Abstract:
AC to DC inverters that are formed from electronic switches, such as MOSFETs, that are controlled by gating pulses obtained from comparators. The comparators compare a varying signal against two closely spaced reference voltages so as to provide gating pulses with delays needed to prevent shoot-through in the electronic switches.

Description:
BACKGROUND 
     In many cases it is necessary or desirable to power AC synchronous motors from a single voltage DC supply voltage. Typically, MOSFETs or other electronic switches are used to invert DC to AC power. Conventionally, inverters include driver circuits that require dual polarity supply voltages that may not be readily available in some applications. 
     Driver circuits must provide protection against shoot-through for the inverter to operate safely and efficiently. 
     There is a need for a driver circuit that can operate efficiently from a single polarity DC supply voltage with a minimum of components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of one preferred embodiment of the invention. 
     FIG. 2 is a block diagram of a second preferred embodiment of the invention. 
     FIG. 3 is an idealized timing diagram of signals that would be measured in the block diagrams of FIGS. 1 and 2 during operation. 
     FIGS. 4A,  4 B,  4 C, and  4 D are variants of the block diagram of FIG. 2 in which MOSFETs are used as electronic switches. 
     FIGS. 5,  6 ,  7 , and  8  are an idealized timing diagrams of signals that would be measured in the block diagrams of FIGS. 4A,  4 B,  4 C, and  4 D, respectively, during operation. 
     FIG. 9 is a block diagram of a third preferred embodiment of the invention. 
     FIG. 10 is an idealized timing diagram of signals that would be measured in the block diagram of FIG. 9 during operation. 
     FIG. 11 is an exemplary schematic circuit diagram corresponding to the block diagram of FIG.  1 . 
     FIG. 12 is an exemplary schematic circuit diagram corresponding to the block diagram of FIG.  4 B. 
     FIG. 13 is an exemplary schematic circuit diagram corresponding to the block diagram of FIG.  9 . 
    
    
     DETAILED DESCRIPTION 
     The circuits shown in block diagram form in FIGS. 1,  2 , and  9  are preferred embodiments of the invention. They are specifically designed to power a small AC synchronous pump operating at a frequency of 60 Hz for use in circulating coolant in a fluid-cooling system for a personal computer, but could be used with advantage for other purposes that will be evident to those skilled in the art. The circuit indicated generally by reference numeral  10  in FIG. 1 is designed to power a pump motor having a center-tapped winding. The circuits indicated generally by reference numeral  12  in FIG. 2, reference numerals  14 ,  15 ,  16 , and  17  in FIGS. 4A-4D, and reference numeral  62  in FIG. 9 are designed to power a pump motor requiring 60 Hz AC across its winding. 
     The circuits  10 ,  12  shown in FIGS. 1 and 2 use what will be referred to herein as “electronic switches”. All that is assumed about the electronic switches in the discussion of FIGS. 1 and 2 is that electronic switches have a control terminal indicated by reference letter G, which when a voltage is applied to it closes a connection between two other terminals that are referred to herein as “switched terminals” and keeps the connection closed until the applied voltage stops. FIGS. 1 and 2 are intended to illustrate the overall concept; as will be discussed below, additional circuit elements may be required to be added to FIG. 2 depending upon the type of electronic switch used. Specifically, modified block diagrams for the use of MOSFETs as electronic switches are provided in FIGS. 4A-4D and in FIG.  9 . 
     One limitation of all known electronic switches is that they do not operate instantaneously. For that reason, in many applications a short delay must be provided between the time one switch is turned off and the time another switch is turned on. For example, not providing a delay in the circuit  10  shown in FIG. 1 would cause reduced efficiency in the center-tapped pump motor connected to the output of the circuit because current could flow from the center tap through both halves of the motor winding at the same time in opposite directions if both of the switches were on simultaneously. In the circuits  12 ,  14 ,  15 ,  16 ,  17 ,  62  shown in FIGS. 2,  4 A- 4 D, and  9  lack of a delay could cause excessive current through the switches, possibly destroying them, because a current could flow directly through the both pairs of switches to ground without passing through the motor winding if all of the switches were closed long enough. In all circuits a delay is provided by the combination of a voltage divider  18 , a voltage follower  20 , an oscillator  22 , and a first comparator and a second comparator, labeled with reference numerals  24  and  26 , respectively, in FIGS. 1 and 2, with reference numerals  28  and  30 , respectively, in FIGS. 4A-4D, and with reference numerals  64 ,  66 ,  68 , and  70  in FIG.  9 . 
