High Current Voltage/current Regulator Employing A Plurality Of Parallel Connected Power Transistors

Abstract

In a high current regulator employing high power parallel connected pass transistors, equal current sharing is forced by comparing the voltage drop across small emitter resistors and by means of differential (operational) amplifiers forcing these voltage drops to be substantially equal at all output currents. Three additional quantities are controlled; first, output voltage; second, output current; and third, fold-back overload current characteristics. The overall performance in all major characteristics is enhanced by the liberal use of high gain integrated circuit operational amplifiers.

Description

In high output current series regulated power supplies the regulating pass transistors are used in parallel when the required output current exceeds the rating of available transistors. In order for the use of parallel connected series pass transistors to be effective, the current must be shared equally by the transistors. However, simply connecting transistors in parallel does not produce equal current sharing due to inherent differences in transistor characteristics, particularly, in the base to emitter voltage drops. The higher the current, the more serious this problem becomes. In order to insure at least acceptable equality in the current sharing, it has been standard practice for some time to add resistors in series with the emitters of the transistors to mask the differences in base to emitter voltages of different transistors. In order for this method to be at least partially successful, the resistors are chosen to have a voltage drop at maximum current equal to or greater than the average base to emitter voltage. For example, with silicon transistors this resistor drop is made equal to 0.5 to 1.0 volt at maximum current.

While the current equalizing resistor method is useful, particularly in power supplies rated at moderate currents, it is a method which substantially decreases the efficiency of the power supply, generates undesirable heat and becomes less effective as the current demands are increased. For example, in a 1,000 ampere power supply employing 1.0 volt drop current equalizing resistors, 1,000 watts of undesirable and wasted heat is produced at maximum output. Furthermore, the more transistors that are used in parallel the greater the risk of unequal current sharing due to unequal temperature and other conditions and the more likely becomes failure unless the emitter resistors are made unduly large.

SUMMARY

In accordance with the present invention, the equalization of current in parallel connected series pass transistors is accomplished by using relatively small emitter resistors, of the order of one-tenth the conventional value, and by forcing the voltage drops across all emitter resistors to be the same by means of high gain operational amplifiers. With all emitter resistors of equal value, the voltage drop across each is compared with one which may be considered the reference and the amplifiers drive each of the other transistors to force the voltage drops across all resistors to be the same and hence the current through all transistors must be the same. The efficiency of the system is, by this means, substantially increased. The emitter resistor in this case may typically be one-tenth of the conventional resistor so that wasted power is only one-tenth and the heat generated is only one-tenth. The reduced heat and the ability to keep current sharing more exact permits higher currents to be handled by a given transistor. This may be of considerable importance since high current carrying capacity transistors are expensive and using them efficiently may affect a substantial saving in cost of the power supply. Less heating has many obvious advantages such as in component life expectancy, etc.

In the preferred form of the present invention the voltage drop across one emitter resistor, which may be considered the reference, is amplified by a predetermined factor to establish a suitable reference voltage. As an example, the voltage drop across the reference emitter resistor may be 0.05 volt which is amplified by 20 to provide a reference voltage of 1 volt. The emitter resistor voltages of the other parallel connected transistors is similarly amplified by a factor 20 to provide voltages readily compared with the amplified reference voltage. The amplified emitter resistor voltage of each transistor is summed at the input to an operational amplifier and the output of these amplifiers are connected to predriver transistors for driving the parallel connected transistors degeneratively phased so as to tend to equate the emitter resistor voltage drops of all parallel connected transistors to the emitter resistor drop of the reference transistor. Since the gain in the feed-back loop is very high, the equalization of voltage drops and hence currents carried by the transistors is very precise.

The regulated power supply in accordance with the preferred form of the present invention has a number of other important features. The output voltage is regulated in a voltage bridge circuit using an operational amplifier control amplifier. This is a cross-over power supply and employs a current regulating bridge also using an operational amplifier as the control amplifier. An additional operational amplifier is provided which compares output current with output voltage in a current limiting circuit with the zero current point determined at a finite output voltage to prevent latch-up. A source of constant driving current is provided for the pass transistors and the voltage control. Current control and current limiting amplifiers are connected in current sinking mode to control the driving current actually applied to the pass transistors. The auxiliary power supplies for the control amplifiers have fast responses so that all control functions are established on turn-on before the main source of unregulated power comes on. This prevents troublesome turn-on transients.

In the drawing:

FIG. 1 is a simplified schematic circuit diagram of the prior art method of equalizing current sharing in parallel connected series pass transistor regulators.

FIG. 2 is a simplified schematic circuit partly in block form showing a simplified form of the present invention.

FIG. 3 is a graph of the voltage/current characteristics of a typical regulated power supply in accordance with the present invention.

