AC-AC converter

Abstract

An AC-AC converter is described that includes an input terminal for connection to an AC power source, an output terminal for connection to a load, a transformer having a primary winding and a secondary winding, a primary side switch, a pair of secondary side switches, and a control circuit that controls the switches. Closing the primary side switch causes a primary current to draw from the AC power source and store energy in the transformer through the primary winding. Closing one secondary side switch causes a secondary current to be output at the output terminal to transfer energy from the transformer to the load. The secondary current has the same polarity as the primary current when the first secondary side switch is closed, and the secondary current has an opposite polarity when the second secondary side switch is closed. The control circuit is operable to control the switch such that an alternating current is output at the output terminal.

Description

Technical Field

[0001] The present invention relates to an AC-AC converter. Background Art

[0002] An AC-AC converter may include a rectifier, a power factor correction (PFC) circuit, a DC link capacitor and an inverter. A problem with this arrangement is that the number of components may be high. In addition, the DC link capacitor typically requires a relatively high capacitance. Summary of the invention

[0003] The present invention provides an AC-AC converter, comprising: an input terminal for connecting to an AC power source; an output terminal for connecting to a load; a transformer having a primary winding and a secondary winding; a primary side switch connected between the input terminal and the primary winding; a pair of secondary side switches connected between the output terminal and the secondary winding; and a control circuit for controlling the switches, wherein: the primary side switch and the secondary side switch are bidirectional; closing the primary side switch causes the primary current to be drawn from the AC power source and pass through the primary winding, thereby storing energy in the transformer; closing one of the secondary side switches causes the secondary current to be output at the output terminal to transfer energy from the transformer to the load, and when the first secondary side switch is closed, the secondary current has the same polarity as the primary current, and when the second secondary side switch is closed, has the opposite polarity; and the control circuit is operable to control the switches so that an alternating current is output at the output terminal.

[0004] Thus, the converter provides AC-AC conversion without the use of a rectifier on the primary side or an inverter on the secondary side. Thus, AC-AC conversion can be achieved with a relatively low component count. In fact, AC-AC conversion can be achieved with only three switches. This is achieved by using bidirectional switches, which can be controlled in both directions. That is, each switch can be conducting and non-conducting in both directions. Thus, the primary side switch can be closed and the primary current can be drawn through the primary winding to store energy in the transformer regardless of the polarity of the AC supply. The secondary side switch can then be controlled so that an alternating current is output at the output terminals regardless of the polarity of the primary current.

[0005] In addition to a relatively low component count, the converter is able to achieve AC-AC conversion with relatively little energy storage. In particular, the converter does not include an AC-DC stage and therefore does not require a DC link capacitor. The converter may include an input filter on the primary side to smooth the input current drawn from the AC power supply, and / or an output capacitor on the secondary side to smooth the output current at the output terminals. However, the control circuit may adopt a relatively high switching frequency for the switches so that the input filter and / or the output capacitor need to store relatively little energy.

[0006] The converter acts as a current source. In particular, when one of the secondary side switches is closed, a voltage is induced in the secondary winding which causes a secondary current to be output at the output terminals. The magnitude of the induced voltage is sufficient to produce a secondary current regardless of any reverse voltage across the output terminals. Therefore, the converter can be used to transfer power to a load that presents an opposite voltage. For example, the converter can be used to power a permanent magnet motor. When the rotor of the permanent magnet motor rotates, a back EMF is induced in the phase winding. Therefore, when the output terminals of the converter are coupled to the phase winding, a back EMF appears at the output terminals. The converter, acting as a current source, is still able to drive current and therefore power into the phase winding regardless of the magnitude of the back EMF.

[0007] The secondary side switches may be connected in series to form a branch. The first end of the branch is then connected to the first end of the secondary winding, and the second end of the branch is connected to the second end of the secondary winding. In one arrangement, one output terminal may be connected to the midpoint of the branch, and the other output terminal may be connected to the center tap of the secondary winding. In another arrangement, the converter may include another pair of secondary side switches connected in series to form another branch. The first end of the other branch is then connected to the first end of the secondary winding, the second end of the other branch is connected to the second end of the secondary winding, one output terminal is connected to the midpoint of the branch, and the other output terminal is connected to the midpoint of the other branch. The advantage of the former arrangement is that only one pair of secondary side switches is required, while the advantage of the latter arrangement is that the transformer is simpler (i.e., a transformer with a tapless secondary winding).

[0008] Each secondary side switch can include four states: (1) open, where the switch does not conduct in either direction; (2) closed, where the switch conducts in both directions; (3) a first diode mode, where the switch conducts only in the first direction; and (4) a second diode mode, where the switch conducts only in the second direction. The control circuit can then be operated to: close one of the secondary side switches to output a secondary current; monitor the magnitude of the secondary current; and when the magnitude of the secondary current is less than or equal to a threshold, change the state of the secondary side switch from closed to one of the diode modes. By changing the state of the switch to the diode mode when the secondary current drops to the threshold, polarity reversal of the secondary current can be avoided, thereby improving the efficiency of the converter.

[0009] The control circuit is operable to receive a signal indicative of the polarity of the voltage at the output terminal, and the control circuit is operable to control the secondary side switch so that the secondary current has the same polarity as the voltage. As described above, the load may present an opposite voltage at the output terminal. By ensuring that the secondary current has the same polarity as the voltage at the output terminal, power can be delivered to the load regardless of the voltage produced by the load at the output terminal.

