A battery charger

Abstract

A battery charger includes input terminals for connection to an AC source supplying an alternating input voltage, output terminals for connection to a battery to be charged; and a power converter circuit connected between the input terminals and the output terminals. The power converter circuit is configured to act on an input current drawn from the AC source, generate an output current at the output terminals, and shape the input current such that the output current has a ratio of peak output current to average output current that is no greater than 1.5. The power converter circuit is configured such that, for a given time step, a difference between an instantaneous input power to the power converter circuit and an instantaneous output power of the power converter circuit is no more than 20%

Description

[0001] A BATTERY CHARGER

[0002] BACKGROUND

[0003] Battery chargers for charging domestic appliances may typically comprise power conversion circuitry for converting electrical power drawn from a mains power source into a form suitable for charging the domestic appliance.

[0004] SUMMARY

[0005] A first aspect provides a battery charger comprising: input terminals for connection to an AC source supplying an alternating input voltage; output terminals for connection to a battery to be charged; and a power converter circuit connected between the input terminals and the output terminals, wherein the power converter circuit is configured to: act on an input current drawn from the AC source; generate an output current at the output terminals, and shape the input current such that the output current has a ratio of peak output current to average output current that is no greater than 1.5, and wherein the power converter circuit is configured such that, for a given time step, a difference between an instantaneous input power to the power converter circuit and an instantaneous output power of the power converter circuit is no more than 20%.

[0006] By shaping the input current such that the output current has a ratio of peak output current to average output current that is no greater than 1.5, for example over a time period corresponding to a charging cycle of a battery to be charged by the battery charger, a relatively flat output current profile may be achieved during each half cycle of the AC source. Such an output current profile may be beneficial for charging a battery compared to scenarios where the ratio of peak output current to average output current is greater than 1.5. For example, absent the shaping of the input current, the input current may be generally sinusoidal in form, and can in some scenarios lead to the output current having a generally sin2profile. Such a sin2profile, however, has a relatively large peak to average current ratio, with the peak more than 2 times the average, and may be undesirable for battery charging. The battery charger of the first aspect mitigates for this via the shaping of the input current to provide a reduced peak to average ratio for the output current. This may lead to a reduction in RMS current, which may in turn reduce heating of a battery during charging.

[0007] The power converter circuit may be configured to shape the input current by applying harmonic injection to the input current. Such harmonic injection may also reduce a peak value of the input current compared to examples where the input current is not shaped, and is generally sinusoidal in form.

[0008] The power converter circuit may be configured to apply odd numbered harmonics to the input current. The power converter circuit may be configured to apply at least third, fifth, seventh, and ninth harmonics to the input current. The power converter circuit may be configured to apply odd numbered harmonics up to and including the fifteenth harmonic to the input current. Each harmonic may be in phase with a fundamental of the input current.

[0009] The power converter circuit may be configured to apply a third harmonic to the input current at a ratio of at least 0.6 times, or at least 0.7 times, an amplitude of a fundamental of the input current. The power converter circuit may be configured to apply a third harmonic to the input current at a ratio of around 0.74 times the amplitude of the fundamental of the input current.

[0010] The power converter circuit may be configured to apply a fifth harmonic to the input current at a ratio of at least 0.4 times, or at least 0.5 times, the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply a fifth harmonic to the input current at a ratio of around 0.51 times the amplitude of the fundamental of the input current.

[0011] The power converter circuit may be configured to apply a seventh harmonic to the input current at a ratio of at least 0.2 times, or at least 0.3 times, the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply a seventh harmonic to the input current at a ratio of around 0.32 times the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply a ninth harmonic to the input current at a ratio of at least 0.1 times, or at least 0.15 times, the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply a ninth harmonic to the input current at a ratio of around 0.18 times the amplitude of the fundamental of the input current.

[0012] The power converter circuit may be configured to apply an eleventh harmonic to the input current at a ratio of at least 0.05 times, or at least 0.08 times, the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply an eleventh harmonic to the input current at a ratio of around 0.083 times the amplitude of the fundamental of the input current.

