CA1322576C - Multiple output power supply - Google Patents
Multiple output power supplyInfo
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- CA1322576C CA1322576C CA000560138A CA560138A CA1322576C CA 1322576 C CA1322576 C CA 1322576C CA 000560138 A CA000560138 A CA 000560138A CA 560138 A CA560138 A CA 560138A CA 1322576 C CA1322576 C CA 1322576C
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Abstract
ABSTRACT
A multiple output power supply is provided comprised of an input circuit for receiving an input signal, an auxiliary output circuit for carrying a self-regulated auxiliary output signal, and a main output circuit carrying a main power output signal. A
feed forward compensation circuit receives and subtracts the auxiliary output signal from the input signal and in response generates a difference signal.
A pulse width modulator receives and compares the main power output signal with the difference signal and in response gates the input signal, thereby regulating the main output signal and enabling current mode operation of the power supply.
A multiple output power supply is provided comprised of an input circuit for receiving an input signal, an auxiliary output circuit for carrying a self-regulated auxiliary output signal, and a main output circuit carrying a main power output signal. A
feed forward compensation circuit receives and subtracts the auxiliary output signal from the input signal and in response generates a difference signal.
A pulse width modulator receives and compares the main power output signal with the difference signal and in response gates the input signal, thereby regulating the main output signal and enabling current mode operation of the power supply.
Description
1322~7~
02 This invention relates in general to power 03 supplies, and more particularly to a method for 04 eliminating oscillation in a current mode multiple 05 output power supply by means of feed forward current 06 compensation.
07 Well known prior art isolated switching 08 current mode power supplies are typically comprised of 09 an input circuit for receiving and switching or gating 10 an input signal, an output circuit transformer coupled 11 to the input circuit for generating an output signal, 12 and a pulse width modulator for receiving the output 13 and input signals, and in response controlling the 14 duty eycle of periodie switching or gating of the 15 input signal, thereby regulating the output signal 16 amplitude.
~17 Such prior art power supplies operate in 18 two modes; voltage mode and current mode. In the case 19 of voltage mode operation, there is no signal feedback from the input circuit to the pulse width modulator, 21 whereas in current mode there is a large amount of 22 signal feedback from the input circuit to the 23 modulator, such that the power supply goes into 24 unconditional oscillation. In practice, a hybrid mode (i.e. a mixture of voltage and current mode) is 26 utilized.
27 Multiple output power supplies typically 28 utilize a swinging choke or push-pull transformer for 29 coupling the input circuit to the main output and auxiliary output circuits. The auxiliary output 31 circuit may be made self-regulating by means of a 32 magnetic amplifier, as described in greater detail 33 below. In order to prevent saturation of the swinging 34 choke, the main output circuit is operated in current mode with current sensing (i.e. feedback) in the input 36 circuit.
37 In a multiple output power supply current 38 is sensed in the transformer primary coil for ,~
~322~7~
02 controlling the pulse width modulator. This current 03 represents the sum of the currents flowing in each of 34 the output circuits (i.e. flowing in separate 05 secondary coils of the transformer). However, in the 06 event that one of the auxiliary output circuits is 07 self-regulating by means of either current or voltage 08 control, the regulation in the auxiliary circuit 09 competes with the primary or input circuit regulation for control of power supplied to the auxiliary output 11 circuit.
12 In the event further auxiliary outputs of 13 the supply have loads connected thereacross, they 14 serve to dampen the effect of contention between the input and self-regulated auxiliary output by absorbing 16 energy and providing additional current to the 17 self-regulated auxiliary output when required. This 18 results in poor cross-regulation of the multiple 19 output circuits.
Furthermore, it has been found that a 21 power supply operating in current mode will start 22 oscillating if the equivalent impedance of the main 23 output circuit becomes greater than the input 24 impedance of the auxiliary output circuit.
According to the prior art, a number of 26 techniques have been utilized to prevent such 27 oscillation. One technique involves forcing the 28 circuit to operate in a hybrid mode (approaching 29 voltage mode). Another technique involves lowering the feedback gain between the main output circuit and 31 the pulse width modulator.
32 According to the present invention, 33 current mode feed forward current compensation is 34 provided for addressing the problem of oscillation in a multiple output power supply. A portion of the 36 signal carried by the auxiliary circuit is subtracted 37 from the signal sensed in the input circuit, and the 38 difference signal is transmitted for controlling the 1322~76 01 _ 3 _ 02 current mode pulse width modulator. Accordingly, 03 the auxiliary output circuit i5 made virtually 04 transparent to the input circuit pulse width 05 modulator, thereby eliminating oscillation due to 06 contention between the input circuit and 07 self-regulated output circuit.
08 From small signal equivalent model 09 analysis, it has been discovered that, according to the technique of the present invention, the equivalent 11 auxiliary output circuit impedance approaches infinity 12 as a result of current compensation provided by 13 subtracting the auxiliary signal from the primary 14 input signal.
There are two additional benefits 16 resulting from use of the feed forward current 17 compensation technique of the present invention.
18 Firstly, transient loads connected to the auxiliary 19 output circuit do not affect operation of the main or other output circuit, resulting in improved 21 cross-regulation. By subtracting the current 22 information of the auxiliary output connected to a 23 transient load from the input current, the auxiliary 24 output circuit is made transparent to the current mode pulse width modulator such that the input circuit can 26 deliver more current to the auxiliary output circuits.
27 Secondly, the feedback path from the 28 main output circuit to the pulse width modulator 29 eliminates the effects of impedance variations in the input circuit, resulting in enhanced auxiliary output 31 circuit operation.