[0010] The control circuit is operable to control the switch so that during a switching cycle, the switch is configured as: a first configuration in which the primary current is drawn from the AC power source to store energy in the transformer; a second configuration in which the secondary current is output at the output terminal to transfer energy from the transformer to the load; and a third configuration in which the energy in the transformer is circulated around the freewheeling circuit or transferred from the transformer to another load. Therefore, the converter is able to deliver unbalanced energy transfer. That is, the energy drawn from the AC power source and stored in the transformer when the switch is in the first configuration may be different from the energy transferred from the transformer to the load when the switch is in the second configuration. Then, the difference in energy is managed by configuring the switch to the third configuration, which allows the energy in the transformer to circulate around the freewheeling circuit or be transferred to a different load. The use of unbalanced energy transfer makes the shape of the instantaneous output power different from the shape of the instantaneous input power. Therefore, the efficiency of the converter can be improved. For example, the instantaneous output power can be better shaped to avoid or reduce excessive output current, while the instantaneous input power can be shaped to obtain a good power factor.

[0011] The control circuit is operable to: monitor the primary current; switch the switch from the first configuration to the second configuration when the primary current meets the first criterion; monitor the secondary current; and switch the switch from the second configuration to the third configuration when the secondary current meets the second criterion. Thus, both the input current and the output current, as well as the input power and the output power, can be independently shaped. In particular, the first configuration can be employed for an arbitrary time period until the primary current meets the first criterion, and the second configuration can be employed for an arbitrary time period until the secondary current meets the second criterion. These arbitrary time periods, which are shorter in total than the total period of the switching cycle, are made possible by the third configuration, which provides a path for the excess energy stored in the transformer. The first criterion and the second criterion can then be defined so that a specific profile is achieved for the primary current (and therefore the input current drawn from the AC power source) and the secondary current (and therefore the output current applied to the load).

[0012] The control circuit is operable to switch the switch from the second configuration to the third configuration when the integral of the secondary current is greater than or equal to the average current reference.This form of current control may better approximate a desired profile (eg, sinusoidal) of the output current.

[0013] When the integral of the primary current is greater than or equal to the average current reference, the control circuit is operable to switch the switch from the first configuration to the second configuration. As described in the previous paragraph, this form of current control can better approximate the desired profile of the input current (e.g., sinusoidal). The control circuit can use the same form of current control (i.e., average current control) for both the input current and the output current.

[0014] The freewheeling circuit may comprise one of the primary winding and the secondary winding. This has the advantage that the residual energy in the transformer can be managed in a cost-effective manner. Furthermore, the losses due to the leakage inductance may be reduced compared to a freewheeling circuit with an auxiliary winding.

[0015] The present invention also provides a motor system comprising an AC-AC converter as described in the preceding paragraph and a permanent magnet motor, wherein a phase winding of the permanent magnet motor is connected to the output terminals.

[0016] Conventional AC power supplies for permanent magnet motors typically include a rectifier, a PFC circuit, and a DC link capacitor that outputs a stable DC voltage, and an inverter that converts the DC voltage into an AC voltage that is applied to the phase winding. In addition to having a relatively high number of components and a high-capacitance DC link capacitor, the problem with this arrangement is that it is difficult to control the current and, therefore, the power delivered to the motor. The magnitude of the back EMF induced in the phase winding is opposite to the applied voltage, and it varies with the position and speed of the rotor. Therefore, when controlling the inverter, a relatively complex scheme is usually required to ensure that the required current and power are driven into the phase winding. On the other hand, the present converter acts as a current source. When a secondary-side switch is closed, a voltage is induced on the secondary winding, which applies a secondary current to the phase winding regardless of the magnitude of the back EMF. Therefore, compared to conventional AC power supplies, a much simpler control scheme can be used to drive current and power into the phase winding.

[0017] The control circuit can be used to control the switches so that the alternating current at the output terminals has the same frequency as the back EMF induced in the phase winding. This improves the efficiency of the motor system.

[0018] The control circuit is operable to control the switches at a switching frequency that is higher than the frequency of the back EMF induced in the phase windings. This has the advantage that the input current drawn from the AC supply and / or the output current produced at the output terminals can be smoothed using less energy storage. If the transformer does not need to store any residual energy at the end of each switching cycle (for example, because the energy transfer is balanced, or because any residual energy is delivered to different loads), operating at a higher switching frequency has the further benefit that the transformer needs to store less energy and therefore a smaller transformer can be used.

[0019] The motor can output a signal indicating the polarity of the back EMF induced in the phase winding, and the control circuit can receive the signal and control the secondary side switch so that the secondary current has the same polarity as the back EMF. This improves the efficiency of the motor system.

[0020] The control circuit can monitor the speed of the motor and control the primary side switch to adjust the magnitude of the primary current in response to changes in the motor speed. Therefore, the input power can be adjusted in response to changes in the rotor speed, thereby adjusting the output power. During motor startup, the magnitude of the back EMF induced in the phase winding is relatively small. Therefore, for a given primary current, the secondary current will be relatively high. By adjusting the magnitude of the primary current in response to changes in the motor speed, excessive secondary currents can be avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a circuit diagram of a first example of an AC-AC converter;

[0022] Figure 2 is a circuit diagram of a second example of an AC-AC converter;

[0023] Figure 3 is a circuit diagram of a third example of an AC-AC converter;

[0024] Figure 4 shows an example of current flow in a transformer of a third AC-AC converter;

[0025] Figure 5 is a circuit diagram of a fourth example of an AC-AC converter;

[0026] Figure 6 is a circuit diagram of a fifth example of an AC-AC converter;

[0027] Figure 7 is a circuit diagram of a sixth example of an AC-AC converter; and

[0028] Figure 8 This is the circuit diagram of the motor system. DETAILED DESCRIPTION

[0029] Figure 1 The AC-AC converter 10 includes input terminals 12 , 13 , an input filter 14 , a primary side switch 20 , a transformer 22 , a pair of secondary side switches 30 , 32 , an output capacitor 40 , output terminals 42 , 43 and a control circuit 50 .