[0013] The power converter circuit may be configured to apply a thirteenth harmonic to the input current at a ratio of at least 0.01 times, or at least 0.02 times, the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply a thirteenth harmonic to the input current at a ratio of around 0.03 times the amplitude of the fundamental of the input current.

[0014] The power converter circuit may be configured to apply a fifteenth harmonic to the input current at a ratio of at least 0.002 times, or at least 0.003 times, the amplitude of the fundamental of the input current. The power converter circuit may be configured to apply a fifteenth harmonic to the input current at a ratio of around 0.004 times the amplitude of the fundamental of the input current.

[0015] The ratio of peak output current to average output current may be no greater than 1.4, no greater than 1.3, or no greater than 1.2. The ratio of peak output current to average output current may be around 1.15.

[0016] The power converter circuit may be configured to generate the output current such that the output current has 100% ripple. The power converter circuit may be configured such that, for the given time step, a difference between the instantaneous input power to the power converter circuit and the instantaneous output power of the power converter circuit is no more than 10%, no more than 5%, or no more than 1%. The difference between the instantaneous input power to the power converter circuit and the instantaneous output power of the power converter circuit may neglect losses.

[0017] The power converter circuit may comprise an energy storage capacity of less than 900nF. Thus the battery charger may employ capacitors of relatively small capacitance, thereby reducing a cost and size of the battery charger.

[0018] The power converter circuit may be configured to generate the output current such that the output current has a substantially trapezoidal waveform, for example over a half cycle of the AC source. The power converter circuit may be configured to generate the output current such that a value of the output current varies by no more than 10% over a period corresponding to at least 75%, at least 80%, or at least 90%, of a half cycle of the AC source.

[0019] The power converter circuit may be configured to shape the input current such that a waveform of the input current comprises two local maxima, and a local minima between the two local maxima, wherein the value of the local minima is no more than 0.5 times the value of the local maxima.

[0020] The power converter circuit may be configured to shape the input current such that, at a peak of the AC source, the peak output current has a value that is no greater than 30% more than a value of the average output current.

[0021] The battery charger may have an output power of at least 100W, for example around 150W.

[0022] The power converter circuit may comprise an inductive circuit element, and a switch configured to control flow of current through the inductive circuit element. The power converter circuit may comprise a controller configured to control a duty cycle of the switch to shape the input current. The controller may utilise a lookup table to determine a ratio of a harmonic to be applied to the input current.

[0023] The power converter circuit may have a single power conversion stage.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure l is a schematic diagram of a battery charger;

[0026] Figure 2 is a circuit diagram illustrating a single-ended primary inductor converter (SEPIC) of the battery charger of Figure 1;

[0027] Figure 3 is a table showing illustrative values of harmonic coefficients utilised in operation of the battery charge;

[0028] Figure 4 is a schematic plot of a waveform of an input current of the battery charger of Figure 1; and

[0029] Figure 5 is a schematic plot of a waveform of an output current of the battery charger of Figure 1.

