HK1015085B - Opposed current power converter and method for operating thereof - Google Patents

Description

Butt-joint current power converter and operation method thereof

Technical Field

The present invention relates to current-interfaced pulse width modulation ('PWM') power converters and methods of operating the same.

Background

Power converters are widely used in industrial and commercial applications. Such power converters may be used to convert Direct Current (DC) to Alternating Current (AC) for use as an AC power source or battery charger/discharger, motor controller, or the like. Power converters can also be used as amplifiers in entertainment (sound amplifiers) and in industry. Prior art Pulse Width Modulation (PWM) converters employ a pair of switches to alternately connect a load to a dc power source of opposite polarity. The modulator alternately (not simultaneously conducting) opens and closes the switches to generate a pulse width modulated output signal, which is then filtered by a low pass filter before being delivered to the load. In prior art devices, the problem that must be emphasized is to ensure that the switches cannot be closed simultaneously, but for high fidelity management of the system, it is required that the opening and closing times of the switches are as close as possible. In order to manage the instantaneous current during switching (referred to as "rush current"), an inductor is connected between the switches. And the use of an "underlap" circuit creates a small, controllable time interval between switch conduction times. The opening and closing of the switch superimposes a so-called "ripple" frequency on the output waveform at a frequency equal to the switching frequency of the switch. Since most common input values are zero in speech and sound applications, the amplitude of the ripple frequency needs to be reduced as much as possible, especially at the zero output of the power converter, where most such devices are calibrated.

Disclosure of Invention

It is an object of the present invention to provide a power converter comprising a pair of switches that can be activated simultaneously to provide opposite current signals to a common output, thereby minimizing ripple and power consumption and accommodating periods of high power demand. It is another object of the present invention to provide a full bridge power converter having the functionality of the power converter described above. It is a further object of the present invention to provide a method of operating the above power converter.

Thus, according to one aspect of the present invention, there is provided a power converter comprising: a pair of switches (50, 56), each of said switches being connected between two constant voltage sources (54, 58) for controlling the current passing therethrough and an output node (52), said output node (52) being connected to a load (74), said currents being summed at said output nodes; and modulation means (82) for sequentially opening and closing the switches, wherein the switches are opened and closed by the pair in a sequence that activates in opposition to each other in the currents controlled by the switches.

According to another aspect of the invention, there is provided a full bridge power converter comprising a high side half bridge (48A) and a low side half bridge (48B), a load (74) connected between the half bridges, each half bridge comprising a pair of switches (50A, 50B, 56A, 56B), each switch of each half bridge being connected between a constant voltage source (54, 58) which controls the current passing therethrough, and a common output node, the currents being summed at the respective output nodes of each half bridge, the output node of each half bridge being connected to the load; and modulation means (82) for opening and closing the switches of the half-bridges, wherein the switches are opened and closed in a sequence in which the currents controlled by the switches of each half-bridge are activated in opposition to each other.

According to another aspect of the invention there is provided a method of operating a power converter comprising the steps of: the constant voltage source is connected to an output node (52) by means of switches (50, 56) connected between the constant voltage source and a common output node, wherein the switches are opened and closed in sequence, the switches are opened and closed in a sequence in which the currents controlled by the switches are activated in opposition to each other, and the currents are summed at the output node (52).

In the present invention, rather than having the switches have a conductive crossover, the crossover is carefully designed to be maximized. In the present invention, if zero output is required, S1 and S2 are closed simultaneously and the duty cycle is made at least less than 50%. If positive input is required, the duty cycle of the switch connecting the positive DC voltage source to the load is greater than 50%, and the duty cycle of the switch connecting the negative DC voltage source to the load is correspondingly less than 50%. If only the sum of the "on times" of the switches is required to not exceed 100% duty cycle during normal operation of the device, the device may operate in an under-folded or an overlapped state when the sum of the on times of the switches is greater or less than 100% duty cycle. For example, to minimize ripple and power consumption during the no-input period of the converter, an "under-overlap" condition may be employed. The "overlap" condition may satisfy the usage requirements when the converter momentarily exceeds the rated power.

Brief description of the drawings

Various advantages of the present invention will be further understood from the following description of the invention in conjunction with the accompanying drawings.