[0030] The input terminals 12 , 13 are connected to an AC power source 60 , and the output terminals 42 , 43 are connected to a load 70 .

[0031] The input filter 14 smoothes the current drawn from the AC power source 60 and, in this example, includes an inductor 16 and a capacitor 18 .

[0032] The primary side switch 20 and the secondary side switches 30, 32 are bidirectional switches (e.g., gallium nitride switches) that can be controlled in both directions. That is, each switch can be conducting and non-conducting in both directions. Therefore, the switches are different from MOSFETs that have body diodes, which can conduct in both directions but only non-conduct in one direction. In this particular example, each switch has four possible states: (1) open, in which the switch does not conduct in either direction; (2) closed, in which the switch conducts in both directions; (3) a first diode mode, in which the switch conducts in only one direction; and (4) a second diode mode, in which the switch conducts in only the other direction.

[0033] The transformer 22 includes a primary winding 24 and a secondary winding 26. The secondary winding 26 is a center-tapped winding having a first end 27, a second end 28, and a center tap 29 located between the two ends 27,28.

[0034] The primary side switch 20 and the primary winding 24 are connected in series between the input terminals 12, 13. The secondary side switches 30, 32 are connected in series to form a branch. Then, the first end of the branch is connected to the first end 27 of the secondary winding 26, and the second end of the branch is connected to the second end 28 of the secondary winding 26. One output terminal 42 is connected to the middle point of the branch (i.e., the point between the two secondary side switches 30, 32), and the other output terminal 43 is connected to the center tap 29 of the secondary winding 26.

[0035] The output capacitor 40 is connected in parallel across the output terminals 42 , 43 for smoothing the current output at the output terminals 42 , 43 , as described in more detail below.

[0036] The control circuit 50 is responsible for controlling the switches 20, 30, 32 and outputs gate signals to control the state of each switch 20, 30, 32. The control circuit 50 includes a voltage sensor 52, a primary side current sensor 54, and a secondary side current sensor 56. The voltage sensor 52 provides a voltage measurement of the AC power source 60. The primary side current sensor 54 provides a measurement of the primary current in the primary winding 24, and the secondary side current sensor 55 provides a measurement of the secondary current in the secondary winding 26. The control circuit 50 then uses the sensor measurements to control the state of the switches 20, 30, 32.

[0037] The operation of the converter 10 will now be described.

[0038] For each switching cycle, the control circuit 50 begins by opening the secondary side switches 30, 32 and closing the primary side switch 20. Closing the primary side switch 20 causes a primary current to be drawn from the AC power source 60 and through the primary winding 24. With the secondary side switches 30, 32 open, the primary current and magnetic flux in the transformer 22 increase, thereby storing energy in the transformer 22.

[0039] The control circuit 50 monitors the primary current. When the primary current meets a criterion (described below), the control circuit 50 opens the primary side switch 20 and closes one of the secondary side switches 30, 32, which causes the secondary current to be output at the output terminals 42, 43, thereby powering the load 70 and charging the output capacitor 40. As a result, energy is transferred from the transformer 22 to the load 70 (and the output capacitor 40), and the magnetic flux in the transformer 22 is reduced.

[0040] The control circuit 50 monitors the secondary current. If the secondary current decreases to zero, the control circuit 50 opens the secondary side switches 30, 32 to prevent the secondary current from reversing. Regardless of the magnitude of the secondary current, the control circuit 50 opens the secondary side switches 30, 32 and closes the primary side switch 20 at the end of the switching cycle.

[0041] The control circuit 50 then repeats the switching cycle described above.

[0042] When the first secondary side switch 30 is closed, the secondary current has the same polarity as the primary current. Conversely, when the second secondary side switch 32 is closed, the secondary current has the opposite polarity to the primary current. Then, the control circuit 50 controls the secondary side switches 30, 32 so as to output the alternating current at the output terminals 30, 32. The primary current has the same polarity as the AC power source 60. Therefore, the control circuit 50 controls the secondary side switches 30, 32 based on the polarity of the AC power source 60 determined by the voltage sensor 52.

[0043] As described above, when the primary current meets the criterion, the control circuit 50 opens the primary side switch 20 and closes one of the secondary side switches 30, 32. In one example, the criterion can be met when the instantaneous value of the primary current is greater than or equal to the peak primary current reference. In another example, the criterion can be met when the integral of the primary current is greater than or equal to the average primary current reference. The control circuit 50 can generate the primary current reference by scaling the signal V_AC of the voltage sensor 52. Therefore, the primary current reference is a sine wave in phase with the voltage of the AC power supply 60. Therefore, the input current drawn from the AC power supply 60 is similar to a sine curve in shape and is in phase with the voltage of the AC power supply 60. Therefore, a relatively good power factor can be achieved.

[0044] The control circuit 50 employs a switching cycle with a constant period, and therefore a constant switching frequency. The portion of the switching cycle during which the primary side switch 20 is closed and energy is stored in the transformer 22 may be referred to as the on-time. The remainder of the switching cycle may be referred to as the off-time, during which the primary side switch 20 is open and energy is transferred from the transformer 22. During the off-time, the secondary side switches 30, 32 may be closed or open (e.g., if the secondary current is reduced to zero).

