terms. 
     Current feedback is provided for control and protection of the disclosed power processor. With the positive load sharing provided, it is feasible to provide a current feedback signal from only one module, knowing the other modules cannot vary from this amount by more than a predetermined amount. This current feedback signal represents, within the limits of load sharing, current in any of the other modules not sampled. Hence, with proper scaling, a signal representative of total load current can be derived from the signal obtained from the sample module. In addition, the temperature of the sample module from which the feedback signal is derived can also be utilized for over temperature monitoring. 
     In another embodiment of the invention, the current feedback signal can be derived from a composite signal of the current in all of the power modules. Each power module can be provided with a sampling resistor, the output of which is combined to form a composite current feedback signal. 
     In another embodiment, the output of the power controller is connected in a bridge configuration. Individual power modules are utilized for forming each side of the bridge. Thus, each side of the bridge can be formed from a plurality of power output modules. Power output modules on each side of the bridge are constructed to provide for load sharing. 
     It is an object of this invention to teach a power processor constructed to accommodate a plurality of power output modules which can be easily added or removed to vary the output capacity. 
     It is another object of this invention to teach a power controller having a plurality of power output modules wherein load sharing among the modules is present and a feedback signal representative of current flowing through each module is derived from only one of the modules. 
     It is yet another object of this invention to teach a power controller having a plurality of output modules wherein an etched copper resistor having a positive temperature coefficient is utilized in each module to enhance load sharing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference may be had to the preferred embodiments exemplary of the invention shown in the accompanying drawings in which: 
     FIG. 1 is a block diagram of a servo controller utilizing the teaching of the present invention showing a current feedback signal derived from one module; 
     FIG. 2 is a block diagram of a servo controller utilizing another embodiment of the present invention wherein a current feedback signal is derived from each module; 
     FIG. 3 is a top view of a portion of the power output section of a servo controller, connected in a bridge configuration, showing the power modules; 
     FIG. 4 is a side view of a portion of the power output section shown in FIG. 3; 
     FIG. 5 is a circuit schematic of the first output power module which also includes the driver circuit for the other output power modules; 
     FIG. 6 is a schematic of a power output module without the driver circuit; 
     FIG. 7 is a view of a servo controller constructed according to the teaching of the present invention with the power output modules connected in a bridge configuration; 
     FIGS. 8A, 8B, 8C and 8D show a detailed circuit of a servo controller shown in FIG. 7 utilizing the teachings of the present invention; 
     FIG. 9 shows a complete output power and driver module mounted to the associated heat sink; 
     FIG. 10 is a top view of the output and driver module shown in FIG. 9; 
     FIG. 11 shows a complete output module with the associated heat sink; 
     FIG. 12 is a top view of the output module shown in FIG. 11; and 
     FIG. 13 is a block diagram of a servo controller connected with a bridge output showing a current feedback signal derived from shunt resistors in each leg of the bridge. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and FIG. 1 in particular, there is shown a servo controller utilizing the teachings of the present invention. Servo controller 10 is provided with an input 12 and an output 14 which can feed various loads. Input 12 is connected to control circuit 16 which feeds predriver circuits 18. The output of predriver circuits 18 feeds a driver and power module 20. Driver and power module 20 provides the driver signal to drive additional modules 22 when required. The output of the driver and power module 20 and the connected power modules 22 are combined by output combining means 24. Output combining means 24 provides mechanical support for the output and driver module 20 and the power output modules 22 through the electrical connections to the associated modules. Each module 20 or 22 is constructed to provide a proportional share of the power output of servo controller 10. A current feedback signal is provided by current feedback circuits 26 to control operation of servo controller 10. The feedback signal through current feedback circuits 26 is derived only from the power output and driver module 20. Since load sharing among various modules 20 or 22 is provided, the feedback signal through current feedback circuits 26 is representative of the current flow through the other power modules within the limits of the load sharing. The temperature rise of driver module 20 can be monitored and used for protection of associated apparatus and this should also be representative of the maximum temperature of other modules 22. By virtue of the sharing circuits in modules 20 or 22, it is known that current in any other module cannot vary by more than 10% from the proportional share. The capacity of servo controller circuit 10 can be increased by adding additional power modules 22 as desired. By sampling feedback current from only one module 20, circuit simplification is obtained. 
     Referring now to FIG. 2, there is shown another servo controller circuit 100 utilizing a different embodiment of the present invention. Servo controller 100 is provided with control circuit 16, current feedback circuits 26, predriver circuits 18, output and driver modules 20, output modules 22, and output combining means 24 as described above. Servo controller 100 additionally includes a feedback combining means which samples a current feedback signal from each module 20 or 22. Feedback combining means 28 can form either a composite feedback signal of all the current outputs or can provide a signal representative of the highest current supplied by any module 20 or 22. Power modules 22 can be added or subtracted to vary the output capacity of servo controller 100. The output combining means also provides a common mechanical structure for supporting the various modules 20 or 22. The output of each module 20 or 22 is combined by the output combining means in an output summation which yields a composite output which is generally referred to as the amplifier output. The feedback signal can include both voltage and current and other parameters which may be important for various amplifier constructions. 
     Referring now to FIGS. 3 and 4, there is shown the output combining means 24 and a plurality of modules 20 or 22 mounted thereto. As can be seen best in FIG. 4, output combining means 24 includes spaced apart conducting rails 30, 31 and 32 to which the electrical output connection of modules 20 or 22 are attached. These electrically conducting rails 30, 31 and 32 also provide mechanical support for the modules 20 or 22. Driver module 20 includes a driver portion and a power module portion which is similar to that contained in power module 22. The output of combining means 24 is fed through inductors L1 or L2 which limit the rate of change of output current to provide some system protection. 