Fig. 1 is a circuit diagram of a half-bridge power converter made in accordance with the prior art;

FIG. 2 is a timing diagram illustrating the operation of the switches of the circuit of FIG. 1;

FIG. 3 is a circuit schematic of a converter made in accordance with the present invention;

FIG. 4 is a timing diagram of the circuit switch of FIG. 3 when open and closed;

FIG. 5 is a schematic diagram of the circuit switch of FIG. 3 providing current and their sum to an output node;

FIG. 6 is a circuit diagram of a full bridge converter made in accordance with the present invention;

FIG. 7 is a schematic diagram of a circuit used by a modulator to control the converter shown in FIG. 3 and made in accordance with the present invention;

fig. 8-12 are switching diagrams of the modulator of fig. 7 for generating switching pulses in the normal state and in the overlap and underlap states.

Referring now to fig. 1 and 2, which illustrate prior art devices, a prior art transducer or amplifier circuit, generally indicated at 10, is located between the modulator and the load (e.g., a speaker). Power converter 10 includes a switch 16 connected between a forward current source 18 and a node 20 and a switch 22 connected between node 20 and a negative DC power source 24. A time varying signal is applied to node 20. Inductive coils 26 and 28 are connected between the switches 16, 22 and the node 20, respectively, to manage inrush short circuit current when the switches 16, 22 are simultaneously open and closed. Switches 16 and 22 are governed by an output signal that is generated by modulator 12 and applied to switches 16 and 22 via lines 30 and 32. The modulator receives a bias signal on an input 34 and a feedback signal from an output node 36 which is supplied to the modulator 12 via line 38. The output node 36 is connected to the load 14. A low pass filter consisting of an inductor 40 and a capacitor 42 is connected between the output node 36 and the node 20 to filter the switching signal generated when the switches 16 and 22 are opened and closed. The switches 16 and 22 may be mosfets or IGBTs (insulated gate bipolar transistors) which are well known to those skilled in the art and which are controlled by switching signals delivered via lines 30 and 32.

Referring now to fig. 2, the modulator produces a waveform 44 that is delivered on line 32 to operate switch 16, and a waveform 46 that is delivered on line 32 to operate switch 22. It can be seen that the pulses are alternately delivered via lines 32 and 32; i.e., the signal on line 30 that closes switch 16 never allows closing switch 16 to open unless switch 22 is open, and vice versa. However, as noted above, for high fidelity and speech amplification, it is desirable to close switch 22 immediately after switch 16 is opened, and vice versa. The induction coils 26 and 28 are used to manage a "surge short," which is the instantaneous current generated when the switches 16 and 22 are opened and closed substantially simultaneously.

Referring now to fig. 3 and 4, a power converter circuit in accordance with the present invention is shown generally at 48 and includes a switch 50 connecting an output node 52 to a positive DC power source 54 and a switch 56 connecting the output node 52 to a negative DC power source 58. When the switch 50 is reopened after closing, the diode 60 allows current to freewheel through the output node 52 to the negative DC power supply 58, while the diode 62 allows current to freewheel through the output node 52 to the positive DC power supply 54. The signal applied to output node 52 through switch 50 is filtered by a low pass filter consisting of inductor 64 and capacitor 66, while a similar low pass filter consisting of inductor 68 and capacitor 66 filters the current resulting from the closing and opening of switch 56. Switches 50 and 56 are controlled by a modulator discussed in detail below with respect to fig. 8. As described above, switches 50 and 56 may be implemented with MOSFETs, IGBTs, or other similar devices.

Referring now to fig. 4, waveform 70 represents the signal from the modulator (not shown) operating switch 50 and waveform 72 represents the signal from the modulator operating switch 56. As shown, the duty cycles of the waveforms 70, 72 are both 50%. So that when they are added, the output at output node 52 is zero since switch 50 connects the output node to the positive DC supply 54 and switch 56 connects the output node 52 to the negative DC supply 58. The small arrows on waveforms 70 and 72 indicate the direction of modulation on common supply node 52 to increase the forward output connected to load 74. The center portions of the waveforms 70, 72 overlap each other, and the width of the waveform 72 decreases and the width of the waveform 70 increases as the forward output increases.