[0045] The current reference for regulating the primary current can be defined so that in each switching cycle, all the energy stored in the transformer 22 during the on-time is transferred from the transformer 22 during the off-time. Therefore, the instantaneous input power drawn from the AC power source 60 and the instantaneous output power driven to the load 70 are the same. Alternatively, the current reference can be defined so that in a particular switching cycle, the energy stored in the transformer 22 during the on-time is greater or less than the energy transferred from the transformer 22 during the off-time. Therefore, the instantaneous input power drawn from the AC power source 60 and the instantaneous output power driven to the load 70 are different. The difference in instantaneous energy is then stored by the transformer 22. The advantage of this is that the shape of the output power can be different from the shape of the input power while still maintaining a good power factor. However, the current reference for regulating the primary current is defined so that in each cycle of the AC power source 60, the total input power and the total output power are the same. Therefore, the net energy stored by the transformer 22 in each cycle of the AC power source 60 is zero.

[0046] Converter 10 is similar to a flyback converter and operates in a manner similar to a flyback converter. Specifically, transformer 22 behaves as a coupled inductor and combines the functions of energy storage, energy transfer, and isolation. The two behaviors described in the previous paragraph can be considered as discontinuous and continuous conduction modes of a conventional flyback converter. However, for a conventional flyback converter, even when operating in continuous conduction mode, the instantaneous energy transfer is always balanced. That is, the energy stored during the on-time is the same as the energy transferred during the off-time. In contrast, for the converter 10 described herein, the instantaneous energy transfer may be unbalanced. In particular, the energy stored during the on-time of the switching cycle may be greater or less than the energy transferred during the off-time.

[0047] Conventional flyback converters output a direct current, whereas the converter 10 described herein outputs an alternating current. In addition, the converter 10 provides AC-AC conversion without requiring a rectifier on the primary side or an inverter on the secondary side. Therefore, the AC-AC conversion requires a relatively small number of components. This can be achieved by using bidirectional switches 20, 30, 32 and a center-tapped secondary winding 26. In particular, by using bidirectional switches 30, 32 and a center-tapped winding 26 on the secondary side, an alternating current can be output at the output terminals 42, 43 regardless of the polarity of the primary current.

[0048] In addition to a relatively low component count, converter 10 is able to achieve AC-AC conversion with relatively little energy stored on the primary or secondary side. For example, control circuit 50 can employ a relatively high switching frequency (e.g., a kHz frequency, such as between 40 and 150 kHz) so that input filter 14 and output capacitor 40 need to store relatively little energy.

[0049] However, converter 10 is not without disadvantages. In particular, transformer 22 is required to store the electrical energy transferred from the primary side to the secondary side. Therefore, as the required output power increases, the size of transformer 22 must increase. Therefore, converter 10 is particularly suitable for relatively low power loads, such as loads less than 200W.

[0050] The magnitude of the primary current reference defines the magnitude of the input current and, therefore, the input power drawn from the AC power source 60. Therefore, the control circuit 50 can adjust the magnitude of the primary current reference in order to control the input power of the converter 10, thereby controlling the output power of the converter 10. In some examples, the control circuit 50 can receive an input signal indicating a desired output power, and the control circuit 50 can control the magnitude of the primary current reference in response to the input signal.

[0051] The primary current reference used by the control circuit 50 has a sinusoidal shape. Specifically, the control circuit scales the V_AC signal to generate a primary current reference. Therefore, the input current drawn by the converter 10 is substantially sinusoidal in shape and is in phase with the voltage of the AC power supply 60. This has the advantage that a relatively high power factor can be achieved. However, a potential disadvantage of drawing a sinusoidal input current is that for a given average input power, the peak input power and the peak input current are relatively high. Therefore, the control circuit 50 can adopt a primary current reference with an alternative shape, in particular a primary current reference that reduces the peak input power and / or the peak input current. For example, the control circuit 50 can adopt a limited sine wave or trapezoidal waveform as the primary current reference. The advantage of this is that the converter 10 can use components rated for lower current and / or lower power. Any deviation from a sinusoidal waveform will inevitably reduce the power factor and increase the harmonic content of the input current. Regulations in many countries (such as IEC61000-3-2) impose strict restrictions on the harmonic content of the input current that can be drawn from the mains power supply. However, when converter 10 is used to power relatively low power loads, a waveform for the primary current reference may be employed that reduces the peak input power and / or peak input current while remaining within harmonic limits imposed by regulations.

[0052] When the secondary side switches 30, 32 are closed, the control circuit 50 monitors the secondary current and, if the secondary current decreases to zero, opens the secondary side switches 30, 32. The delay between sensing the secondary current and opening the secondary side switches 30, 32 may mean that the secondary current transitions through zero and reverses polarity, which reduces the efficiency of the converter 10. As described above, each of the switches 20, 30, 32 is capable of operating in diode mode, in which the switch is turned on in only one direction. This can be used to avoid the reversal of the secondary current. For example, the control circuit 50 can monitor the secondary current, and in the case where the secondary current is less than a threshold value, the control circuit 50 can change the state of the closed secondary side switches 30, 32 to diode mode. Therefore, the efficiency of the converter 10 can be improved.

[0053] In the above example, the secondary winding 26 is center-tapped and the converter 10 includes two secondary-side switches 30 , 32 . Figure 2 A second example of an AC-AC converter 200 is shown, wherein the transformer 22 includes a tapless secondary winding 26, and the converter 200 includes another pair of secondary side switches 34, 36, which are connected in series to form another branch. Then, the two branches 30, 32 and 34, 36 are arranged as an H bridge, with the output terminals 42, 43 and the output capacitor 40 located in the center. The mode in which the converter 200 operates is substantially unchanged from the above-described mode. However, when energy is transferred from the transformer 22, the control circuit 50 now closes two of the secondary side switches 30, 36. When the first pair of secondary side switches 30 and 36 are closed, the secondary current has the same polarity as the primary current. In contrast, when the second pair of secondary side switches 32 and 34 are closed, the secondary current has a polarity opposite to the primary current.