32 Furthermore, in order to implement the 33 feed forward current compensation technique according 34 to the present invention, no additional components are required over standard prior art multiple output power 36 supplies. The reason for this is th~t current flowing 37 in the auxiliary circuit is usually detected in prior 38 art power supplies in order to provide auxiliary 39 overload protection.
1322~76 01 - 3a -02 According to the present invention there 03 is provided a feed forward current compensation 04 circuit for use in a multiple output power supply 05 operating in current mode. The power supply is 06 comprised of an input circuit, a main output circuit, 07 a regulator for feeding back a signal representative 08 of current flowing in the main output circuit to the 09 input circuit for regulating the input circuit, and at least one self-regulated auxiliary output circuit 11 which affects operation of the regulator. The feed 12 forward current compensation circuit is comprised of 13 circuitry for detecting current flowing in the 14 auxiliary output circuit and generating a control signal representative thereof, and circuitry for 16 subtracting the control signal from the signal 17 representative of current flowing in the main output 18 circuit, whereby the effect of the self-regulated 19 auxiliary output circuit on the regulator is cancelled, thereby preventing oscillation of the power 21 supply.
22 According to a preferred embodiment of the 23 present invention, there is provided a multiple output 24 current mode power supply comprised of an input circuit for receiving an input signal, a main output 26 circuit transformer coupled to the input circuit, for 27 generating a main output signal, and circuitry for 28 detecting the main output signal and in response 29 generating a control voltage signal proportional to the amplitude thereof. A pulse width modulator is 31 provided for receiving the control voltage signal and 32 in response switching the input signal according to a 33 predetermined pulse width, thereby regulating the 34 input signal. A self-regulated auxiliary output circuit transformer is coupled to the input circuit, 36 for generating an auxiliary output signal, and a feed 37 forward compensation circuit is provided for receiving 1322~7~
01 - 3b -02 and subtracting the auxiliary output signal from the 03 input signal, thereby generating a difference signal, 04 and for subtracting the difference signal from the 05 control voltage signal, whereby the effect of the 06 self-regulated auxiliary output circuit on regulation 07 of the input signal is cancelled.
132257~
01 _ 4 _ 02 A better understanding of the present 03 invention and the prior art will be obtained with 04 reference to the detailed description below in 05 conjunction with the following drawings, in which:
06 Figure 1 is a simplified schematic block 07 diagram of a multiple output power supply according to 08 the prior art, 09 Figure 2 is a small signal equivalent model for describing normal current mode operation of 11 the power supply shown in Figure 1, 12 Figure 3 is a functional block schematic 13 diagram of a pulse width modulator with feed forward 14 current compensation, according to the present invention, 16 Figure 4 is a small signal equivalent 17 model for describlng current mode operation of the 18 multiple output power supply shown in Figure 1 with 19 feed forward current compensation, according to the present invention, and 21 Figure 5 is a block schematic diagram of a 22 multiple output power supply with feed forward current 23 compensation according to a preferred embodiment of 24 the present invention.
With reference to Figure 1, a multiple 26 output push-pull power supply is illustrated having a 27 main feedback path between the main output circuit and 2~ input circuit, and having an auxiliary output 29 independently regulated by a magnetic amplifier.
An input signal (Vi) is applied to the 31 primary coil lA of a swinging choke 1, and coupled 32 thereacross to a pair of secondary coils lB and lC
33 connected to main and auxiliary output circuits 34 respectively. The main output signal VO is carried by a pair of output terminals 3 and fed back by means of 36 an amplifier 5 to the control input of a pulse width 37 modulator 7, in a well known manner. The pulse width 132257~
01 ~ 5 -02 modulator 7 generates control signals for gating or 03 switching the input signal Vi via a pair of power 04 switches 9 and 11.
05 The output circuit is further comprised of 06 a pair of diodes 13 and 15, having annodes thereof 07 connected to opposite end termin~ls of secondary coil 08 lB and cathodes thereof connected together and to an 09 output choke 17 connected to one of the output terminals 3. Also, a capacitor 19 is connected across 11 the pair of output terminals 3 in a well known manner.
12 A magnetic amplifier power switch 21 is 13 provided in the auxiliary output circuit, and is 14 comprised basically of a small saturable choke in the form of a pair of coils 21A and 21B having no gap 16 therebetween. The choke 21 is characterized by very 17 large inductance in order that it can be saturated 18 with a very small DC current.
19 In response to the switched or gated input signal being applied to the primary coil lA, current 21 begins flowing through secondary coil lC. This 22 current is designated as the "on" time. Initially, 23 the choke 21 is typically not fully saturated, and 24 behaves like an open switch because of the large inductance. After a short delay, which is made less 26 than the "on" time, the choke 21 saturates and behaves 27 like a closed switch. The delay between the "on" time 28 and the time at which the choke saturates, depends 29 upon the degree of saturation of the choke 21 during the previous "off" time, (i.e. when no current is 31 flowing through secondary coil lB as a result of 32 switches 9 and 11 being switched off by pulse width 33 modulator 7). If during the "off" time the choke 21 34 had been kept in saturation, at the next "on" time, the magnetic switch will already be in a "closed"
36 state and there will be no delay. However, if during 37 the previous "off" time, the choXe 21 had been pulled 38 out of saturation, then there will be a delay prior to 1322~76 02 saturation of choke 21, where the delay is inversely 03 proportional to the extent of saturation of the choke 04 21 during the previous "off" time. The extent of 05 saturation is controlled by the amount of auxiliary 06 output signal being reapplied to the choke 21 from 07 output terminals 23 via amplifier 25.