     A distinction is drawn between the comparators  24 ,  26  used in FIGS. 1 and 2 and the comparators  28 ,  30  used in FIGS. 4A-4D. In the circuit  10 ,  12  shown in FIGS. 1 and 2, the comparators  24 ,  26  also function as buffers and are hereinafter referred to as “comparator/buffers  24 ,  26 ”. For the same reason, the comparators  64 ,  66 ,  68 ,  70  shown in FIG. 9 are hereinafter referred to as “comparator/buffers  64 ,  66 ,  68 ,  70 ”. 
     In each circuit  10 ,  12 ,  14 ,  15 ,  16 ,  17 ,  62  the voltage divider  18  provides three closely spaced voltages from the DC supply voltage: an offset voltage half-way between the supply voltage and ground and two reference voltages bracketing the offset voltage and differing from it by approximately 1%. Specifically, if the supply voltage is 12 VDC, then the offset voltage is 6 VDC and the reference voltages are 5.94 VDC and 6.06 VDC. 
     The voltage follower  20  is connected between the offset voltage provided by the voltage divider  18  and the oscillator  22  so as to provide a low impedance 6 VDC source for the oscillator  22 . The oscillator  22  is operational amplifier configured as a relaxation oscillator so as to produce a waveform that is approximately a triangular wave signal varying from approximately 4.5 volts to 7.5 volts. In general, the waveforms shown in the drawings and discussed herein are idealizations of the actual waveforms that would be observed in the circuits described herein. In particular, the spacing between the reference voltages shown in FIGS. 5A,  6 A,  7 A,  8 A, and  10 A is greatly exaggerated so that the resulting delays are more clearly visible in the drawings. 
     In all circuits  10 ,  12 ,  14 ,  15 ,  16 ,  17 , the triangular waveform signal produced by the oscillator  18  is provided to both comparators  24 ,  26  or comparator/buffers  28 ,  30 . In circuits  10 ,  12  shown in FIGS. 1 and 2, the triangular wave signal is provided to the non-inverting input terminal of the first comparator/buffer  24  and to the inverting input terminal of the second comparator/buffer  26 . In the circuit  14  shown in FIG. 4A, the triangular wave signal is provided to the non-inverting input terminal of the first comparator  28  and to the inverting input terminal of the second comparator  30 . In the circuit  15  shown in FIG. 4B, the triangular wave is provided to the non-inverting input terminals of both comparators  28 ,  30 . In the circuit  16  shown in FIG. 4C, the triangular wave is provided to the inverting input terminals of both comparators  28 ,  30 . In the circuit  17  shown in FIG. 4D, the triangular wave signal is provided to the inverting input terminal of the first comparator  28  and to the non-inverting input terminal of the second comparator  30 . The corresponding connections in circuit  62  shown in FIG. 9 are discussed below. 
     The input terminal of each of comparators or comparator/buffers not connected to the oscillator  22  is connected to one or the other of the reference voltages provided by the voltage divider  18 . In the circuits  10 ,  12  shown in FIGS. 1 and 2, the inverting input terminal of the first comparator/buffer  24  is connected to the 6.06 VDC reference voltage and the non-inverting input terminal of the second comparator/buffer  26  is connected to the 5.94 VDC reference voltage. In the circuit  14  shown in FIG. 4A, the inverting input terminal of the first comparator  28  is connected to the 6.06 VDC reference voltage and the non-inverting input terminal of the second comparator  30  is connected to the 5.94 VDC reference voltage. In the circuit  15  shown in FIG. 4B, the inverting input terminal of the first comparator  28  is connected to the 6.06 VDC reference voltage and the inverting input terminal of the second comparator  30  is connected to the 5.94 VDC reference voltage. In the circuit  16  shown in FIG. 4C, the non-inverting input terminal of the first comparator  28  is connected to the 6.06 VDC reference voltage and the non-inverting input terminal of the second comparator  30  is connected to the 5.94 VDC reference voltage. In the circuit  17  shown in FIG. 4D, the non-inverting input terminal of the first comparator  28  is connected to the 6.06 VDC reference voltage and the inverting input terminal of the second comparator  30  is connected to the 5.94 VDC reference voltage. The corresponding connections in circuit  62  shown in FIG. 9 are discussed below. 