[0054] exist Figure 1 and 2 In both examples shown, the AC-AC converter 10, 200 comprises a pair of secondary side switches 30, 32 which are connected in series to form a branch. One end of the branch is then connected to the first end 27 of the secondary winding 26, the other end of the branch is connected to the second end 28 of the secondary winding 26, and the middle point of the branch is connected to one of the output terminals 42. Figure 1 In the example of , the other output terminal 43 is connected to the center tap 29 of the secondary winding 26. Figure 2 In the example of , the converter 200 includes another pair of secondary side switches 34, 36, which are connected in series to form another branch. Then, one end of the other branch is connected to the first end 27 of the secondary winding 26, the other end of the other branch is connected to the second end 28 of the secondary winding 26, and the middle point of the other branch is connected to another output terminal 43. Figure 1 The converter 10 has the advantage of fewer switches, while Figure 2The converter 200 has the advantage of a simpler transformer.

[0055] Although not shown in the figures, each example converter 10, 200 includes a snubber circuit located on the primary side of the transformer 22 to manage leakage inductance. Because the converter 10, 200 does not include a rectifier on the primary side, the polarity of the primary current changes with each half cycle of the input voltage. Therefore, the diode of the snubber circuit is replaced by a bidirectional switch that operates in diode mode. The control circuit 50 then controls the snubber switch so that it operates in one of two diode modes based on the polarity of the input voltage, as determined from the V_AC signal. For example, when the polarity of the input voltage is positive, the snubber switch can operate in the first diode mode, and when the polarity of the input voltage is negative, the snubber switch can operate in the second diode mode.

[0056] As described above, the instantaneous energy transfer from the primary side to the secondary side of the converter 10, 200 may be unbalanced, with the energy difference being stored in the transformer 22. The benefit of this is that the profile of the output power can be different from the profile of the input power. However, the profile of the output power is still closely related to the profile of the input power. For example, it is not possible to arbitrarily shape the output current without adversely affecting the shape of the input current, which in turn can adversely affect the current harmonics and power factor. For example, consider a sinusoidal output current with a higher frequency than the input current. To achieve this, the zero crossings in the output current would require switching cycles during which no energy is transferred from the transformer 22. This would require the primary side switch 20 to be closed for the entire duration of the switching cycle, resulting in a peak in the input current at the frequency of the output current. An input filter 14 with significantly higher impedance would then be required to handle this ripple. However, it is possible to adjust the converter 10, 100 so that the input current and the output current can be shaped independently, as will now be described with reference to Figures 3 to 7 as described.

[0057] Figure 3 A third example of a converter 300 is shown.

[0058] The converter 300 is similar to the Figure 1 No changes are shown. First, the converter 300 includes the freewheeling circuit 100. Second, the control circuit 50 of the converter 300 uses a secondary current reference to regulate the secondary current.

[0059] The freewheeling circuit 100 is relatively simple and includes a freewheeling switch 102 connected in parallel with the primary winding 24. Figure 3 The circuit is drawn with thick lines to better identify it.

[0060] For each switching cycle, the control circuit 50 begins by opening the secondary side switches 30, 32 and the freewheeling switch 102, and closing the primary side switch 20. Closing the primary side switch 20 causes the primary current to flow through the primary winding 24 to store energy in the transformer 22. The control circuit 50 monitors the primary current, and when the primary current meets a criterion (as described above), the control circuit 50 opens the primary side switch 20 and closes one of the secondary side switches 30, 32, and then outputs the secondary current at the output terminals 42, 43, and energy is transferred from the transformer 22 to the load 70. The control circuit 50 monitors the secondary current, and when the secondary current meets another criterion (as described below), the control circuit 50 opens the secondary side switches 30, 32 and closes the freewheeling switch 102. With the freewheeling switch 102 closed, the current and energy in the transformer 22 freewheel around the freewheeling circuit 100. Finally, at the end of the switching cycle, the control circuit 50 opens the freewheeling switch 102 and closes the primary side switch 20.

[0061] Figure 4 The current in the windings 24, 26 of the transformer 22 during a switching cycle is shown. At time T1, the primary side switch 20 is closed (the secondary side switches 30, 32 and the freewheeling switch 102 are open), and current is drawn from the AC power source 60. Therefore, the current in the primary winding 24 rises and energy is stored in the transformer 22. In this example, at time T1, energy has been stored in the transformer 22. Therefore, when the primary side 20 switch is closed, the primary current rises from a non-zero starting value. At time T2, the primary side switch 20 is open and one of the secondary switches 30, 32 is closed. Therefore, current flows in the secondary winding 26 and energy is transferred from the transformer 22. For simplicity, the turns ratio (Np / Ns) of the transformer 22 is 1. Therefore, the primary current and the secondary current have the same magnitude at time T2. As energy is transferred from the transformer 22 to the load 70, the secondary current decreases over time. At time T3, the secondary side switches 30, 32 are open and the freewheeling switch 102 is closed. Therefore, the current freewheels around the freewheeling circuit 100 until time T4, at which point the freewheeling switch 102 is opened and the primary side switch 20 is closed.