08 Additional diodes 27 and 29 are connected 09 to coils 21A and 21B as well as to a further diode 31 and output inductor 33, in a well known manner. Zener 11 diodes 35 and 37 are connected to second inputs of 12 feedback amplifiers 25 and 5, respectively. In 13 addition, a capacitor 39 is connected across auxiliary 14 output terminals 23 in a well known manner.
When the power supply of Figure 1 is 16 operating in pure voltage mode, then the input 17 resistance Rs=O. Hence, the auxiliary input is in 18 parallel with the source of input voltage signal tvi), 19 and the input impedance of the auxiliary circuit can be ignored. Consequently, the auxiliary output 21 circuit has no effect on the main output circuit 22 behaviour.
23 However, this situation is entirely 24 different for current mode operation of the power supply. With reference to Figure 2, the equivalent 26 small signal model of the push-pull multiple output 27 power supply of Figure 1, is shown. An AC signal 28 source 40 generates a signal having an amplitude 29 envelope characterized by Dvi, where D is the duty cycle of the control signal output from pulse width 31 modulator 7, and Vi is the small signal input voltage 32 received by the input circuit. A small signal DC
33 current source 42 is connected to the signal source 40 34 and in parallel to an equivalent input resistor 54 having input resistance Rc, where 36 Rs(Vi/A) 37 Rc=
38 l-ML/M
1322~7~
01 ~ 7 -02 ML is the rate of change in current flowing through 03 output inductor 17 and M is the rate of change of the 04 ramp signal amplitude generated within pulse width 05 modulator 7. The current source 42 generates a 06 current vc/Rs~ where VC is the small signal voltage 07 output from amplifier 25 (Figure 1) and fed back to 08 the pulse width modulator 7, and Rs is the input 09 resistance of the input circuit.
The input impedance of the auxiliary 11 output circuit is characterized by a small signal 12 equivalent circuit comprising a resistor 46 having 13 negative resistance -rai in parallel with a current 14 source 48 generating a small signal current i'a.
It is apparent from Figure 2 that the 16 power supply will begin oscillating when the 17 equivalent impedance R of the load 50 connected to the 18 main output circuit is greater than the input 19 impedance -rai of the auxiliary circuit.
According to the present invention, feed 21 forward compensation circuitry is provided for 22 subtracting within the current mode modulator 7, a 23 portion of the auxiliary current (Ia) from the 24 detected input current Ipri With reference to Figure 3, a functional 26 diagram of the pulse width modulator with feed forward 27 current compensation, is shown according to the 28 present invention. The auxiliary current signal (Ia) 29 is detected across the equivalent input resistance Rs, and subtracted in difference circuit 70 from the input 31 current signal (Ipri) detected across the input 32 resistance. Circuit 70 generates a difference signal 33 for application to summing circuit 58, which sums the 34 signal fed back from the circuit 58 with the signal output from ramp generator 57, for application to a 36 latch circuit 54 by means of comparator 51.
37 With reference to the small signal 38 equivalent model of Figure 4, the results of feed ` 132~76 02 forward current compensation are shown. In 03 particular, the equivalent impedance of the auxiliary 04 output circuit approaches infinity as a result of the 05 current compensatlon (~Ia) provided by current source 06 80. Thus, the network of Figure 4 reduces to the 07 familiar form of a single output small signal hybrid 08 mode pulse width modulator.
09 With reference to Figure 5, a multiple output power supply is illustrated, according to a 11 successful prototype of the present invention, 12 comprised of parallel input circuits 100 and 102, each 13 comprised of a pair of power switches 104, 105 and 14 106, 107 and a pair of input current sensing transformers 109 and 111. Pulse width modulators 112 16 and 114 are connected to the power switches 104-107 in 17 a well known manner. Pulse width modulators 112 and 18 114 are preferably in the form of integrated circuits, 19 such as the well known model 1526 pulse width modulator.
21 Transformer 109 has a secondary coil lO9B
22 thereof connected to an input of pulse width modulator 23 112 via a diode 116 and shunt resistor 118.
24 Similarly, a secondary coil lllB of transformer 111 i5 connected to a control input of 26 pulse width modulator 114 via a diode 120 and shunt 27 resistor 122.
28 A ramp signal is received from a ramp 29 signal generator (not shown), such as ramp signal generator 17 discussed above, and coupled to the 31 transformer secondary coils lO9B and lllB via an AC
32 coupling capacitor 124.
33 A pair of swinging chokes (or push-pull 34 converters), 123 and 125 are provided for coupling the input circuits 100 and 102 to respective auxiliary 36 output circuits 127 and 129 and to a main output 37 circuit 126 of the power supply.
1322~76 01 _ 9 _ 02 According to the preferred embodiment, the 03 two swinging chokes 123 and 125 function in a 04 proportional current sharing mode for the main output 05 circuit 126 which generates a 5 volt, 100 amp output 06 signal. Output circuit 129 delivers a 12 volt, 8 amp 07 auxiliary output signal and 40~ of the main output 08 signal, while output circuit 127 delivers a -12 volt, 09 1 amp auxiliary output signal and 60~ of the main 5 volt output signal.
11 By using stepped gap chokes 123 and 125, 12 the auxiliary output circuits are capable of 13 delivering power with as little as 1 amp total current 14 flowing from the main output circuit 126.
Voltage control feedback is provided from 16 the main output circuit 126 to respective control 17 inputs of pulse width modulators 112 and 114 by means 18 of a closed optocoupler 128 for receiving the main 19 output signal from a feedback amplifier arrangement comprised of buffer amplifier 130 and zener diode 132.