[0030] DETAILED DESCRIPTION

[0031] Figure 1 shows a schematic diagram of a battery charger 10. The battery charger 10 includes a single-ended primary inductor converter (SEPIC) 12 and a controller 14. It will be appreciated that, in practice, the SEPIC 12 and the controller 14 can be considered part of the same circuitry. Both the SEPIC 12 and the controller 14 are provided in a housing 16 of the battery charger 10. The battery charger 10 includes a plug 18 for plugging the battery charger 10 into an external power source, such as a mains power socket. The plug 18 is electrically connected to the SEPIC 12, to transmit power (e.g. current and / or voltage) received to the SEPIC 12. An output of the SEPIC 12 is electrically connected to a connector 20 of the battery charger 10, which can be used for connecting the battery charger 10 to an external device 22 via a cable 24. The external device 22 may include an internal battery for powering the device 22. The SEPIC 12 is configured to convert power received from the external power source (via the socket 18) to a suitable voltage and / or current level for use by the external device 22. In particular, the SEPIC 12 is configured as an AC-to-DC power converter, to convert AC power received from an AC power source (e.g. mains power) to DC power for use by the external device. The SEPIC 12 is configured to operate as a buck-boost converter, to facilitate use of the power supply for charging battery powered devices. Moreover, the SEPIC 12 is configured to enable ‘hybrid’ operation of the external device 22, such that the device 22 can be powered a) from its internal battery only, b) from the battery charger 10 only, c) from the power supply whilst charging the battery with the battery charger 10, and d) from both the power supply and the battery. The controller 14 is connected to the SEPIC 12 and configured to control operation of the SEPIC 12. In particular, the controller 12 is configured to control the states of multiple switches in the SEPIC 12 as described in more detail below, to enable efficient power conversion.

[0032] Figure 2 shows a circuit diagram of the SEPIC 12. The SEPIC 12 comprises a primary circuit 28 and a secondary circuit 30 which are coupled to one another via a coupled inductor 32. The coupled inductor 32 is configured to provide galvanic isolation between the primary circuit 28 and the secondary circuit 30. The coupled inductor 32 includes a primary coil 34 which is connected in the primary circuit 28, and a secondary coil 36 which is connected in the secondary circuit 30. A ratio of turns in the primary coil 34 and the secondary coil 36 can be selected to provide a desired voltage ratio between the primary coil 34 and the secondary coil 36. The primary coil 34 may have a larger number of turns compared to the secondary coil 36, e.g. to enable charging of a battery having a lower voltage than mains supply. For example, the primary coil 34 and the secondary coil may have a turns ratio of 4: 1. The coupled inductor 32 further includes a magnetic core (not shown), around which the primary coil 34 and the secondary coil 36 are wound. The magnetic core includes an air gap (i.e. a discontinuity), which serves to increase an ability of the coupled inductor 32 to store energy before saturation is reached. For example, a planar core structure with an air gap could be used, although other core shapes could be selected, e.g. based on a desired switching frequency of the SEPIC 12. The primary circuit 28 includes a first input terminal 38a and a second input terminal 38b, which are connectable to a power source 40, which is an AC power source, via the plug 18. The primary circuit 28 further includes a first line 42 of series-connected components between the first input terminal 38a and a first end of the primary coil 34. The first line 42 includes a first inductor LI and a first capacitor Cl, which are connected in series between the first input terminal 38a and the first end of the primary coil 34. The primary circuit 28 also includes a second line 44, which is connected between the second input terminal 38b and a second end of the primary coil 34. The second line 44 may, for example, correspond to a conductive wire or trace which electrically connects the second input terminal 38b to the second end of the primary coil 34. A first connecting line 46 is connected between the first line 42 and the second line 44, with a first switch SW1 being arranged on the first connecting line 46. The first switch SW1 is thus arranged to control a flow of current along the first connecting line 46 between the first line 42 and the second line 44. A second connecting line 48 is connected between the first line 42 and the second line 44, with a second switch SW2 and a second capacitor C2 being connected in series on the second connecting line 48. The second switch SW2 is thus arranged to control a flow of current along the second connecting line 48, and hence charging and discharging of the second capacitor C2. Figure 2 also depicts a possible leakage inductance LpL of the primary coil 34. The first line 42 may correspond to a ‘live’ line of the primary circuit 28, whilst the second line 44 may correspond to a ‘neutral’ line of the primary circuit 28 (or vice versa).