[0062] exist Figure 4 In the illustrated switching cycle, the energy transferred from the transformer 22 when the secondary side switches 30, 32 are closed is greater than the energy stored when the primary side switch 20 is closed. Therefore, the magnitude of the primary current at the end of the switching cycle (i.e., at time T4) is less than the magnitude at the beginning of the switching cycle (i.e., at time T1).

[0063] When the secondary current meets the criterion, the control circuit 50 opens the secondary side switches 30, 32 and closes the freewheeling switch 102. In one example, the criterion can be met when the instantaneous value of the secondary current is less than or equal to the secondary current reference. In another example, the criterion can be met when the integral of the secondary current is greater than or equal to the average secondary current reference. In both examples, the secondary current reference can be defined so that a specific profile is obtained for the secondary current and therefore the output current generated at the output terminals 42, 43.

[0064] Therefore, the converter 300 is able to shape the input current and the output current. This is made possible by the freewheeling circuit 100, which provides a path for the residual energy stored in the transformer 22. Therefore, when there is still energy in the transformer 22, the primary side switch 20 and the secondary side switches 30, 32 can be disconnected. Therefore, the opening and closing of the secondary side switches 30, 32 are decoupled from the opening and closing of the primary side switch 20. In particular, the primary side switch 20 can be closed for a desired time period so that the primary current meets the first standard, and the secondary side switches 30, 32 can be closed for a desired time period so that the secondary current meets the second standard. Therefore, both the input current and the output current can be shaped independently. The only requirement is that the input current and the output current are shaped so that in each cycle of the AC power supply 60, the total input power (i.e., the total energy stored in the transformer 22 during the on time) is the same as the total output power (i.e., the total energy transferred from the transformer 22 during the off time). Therefore, the net energy stored by the transformer 22 in each cycle of the AC power supply 60 is zero.

[0065] Likewise, despite Figure 3 , the converter 300 includes a snubber circuit connected in parallel with the primary winding 24 (and therefore in parallel with the freewheeling switch 102). Although both the snubber circuit and the freewheeling circuit 100 are used to manage energy in the transformer 22, and each includes a bidirectional switch, the operation of the switches is very different. The snubber switch operates only in diode mode and changes state only once per half cycle of the input voltage. In contrast, the freewheeling switch 102 changes state with each switching cycle. Therefore, the freewheeling switch 102 operates at the same switching frequency as the primary side switch 20 and the secondary side switches 30, 32.

[0066] Figure 5 A fourth example of a converter 400 is shown.

[0067] Figure 5 The converter 400 is similar to Figure 3 However, with Figure 3In contrast to the example of FIG. 2 , the energy in the transformer 22 is freewheeling around a freewheeling circuit 110 that includes a secondary winding 26 instead of a primary winding 24. Again, to better identify the freewheeling circuit 110, the circuit is Figure 5 Draw in thick lines.

[0068] The converter 400 includes an additional secondary side switch 38 connected between the center tap 29 of the secondary winding 26 and the output terminal 43. In order to better distinguish the secondary side switches 30, 32, 38 in the following discussion, the additional secondary side switch 38 will be referred to as a common switch 38, and the other two secondary side switches 30, 32 will be referred to as branch switches 30, 32.

[0069] For each switching cycle, the control circuit 50 starts by opening the secondary side switches 30, 32, 38 and closing the primary side switch 20. Closing the primary side switch 20 causes the primary current to flow through the primary winding 24 to store energy in the transformer 22. The control circuit 50 monitors the primary current, and when the primary current meets the criteria, the control circuit 50 opens the primary side switch 20, closes the common switch 38 and one of the branch switches 30, 32 on the secondary side. The secondary current is then output at the output terminals 42, 43, and energy is transferred from the transformer 22 to the load 70. The control circuit 50 monitors the secondary current, and when the secondary current meets another criteria, the control circuit 50 opens the common switch 38 and closes the other of the branch switches 20, 32, with the two branch switches 30, 32 on the secondary side now closed, and the current and energy in the transformer 22 are freewheeling around the freewheeling circuit 110 including the secondary winding 26 and the two branch switches 30, 32. Finally, at the end of the switching cycle, the control circuit 50 opens the branch switches 30 , 32 and closes the primary side switch 20 .

[0070] The specific configuration of the switches described in the previous paragraphs is only an example, and other configurations that achieve the same behavior are possible. For example, one of the branch switches 30, 32 can remain closed throughout the switching cycle. The selection of the branch switch 30, 32 to remain closed will depend on the desired polarity of the secondary current. The branch switches 30, 32 can remain closed for multiple switching cycles, for example, until the polarity of the secondary current needs to be changed. Therefore, switching losses can be reduced, thereby improving the efficiency of the converter 400.

[0071] Therefore, the operation of converter 400 is similar to that of the above combined Figure 3 In particular, each switching cycle includes a first period during which energy is stored in the transformer 22 , a second period during which energy is transferred from the transformer 22 , and a third period during which energy is freewheeled in the transformer 22 .

[0072] use Figure 2The converter 200 can also achieve freewheeling around the secondary side. As described above, the secondary side switches 30, 32, 34, 36 of the converter 200 are arranged as an H-bridge. Then, by closing the high-side switches 30, 34 of one branch and the low-side switches 32, 36 of the other branch, energy is transferred from the transformer 22 to the load 70. Then, the energy in the transformer 22 is freewheeled by closing two switches of one branch (e.g., switches 30 and 32) and opening two switches of the other branch (e.g., switches 34 and 36). Similar to Figure 5 , the energy in the transformer 22 then freewheels around a freewheeling circuit including the secondary winding 26 and two closed switches (eg, switches 30 and 32 ).