21 A magnetic amplifier 133 is provided in 22 the second auxiliary output circuit 129 for receiving 23 a portion of the auxiliary output signal via an 24 amplifier 134 connected to a zener diode 135, and maintaining a self-regulated auxiliary output signal, 26 as discussed above with reference to Figure 3.
27 Feed forward compensation is provided by 28 means of a further current sensing transformer coil 29 149A disposed in the auxiliary output circuit 129, coupled to a secondary coil 149B connected with 31 opposite polarity to the secondary coil lllB of input 32 current sensing transformer 111. A diode 510 and 33 shunt resistor 152 are connected across coil 149B in a 34 well known manner.
In operation, auxiliary output current 36 flowing through coil 149A results in generation of a 37 voltage signal across resistor 152 connected across 38 coil 149B, as a result of transformer coupling. The 1322~7~
02 voltage signal developed across resistor 152 is in 03 opposite polarity to the voltage appearing across 04 resistor 122 as a result of transformer coupling of 05 the input current signal flowing through transformer 06 coil 111. Thus, the net signal applied to the control 07 input of pulse width modulator 114 is the difference 08 of the input current signal minus the auxiliary output 09 current signal.
Twelve volt current limiting is provided 11 by an amplifier 154 connected to secondary coil lllB
12 and a tapped resistor 156 connected to the ramp signal 13 source via capacitor 124. Primary current limiting is 14 provided by a potentiometer 158 connected to the 15 output of optocoupler 128, and to the voltage control 16 input of pulse width modulator 114.
17 Auxiliary output circuit 127 is provided 18 with a pair of protective diodes 160 and 162 connected 19 across the secondary coil of swinging choke 123 and to 20 an output choke 164 connected in parallel with an 21 output filtering capacitor 166. The secondary coil of 22 choke 123 is center tapped for connection to an output 23 regulator 170 having a further input connected to 24 choke 164 and capacitor 166, and an output thereof for 25 generating the -12 volt auxiliary output signal.
26 Similarly, auxiliary output circuit 129 is 27 provided with a pair of protective diodes 180 and 182 28 connected to magnetic amplifier power switch 133, and 29 a further diode 186. The cathodes of diodes 180, 182 30 and 186 are connected together and to one terminal of 31 an output choke 188 having an opposite terminal 32 connected to an output capacitor 190 carrying the 12 33 volt auxiliary output signal.
34 Main output circuit 126 is also provided 35 with protective diodes 172, 174 and 192, 194, output 36 chokes 176 and 196, and output capacitors 178 and 198 37 all connected in a well known manner.
1322~7~
02 Due to design optimization in driving the 03 magnetic amplifier power switch 133, the 12 volt, 8 04 amp auxiliary output signal has a power density of 05 more than 10 watts per cubic inch and an efficiency of 06 more than 86~. By means of comparison, prior art 07 supplies are typically characterized by auxiliary 08 output circuits which deliver only 1 amp and dissipate 09 more power than the 8 amp output of the preferred embodiment.
11 According to the successful prototype, a 12 600 watt power supply was built for receiving a 48 13 volt DC input signal (Vi) and generating the 14 aforementioned main output signal at 5 volts and 100 amps, and the auxiliary output signals at 12 volts and 16 8 amps, and -12 volts at 1 amp, respectively.
17 A person understanding the present 18 invention may conceive of other embodiments or 19 variations thereof. All such embodiments or variations are believed to be within the sphere and 21 scope of the present invention as defined by the 22 claims appended hereto.
02 This invention relates in general to power 03 supplies, and more particularly to a method for 04 eliminating oscillation in a current mode multiple 05 output power supply by means of feed forward current 06 compensation.
07 Well known prior art isolated switching 08 current mode power supplies are typically comprised of 09 an input circuit for receiving and switching or gating 10 an input signal, an output circuit transformer coupled 11 to the input circuit for generating an output signal, 12 and a pulse width modulator for receiving the output 13 and input signals, and in response controlling the 14 duty eycle of periodie switching or gating of the 15 input signal, thereby regulating the output signal 16 amplitude.
~17 Such prior art power supplies operate in 18 two modes; voltage mode and current mode. In the case 19 of voltage mode operation, there is no signal feedback from the input circuit to the pulse width modulator, 21 whereas in current mode there is a large amount of 22 signal feedback from the input circuit to the 23 modulator, such that the power supply goes into 24 unconditional oscillation. In practice, a hybrid mode (i.e. a mixture of voltage and current mode) is 26 utilized.
27 Multiple output power supplies typically 28 utilize a swinging choke or push-pull transformer for 29 coupling the input circuit to the main output and auxiliary output circuits. The auxiliary output 31 circuit may be made self-regulating by means of a 32 magnetic amplifier, as described in greater detail 33 below. In order to prevent saturation of the swinging 34 choke, the main output circuit is operated in current mode with current sensing (i.e. feedback) in the input 36 circuit.
37 In a multiple output power supply current 38 is sensed in the transformer primary coil for ,~
~322~7~
02 controlling the pulse width modulator. This current 03 represents the sum of the currents flowing in each of 34 the output circuits (i.e. flowing in separate 05 secondary coils of the transformer). However, in the 06 event that one of the auxiliary output circuits is 07 self-regulating by means of either current or voltage 08 control, the regulation in the auxiliary circuit 09 competes with the primary or input circuit regulation for control of power supplied to the auxiliary output 11 circuit.