Referring to the waveforms shown in fig. 5, waveform 76 represents the current in inductor 64, waveform 78 represents the current in inductor 68, and waveform 80 represents the sum of the upper and lower waveforms shown in fig. 5, which is the current at output node 52. It will be seen that the waveforms 76 and 78 include a ripple superimposed on the waveforms, the ripple frequency or superimposed waveform being the result of the closing and opening of switch 50 in the case of waveform 76 and the closing and opening of switch 56 in the case of waveform 78. It will be seen that a similar waveform is superimposed on waveform 80 but the frequency of the waveform is twice the frequency of the waveform superimposed on curves 76 and 78. This is because switches 50 and 56 are activated at different times, so the ripple superimposed on the sum of the waveforms doubles the frequency of any single switch. This is desirable because higher frequency ripples are easily filtered out. It will be seen that at zero output, represented by X in figure 5, the ripple amplitude is substantially zero. This is because switches 50 and 56 are activated at approximately the same time at zero output current as shown in fig. 4. The ripple is therefore zero at zero output. As mentioned above, the test standard for DC-AC converters is generally specified by a ripple at zero output. By using the invention, the ripple amplitude under zero output is minimum. The ripple indicator at zero output is chosen because the ripple overlap noise is most noticeable and annoying at pauses without output when amplifying sound. In speech amplification, this is most common due to pauses between words.

It will be appreciated that in the present invention, small inductors such as inductors 26 and 28 shown in fig. 1 to manage surge shorts are not required because the switches are isolated by inductors 64, 68 which act as low pass filters. Also, as described below, underlap, which can superimpose a zero output over a longer time interval, is easier to achieve by the modulator needed to operate the converter 48. The modulator is also greatly simplified in construction compared to that required by the prior art and requires no circuitry to achieve underlap but by simply shifting a DC bias on the triangular waveform, as will be described in more detail below.

Referring now to fig. 6, a load 74 is connected to a full-bridge converter consisting of two half-bridge converters 48A, 48B, each of which is identical to the half-bridge converter shown in fig. 3. The cells of each half-bridge converter 48A, 48B therefore retain the same reference numerals, but with the addition of the letters "a" and "B". It is well known to shift the operating phase of the switches of one half-bridge of a full-bridge converter relative to the other half-bridge. The current ripple frequency to the load 79 is therefore twice that on each half bridge. Adding the half-bridge of the present invention will increase the ripple frequency of each. As mentioned above, high frequency ripples are more easily filtered.

Referring now to fig. 7, the modulator, generally designated 82, includes an error amplifier 84 that sums the difference between an input signal at an output node representing a desired level (fed back to an input 86 of the internal amplifier 84) and a feedback signal received at an input 88 of the internal amplifier 84. The feedback signal on input 88 is taken from output node 52. The output of error amplifier 84 is supplied to inverting input 87 of comparator 89 whose output is supplied on line 90 to operate switch 56. The output of error amplifier 84 is also fed through inverter 29 and then to inverting input 94 of comparator 96, whose output 98 is connected to operating switch 50.

The modulator 82 also includes a triangular wave generator, generally indicated by the numeral 100, which receives a square wave input, generally generated in a conventional manner, at 102 and produces a triangular wave output on line 104 which is then passed to a positive input 106 of the comparator 89 and a positive input 108 of the comparator 96. The triangular wave generator 100 is a general device and includes a bias control device 110 that adjusts the DC level of the waveform generated by the triangular wave generator up and down. The bias control device 110 may respond to dynamic conditions such as pauses in voice delivery and temporary power demands and access typically provided by the transducer.

Referring now to fig. 8-12, the operation of the modulator 82 and the converter 48 is described below. Referring to fig. 8, the triangular wave T generated by the triangular wave generator 100 is centered at zero. Assume that the output of error amplifier 84 is also zero. In this case, the comparators 89 and 96 are on at point a and off at point B. Thus producing waveforms 70 and 72 as shown in fig. 4. The waveforms 70 and 72 are on and off simultaneously and the period is exactly equal to half the duty cycle (duty cycle between the dots on the triangle wave T, e.g., between points a and a or points B and B). As described above, switch 50, which is connected to the positive DC power supply, is turned on and off by waveform 70, while switch 56, which is connected to the negative DC power supply, is turned on and off by waveform 72. Since the period of the switches 50 and 56 is exactly equal to half the duty cycle, the voltage to the switches is zero after summing on the output node 52.