     In the circuit  10  shown in FIG. 1, the comparator/buffer  24  is connected to the control terminal of a first electronic switch  32  and the comparator/buffer  26  is connected to the control terminal of a second electronic switch  34 . The electronic switches  32 ,  34  each have two switched terminals that are effectively connected together when a control voltage is applied to their control terminals. One switched terminal of first electronic switch  32  is connected to a first output terminal  36  and the other switched terminal of first electronic switch  32  is connected to ground. Similarly, one switched terminal of second electronic switch  34  is connected to a second output terminal  38  and the other switched terminal of second electronic switch  28  is connected to ground. A third output terminal  40  is connected to the 12VDC supply voltage. In the application for which the circuit  10  of FIG. 1 was designed, the first and second output terminals  36 ,  38  are for connection to opposite ends of the winding of a pump motor (not shown in FIG.  1 ). The third output terminal  40  is for connection to a center tap of the winding of the pump motor. 
     In the circuit  12  shown in FIG. 2, the comparator/buffer  24  is connected to the control terminal of a second electronic switch  44  and to the control terminal of a third electronic switch  46 . The comparator/buffer  26  is connected to the control terminal of a first electronic switch  42  and to the control terminal of a fourth electronic switch  48 . The electronic switches  42 ,  44 ,  46 ,  48  each have two switched terminals that are effectively connected together when a control voltage is applied to their control terminals. One switched terminal of first electronic switch  42  and one switched terminal of the third electronic switch  46  are connected to the 12VDC supply voltage. One switched terminal of second electronic switch  44  and one switched terminal of the fourth electronic switch  48  are connected to ground. The other switched terminals of the first electronic switch  42  and the second electronic switch  44  are connected to a first output terminal  50 . The other switched terminals of the third electronic switch  46  and the fourth electronic switch  48  are connected to a second output terminal  52 . The resulting circuit configuration of electronic switches  42 ,  44 ,  46 ,  48  is commonly referred to as an H-bridge. In the application for which the circuit  12  of FIG. 2 was designed, the first and second output terminals  50 ,  52  are for connection to a pump motor. 
     FIG. 3 is a combined timing diagram for the circuit  10  shown in FIG. 1 that also applies to the circuit  12  shown in FIG.  2 . The output of the oscillator  22  is shown in FIG.  3 A and the outputs of the first and second comparator/buffers  24 ,  26  are shown in FIGS. 3B and 3C, respectively, when the circuit  10 ,  12  is connected to a 12 VDC supply. The comparator/buffer  24  produces at its output terminal a train of positive-going pulses shown in FIG. 3B each of which lasts from the time that the triangular wave signal shown in FIG. 3A from the oscillator  22  rises above 6.06 VDC and ends when that signal drops below 6.06 VDC The comparator/buffer  26  produces at its output terminal a train of positive-going pulses each of which lasts from the time that the triangular wave signal shown in FIG. 3A from the oscillator  22  drops below 5.94 VDC and ends when that signal rises above 5.94 VDC. As can be seen from FIGS. 3B and 3C, the two trains of pulses alternately go positive and are spaced so that the rise of a pulse from one train is delayed following the fall of the last pulse from the other train. 
     When the circuit  10  shown in FIG. 1 is provided with a 12 VDC supply voltage and connected to a center-tap winding pump motor at the output terminals  36 ,  38 ,  40 , then whenever a voltage pulse arrives from the first comparator/buffer  24  at the control terminal of the first electronic switch  32 , the first electronic switch  32  closes so as to connect the end of the winding of the pump motor that is connected to output terminal  36  to ground. Conversely, whenever a voltage pulse arrives from the second comparator/buffer  26  at the control terminal of the second electronic switch  34 , then the second electronic switch  34  closes so as to connect the end of the winding of the pump motor that is connected to output terminal  38  to ground. The result is that a current will flow, induced by the application of the 12 VDC supply voltage, through half of the winding of the pump motor and to ground through the electronic switch  32 ,  34  that is closed for as long as the pulse presented to the control terminal of the electronic switch  32 ,  34  lasts. In operation, a voltage is applied alternately to the control terminals of the electronic switches  32 ,  34 , so that current will alternately flow in opposite directions through alternate halves of the pump motor winding. Due to the delay discussed above, the electronic switches  32 ,  34  will not be closed at the same time, thereby preventing current from flowing in both halves of the winding at the same time, a situation that would tend to reduce the efficiency of the pump motor. If the pump motor winding were a purely resistive load, the resulting voltage across the output terminals  36 ,  38  would be as is shown in FIG.  3 D. 