[0073] Figure 6 A fifth example of a converter 500 is shown.

[0074] In addition to the freewheeling circuit 120, the converter 500 and Figure 3 In this example, the freewheeling circuit 120 includes a freewheeling switch 122 and an auxiliary winding 124 wound around the core of the transformer 22. The control circuit 50 is combined with the above Figure 3 The switches 20, 30, 32, 122 are controlled in the same manner as described for the converter 300. Therefore, each switching cycle includes a first period during which energy is stored in the transformer 22, a second period during which energy is transferred from the transformer 22, and a third period during which energy is freewheeled in the transformer 22. Figure 3 and 5 In contrast to the example of FIG. 1 , energy in the transformer 22 is freewheeling around the auxiliary winding 124 rather than around the primary winding 24 or the secondary winding 26 .

[0075] Although providing the auxiliary winding 124 may increase leakage inductance, the auxiliary winding 124 may have a higher number of turns than the primary winding 24 and the secondary winding 26, so that a lower current is freewheeling around the freewheeling circuit 120. Thus, an overall improvement in efficiency may be achieved.

[0076] Figure 7 A sixth example of a converter 600 is shown.

[0077] Converter 600 is similar to Figure 6 However, the freewheeling circuit 120 is replaced by an AC-DC circuit 130. The AC-DC circuit 130 includes a switch 132, an auxiliary winding 134 wound on the core of the transformer 22, a bridge rectifier 136, a DC link capacitor 138 and a pair of output terminals 140 for connecting to another load 670.

[0078] The switches 20, 30, 32, 132 of the converter 600 are combined with the above Figure 3 and6 The AC-DC circuit 130 is controlled in the same manner as described, and the switch 132 of the AC-DC circuit 130 assumes the role of the freewheeling switches 102 and 122. When the switch 132 of the AC-DC circuit 130 is closed, a current is induced in the auxiliary winding 134. The current is then rectified by the bridge rectifier 136 and charges the DC link capacitor 138. Therefore, a DC voltage is output at the output terminal 140. Therefore, the converter 600 is used as both an AC power source and a DC power source. Specifically, the converter 600 provides an alternating current to the load 70 and a direct current to another load 670.

[0079] In this example, a bridge rectifier 136 is used to rectify the current induced in the auxiliary winding 134. Therefore, the switch 132 does not need to be a bidirectional switch and is a MOFSET in this example. It should be understood that other topologies of the AC-DC circuit are also possible. By way of example only, the auxiliary winding 134 can be center-tapped and the bridge rectifier 136 can be replaced with a pair of reverse diodes connected in series to form a diode branch. One end of the diode branch is connected to one end of the auxiliary winding and the other end of the diode branch is connected to the other end of the auxiliary winding. One of the output terminals 140 of the AC-DC circuit 130 is then connected to the center tap of the auxiliary winding 134, and the other of the output terminals 140 is connected to the midpoint of the diode branch.

[0080] Figure 3 , 5 Each converter 300, 400, 600 shown in Figures 6 and 7 includes a freewheeling circuit 100, 110, 120, and the remaining energy in the transformer 22 is freewheeling around the freewheeling circuit. Figure 7 The converter 600 includes an AC-DC circuit 130 that transfers the remaining energy from the transformer 22 to another load 670. However, each converter manages the remaining energy in the transformer 22 so that the input current and the output current can be shaped independently.

[0081] Common to each of these converters 300, 400, 600, 700 is the way in which the switches of the converter are controlled. In particular, the control circuit 50 controls the switches so that in each switching cycle, the switches are configured as: (i) a first configuration in which the primary current is drawn from the AC power source to store energy in the transformer 22; (ii) a second configuration in which the secondary current is output at the output terminals 42, 43 to transfer energy from the transformer 22 to the load 70; and (iii) a third configuration in which the remaining energy in the transformer 22 is freewheeling around the freewheeling circuit 100, 110, 120 or transferred from the transformer 22 to another load 670. The control circuit 50 monitors the primary current and switches the switch from the first configuration to the second configuration when the primary current meets the first criterion. Then, the control circuit 50 monitors the secondary current and switches the switch from the second configuration to the third configuration when the secondary current meets the second criterion.

[0082] Figure 8 A motor system 700 is shown, which includes Figure 3 The converter 300 and the permanent magnet motor 770 are provided.

[0083] The motor 770 includes a phase winding connected to the output terminals 42, 43 of the converter 300. The motor 770 outputs a signal BEMF that represents the back EMF induced in the phase winding. For example, the motor 770 may include a position sensor (e.g., a Hall effect sensor or an optical encoder) that outputs a signal indicating the speed and position of the rotor, from which the back EMF induced in the phase winding can be inferred. In another example, the motor 770 may include an auxiliary winding that outputs a signal indicating the back EMF induced in the auxiliary winding, which can then be used to infer the back EMF induced in the phase winding.

[0084] The control circuit 50 then uses the BEMF signal output by the motor 770 to control the switches 20, 30, 32, 102. In particular, the control circuit 50 controls the secondary side switches 30, 32 so that the output current has the same polarity as the back EMF induced in the phase current. Therefore, the selection of which secondary side switch 30, 32 to close depends on the polarity of the input voltage and the polarity of the back EMF. The control circuit 50 uses a secondary current reference that matches the shape of the back EMF and is in phase. Therefore, the output current driven into the phase winding of the motor 770 has the same shape and phase as the back EMF. This maximizes the efficiency of the motor system 700 because the torque per ampere is maximized when the waveform of the phase current matches the waveform of the back EMF.