FIG. 4 is a complete schematic circuit diagram of the preferred form of the present invention.

FIG. 1 shows a simplified circuit diagram of a regulated power supply as practiced in the prior art. A source of unregulated voltage as represented by battery 1 feeds a load 2 through a plurality of parallel connected series regulated transistors 3, 4, 5, 6 and 7 driven in parallel by control amplifier 8 and connected between line 14 through individual emitter resistors 9, 10, 11, 12 and 13 and line 15. Since transistors vary with respect to base to emitter voltage at a given current, transistors connected in parallel without any equalizing means will carry unequal currents. For this reason the emitter resistors are provided and of sufficient resistance value to swamp the variations in base to emitter voltages encountered. With silicon transistors exhibiting base to emitter voltages in the range of 0.5 to 1.0 volt, resistors are used generally having values such as to cause a voltage drop of from 0.5 to 1.0 volt at the maximum rated current of the regulated power supply.

FIG. 2 shows a greatly improved circuit over the prior art of FIG. 1. Here transistors 3, 4, 6 and 7 are connected through relatively small emitter resistors 16, 17, 18 and 19 to line 14 and operational amplifiers 20, 21 and 22 compare the voltage across emitter resistor 16 with that across resistors 17, 18 and 19 respectively and by degenerative feedback to bases 23, 24 and 25 respectively constrain transistors 4, 6 and 7 to carry the same current as transistor 3. Taking operational amplifier 20 as an example. This may be any one of many suitable amplifiers such as the 741 integrated circuit operational amplifier shown symbolically (bias voltage connections not shown) (or discrete component amplifier such as the Analog Devices AD-106) and having a non-inverting input terminal 26, an inverting input 27 and an output terminal 28. Non-inverting input terminal 26 is connected to the junction between resistor 16 and emitter 29; inverting input 27 is connected to the junction between resistor 17 and emitter 30; and output terminal 28 is connected to base 23 of transistor 4. Now, the elements of a voltage bridge power supply are shown in simplified form with operational amplifier 31 (control amplifier) having a reference voltage represented by zener diode 32 connected through reference resistor 33 to non-inverting input 35, voltage control feedback resistor 34 connected from the high side of load 2 to this same non-inverting input 35, non-inverting input 36 being common and connected over lead 37 to common line 14 and output terminal 38 connected to base 39 of transistor 3. (This will be seen to be the basic bridge regulator as well - described in U.S. Pat. No. 3,028,538) Now, amplifier 31 drives transistor 3 to call for an output voltage equal to the reference voltage times the resistance of feedback (voltage control) resistor 34 divided by the resistance of reference resistor 33. The resulting current provided by transistor 3 flows through emitter resistor 16 and produces a voltage drop thereacross. Operational amplifier 20 receiving a voltage at non-inverting input terminal 26 but none at inverting input terminal 27, since transistor 4 has received no signal to conduct, provides a positive output at base 23 causing transistor 4 to conduct current which is driven to increase until input terminal 27 has a potential equal to the potential of terminal 26 (the null input characteristic of operational amplifier action). In this way transistor 4 is forced to pass or carry a current precisely equal to the current passed or carried by transistor 3 (provided resistors 16 and 17 are accurately equal). In a similar manner amplifier 21 causes transistor 6 to pass a current equal to the current passed by transistor 3 (the reference transistor) and amplifier 22 causes transistor 7 to pass a current equal to the current passed by transistor 3 (all assuming resistors 18 and 19 are equal to resistor 16). In this way all four parallel connected pass transistors 3, 4, 6 and 7 are constrained to carry equal currents. In the same way if more or less voltage is called for across the load by changing the setting of variable voltage control resistor 34, the four transistors are automatically adjusted to carry equal parts of the new current. While only four parallel connected transistors are shown and described above it will be evident that the system can be extended to a very large number of parallel transistors while providing equal current sharing for all. The value of emitter resistors 16, 17, 18 and 19 is no longer dictated by the base to emitter voltages of the transistors but can, in fact, be much lower resistance than would be required for this purpose with a consequent increase in efficiency of the power supply and decrease in heat generated.