     The operation of the circuit  12  shown in FIG. 2 is somewhat different from that of the circuit  10  shown in FIG. 1, although the timing diagram of FIG. 3 also applies. When the circuit  12  shown in FIG. 2 is provided with a 12 VDC supply voltage and connected to a pump motor at the output terminals  50 ,  52 , then whenever a voltage pulse arrives from the first comparator/buffer  24  at the control terminals of the second and third electronic switches  44 ,  46 , the second electronic switch  44  closes so as to connect the first output terminal  50  to ground and the third electronic switch  46  also closes to connect the second output terminal  52  to the 12 VDC supply voltage. Conversely, whenever a voltage pulse arrives from the second comparator/buffer  26  at the control terminals of the first and fourth electronic switches  42 ,  48 , the fourth electronic switch  48  closes so as to connect the second output terminal  52  to ground and the first electronic switch  42  also closes to connect the first output terminal  50  to the 12 VDC supply voltage. In operation, current will alternately flow in opposite directions through the pump motor winding. Due to the delay discussed above, the first and fourth electronic switches  42 ,  48  will not be closed at the same time that the second and third electronic switches  44 ,  46  are closed, thereby preventing current from bypassing the pump motor and flowing directly through in two paths, one through both the first and second electronic switches  42 ,  44  and the other through both the third and fourth electronic switches  46 ,  48 , a situation that could destroy the electronic switches  42 ,  44 ,  46 ,  48 . If the pump motor winding were a purely resistive load, the resulting voltage across the output terminals  50 ,  52  would be as is shown in FIG.  3 D. 
     However, the circuit  12  shown in FIG. 2 will not operate properly if all N-channel MOSFETs are used as electronic switches  42 ,  44 ,  46 ,  48  because the circuit cannot supply sufficient positive gate voltage. Preferably, P-channel MOSFETs should be used for first and third electronic switches  42 ,  46 , as will now be described in relation to FIGS. 4A,  4 B,  4 C, and  4 D. 
     The circuit  14  shown in FIG. 4A is a modification of the circuit  12  shown in FIG. 2 in which first and third electronic switches  42 ,  46  are P-channel MOSFETs and the second and fourth electronic switches  44 ,  48  are N-channel MOSFETs. In addition, a first buffer  54  has been added between the comparator  28  and the second electronic switch  44 , a first buffer/inverter  56  has been added between the comparator  28  and the third electronic switch  46 , a second buffer/inverter  58  has been added between the comparator  30  and the first electronic switch  42 , and a second buffer  60  has been added between the comparator  30  and the fourth electronic switch  48 . 
     The operation of the circuit  14  shown in FIG. 4A is similar to that of the circuit  12  shown in FIG. 2, with the exception that pulses to the P-channel MOSFETs used as the first and third electronic switches  42 ,  46  must be inverted due to the characteristics of P-channel MOSFETs. The first and second buffer/inverters  56 ,  58  provide the inversion as well as buffering. The first and second buffers  54 ,  60  simply provide buffering between the comparators  28 ,  30  and the second and fourth electronic switches  44 ,  48  as the N-channel MOSFETs used for those electronic switches do not require inversion. FIG. 5 is a timing diagram for the circuit  14 . FIG. 5A shows the output of oscillator  22 . FIGS. 5B and 5C show the outputs of the first and second comparators  28 ,  30 . FIGS. 5D,  5 E,  5 F, and  5 G show the inputs to the control terminals of the second, third, fourth and first electronic switches, respectively. FIG. 5H shows the output voltage across a resistive load connected between the output terminals  50 ,  52 . 