Referring now to fig. 9, the triangular wave T is still centered at zero, but the output of amplifier 84 is +1 units. The output is provided to a comparator 96 via an inverter 92. Comparator 96 is therefore turned on when the triangular wave exceeds-1 unit as shown by point C and turned off when the triangular wave falls below one unit as shown by F. Also comparator 98 turns on when the triangle wave exceeds +1 unit as shown by point D and turns off when the triangle wave falls below one unit as shown by E. Waveform 70 is therefore on at C and off at F, and waveform 72 is on at D and off at E. As shown in fig. 9, the centers of the waveforms 70 and 72 coincide with each other. Thus, since waveforms 70 and 72 are summed at output node 52, there is a positive output provided to load 74. Referring to fig. 10, assuming that the output of amplifier 84 is negative by one unit, the "on" times of switches 50 and 56 are the opposite of those of waveforms 70 and 72 shown in fig. 9. Switch 56 will therefore be on at point G and off at point J, and switch 50 will be on at point H and off at point I. The sum of the conduction times of the waveforms 70, 72 is still equal to one period of the triangular wave T.

Fig. 11 shows the operation of the modulator 82 and the converter 48 in a so-called overlap condition, which occurs for example in the case of a corresponding instantaneous high power demand. The output of amplifier 84 is assumed to be zero. In the overlapped state, the bias control device 110 is operated to shift the waveform T upward by one unit so that the center is one unit above zero instead of the center being zero in fig. 8-10. The dashed lines in fig. 11 and 12 include the positions of the triangular waveforms when the DC bias is not displaced. Thus both comparators are on at point K and off at point L, producing the same waveforms as waveforms 70 and 72 to operate switches 50 and 56. It will be seen, however, that the sum of the "on" times of switches 50 and 56 shown by waveforms 70 and 72 exceeds one duty cycle of the triangular wave T. Referring to fig. 12, it shows an under-folded condition in which the switches 50, 56 are conducting for a total period less than the duty cycle of the triangular wave T. The under-folded state is employed when, for example, the converter output is not needed for a period of time. The underlap state is achieved by operating the bias control device 110 to displace the triangular wave T upward by one unit. As shown in fig. 12, the triangular wave T is displaced downward by one unit from having its center located one unit below zero. The output of amplifier 48 is assumed to be zero. In this case, as shown in FIG. 12, waveforms 70 and 72 are generated when the triangle wave is greater than zero, see point M, and end when the triangle wave is below point N. The waveforms 70 and 72 are therefore both on and off, but the overall duty cycle is less than one duty cycle of the triangular wave T, since the dc bias of the triangular wave T is shifted downwards.

During power converter operation, a constant voltage source is first connected to an output node (52) by means of switches (50, 56) connected between the constant voltage source and a common output node, wherein the switches are opened and closed in sequence, the switches are opened and closed in a sequence such that currents controlled by the switches activate opposite to each other, and the currents are summed at the output node (52). The method may further include the step of opening and closing the switches (50, 56) at a controlled duty cycle. Alternatively, the output of the output node (52) may be controlled by increasing or decreasing the duty cycle of one of the switches (50, 56) and correspondingly decreasing or increasing the duty cycle of the other of the switches (50, 56). Further, the step of closing one of the switches, closing the other switch and opening the other switch before opening the switches may be performed. Finally, one of the switches may be closed and then the other switch closed before opening the switch.

Claims (19)

1. A power converter, comprising: a pair of switches (50, 56), each of said switches being connected between two constant voltage sources (54, 58) for controlling the current passing therethrough and an output node (52), said output node (52) being connected to a load (74), said currents being summed at said output nodes; and modulation means (82) for sequentially opening and closing the switches, wherein the switches are opened and closed by the pair in a sequence that activates in opposition to each other in the currents controlled by the switches.

2. A power converter as claimed in claim 1, characterized in that said modulating means (82) comprises means (84, 89, 96, 100) for opening and closing each of said switches (50, 56) within a controllable duty cycle.

3. A power converter as claimed in claim 2, characterised in that said modulating means (82) comprises means (100, 89, 86) for controlling the output at the output node by increasing or decreasing the duty cycle of one of said switches and correspondingly decreasing the duty cycle of the other of said switches.

4. A power converter as claimed in claim 3, characterised in that the constant voltage source comprises a positive voltage source (54) and a negative voltage source (58), one of the switches (50) being connected between the positive voltage source and the common output node (52) and the other switch (56) being connected between the negative voltage source (58) and the common output node (52), a first one-way conducting means (60) being connected between the one switch (50) and the negative voltage source (58) to allow current to be drawn from the common output node (52) when the switch (50) is open, and a second one-way conducting means (62) being connected between the other switch (56) and the positive voltage source (54) to allow current to be drawn from the common output node (52) when the other switch (56) is open.