[0085] A conventional AC power supply for a permanent magnet motor typically includes a rectifier, a PFC circuit, and a DC link capacitor (i.e., the first stage) that outputs a stable DC voltage, and an inverter (i.e., the second stage) that converts the DC voltage into an AC voltage applied to the phase winding. This arrangement has a relatively high number of components, and the DC link capacitor typically requires a relatively high capacitance. Another problem with this arrangement is that it is difficult to control the current, and thus the power delivered to the motor. The magnitude of the back EMF induced in the phase winding is opposite to the applied voltage, and it varies with the position and speed of the rotor. Therefore, when controlling the inverter, a relatively complex scheme is often required to ensure that the required current and power are driven into the phase winding in an efficient manner. On the other hand, the converter 300 described herein acts as a current source. In particular, when the secondary side switches 30, 32 are closed, a voltage is induced on the secondary winding 26, forcing the secondary current to flow. Importantly, the induced voltage drives the secondary current, regardless of the magnitude of the back EMF on the output terminals 42, 43. Therefore, compared to conventional AC power supplies, a much simpler control scheme can be used to drive current and power into the phase winding in an efficient manner. Specifically, by adopting a secondary current reference that matches the back EMF, excessive phase current can be avoided and the efficiency of the motor system 700 can be improved.

[0086] Although specific examples have been described so far, it should be understood that various modifications may be made without departing from the scope of the invention as defined by the claims. By way of example only, each of the above converters includes an input filter and an output capacitor, which smooth high frequency ripples in the input current and the output current, respectively. However, it will be apparent from the above discussion that neither the input filter nor the output capacitor is necessary for the primary function of the AC-AC converter, which is to output an alternating secondary current.

Claims

1. An AC-AC converter, comprising: Input terminal for connecting to AC power source; Output terminals, for connection to a load; a transformer having a primary winding and a secondary winding; a primary-side switch connected between the input terminal and the primary winding; a pair of secondary-side switches connected between the output terminals and the secondary winding; and Control circuit, used to control the switch, in: The primary side switch and the secondary side switch are bidirectional; Closing the primary side switch causes a primary current to be drawn from the AC power source and through the primary winding to store energy in the transformer; Closing one of the secondary side switches causes the secondary current to be output at the output terminal to transfer energy from the transformer to the load, the secondary current having the same polarity as the primary current when the first secondary side switch is closed and having an opposite polarity to the primary current when the second secondary side switch is closed; and The control circuit is operable to control the switch so that an alternating current is output at the output terminal.

2. The AC-AC converter of claim 1 , wherein the control circuit is operable to control the switch such that during a switching cycle, the switch is configured to: a first configuration wherein the primary current is drawn from the AC power source to store energy in the transformer; a second configuration, wherein the secondary current is output at the output terminal to transfer energy from the transformer to the load; and A third configuration wherein energy in the transformer is freewheeled around a freewheeling circuit or transferred from the transformer to another load.

3. The AC-AC converter of claim 2, wherein the control circuit is operable to: monitoring the primary current; switching the switch from the first configuration to the second configuration when the primary current satisfies a first criterion; monitoring the secondary current; and When the secondary current meets a second criterion, the switch is switched from the second configuration to the third configuration.

4. An AC-AC converter according to claim 3, wherein the control circuit is operable to switch the switch from the second configuration to the third configuration when the integral of the secondary current is greater than or equal to an average current reference. 5 . The AC-AC converter according to claim 2 , wherein the freewheeling circuit comprises one of the primary winding and the secondary winding.

6. An AC-AC converter according to any one of the preceding claims, wherein the secondary side switches are connected in series to form a branch, a first end of the branch is connected to a first end of the secondary winding, a second end of the branch is connected to a second end of the secondary winding, one of the output terminals is connected to a midpoint of the branch, and another of the output terminals is connected to a center tap of the secondary winding.

7. An AC-AC converter according to any one of the preceding claims, wherein each secondary side switch comprises four states: (1) open, wherein the switch does not conduct in either the first direction or the second direction; (2) closed, wherein the switch conducts in both the first direction and the second direction; (3) a first diode mode, wherein the switch conducts only in the first direction; and (4) a second diode mode, wherein the switch conducts only in the second direction; and the control circuit is operable to: closing one of the secondary-side switches to output the secondary current; monitoring the magnitude of the secondary current; and When the magnitude of the secondary current is less than or equal to a threshold, the state of the secondary side switch is changed from closed to one of the diode modes.

8. An AC-AC converter according to any one of the preceding claims, wherein the control circuit is operable to receive a signal indicative of the polarity of the voltage at the output terminal, and the control circuit is operable to control the secondary side switch so that the secondary current has the same polarity as the voltage.

9. A motor system comprising a permanent magnet motor and an AC-AC converter according to any one of the preceding claims, wherein a phase winding of the permanent magnet motor is connected to the output terminals.

10. The motor system of claim 10, wherein the control circuit is operable to control the switches so that the alternating current at the output terminals has the same frequency as the back EMF induced in the phase winding.

11. A motor system according to claim 10 or 11, wherein the control circuit is operable to control the switches at a switching frequency that is higher than the frequency of the back EMF induced in the phase winding.

12. A motor system according to any one of claims 10 to 12, wherein the motor outputs a signal representing the polarity of the back EMF induced in the phase winding, and the control circuit is operable to receive the signal and control the secondary side switch so that the secondary current has the same polarity as the back EMF.

13. The motor system of any one of claims 10 to 13, wherein the control circuit monitors motor speed, and the control circuit controls the primary side switch to adjust the magnitude of the primary current in response to changes in motor speed.

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