12 In the event further auxiliary outputs of 13 the supply have loads connected thereacross, they 14 serve to dampen the effect of contention between the input and self-regulated auxiliary output by absorbing 16 energy and providing additional current to the 17 self-regulated auxiliary output when required. This 18 results in poor cross-regulation of the multiple 19 output circuits.
Furthermore, it has been found that a 21 power supply operating in current mode will start 22 oscillating if the equivalent impedance of the main 23 output circuit becomes greater than the input 24 impedance of the auxiliary output circuit.
According to the prior art, a number of 26 techniques have been utilized to prevent such 27 oscillation. One technique involves forcing the 28 circuit to operate in a hybrid mode (approaching 29 voltage mode). Another technique involves lowering the feedback gain between the main output circuit and 31 the pulse width modulator.
32 According to the present invention, 33 current mode feed forward current compensation is 34 provided for addressing the problem of oscillation in a multiple output power supply. A portion of the 36 signal carried by the auxiliary circuit is subtracted 37 from the signal sensed in the input circuit, and the 38 difference signal is transmitted for controlling the 1322~76 01 _ 3 _ 02 current mode pulse width modulator. Accordingly, 03 the auxiliary output circuit i5 made virtually 04 transparent to the input circuit pulse width 05 modulator, thereby eliminating oscillation due to 06 contention between the input circuit and 07 self-regulated output circuit.
08 From small signal equivalent model 09 analysis, it has been discovered that, according to the technique of the present invention, the equivalent 11 auxiliary output circuit impedance approaches infinity 12 as a result of current compensation provided by 13 subtracting the auxiliary signal from the primary 14 input signal.
There are two additional benefits 16 resulting from use of the feed forward current 17 compensation technique of the present invention.
18 Firstly, transient loads connected to the auxiliary 19 output circuit do not affect operation of the main or other output circuit, resulting in improved 21 cross-regulation. By subtracting the current 22 information of the auxiliary output connected to a 23 transient load from the input current, the auxiliary 24 output circuit is made transparent to the current mode pulse width modulator such that the input circuit can 26 deliver more current to the auxiliary output circuits.
27 Secondly, the feedback path from the 28 main output circuit to the pulse width modulator 29 eliminates the effects of impedance variations in the input circuit, resulting in enhanced auxiliary output 31 circuit operation.
32 Furthermore, in order to implement the 33 feed forward current compensation technique according 34 to the present invention, no additional components are required over standard prior art multiple output power 36 supplies. The reason for this is th~t current flowing 37 in the auxiliary circuit is usually detected in prior 38 art power supplies in order to provide auxiliary 39 overload protection.
1322~76 01 - 3a -02 According to the present invention there 03 is provided a feed forward current compensation 04 circuit for use in a multiple output power supply 05 operating in current mode. The power supply is 06 comprised of an input circuit, a main output circuit, 07 a regulator for feeding back a signal representative 08 of current flowing in the main output circuit to the 09 input circuit for regulating the input circuit, and at least one self-regulated auxiliary output circuit 11 which affects operation of the regulator. The feed 12 forward current compensation circuit is comprised of 13 circuitry for detecting current flowing in the 14 auxiliary output circuit and generating a control signal representative thereof, and circuitry for 16 subtracting the control signal from the signal 17 representative of current flowing in the main output 18 circuit, whereby the effect of the self-regulated 19 auxiliary output circuit on the regulator is cancelled, thereby preventing oscillation of the power 21 supply.
22 According to a preferred embodiment of the 23 present invention, there is provided a multiple output 24 current mode power supply comprised of an input circuit for receiving an input signal, a main output 26 circuit transformer coupled to the input circuit, for 27 generating a main output signal, and circuitry for 28 detecting the main output signal and in response 29 generating a control voltage signal proportional to the amplitude thereof. A pulse width modulator is 31 provided for receiving the control voltage signal and 32 in response switching the input signal according to a 33 predetermined pulse width, thereby regulating the 34 input signal. A self-regulated auxiliary output circuit transformer is coupled to the input circuit, 36 for generating an auxiliary output signal, and a feed 37 forward compensation circuit is provided for receiving 1322~7~
01 - 3b -02 and subtracting the auxiliary output signal from the 03 input signal, thereby generating a difference signal, 04 and for subtracting the difference signal from the 05 control voltage signal, whereby the effect of the 06 self-regulated auxiliary output circuit on regulation 07 of the input signal is cancelled.
132257~
01 _ 4 _ 02 A better understanding of the present 03 invention and the prior art will be obtained with 04 reference to the detailed description below in 05 conjunction with the following drawings, in which:
06 Figure 1 is a simplified schematic block 07 diagram of a multiple output power supply according to 08 the prior art, 09 Figure 2 is a small signal equivalent model for describing normal current mode operation of 11 the power supply shown in Figure 1, 12 Figure 3 is a functional block schematic 13 diagram of a pulse width modulator with feed forward 14 current compensation, according to the present invention, 16 Figure 4 is a small signal equivalent 17 model for describlng current mode operation of the 18 multiple output power supply shown in Figure 1 with 19 feed forward current compensation, according to the present invention, and 21 Figure 5 is a block schematic diagram of a 22 multiple output power supply with feed forward current 23 compensation according to a preferred embodiment of 24 the present invention.
With reference to Figure 1, a multiple 26 output push-pull power supply is illustrated having a 27 main feedback path between the main output circuit and 2~ input circuit, and having an auxiliary output 29 independently regulated by a magnetic amplifier.