FIG. 4 is a complete schematic circuit diagram, with a few simplifications, of the preferred embodiment of the present invention as applied to a high current voltage/current cross-over bridge regulated power supply with fold-back current limiting. As in FIG. 2, the source of power to be regulated is represented by battery 1, line 15 is the positive line to the series/parallel regulating transistors and line 14 is the positive common line. The output or load terminals are negative terminal 40 connected directly to the negative end of the source of power to be regulated and positive terminal 41 connected through current sensing resistor 42 to positive common line 14. The voltage control bridge is numbered to correspond with the numbering in FIG. 2 and described above and includes variable output voltage control resistor 34, reference voltage zener diode 32 (supplied from a suitable source as battery 43 through resistor 44), reference resistor 33 and operational (control) amplifier 31 including non-inverting input terminal 35, inverting input terminal 36 and output terminal 38. Four pass transistors 3, 4, 6 and 7 (as in FIG. 2) are connected between positive line 15 on one side and through emitter resistors 16, 17, 18 and 19 to common line 14 on the other side. Control point 48, to be described in more detail below, drives reference transistor 3 through an emitter to base chain of current amplifiers 45, 46 and 47. Constant current is supplied to control point 48 by means of constant current source transistor 51. Voltage regulation is exerted by voltage control amplifier 31 acting as a current sink through gate diode 49 - 50.

The current equalizing circuit of FIG. 4 differs from that shown in FIG. 2 and described above in that the voltage drop across the emitter resistors is first amplified by a precisely determined factor and then the controlling comparison is made. An amplified reference voltage is provided by operational amplifier 52 which includes inverting input terminal 53, non-inverting input terminal 54 and output terminal 55. Input resistor 56 is connected between inverting input terminal 53 and common line 14 and feedback resistor 57 is connected between output 55 and inverting input 53. In accordance with standard operational amplifier operation the gain of this circuit at non-inverting input terminal 54 is equal to the sum of the resistance of resistors 56 and 57 divided by the resistance of resistor 56. (See Handbook of Operational Amplifier Applications, Burr-Brown 2nd Edition 1963 bottom of page 7, FIG. 11.) For purposes of illustration we will assume this ratio to be 100 so that the amplifier has a predetermined and precise gain of 100 times the input voltage applied to non-inverting input terminal 54. Non-inverting input terminal 54 is connected to the junction between resistor 16 and emitter 29 over lead 58 so that the voltage across emitter resistor 16 appears at output terminal 55 multiplied by 100. For illustration purposes, assume the voltage across emitter resistor 16 at full current is 50 millivolts; then the reference voltage at output terminal 55 and on reference line 59 will be 5 volts. In the same way operational amplifier 60 provides a 100 times amplification of the voltage across emitter resistor 17 at output terminal 61 and on line 62. This latter voltage is compared with the amplified reference voltage on line 59 by means of operational amplifier 63. Operational amplifier 63 includes a non-inverting input terminal 64, an inverting input terminal 65 and an output terminal 66. The amplified reference voltage on line 59 is applied to inverting input terminal 65 through input resistor 67 and the amplified voltage to be compared and regulated on line 62 is applied to this same terminal through input resistor 68. The output which results if these two input voltages are not equal appearing at output terminal 66 is applied over lead 69 to the base of transistor 70. Emitter 71 is connected over lead 72 to the base of transistor 73 which in turn is emitter coupled to the base of transistor 74 which again is emitter coupled to the base of transistor 4. This high gain current amplifying chain causes transistor 4 to pass current until the voltage drop across emitter resistor 17 and amplified by amplifier 60 equals the amplified reference voltage at output terminal 55 of amplifier 52. At this point the voltages at inverting input terminal 65 are equal and opposite, i.e. exactly balanced and no further output is produced by amplifier 63. If the current passed by transistor 4 tends to increase above this regulated value the action reverses and the current is decreased so that at all times the current passed by transistor 4 is precisely equal to the current passed by reference transistor 3.

In a similar manner the voltages across emitter resistors 18 and 19 are amplified by operational amplifiers 75 and 76 respectively and the amplified voltages thus provided on lines 77 and 78 respectively are compared with the amplified reference voltage at the inverting inputs to amplifiers 79 and 80 respectively and corrective action initiated through transistors 81 and 82 and associated current amplifiers driving transistors 6 and 7 respectively.

The collector voltage for the intermediate or driver amplifiers 46, 47, 73, 74 and so on is supplied by a suitable auxiliary source represented simply by battery which is returned directly to common line 14 over lead 84 so that return current does not pass through current sensing resistor 42.

In the power supply of FIG. 4, in addition to the voltage regulation described above, improved current regulation is also supplied. The voltage drop, due to load current, across current sensing resistor 42 is amplified by operational amplifier 85 at a gain preset by the choice of input resistor 86 and feedback resistor 87 and the resulting output is applied over lead 89 to current control bridge junction 88. The current control bridge includes a reference voltage source applied across zener reference diode 90, current control bridge reference resistor 91, current control variable resistor 92 and the output of amplifier 85. The current bridge null junction 93 is, as usual, the junction between the reference resistor 91 and the control resistor 92 and is connected to the inverting input terminal 98 of control (operational) amplifier 94. This current control bridge regulates, as does the voltage control bridge, by providing a current sink at the output of amplifier 94 acting over lead 95 and through gate diode 96-97 to the base of transistor 45 at junction point 48. When the voltage drop across current sensing resistor 42 as amplified by amplifier 85 is equal to the voltage drop across current control resistor 92, the current bridge is in balance and a null exists at the input to control amplifier 94. If the output current now tends to increase above this value which is in balance, a signal appears at the input to amplifier 94 of such a polarity as to cause the voltage at its output (line 95) to decrease, causing gate 96-97 to conduct and robbing driving current from the input to transistor 45. This in effect tends to decrease the output current and to restore the balance.