[0033] The first inductor LI is connected between the first input terminal 38a and a first junction 50 between the first connecting line 46 and the first line 42. This location of the first inductor LI enables the first inductor to continuously draw current from the power source 40. The first capacitor Cl is connected between the first inductor LI and the first end of the primary coil 34, between the first junction 50 and a second junction 52 between the second connecting line 48 and the first line 42. In this manner, the second line 48 (including SW2 and C2) is connected in parallel with the primary coil 34. This arrangement facilitates complementary switching of SW 1 and SW2, an example of which is discussed below. However, in other examples (not shown), the first capacitor Cl may instead be connected between the second junction 52 and the first end of the primary coil 34, such that the second line 48 (including SW2 and C2) is connected in parallel with the first switch SW 1. It should be noted that, whilst the first inductor LI and the first capacitor Cl are shown as being on the first line 42, in other examples one or both of the first inductor LI and the first capacitor Cl may be on the second line 44. Additionally, it is possible to split the first inductor LI and / or the first capacitor Cl between the first line 42 and the second line 44.

[0034] The secondary circuit 30 includes a first output terminal 54a and a second output terminal 54b for outputting a voltage from the secondary circuit 30. The output terminals 54a, 54b are connectable to a battery 56 which is to be charged. For example, the first output terminal 54a may be a positive output terminal, and the second output terminal 54b may be a negative output terminal. In the context of the battery charger 10 described above, the output terminals 54a, 54b may be connected to the connector 20, to enable the output of the SEPIC 12 to be provided to the external device 22. The secondary circuit 30 further includes a third line 58 connected between a first end of the secondary coil 36 and the first output terminal 54a, the third line 56 including a first diode DI and a second inductor L2, which are connected in series between the first end of the secondary coil 36 and the first output terminal 54a. A fourth line 60 is connected between a second end of the secondary coil 36 and the second output terminal 54b, such that the second end of the secondary coil 36 is electrically connected to the second output terminal 54b. Additionally, a third connecting line 62 is connected between the third line 58 and the fourth line 60, the third connecting line 62 including a third capacitor C3. The second inductor L2 is arranged such that it is connected between a junction 64 between the third line 58 and the third connecting line 62, and the first output terminal 54a. In other examples, the second inductor L2 may instead be on the fourth line 60, connected between a junction 66 between the fourth line 60 and the third connecting line 62. Accordingly, the second inductor L2 and the third capacitor C3 are arranged to act as an LC filter for the output of the secondary circuit 30. Note Figure 2 also depicts a possible leakage inductance LsL of the secondary coil 36.

[0035] The first diode DI is located on the third line 58, connected between the first end of the secondary coil 36 and the junction 64. Alternatively, the first diode DI may be arranged on the fourth line 60, such that it is connected between the second end of the secondary coil 36 and the junction 66. In either of these arrangements, the first diode DI only allows a unipolar current output from the secondary coil 36, i.e. the diode DI only allows current out of the secondary coil when the first diode DI is forward-biased. As a result, energy is stored in the coupled inductor 32 when current cannot be released from the secondary coil 36. This reduces output filtering requirements on the second inductor L2 and the third capacitor C3, thus enabling a size of L2 and C3 to be reduced. For example, in some cases this allows an inductance of L2 to be reduced so that it can be as small as 33% of the original value, compared to other converter arrangements.

[0036] As an example, values for components of the SEPIC 12 may be as follows:

[0037] • LI : less than 1 mH, in some cases less than 0.5 mH;

[0038] • Cl and C2 may have similar values of capacitance, for example less than 1 pF, in some cases less than 470 nF;

[0039] • L2: around 10 pH;

[0040] • C3: around 4.7 pF.

[0041] The above example values may be suitable for making a 150W battery charger. Of course, the values of the components used in the SEPIC 12 may be adapted based on an intended usage of the SEPIC 12.

[0042] In the example of Figure 2, the first diode DI is implemented as a conventional diode. However, in other examples, the diode DI may be implemented using a MOSFET. This can improve efficiency of the SEPIC 12, as the MOSFET can be controlled to be in a fully conductive state when the diode DI would be forward-biased, thus avoiding losses associated with the diode. Where a MOSFET is used, a controller (e.g. a localised control integrated circuit) may similarly be used to control the MOSFET, e.g. by controlling generation of a gate voltage for controlling the MOSFET.