5. A power converter as claimed in claim 1, characterized in that said modulating means comprises means (100, 89, 86) for closing one of said switches, closing the other switch and opening the other switch before opening said switch.

6. A power converter as claimed in claim 1, wherein one of said switches (50) is connected to said output node through a first inductor (64) and the other switch (56) is connected to said output node through a second inductor (68).

7. A power converter as claimed in claim 1, characterised in that said modulator means (82) comprises means (100, 89, 96) for closing one of said switches and subsequently closing the other switch before opening the switch.

8. A power converter as claimed in claim 1, characterized in that said modulating means (82) comprises means (89, 96) for generating a first electrical signal for controlling said one switch and a second electrical signal for controlling the other switch; a triangular wave generating means (100) for generating a triangular wave signal; and means (84, 92) for generating said first and second electrical signals as a function of the same triangular wave signal generated by said triangular wave generating means.

9. A power converter as claimed in claim 8, characterized in that said modulating means (82) comprises a pair of comparators (89, 96) for generating said first and second electrical signals, one of said comparators (89) comparing said triangular wave signal with a reference signal and the other comparator (96) comparing said triangular wave signal with an inverted signal of said reference signal.

10. A power converter as claimed in claim 8, characterized in that said modulating means (82) comprises means (110) for shifting the DC level of said triangular wave signal to form an under-crossover between said switching outputs.

11. A full bridge power converter comprising a high side half bridge (48A) and a low side half bridge (48B), a load (74) connected between the half bridges, each half bridge comprising a pair of switches (50A, 50B, 56A, 56B), each switch of each half bridge being connected between a constant voltage source (54, 58) which controls the current passing through it, and a common output node, the currents being summed at a respective output node of each half bridge, the output node of each half bridge being connected to the load; and modulation means (82) for opening and closing the switches of the half-bridges, wherein the switches are opened and closed in a sequence in which the currents controlled by the switches of each half-bridge are activated in opposition to each other.

12. Full bridge power converter as claimed in claim 11, characterized in that said modulator means (82) comprises means (100, 110) for phase shifting the operation of one half bridge switch with respect to the operation of the other half bridge switch.

13. Full bridge power converter as claimed in claim 11, wherein said modulating means (82) comprises means for controlling the output of the output node (52) of each half bridge (48A, 48B) by increasing or decreasing the duty cycle of said one of the switches of each half bridge and correspondingly decreasing or increasing the duty cycle of the other of said switches of each half bridge.

14. A full-bridge power converter as claimed in claim 13, wherein said constant voltage sources include a positive voltage source (54A, 54B) and a negative voltage source (58A, 58B), one of said switches of each half-bridge being connected between said positive voltage source and said common output node and the other switches of each half-bridge being connected between said negative voltage source and said common output motor, a first one-way conducting means being connected between said one switch of each half-bridge and said negative voltage source to permit current to be drawn from said common output node when said switch of each half-bridge is open, and a second one-way conducting means being connected between said other switch of each half-bridge and said positive voltage source to permit current to be drawn from said common output node when said other switch of each half-bridge is open.

15. A method of operating a power converter, comprising the steps of: the constant voltage source is connected to an output node (52) by means of switches (50, 56) connected between the constant voltage source and a common output node, wherein the switches are opened and closed in sequence, the switches are opened and closed in a sequence in which the currents controlled by the switches are activated in opposition to each other, and the currents are summed at the output node (52).

16. A method of operating a power converter as claimed in claim 15, characterized in that the method comprises the step of opening and closing the switches (50, 56) in a period of a controllable duty cycle.

17. A method of operating a power converter as claimed in claim 15, characterized in that the method includes the step of controlling the output at the output node (52) by increasing or decreasing the duty cycle of one of said switches (50, 56) and correspondingly decreasing or increasing the duty cycle of the other of said switches (50, 56).

18. The method of operating a power converter as recited in claim 15 further comprising the steps of closing one of said switches, closing the other switch and opening the other switch before opening said switch.

19. A method of operating a power converter as claimed in claim 1, characterized in that said method comprises the step of closing one of said switches and subsequently closing the other switch before opening the switch.