     Referring now to FIG. 6, there is shown a power output module 22. Power output module 22 includes a PNP transistor Q1 and an NPN transistor Q2 which are complementary power transistors that act as power switches conducting a proportional share of the total controller 10 output current. Resistors R1 and R5 are emitter resistors that act to force the transistors Q1 and Q2 to conduct a proportional amount of the output current. That is, when a plurality of power output modules 22 are connected in parallel, resistors R1 and R5 assure that current sharing among the various output modules will be fairly even. The value of R1 and R5 are selected to assure that the load sharing among the various modules will be within 10%. Resistor R1 is an etched copper resistor formed integral with the printed circuit boards on which the components are mounted. Resistor R1 has a positive temperature coefficient which enhances load sharing. R2 and R3 are base-emitter resistors that act to bypass leakage current around the associated transistor in such a manner as to prevent false conduction or premature voltage breakdown. Resistors R2 and R3 are especially important at elevated operating temperatures. Diodes D1 and D2 are free-wheeling diodes that act to conduct inductive load currents during periods of time when Q1 and Q2 are turned off. That is, when load current is flowing through either transistor Q1 or Q2 and they are switched off, inductance in the circuit will prevent the current from instantaneously being driven to zero and to prevent damage to the circuit, it will be shunted through one of the free-wheeling diodes D1 or D2. R4 is part of a composite current limiting resistor for the predriver transistors Q26 or Q37 shown in FIG. 8B. Resistor R4 acts in effective parallel with other similar resistors in other parallel modules 22 to limit current from predriver transistors Q26 or Q37 to a safe value. Terminal 40 is provided for making a positive connection to a positive voltage bus 30 or line 111. Terminal 42 is provided for making a connection to the output bus 31 of combining means 24, from which the load connections are taken. Terminal 44 is provided for making a connection to the common circuit bus 32 or line 112. Each power module is provided with three input connections 46, 48 and 50. Each input connection 46, 48 or 50 is provided with two connection points for easy connection to adjacent modules. A driver signal can be provided on connectors 46 and 48 for transistors Q1 or Q2, respectively. Connector 50 completes a circuit to resistor R4 which limits current from driver transistors Q37 or Q26, when used in a bridge connection as shown in FIG. 8B. Capacitor C1 is an integral capacitor that acts to smooth the voltage on the DC bus at each module. 
     Referring now to FIG. 5, there is shown a circuit for a combined power output and driver module 20. Power and driver module 20 includes a power output circuit 22 as shown in FIG. 6 and described in detail above. Power output and driver module 20 also includes a temperature sensing portion, a driver portion, and a fault sensing portion. Transistors Q3 and Q4 are NPN/PNP complementary transistors acting as driver transistors for transistors Q1 and Q2 of the integral output circuit 22 and any other connected power module circuits 22. Resistors R7 and R8 are base-emitter resistors associated with transistors Q3 and Q4, respectively, for conducting leakage current around the associated devices. Resistors R3 and R4 prevent false conduction and premature voltage breakdown, especially at elevated device temperatures. Diode D4 is a fast recovery-type diode that acts to prevent the conduction of transistor Q3 when transistor Q4 is conducting, and conversely to prevent transistor Q4 from conducting when transistor Q3 is conducting. Output connections 46, 48 and 50 are provided, each having two terminals to facilitate connection to adjacent modules 20 or 22. Driver transistors Q3 and Q4 drive all transistors Q1 and Q2 in the parallel output modules which are connected. A temperature sensing switch T1 is provided on circuit 20 in proximity to the primary temperature generating elements which are transistors Q1, Q2, Q3 and Q4. Thermal switch T1 is part of the output driver module and acts to monitor the temperature of this module. When temperature switch T1 operates, the associated power processor is disabled. Power and driver module 20 is provided with a ten terminal connector J1 having output terminal connections J11 through J20. J19 and J20 provide connections to thermal switch T1. J11 provides a connection to the positive bus from the printed circuit board. J12 provides a connection to the overvoltage protection portion of module 20. J13, J15 and J17 provide connections to module 20 from the predriver circuit. Connectors J16 and J18 provide connections for the current feedback signal. 
     In FIG. 8B, resistor R134 provides current limiting for predriver transistor Q30 and resistor R112 provides current limiting for predriver transistor Q19. Resistor R4, rather than using a current limiting resistor such as R134 or R112, is placed in each module 20 or 22 to limit the current through predriver transistor Q26 or Q37. Resistor R4 also functions to evenly distribute the current from the predriver transistor Q26 or Q37 to each module 20 or 22 through each modules sensing resistor R5. An alternate to this construction would be to use a resistor such as R134 connected to the collector of transistor Q37 and also tie the emitter of transistor Q37 to the common line 112. This could by-pass a substantial amount of current around current sampling resistor R5. With the addition of R4, the current which flowed through transistor Q37 is limited and all current flowing through Q37 proportionately flows through each sampling resistor R5. 
     Driver module circuit 20 is also provided with a fault sensing portion consisting of diode D3 and resistor R6. As current flows through resistor R1, line 110 becomes negative with respect to positive line 111, due to the current flow through resistor R1. Current flows through network D3 and R6. When this becomes sufficient, due to the current flow through R1, an over current fault circuit located on the controller main printed circuit board is activated. This circuit acts to disable the servo controller when the current flow exceeds a predetermined overload value. R6 is also used to combine the signal sensed from R1 with similar signals from the output driver module connected on the opposite side of the bridge. Diode D3 acts to compensate for temperature caused changes in the resistance of R1 which is an etched foil resistor. 
     Resistor R1, which is formed from copper, has a positive temperature coefficient which improves current sharing module to module. The concept of readily removable power output modules requires good load sharing to be viable. A load sharing resistor having a positive temperature coefficient provides a means for reducing the power output of each module should the ambient temperature increase or the cooling air flow decrease. The load sharing resistor R1 is formed directly on the PC-board and placed in the air flow close to the semiconductors being controlled. The etched copper resistor R1 regulates current flow through the associated module, 20 or 22, as a function of temperature. 
     The beneficial aspects of the etched copper resistor can best be understood by considering the following analysis. The bases of all the transistors, Q1 or Q2, through which load sharing is desired are tied together. Thus, the voltage, V base, on each base is the same. Considering a constant potential applied to each base, the following equation defines the approximate behavior of the transistor, Q1 or Q2. 
     