FIG. 4 also shows a current fold-back circuit means incorporating operational amplifier 99 and acting as a current sink through diode gate 100-101 on the base of transistor 45. The fold-back circuit acts to reduce the output current in accordance with the output voltage (see the fold-back as plotted in FIG. 3) and to provide a predetermined minimum output current at zero output voltage so that the system will not latch as it may if the output current goes to zero at zero output voltage. The output voltage on line 102 is compared through resistor 103 with the amplified output current voltage drop at terminal 88 through resistor 104 to inverting input terminal 105 of fold-back control amplifier 99. This comparison causes the output current to track the output voltage along a line, as shown in the fold-back portion of FIG. 3. A small positive voltage is provided at junction point 115 by means of a voltage divider comprising resistors 107 and 106 connected from the positive line 109 and common line 14. This small positive voltage is applied over lead 108 to the non-inverting input 110 of amplifier 99. Thus, a reference is supplied at the input to amplifier 99 and when the amplified output current as divided by resistors 103 and 104 reaches the same value at inverting input 105, no further reduction in output current is forced by amplifier 99 as shown by the intersection of the fold-back line with the zero voltage line of FIG. 3.

Suitable positive and negative operating voltages for all of the operational amplifiers are provided by suitable positive and negative voltage auxiliary supplies represented by batteries 111 and 112 respectively. The common junction between these auxiliary supplies is connected to common lead 14 (on the input side of current sensing resistor 42); the positive supply is connected to all positive supply points of the operational amplifiers over common positive line 109; and the negative supply is connected to all negative supply points of the operational amplifiers over common negative line 113. Zener diode 90 is supplied from negative line 113 through resistor 114.

Claims

I claim:

1. In an electrical control circuit the combination of;

a plurality of power transistors connected in parallel between a source of power to be regulated and a load to be supplied with power;

feedback amplifier means coupled to one of said transistors;

resistor means connected in series with each of said transistor means;

an operational amplifier including an inverting input, a non-inverting input and an output;

a connection between one of said inputs and one of said resistor means;

a connection between the other of said inputs and another of said resistor means;

and a connection between said output and the input to one of said transistors.

2. In a voltage regulating control circuit the combination of;

two power transistors each including a base, an emitter and a collector;

a source of direct current voltage to be regulated;

a load to be supplied with direct current voltage;

a connection between said collectors and one side of said source of direct current voltage;

a connection between one end of said load and the other side of said source of direct current voltage;

a first resistor connected between the emitter of the first of said transistors and the other end of said load;

a second resistor connected between the emitter of the second of said transistors and the last said end of said load;

an operational amplifier including a non-inverting input, a common terminal and an output;

a connection between said common terminal and the last said end of said load;

a connection between said output and the base of said first of said transistors;

a source of reference voltage and a reference resistor connected in series and between the last said end of said load and said input;

an adjustable voltage control feedback resistor connected between the first said end of said load and said input;

a second operational amplifier including an inverting input, a non-inverting input and an output;

coupling means connected between the emitter of said first transistor and said non-inverting input of said second operational amplifier;

coupling means connected between the emitter of said second transistor and said inverting input of said second operational amplifier;

and coupling means connected between the base of said second transistor and said output of said second operational amplifier;

whereby said first and second transistors are programmed by second operational amplifier to produce substantially equal voltages drops across the emitter resistors and thereby to share current between said voltage source and said load substantially equally.

3. A voltage regulating circuit as set forth in claim 2;

wherein the coupling means connected between the emitter of said second transistor and said inverting input of said second operational amplifier includes a third operational amplifier for amplifying the voltage drop across said emitter resistor.

4. In a voltage regulator, the combination of;

a bridge regulator including two series pass transistors connected in parallel;

equal resistors connected in series with said transistors;

and an operational amplifier connected to compare the voltage drops across said resistors and to program one of transistors degeneratively to maintain said drops substantially equal.

5. In a voltage regulator, the combination of;

a bridge regulator including two series pass transistors connected in parallel through two emitter resistors;

two equal gain operational amplifiers connected to amplify the voltage drops across said resistors by a predetermined factor;

and an operational amplifier connected to compare said amplified voltage drops and to program one of said transistors degeneratively to maintain said amplified voltage drops substantially equal.