[0043] Returning to the primary circuit 28, the first switch SW1 and the second switch SW2 are implemented using bipolar switches. In other words, the first switch SW 1 and the second switch SW2 each have four states, namely open (i.e. current in both directions is blocked from flowing through the switch), closed (i.e. current can flow through the switch in both directions), up-diode (i.e. current can only flow through the switch in a first direction), and down-diode (i.e. current can only flow through the switch in a second, opposite direction). For example, the first switch SW1 and the second switch SW2 may each comprise a bidirectional gallium-nitride (BiGaN) switch. An example bidirectional switch is disclosed in EP2309633A1 (see in particular Figs. 2 and 3 of that document). By using bidirectional switches, it is no longer necessary to use a bridge rectifier for converting AC power received from the power supply 40 to DC power, enabling a size of the SEPIC 12 to be reduced. In particular, the switches SW1 and SW2 provide full directional control (e.g. up- diode and down diode states), the switches SW1 and SW2 can be controlled to act as a rectifier. In general terms, the first switch SW1 can be used to control an amount of current received from the power source 40. In particular, when the first switch SW1 is closed, current is drawn into the primary circuit 28 via the first inductor LI. The second switch serves to control a voltage seen across the primary coil 34, as well as charging and discharging of the second capacitor C2. The second switch SW2 and second capacitor C2 also serve to manage leakage inductance from the primary coil 34. For example, the second switch SW2 can be closed to allow the second capacitor C2 to discharge. The states of the switches SW1 and SW2 are controlled via the controller 14, with the controller 14 configured to control generation of gate voltages for controlling the switches SW1 and SW2.

[0044] It will be appreciated from the discussion above that there are no significant energy storage components present in the SEPIC 12 that impact a relationship between the instantaneous input power of the SEPIC 12 and the instantaneous output power of the SEPIC 12. This means that, neglecting losses, the instantaneous output power of the SEPIC 12 is substantially equal to the input power of the SEPIC 12, although embodiments where a difference between the instantaneous input power of the SEPIC 12 and the instantaneous output power of the SEPIC 12 are no more than 20% are also envisaged. If the SEPIC 12 were to draw a sinusoidal input current from the power source 40, then an output current of the SEPIC 12 would have a sin2profile. This could, however, provide a relatively high RMS current, which can lead to excessive heating of the battery 56 during charging. To mitigate for this, the controller 14 controls the first switch SW1 of the SEPIC 12 to shape the input current, such that the output current of the SEPIC 12 has a reduced ratio of peak output current to average output current, relative to a sin2output current, of less than 1.5.

[0045] To achieve this, harmonic injection is applied to the input current in line with the below formula: where:

[0046] Iref is the current reference waveform;

[0047] Iso is the peak value of the fundamental component; and

[0048] Coefn is a harmonic coefficient to be applied to the fundamental component.

[0049] The applied harmonics are odd harmonics up to the fifteenth harmonic, and are all in phase with the fundamental. The harmonic coefficients are fixed multipliers accessed by the controller 14 from a lookup table indexed by the input power demand for the fundamental frequency amplitudes. The controller 14 utilises the harmonic coefficients to recreate a reference waveform, and feed the reference waveform to a PWM modulator (not shown) to determine a duty cycle for the first switch SW1. The controller 14 then controls generation of a gate voltage to operate the first switch SW1 to obtain the desired profile on input current.

[0050] Example harmonic coefficients for operation of the battery charger 10 at 150W are shown in Figure 3. By utilising such harmonic injection, the input current 100 for the SEPIC 12 takes the form shown in Figure 4, whilst the output current 200 of the SEPIC 12 takes the form shown in Figure 5.

[0051] As can be seen in Figure 4, the input current 100 is not sinusoidal, and for each half cycle of the power source 40 the input current 100 rises to a first local maxima 102, decreases toward a local minima 104 at a halfway point of the half cycle of the power source 40, increases toward a second local maxima 106, and decays to zero at the end of the half cycle of the power source 40. The local minima 104 has a value that is just under a third of the value of the first 102 and second 104 local maxima.