         (v.sub.b - v.sub.be /r) =0 i.sub.l + i.sub.b               (1) 
    
     where, 
     V BE  is the base to emitter voltage 
     V B  is the base voltage 
     I L  is the load current 
     I B  is base current Since (I L  /I B ) = H FE  of the transistor, equation (1) can be rewritten as ##EQU1## 
     Since H FE  is usually greater than 10, and more typically 15 or 20, the contribution of 1/H FE  is relatively small, and I L  can be approximated as 
     
         I.sub.L ≃ (V.sub.B - V.sub.BE /R)            (4) 
    
     the temperature rise of resistor R1 is related to the amount of power it dissipates. Since the actual load current and transistor base current pass through the resistor R1, the dissipated power P R1  can be expressed as ##EQU2## By differentiating equation (3) with respect to R, the rate of change in load current, I L , as a function of change in R can be determined. ##EQU3## or 
     
         dI.sub.L = [-I.sub.L ] [(1/R] dR 8 
    
     since 
     
         dI.sub.L ≃ εI, and 
    
     
         dR ≃ ΔR ##EQU4## thus the load current will change by a ratio that is equal to the ratio change in the resistance, R. 
    
     The temperature T R  of the resistor, R, can be expressed as ##EQU5## Note that due to the squared term, of I L , the temperature of the resistor is greatly affected by variations in I L . If the material utilized in the construction of resistor, R, possesses a positive temperature coefficient, such as copper, steel or the like, the change in resistance as a function of temperature can be expressed as 
     
         ΔR/R = K ΔT                                    (12) 
    
     in the case of copper, which can be directly formed on the printed circuit board, by well known photographic etching techniques, the change in resistance as a function of temperature can be expressed as 
     
         ΔR/R = K.sub.1 ΔT                              (13) 
    