An input signal (Vi) is applied to the 31 primary coil lA of a swinging choke 1, and coupled 32 thereacross to a pair of secondary coils lB and lC
33 connected to main and auxiliary output circuits 34 respectively. The main output signal VO is carried by a pair of output terminals 3 and fed back by means of 36 an amplifier 5 to the control input of a pulse width 37 modulator 7, in a well known manner. The pulse width 132257~
01 ~ 5 -02 modulator 7 generates control signals for gating or 03 switching the input signal Vi via a pair of power 04 switches 9 and 11.
05 The output circuit is further comprised of 06 a pair of diodes 13 and 15, having annodes thereof 07 connected to opposite end termin~ls of secondary coil 08 lB and cathodes thereof connected together and to an 09 output choke 17 connected to one of the output terminals 3. Also, a capacitor 19 is connected across 11 the pair of output terminals 3 in a well known manner.
12 A magnetic amplifier power switch 21 is 13 provided in the auxiliary output circuit, and is 14 comprised basically of a small saturable choke in the form of a pair of coils 21A and 21B having no gap 16 therebetween. The choke 21 is characterized by very 17 large inductance in order that it can be saturated 18 with a very small DC current.
19 In response to the switched or gated input signal being applied to the primary coil lA, current 21 begins flowing through secondary coil lC. This 22 current is designated as the "on" time. Initially, 23 the choke 21 is typically not fully saturated, and 24 behaves like an open switch because of the large inductance. After a short delay, which is made less 26 than the "on" time, the choke 21 saturates and behaves 27 like a closed switch. The delay between the "on" time 28 and the time at which the choke saturates, depends 29 upon the degree of saturation of the choke 21 during the previous "off" time, (i.e. when no current is 31 flowing through secondary coil lB as a result of 32 switches 9 and 11 being switched off by pulse width 33 modulator 7). If during the "off" time the choke 21 34 had been kept in saturation, at the next "on" time, the magnetic switch will already be in a "closed"
36 state and there will be no delay. However, if during 37 the previous "off" time, the choXe 21 had been pulled 38 out of saturation, then there will be a delay prior to 1322~76 02 saturation of choke 21, where the delay is inversely 03 proportional to the extent of saturation of the choke 04 21 during the previous "off" time. The extent of 05 saturation is controlled by the amount of auxiliary 06 output signal being reapplied to the choke 21 from 07 output terminals 23 via amplifier 25.
08 Additional diodes 27 and 29 are connected 09 to coils 21A and 21B as well as to a further diode 31 and output inductor 33, in a well known manner. Zener 11 diodes 35 and 37 are connected to second inputs of 12 feedback amplifiers 25 and 5, respectively. In 13 addition, a capacitor 39 is connected across auxiliary 14 output terminals 23 in a well known manner.
When the power supply of Figure 1 is 16 operating in pure voltage mode, then the input 17 resistance Rs=O. Hence, the auxiliary input is in 18 parallel with the source of input voltage signal tvi), 19 and the input impedance of the auxiliary circuit can be ignored. Consequently, the auxiliary output 21 circuit has no effect on the main output circuit 22 behaviour.
23 However, this situation is entirely 24 different for current mode operation of the power supply. With reference to Figure 2, the equivalent 26 small signal model of the push-pull multiple output 27 power supply of Figure 1, is shown. An AC signal 28 source 40 generates a signal having an amplitude 29 envelope characterized by Dvi, where D is the duty cycle of the control signal output from pulse width 31 modulator 7, and Vi is the small signal input voltage 32 received by the input circuit. A small signal DC
33 current source 42 is connected to the signal source 40 34 and in parallel to an equivalent input resistor 54 having input resistance Rc, where 36 Rs(Vi/A) 37 Rc=
38 l-ML/M
1322~7~
01 ~ 7 -02 ML is the rate of change in current flowing through 03 output inductor 17 and M is the rate of change of the 04 ramp signal amplitude generated within pulse width 05 modulator 7. The current source 42 generates a 06 current vc/Rs~ where VC is the small signal voltage 07 output from amplifier 25 (Figure 1) and fed back to 08 the pulse width modulator 7, and Rs is the input 09 resistance of the input circuit.
The input impedance of the auxiliary 11 output circuit is characterized by a small signal 12 equivalent circuit comprising a resistor 46 having 13 negative resistance -rai in parallel with a current 14 source 48 generating a small signal current i'a.
It is apparent from Figure 2 that the 16 power supply will begin oscillating when the 17 equivalent impedance R of the load 50 connected to the 18 main output circuit is greater than the input 19 impedance -rai of the auxiliary circuit.
According to the present invention, feed 21 forward compensation circuitry is provided for 22 subtracting within the current mode modulator 7, a 23 portion of the auxiliary current (Ia) from the 24 detected input current Ipri With reference to Figure 3, a functional 26 diagram of the pulse width modulator with feed forward 27 current compensation, is shown according to the 28 present invention. The auxiliary current signal (Ia) 29 is detected across the equivalent input resistance Rs, and subtracted in difference circuit 70 from the input 31 current signal (Ipri) detected across the input 32 resistance. Circuit 70 generates a difference signal 33 for application to summing circuit 58, which sums the 34 signal fed back from the circuit 58 with the signal output from ramp generator 57, for application to a 36 latch circuit 54 by means of comparator 51.
37 With reference to the small signal 38 equivalent model of Figure 4, the results of feed ` 132~76 02 forward current compensation are shown. In 03 particular, the equivalent impedance of the auxiliary 04 output circuit approaches infinity as a result of the 05 current compensatlon (~Ia) provided by current source 06 80. Thus, the network of Figure 4 reduces to the 07 familiar form of a single output small signal hybrid 08 mode pulse width modulator.