[0052] As can be seen in Figure 5, the output current 200 is not sin2in form, and is instead generally trapezoidal in form. The output current 200 has a peak value of just less than 15 A, which is reduced relative to the roughly 25 A that would be expected for a sin2output current if the harmonic injection were not performed. The output current 200 is substantially constant for over 80% of the half cycle of the power source 40. A ratio of the peak output current to the average output current is around 1.157, which is reduced relative to a value of 2.013 that would be expected for a sin2output current if the harmonic injection were not performed. This may lead to a reduction in RMS current, which may in turn reduce heating of the battery 56 during charging.

[0053] It will be appreciated that the harmonic coefficients may vary in practice depending on the power that the battery charger 10 is intended to provide. For example, battery chargers may be considered Class A products under IEC61000-3 -2 as of the priority date of the present application, and there are limits on the harmonic current amplitude for odd numbered harmonics that are permissible for Class A products. As the input power of the battery charger 10 increases, the harmonic coefficients in the table of Figure 3 may lead to current amplitudes for certain harmonics, such as the fifth and the ninth harmonics, reaching the permissible limits. In such a scenario the harmonic coefficients for the fifth and ninth harmonics can be fixed, whilst the remaining harmonic coefficients can be modified, to still shape the input current such that the output current has a ratio of peak output current to average output current that is no greater than 1.5.

[0054] Whilst particular examples have been described, it should be understood that these are illustrative examples only and that various modifications may be made without departing from the scope of the claims.

Claims

CLAIMS1. A battery charger comprising: input terminals for connection to an AC source supplying an alternating input voltage; output terminals for connection to a battery to be charged; and a power converter circuit connected between the input terminals and the output terminals, wherein the power converter circuit is configured to: act on an input current drawn from the AC source; generate an output current at the output terminals, and shape the input current such that the output current has a ratio of peak output current to average output current that is no greater than 1.5, and wherein the power converter circuit is configured such that, for a given time step, a difference between an instantaneous input power to the power converter circuit and an instantaneous output power of the power converter circuit is no more than 20%.

2. A battery charger as claimed in Claim 1, wherein the power converter circuit is configured to shape the input current by applying harmonic injection to the input current.

3. A battery charger as claimed in Claim 2, wherein the power converter circuit is configured to apply odd numbered harmonics to the input current.

4. A battery charger as claimed in Claim 2 or Claim 3, wherein the power converter circuit is configured to apply at least third, fifth, seventh, and ninth harmonics to the input current.

5. A battery charger as claimed in any one of Claims 2 to 4, wherein the power converter circuit is configured to apply a third harmonic to the input current at a ratio of at least 0.6 times, or at least 0.7 times, an amplitude of a fundamental of the input current.

6. A battery charger as claimed in any one of the preceding claims, wherein the ratio of peak output current to average output current is no greater than 1.4, no greater than 1.3, or no greater than 1.2.

7. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit is configured to generate the output current such that the output current has 100% ripple.

8. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit comprises an energy storage capacity of less than 900nF.

9. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit is configured to generate the output current such that the output current has a trapezoidal waveform.

10. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit is configured to shape the input current such that a waveform of the input current comprises two local maxima, and a local minima between the two local maxima, wherein the value of the local minima is no more than 0.5 times the value of the local maxima.

11. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit is configured to shape the input current such that, at a peak of the AC source, the peak output current has a value that is no greater than 30% more than a value of the average output current.

12. A battery charger as claimed in any one of the preceding claims, wherein the battery charger has an output power of at least 100W.

13. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit comprises an inductive circuit element, and a switch configured to control flow of current through the inductive circuit element.

14. A battery charger as claimed in Claim 13, wherein the power converter circuit comprises a controller configured to control a duty cycle of the switch to shape the input current.

15. A battery charger as claimed in any one of the preceding claims, wherein the power converter circuit has a single power conversion stage.

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