     where K 1  ≃ 0.00393 
     T is temperature in degrees celsius The use of a positive temperature coefficient material for resistor R1 provides improved regulation of the load current through a given transistor. If the ambient temperature increases, R1 also increases in value reducing I L . This provides improved load sharing. Furthermore, since most power amplifying devices employ forced air cooling, the resistor, R1, can also be cooled by this same air flow. Should the air flow be reduced, due to low voltage, dirty filters, or the like, the control resistor, R1, will increase in value and reduce the current from the amplifier to a safe level. Load sharing resistors, R1 and R5, also provide for load sharing among the paralleled diodes D1 and D2. With inductive loads when transsitor Q1 or Q2 shuts off, current will momentarily continue to flow through the appropriate diode D1 or D2. By virtue of load sharing, each parallel diode D1 or D2 carries a proportional amount of current. If there was not load sharing among the diodes D1 or D2, diodes having a large current rating in parallel with all the modules 20 or 22 would be required. 
     Referring now to FIG. 7, there is shown a more detailed block diagram of a servo controller 11 having the output of modules 20 or 22 connected in a bridge configuration. The servo controller 11 shown in FIG. 7 includes a power bridge output section 60, a current feedback section 62, delay and drive sections 64 and 65, a synchronization and comparator section 66, a preamplifier 68, output inductors L1 and L2, thermo switches THS1 and THS2, fault sensing section 70, amplifier disable circuitry 72, and a bias supply 74. Input terminals T1, T2 and T3 are also provided for making electrical connections to the servo controller 11. A fan 76 is provided for cooling the power semiconductors utilized in controller 11. A fuse 78 provides protection for the fan 76 and the bias supply circuit 74 of controller 11. 
     Preamplifier 68 which has input connections to terminals T1, T2 and T3 amplifies and combines various input signals including a tachometer feedback signal, for presentation to the power sections of controller 11. Preamplifier 68 filters and scales the various input signals and amplifies the combined error signal. Preamplifier 68 processes the combined error signal through a servo compensation network and also limits the maximum value of positive and negative voltage signals at its output so as to limit the ultimate current flow through the power output section and load. Preamplifier 68 has appropriate adjustment potentiometers to accomplish the desired compensation, voltage limiting, and scaling functions. 
     The synchronization and comparator section 66 includes a synchronization oscillator 80 and a comparator network 82. Oscillator 80 provides a synchronization signal in the form of a triangular wave shape. This triangular signal acts to modulate the signal obtained from the preamplifier section, along line 81, to form a third signal which is fed to comparator section 82. The combined signal fed along line 81 to comparator section 82 is combined algebraicly with the current feedback signal from current feedback sections 62. The total combined signal acts to modulate the switching point of comparator section 82. The output of comparator section 82 is fed to delay and driver sections 64 and 65. Section 66 supplies identical digital signals to delay and driver sections 64 and 65. 
     The output of delay and driver section 64 and 65 are in turn fed to the input of power bridge 60. Delay and driver sections 64 and 65 are supplied with identical digital signals from comparator 82. Delay and driver sections 64 and 65 are the same except for an additional inverter circuit at the input to delay and driver 65. The function of the inverter in delay and driver 65 is to supply a logic negative signal to that delay and driver section 65. The function of the delay and driver section is to delay by a controlled amount the positive, ON signal; but not delay the negative, OFF signal as these signals pass through associated delay and driver sections. In addition to ON delay sections 64 and 65 provide the required circuitry to provide the appropriately phased and amplified signals to the output bridge section 60. 
     The power bridge output section 60 comprises two output and driver modules 20 which are connected in a bridge output configuration. Up to seven output modules 22 can be connected in parallel with each output and driver module 20. Thus, each side of the power bridge 60 has one output and driver module 20 and up to seven output modules 22 connected in parallel. Current from the associated output circuits 20 flows through combining means 41 which comprises mechanical supports and current collectors 30, 31 and 32. The modules 22 or 20 are mounted to a mechanical structure, part of which consists of aluminum current collector rails 30, 31 and 32 which acts as the combining means 41 for the modules. The modules 20 and 22 are configured electrically and mounted mechanically so as to form a bridge or H output power section 60. 