     The circuit  15  shown in FIG. 4B is a variant of the circuit  14  shown in FIG.  4 A. The only differences are that the inputs of the second comparator  30  have been reversed and the output connections of the second buffer/inverter  58  and the second buffer  60  have been interchanged so that the second buffer/inverter  58  is connected to the fourth electronic switch  48  and the second buffer  60  is connected to the first electronic switch  42 . Because the reversal of inputs to the second comparator  30  cancels out the interchanging of the output connections of the second buffer/inverter  58  and the second buffer  60 , the resulting pulse train applied to the electronic switches  42 ,  44 ,  46 ,  48  is unchanged. FIG. 6 is a timing diagram for the circuit  15 . FIG. 6A shows the output of oscillator  22 . FIGS. 6B and 6C show the outputs of the first and second comparators  28 ,  30 . FIGS. 6D,  6 E,  6 F, and  6 G show the inputs to the control terminals of the second, third, fourth and first electronic switches, respectively. FIG. 6H shows the output voltage across a resistive load connected between the output terminals  50 ,  52 . 
     The circuit  16  shown in FIG. 4C is a further variant of the circuit  14  shown in FIG.  4 A. The inputs of both the first comparator  28  and the second comparator  30  have been reversed. To provide the same gate signals as provided in circuits  14  and  15 , the output connection of the first buffer  54  is connected to the first electronic switch  42 , the output connection of the first buffer/inverter  56  is connected to the fourth electronic switch  48 , the output connection of the second buffer  60  is connected to the second electronic switch  44 , and the second buffer/inverter  58  is connected to the third electronic switch  46 . The resulting pulse train applied to the electronic switches  42 ,  44 ,  46 ,  48  is unchanged. FIG. 7 is a timing diagram for the circuit  16 . FIG. 7A shows the output of oscillator  22 . FIGS. 7B and 7C show the outputs of the first and second comparators  28 ,  30 . FIGS. 7D,  7 E,  7 F, and  7 G show the inputs to the control terminals of the second, third, fourth and first electronic switches, respectively. FIG. 7H shows the output voltage across a resistive load connected between the output terminals  50 ,  52 . 
     The circuit  17  shown in FIG. 4D is a further variant of the circuit  16 . The only differences are that the inputs of the second comparator  30  have been reversed and the output connections of the second buffer/inverter  58  and the second buffer  60  have been interchanged so that the second buffer/inverter  58  is connected to the third electronic switch  46  and the second buffer  60  is connected to the second electronic switch  44 . Because the reversal of inputs to the second comparator  30  cancels out the interchanging of the output connections of the second buffer/inverter  58  and the second buffer  60 , the resulting pulse train applied to the electronic switches  42 ,  44 ,  46 ,  48  is unchanged. FIG. 8 is a timing diagram showing for the circuit  17  shown in FIG.  4 D. FIG. 8A shows the output of oscillator  22 . FIGS. 8B and 8C show the outputs of the first and second comparators  28 ,  30 . FIGS. 8D,  8 E,  8 F, and  8 G show the inputs to the control terminals of the second, third, fourth and first electronic switches, respectively. FIG. 8H shows the output voltage across a resistive load connected between the output terminals  50 ,  52 . 
     In the circuits  14 ,  15 ,  16 ,  17  shown in FIGS. 4A-4D, the buffers  54 ,  60  are optional, as is the buffering function provided by the buffer/inverters  56 ,  58 . However, buffering is preferred. 
     FIG. 9 shows another alternative circuit  62  for use with MOSFETs as electronic switches in which four comparators/buffers and no inverters are used. A separate comparator/buffer is used for each MOSFET. The circuit  62  shown in FIG. 9 is identical to the circuits  14 ,  15 ,  16 ,  17  shown in FIGS. 4A,  4 B,  4 C, and  4 D up to the point at which the two reference voltages and the oscillator output are provided to comparators. However, rather than using buffer/inverters  58 ,  60  to obtain proper gate signals for the P-channel MOSFETs, two additional comparator/buffers are added in parallel with the two comparators used in circuits  14 ,  15 ,  16 ,  17 , but with their inputs reversed so as to provide inverted pulses. More specifically, the triangular waveform signal produced by the oscillator  18  is provided to the inverting input terminal of a first comparator/buffer  64 , the inverting input terminal of a second comparator/buffer  66 , the non-inverting input terminal of a third comparator/buffer  68 , and the non-inverting input terminal of a fourth comparator/buffer  70 . The non-inverting input terminal of a first comparator/buffer  64  and the non-inverting input terminal of a second comparator/buffer  66  are connected to the 6.06 VDC reference voltage and the inverting input terminal of a third comparator/buffer  68  and the inverting input terminal of a fourth comparator/buffer  70  are connected to the 5.94 VDC reference voltage. The output of the first comparator/buffer  64  is connected to the gate of the first electronic switch  42 , the output of the second comparator/buffer  66  is connected to the gate of the second electronic switch  44 , the output of the third comparator/buffer  68  is connected to the gate of the third electronic switch  46 , and output of the fourth comparator/buffer  70  is connected to the gate of the fourth electronic switch  48 . As in the circuits  14 ,  15 ,  16 ,  17  shown in FIGS. 4A,  4 B,  4 C, and  4 D, the first and third electronic switches are P-channel MOSFETs and the second and fourth electronic switches are N-channel MOSFETs. 