09 With reference to Figure 5, a multiple output power supply is illustrated, according to a 11 successful prototype of the present invention, 12 comprised of parallel input circuits 100 and 102, each 13 comprised of a pair of power switches 104, 105 and 14 106, 107 and a pair of input current sensing transformers 109 and 111. Pulse width modulators 112 16 and 114 are connected to the power switches 104-107 in 17 a well known manner. Pulse width modulators 112 and 18 114 are preferably in the form of integrated circuits, 19 such as the well known model 1526 pulse width modulator.
21 Transformer 109 has a secondary coil lO9B
22 thereof connected to an input of pulse width modulator 23 112 via a diode 116 and shunt resistor 118.
24 Similarly, a secondary coil lllB of transformer 111 i5 connected to a control input of 26 pulse width modulator 114 via a diode 120 and shunt 27 resistor 122.
28 A ramp signal is received from a ramp 29 signal generator (not shown), such as ramp signal generator 17 discussed above, and coupled to the 31 transformer secondary coils lO9B and lllB via an AC
32 coupling capacitor 124.
33 A pair of swinging chokes (or push-pull 34 converters), 123 and 125 are provided for coupling the input circuits 100 and 102 to respective auxiliary 36 output circuits 127 and 129 and to a main output 37 circuit 126 of the power supply.
1322~76 01 _ 9 _ 02 According to the preferred embodiment, the 03 two swinging chokes 123 and 125 function in a 04 proportional current sharing mode for the main output 05 circuit 126 which generates a 5 volt, 100 amp output 06 signal. Output circuit 129 delivers a 12 volt, 8 amp 07 auxiliary output signal and 40~ of the main output 08 signal, while output circuit 127 delivers a -12 volt, 09 1 amp auxiliary output signal and 60~ of the main 5 volt output signal.
11 By using stepped gap chokes 123 and 125, 12 the auxiliary output circuits are capable of 13 delivering power with as little as 1 amp total current 14 flowing from the main output circuit 126.
Voltage control feedback is provided from 16 the main output circuit 126 to respective control 17 inputs of pulse width modulators 112 and 114 by means 18 of a closed optocoupler 128 for receiving the main 19 output signal from a feedback amplifier arrangement comprised of buffer amplifier 130 and zener diode 132.
21 A magnetic amplifier 133 is provided in 22 the second auxiliary output circuit 129 for receiving 23 a portion of the auxiliary output signal via an 24 amplifier 134 connected to a zener diode 135, and maintaining a self-regulated auxiliary output signal, 26 as discussed above with reference to Figure 3.
27 Feed forward compensation is provided by 28 means of a further current sensing transformer coil 29 149A disposed in the auxiliary output circuit 129, coupled to a secondary coil 149B connected with 31 opposite polarity to the secondary coil lllB of input 32 current sensing transformer 111. A diode 510 and 33 shunt resistor 152 are connected across coil 149B in a 34 well known manner.
In operation, auxiliary output current 36 flowing through coil 149A results in generation of a 37 voltage signal across resistor 152 connected across 38 coil 149B, as a result of transformer coupling. The 1322~7~
02 voltage signal developed across resistor 152 is in 03 opposite polarity to the voltage appearing across 04 resistor 122 as a result of transformer coupling of 05 the input current signal flowing through transformer 06 coil 111. Thus, the net signal applied to the control 07 input of pulse width modulator 114 is the difference 08 of the input current signal minus the auxiliary output 09 current signal.
Twelve volt current limiting is provided 11 by an amplifier 154 connected to secondary coil lllB
12 and a tapped resistor 156 connected to the ramp signal 13 source via capacitor 124. Primary current limiting is 14 provided by a potentiometer 158 connected to the 15 output of optocoupler 128, and to the voltage control 16 input of pulse width modulator 114.
17 Auxiliary output circuit 127 is provided 18 with a pair of protective diodes 160 and 162 connected 19 across the secondary coil of swinging choke 123 and to 20 an output choke 164 connected in parallel with an 21 output filtering capacitor 166. The secondary coil of 22 choke 123 is center tapped for connection to an output 23 regulator 170 having a further input connected to 24 choke 164 and capacitor 166, and an output thereof for 25 generating the -12 volt auxiliary output signal.
26 Similarly, auxiliary output circuit 129 is 27 provided with a pair of protective diodes 180 and 182 28 connected to magnetic amplifier power switch 133, and 29 a further diode 186. The cathodes of diodes 180, 182 30 and 186 are connected together and to one terminal of 31 an output choke 188 having an opposite terminal 32 connected to an output capacitor 190 carrying the 12 33 volt auxiliary output signal.
34 Main output circuit 126 is also provided 35 with protective diodes 172, 174 and 192, 194, output 36 chokes 176 and 196, and output capacitors 178 and 198 37 all connected in a well known manner.
1322~7~
02 Due to design optimization in driving the 03 magnetic amplifier power switch 133, the 12 volt, 8 04 amp auxiliary output signal has a power density of 05 more than 10 watts per cubic inch and an efficiency of 06 more than 86~. By means of comparison, prior art 07 supplies are typically characterized by auxiliary 08 output circuits which deliver only 1 amp and dissipate 09 more power than the 8 amp output of the preferred embodiment.
11 According to the successful prototype, a 12 600 watt power supply was built for receiving a 48 13 volt DC input signal (Vi) and generating the 14 aforementioned main output signal at 5 volts and 100 amps, and the auxiliary output signals at 12 volts and 16 8 amps, and -12 volts at 1 amp, respectively.