     The bridge output connection has advantages over alternate type connections. The bridge output section has desirable features not possessed by the alternate bi-directional type of output section, the so-called single ended amplifier. For power transistors with the same voltage capability the bridge configuration provides twice the load voltage magnitude obtainable with a single ended amplifier. A disadvantage with the single ended switching type amplifier is that a voltage magnitude increase occurs in one of the two power supplied in the single ended amplifier when it provides a constant DC current to an inductive load. The voltage increases if not controlled, will eventually cause breakdown of the power transistors and/or associated components. The bridge type configuration prevents this voltage magnitude increase by providing for regenative feedback from the load. 
     The output from combining means 41 is processed through output inductors L1 and L2. These output inductors L1 and L2 have relatively small values, just being large enough in inductance to limit the rate of rise, di/dt, in the load circuit in the event of a short circuit to the common connection 112. The delayed current rise allows sufficient lag time for the over-current protection circuit to operate and disable the controller before significant damage occurs. This precaution prevents catostrophic failures due to short circuits across the load of from either output to ground or common. 
     Thermal switches THS1 and THS2 provide thermal protection for controller 11. These devices, THS1 and THS2, provide a contact opening signal in the event of an over-temperature condition. Fan 76 provides ambient air flow for cooling of the power semiconductors. If, for example, the cooling fan fails or the ambient temperature increases beyond a predetermined point, thermal switches THS1 and THS2 can disable servo controller 11. The sensor for each thermal switch is connected in associated output and driver modules 20. These modules 20 are mounted the furthest from the cooling fan 76 and hence should be the warmest. By providing thermal protection for these modules 20, the other output modules 22 are also protected. 
     Current feedback section 62 provides a current feedback signal to synchronization section 66. The current feedback signal provided by section 62 can be obtained from the power bridge section 60 by any of the methods described herein. 
     Amplifier disable circuitry 72 is also provided for disabling controller 11 due to a malfunction. Disable circuitry 72 disables the servo controller 11 in response to an externally applied signal from terminal T9, T11 or T13. Amplifier disable circuitry 72 is also connected to be activated by fault sensing circuitry 70. Fault sensing circuitry 70 is connected to power bridge 60 and supplies a disable signal to circuitry 72 upon the occurrence of an overload current flow through power bridge 60. Controller 11 current can be limited in either direction of motor rotation. 
     A bias supply 74 is provided to provide a low power bias supply that provides regulated positive and negative 15 volt DC for the low level controller circuit. The bias supply also supplies an unregulated positive 24 volts DC for part of the delay and driver circuit sections 64 and 65. 
     Referring now to FIGS. 8A, 8B, 8C and 8D there is shown a detailed schematic of a servo controller 11 utilizing the teaching of the present invention. FIG. 8A shows schematically two output and driver sections 22 connected in a power bridge configuration. Each output and driver section 22 has an output section 20 connected in parallel therewith. The output of these sections 20 and 22 are supplied through inductors L1 and L2 to outputs 113 and 114 which is connected to a load which is usually a DC type motor. 
     FIG. 8B shows schematically the connection of delay and drive sections 64 and 65 to power bridge 60. FIG. 8C details schematically the current feedback section 62. The function of circuit 62 is to differentially sample the current signal at each side of the bridge using operational amplifier OA3 for one side and operational amplifier OA2 for the other side. These signals are subsequently combined differentially in operational amplifier OA4. The double differential scheme eliminates common mode voltages from the current feedback signal which would otherwise distort the signal. Because of the differential combining the signal at C is a more accurate representation of the current flowing in the load circuit. The current sampling is accomplished in the two lower legs of the power output bridge 60 near common line 112, thus eliminating a great problem, caused by high common mode voltages, encountered by many prior art amplifiers, which sample current with a shunt disposed in series with the load. 
     FIG. 8D shows schematically the control circuit section where current feedback and current command signals are first combined in operational amplifier OA5. The resultant signal is then combined with a synchronization signal from a triangular wave form generator, consisting of operational amplifier OA9, OA10, and the associated circuitry. The combined signal is fed to comparator OA6, the output of which is fed to each delay and driver section 64 and 65 for further processing. 
     The following are the component values used in a servo controller as shown in FIGS. 8A, 8C and 8D in accordance with the teaching of the present invention. 
     