     FIG. 10 is a timing diagram showing for the circuit  62  shown in FIG.  9 . FIG. 10A shows the output of oscillator  22 . FIGS. 10B,  10 C,  10 D and  10 E show the outputs of the first, second, third, and fourth comparators  64 ,  66 ,  68 ,  70 , respectively, as well as the inputs to the control terminals of the first, second, third, and fourth electronic switches  42 ,  44 ,  46 ,  48 , respectively. FIG. 10F shows the output voltage across a resistive load connected between the output terminals  50 ,  52 . FIG. 10G shows the output voltage across an inductive load connected between the output terminals  50 ,  52 . 
     FIGS. 11,  12 , and  13  are schematic circuit diagrams showing examples of how the circuits  10 ,  15 ,  62  shown in block diagram form in FIGS. 1,  4 B, and  9  may be constructed. The subcircuits corresponding to the blocks of FIGS. 1,  4 B, and  9  are labeled with corresponding reference numerals. All resistors are ¼ watt 1%, unless otherwise indicated. The operational amplifiers are provided by TL084 integrated circuits. In FIG. 11, Q 1  and Q 2  are IRFZ44N MOSFETs in TO220 cases. In FIGS. 12 and 13, Q 1  and Q 3  are IRF5305 MOSFETs and Q 2  and Q 4  are IRFZ44N MOSFETs, all of which are in TO220 cases. Other MOSFETs may be used, as well as other case sizes. The component values shown in the oscillator subcircuit  20  in FIGS. 11,  12 , and  13  are selected to provide a 60 Hz triangular waveform reference signal. The circuits  14 ,  16 ,  17  shown in block diagram form in FIGS. 4A,  4 C, and  4 D may be constructed using the same component values. 
     In the exemplary circuits shown in FIGS. 11,  12 , and  13 , a resistive voltage ladder is used to provide the voltage divider  18 . The resistance of the resistor that connects to the 12V supply and that is shown as a 10K resistor in the resistive voltage ladder in FIGS. 11,  12 , and  13  should be adjusted to obtain a triangular waveform at the output of the oscillator that is rising for approximately the same time as it is falling. Otherwise a net DC voltage may develop across the output terminals. Changing the value of that resistor will, of course, change the values of the reference voltages and the DC-offset voltage, but will have little effect on the difference between the reference voltages. 
     Bipolar transistors may be used rather than MOSFETs in all of the circuits shown. However, as those skilled in the art will understand, external diodes are then required to limit fly-back voltage from an inductive load such as a pump motor winding, and the circuits will be less efficient due to the approximately 0.5 V drop across the bipolar transistors when they are switched on. 
     All circuits  10 ,  12 ,  14 ,  15 ,  16 ,  17 ,  62  shown in FIGS. 1,  2 ,  4 A,  4 B,  4 C,  4 D, and  9  are designed to be powered by a single polarity 12 volt DC supply, but may be adapted to dual polarity supplies by eliminating the voltage follower  20  and connecting the oscillator to ground. The ground connections shown in those drawings would then be connected to the negative polarity DC supply. For example, if a dual polarity 6 V supply with a ground were available, the +12 V terminal would be connected to the +6 V supply, the ground shown would be connected to the −6 V supply, and the oscillator would be connected to the ground. 
     The circuits presented herein may be used with advantage in applications other than providing AC power at a fixed frequency to a pump motor. For example, the frequency of the waveform provided by the oscillator may be controlled so as to vary the frequency of the output and slope of the waveform as it crosses the reference voltages may be varied to vary the power output by varying the time during which all of the electronic switches are off. Possible applications may include control of motor speed and light intensity. 
     Other embodiments will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.