17 A person understanding the present 18 invention may conceive of other embodiments or 19 variations thereof. All such embodiments or variations are believed to be within the sphere and 21 scope of the present invention as defined by the 22 claims appended hereto.
Claims (10)
1. In a multiple output power supply operating in current mode and comprised of an input circuit, a main output circuit, a regulator for feeding back a signal representative of current flowing in said main output circuit to said input circuit for regulating said input circuit, and at least one self-regulated auxiliary output circuit which affects operation of said regulator; a feed forward current compensation circuit comprised of:
(a) means for detecting current flowing in said auxiliary output circuit and generating a control signal representative thereof, and (b) means for subtracting said control signal from said signal representative of current flowing in said main output circuit, whereby the effect of said self-regulated auxiliary output circuit on said regulator is cancelled, thereby preventing oscillation of said power supply.
(a) means for detecting current flowing in said auxiliary output circuit and generating a control signal representative thereof, and (b) means for subtracting said control signal from said signal representative of current flowing in said main output circuit, whereby the effect of said self-regulated auxiliary output circuit on said regulator is cancelled, thereby preventing oscillation of said power supply.
2. A feed forward current compensation circuit as defined in claim 1, wherein said means for subtracting is comprised of first and second transformers having primary coils connected to said input and auxiliary output circuits respectively, and having oppositely poled secondary coils connected in series, said input circuit also being connected to a node connecting said secondary coils.
3. A feed forward current compensation circuit as defined in claim 2, further including a pair of series connected diodes having anodes thereof connected to the secondary coils of said first and second transformers respectively, and each having a cathode connected via a resistor to said node.
4. A multiple output current mode power supply comprised of:
(a) an input circuit for receiving an input signal, (b) a main output circuit transformer coupled to said input circuit, for generating a main output signal, (c) means for detecting said main output signal and in response generating a control voltage signal proportional to the amplitude thereof, (d) a pulse width modulator for receiving said control voltage signal and in response switching said input signal according to a predetermined pulse width, thereby regulating said input signal, (e) a self-regulated auxiliary output circuit transformer coupled to said input circuit, for generating an auxiliary output signal, and (f) a feed forward compensation circuit for receiving and subtracting said auxiliary output signal from said input signal, thereby generating a difference signal, and for subtracting said difference signal from said control voltage signal, whereby the effect of said self-regulated auxiliary output circuit on regulation of said input signal is cancelled.
(a) an input circuit for receiving an input signal, (b) a main output circuit transformer coupled to said input circuit, for generating a main output signal, (c) means for detecting said main output signal and in response generating a control voltage signal proportional to the amplitude thereof, (d) a pulse width modulator for receiving said control voltage signal and in response switching said input signal according to a predetermined pulse width, thereby regulating said input signal, (e) a self-regulated auxiliary output circuit transformer coupled to said input circuit, for generating an auxiliary output signal, and (f) a feed forward compensation circuit for receiving and subtracting said auxiliary output signal from said input signal, thereby generating a difference signal, and for subtracting said difference signal from said control voltage signal, whereby the effect of said self-regulated auxiliary output circuit on regulation of said input signal is cancelled.
5. A power supply as defined in claim 4, wherein said feed forward compensation circuit is comprised of first and second transformers having primary coils disposed in said input and auxiliary output circuits respectively, and oppositely poled series connected secondary coils for generating said difference signal at a node connecting said secondary coils.
6. A power supply as defined in claim 5, wherein said feed forward compensation circuit is further comprised of first and second diodes having anodes thereof connected to opposite end terminals of said first and second transformer secondary coils, and a pair of resistors connected to respective cathodes of said first and second diodes and to said node, the cathode of said first diode also being connected to a control input of said pulse width modulator.
7. A power supply as defined in claim 4, 5 or 6, wherein said auxiliary output circuit is comprised of a magnetic amplifier power switch for regulating current drawn by said auxiliary output circuit from said input circuit.
8. A power supply as defined in claim 4, 5 or 6, wherein said auxiliary output circuit is comprised of a magnetic amplifier power switch formed from a small saturable choke with no gap, and characterized by large inductance, for regulating current drawn by said auxiliary output circuit from said input circuit.
9. A power supply as defined in claim 4, 5 or 6, wherein said means for detecting said main output signal and in response generating said control voltage signal, is comprised of an optocoupler.
10. In a multiple output power supply operating in current mode and comprised of an input circuit, a main output circuit, a regulator for feeding back a signal representative of current flowing in said main output circuit to said input circuit, thereby regulating said input circuit, and at least one self-regulated auxiliary output circuit, a method for preventing oscillation in said power supply due to contention between said regulator and said self-regulated auxiliary output circuit, comprising the steps of detecting current flowing in said auxiliary output circuit and generating a signal representative thereof, and subtracting said signal representative of said current flowing in said auxiliary output circuit from said signal representative of current flowing in said main output circuit, whereby the effect of said self-regulated auxiliary output circuit on said regulator is cancelled, thereby preventing oscillation.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000560138A CA1322576C (en) | 1988-02-29 | 1988-02-29 | Multiple output power supply |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000560138A CA1322576C (en) | 1988-02-29 | 1988-02-29 | Multiple output power supply |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1322576C true CA1322576C (en) | 1993-09-28 |
Family
ID=4137531
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000560138A Expired - Fee Related CA1322576C (en) | 1988-02-29 | 1988-02-29 | Multiple output power supply |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1322576C (en) |
-
1988
- 1988-02-29 CA CA000560138A patent/CA1322576C/en not_active Expired - Fee Related
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