         ______________________________________R1                     .05ΩR2, R3, R7, R8         47ΩR4                     5.1ΩR5                     .1ΩR6, R44, R106,         2KR128,R24, R25, R26, R27, R28, R29,                  10KR30, R31, R32, R33, R109, R116,R121, R131, R139, R144R34, R35               60.4KR37                    2.7KR38, R39, R161         18KR40                    91KR41, R42               39KR45, R47               20KR46                    3.9KR65                    500ΩR67                    500ΩR105, R127             510ΩR43                    82KR108, R130             56KR111, R126, R133, R149 200ΩR112, R134             .5ΩR113                   33KR114, R136, R137       47KR115, R119, R120, R123, R107,R129, R138, R142, R143, R146                  5.1KR117, R140             1.5KR118, R141             4.3KR122, R145             30KR124, R135, R147       15KR125, R148             220ΩR162                   390ΩCl                     30μfC6, C11                3.3pfC7                     0.1μfC8, C9, C10            .1μfC13                    .01μfC21, C22               .0022μfOA6, OA10              748OA2, OA3, OA4, OA5, OA9                  741M1, M2, M3             681D1, D2                 A115BD4                     MR812D3                     IN4006D13                    IN4006Z14, Z15               IN758______________________________________ 
    
     Referring now to FIGS. 9 and 10, there is shown an output and driver module 22 ready for mounting. Module 22 contains two intermediate heat sinks 141 and a final heat sink 140. The heat sinks 140 and 141 provide a good thermal path from the case of power transistors Q1, Q2 Q3 and Q4 to the ambient air which flows passed the finned portion of heat sink 140. Cooling fan 76 provides for an air flow passed the heat sinks 140 which comprise power output section 60. Output transistors Q1 and Q2, and driver transistor Q3 and Q4 are attached to intermediate heat sinks 141 which are connected in turn to the final heat sink 140 by bolts 42. Transistors Q1 and Q2 are attached to opposite sides of a heat sink block 141, while driver transistors Q3 and Q4 are attached to opposite sides of another heat sink block 141. Bolts 42 continue through printed circuit (PC) board 142 to form a stud for mounting to the rail 31 of the combining means 41. Bolt 42 also goes through the intermediate heat sink 141 which is drawn into good thermal contact with outer heat sink 140. Bolt 42 connects to current collector rail 31. A thermal switch THS1 is also connected to heat sink 140 for sensing over temperature conditions in output and driver module 22. Exiting from heat sink 140, at a location between the intermediate heat sinks 141, is a cable 45 which carries signals to and from the main printed circuit board. Module 22 also contains terminal studs 40 and 44 for making contact with the plus 90 volt power rail 30 and the common rail 32, respectively, of the combining structure 41. These studs also provide mechanical support for module 22. Terminal studs are inserted through printed circuit board 142 which provides interconnection between various component parts in the module 22. Resistor R1 is directly etched on printed circuit board 142. FIG. 10 is a top view of the output and driver module 22. Note that this module is double size compared to the output module 20 containing only the power output circuit. FIG. 11 shows a top view of a power output module 20. FIG. 12 shows a side view of power output module 20. Transistors Q1 and Q2 which are power output transistors are attached to intermediate heat sink 151 which is in turn attached to final heat sink 150 by output terminal bolt 42. Bolt 42 also serves as a terminal stud for attaching output modules 20 to the output rail 31 of the combining means 41. Terminal studs 40 and 44 and printed circuit board 142 serve the same purpose as described for FIG. 9 above. 
     FIG. 13 shows a third embodiment for current sampling. A single high power sampling resistor 80 is employed on each side of the bridge to sample current. The resistors used for sampling in the other two embodiments must still be retained since they are used to force current sharing among the various modules.