AC/DC converters having power factor correction

Abstract

An AC/DC converter and conversion method are provided, in which an AC input is rectified and shaped by a waveform shaping capacitor. A current source circuit is used to provide the output current to the output load which has a parallel bulk capacitor. The current source circuit is switched on and off with timing which is dependent on the phase of the AC input signal. This enables a relatively high power factor, for example between 0.7 and 0.9, with low cost circuitry with few components.

Claims

1. An AC/DC converter, comprising: an AC input for receiving an AC input signal and a DC output for supplying power to a load; a rectifier providing a rectified signal between first and second rectifier terminals; a waveform shaping capacitor connected between the first and second rectifier terminals; a current source circuit connected in series with the DC output; and a bulk capacitor connected in parallel with the DC output having a larger capacitance than the waveform shaping capacitor, wherein the current source circuit is adapted to successively switch on and off during a switching period, the switching period being a period wherein a voltage across the waveform shaping capacitor is larger than an actual voltage across the load, and wherein when the current source circuit is switched on, the current is provided to the load and the parallel bulk capacitor, and when the current source circuit is switched off, no current is delivered to the load and the parallel bulk capacitor, but the bulk capacitor maintains a current through the load such that the requirements for IEC62000-3-2 are met.

2. A converter as claimed in claim 1, wherein the current source circuit is adapted to deliver a constant current when switched on.

3. A converter as claimed in claim 1, wherein the current source circuit comprises a linear current source circuit.

4. A converter as claimed in claim 3, wherein the current source circuit comprises a transistor in series with the DC output and a constant voltage source providing a control voltage to a control terminal of the transistor.

5. A converter as claimed in claim 1, wherein the current source circuit comprises a switch mode power converter.

6. A converter as claimed in claim 5, wherein the current source circuit comprises a buck converter.

7. A converter as claimed in claim 1, further comprising a blocking diode in series with the DC output.

8. A converter as claimed in claim 1, adapted to switch on the current source when a phase angle of the AC input signal is smaller than 65 degrees and to subsequently switch off the current source circuit when a phase angle of the AC input signal is greater than 90 degrees.

9. A converter as claimed in claim 1, having a power factor of between 0.7 and 0.9.

10. A converter as claimed in claim 1, wherein the waveform shaping capacitor has a capacitance in the range 100 nF to 1 F and the bulk capacitor has a capacitance in the range 1 F to 100 F.

11. A lighting circuit comprising: an LED driver comprising an AC/DC converter as claimed in claim 1; and an LED load connected to the DC output.

12. The converter as claimed in claim 1, wherein the requirements for IEC62000-3-2 include a third harmonic current, expressed as a percentage of a fundamental current, shall not exceed 86% and a fifth harmonic current shall not exceed 61%.

13. An AC/DC conversion method, comprising: receiving an AC input signal; rectifying the AC input signal; shaping the rectified signal using a waveform shaping capacitor; and providing an output current to an output load with a bulk capacitor in parallel with the output load having a larger capacitance than the waveform shaping capacitor, the output current being delivered from the rectifier and waveform shaping capacitor using a current source circuit, wherein the method comprises switching on and off the current source circuit with timing which is dependent on the phase of the AC input signal such that the requirements for IEC62000-3-2 are met.

14. A method as claimed in claim 13, comprising providing an essentially constant current when the current source circuit is switched on, and comprising switching on the current source circuit when a phase angle of the AC input signal is smaller than 65 degrees and subsequently switching off the current source circuit when a phase angle of the AC input signal is greater than 90 degrees.

15. A method of driving an LED arrangement, comprising providing output current to the LED arrangement using the method of claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a typical passive AC-DC converter;

(3) FIG. 2 shows waveforms to illustrate the operation of the circuit of FIG. 1;

(4) FIG. 3 shows an example of a known so-called passive valley-fill circuit;

(5) FIG. 4 shows waveforms to illustrate the operation of the circuit of FIG. 3;

(6) FIG. 5 shows an example of a so-called active valley-fill circuit;

(7) FIG. 6 shows waveforms to illustrate the operation of the circuit of FIG. 5;

(8) FIG. 7 shows a known AC/DC converter architecture in schematic form;

(9) FIG. 8 shows a first implementation of an AC/DC converter;

(10) FIG. 9 shows waveforms for the circuit of FIG. 8;

(11) FIG. 10 shows a second implementation of an AC/DC converter;

(12) FIG. 11 shows an AC/DC conversion method; and

(13) FIG. 12 shows a current waveform shape in order to explain a set of requirements that may need to be met.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(14) The invention provides an AC/DC converter and conversion method, in which an AC input current is rectified and shaped by a waveform shaping capacitor. A current source circuit is used to provide current to the output load and a bulk capacitor in parallel to the load. The current source circuit is switched on and off with timing which is dependent on the phase of the AC input signal. This enables a relatively high power factor, for example between 0.7 and 0.9, with low cost circuitry with few components.

(15) As explained above, there is a need to increase the power factor above 0.5 (which applies to the circuit of FIG. 1) but without adding excessive circuit complexity.

(16) To improve the power factor, two common solutions are already typically used.

(17) FIG. 3 shows an example of a so-called passive valley-fill circuit, in which an additional passive analog circuit is provided between two output capacitors 14a and 14b and the load, in the form of a network of resistors, diodes and the two capacitors 14a and 14b.

(18) FIG. 4 shows the waveforms corresponding to those in FIG. 2 for the circuit of FIG. 3.

(19) FIG. 5 shows an example of a so-called active valley-fill circuit 50, in which a bulk capacitor 52 is charged to the peak of the rectified mains, and the energy that is stored in the capacitor 52 is only released once the reverse voltage across a diode 54 has reached a preset threshold. The circuitry in parallel with the diode 54 thereby effectively forms a switch that latches once the threshold voltage is exceeded. In this particular example, capacitor 14 is small compared to capacitor 52, and only needed if an EMI filter is required (formed by capacitor 14 and the inductor L1).

(20) FIG. 6 shows the waveforms corresponding to those in FIG. 2 for the circuit of FIG. 5.

(21) The concept in both circuits is to extend the current conduction angle of the classical circuit of FIG. 1 more towards the mains voltage zero crossings, effectively increasing the power factor to the desired value, while at the same time keeping a high voltage at the DC output.

(22) The main problems associated with these solutions are the following: the number of components is still relatively large, especially the active circuit; the valley fill circuit has an intrinsically large voltage ripple of approximately 50% (Vmax:Vmin=2:1); the driver efficiency is reduced, especially if they are designed to meet the special waveform requirements of IEC61000-3-2; the solutions are relatively expensive; and the solutions require a somewhat larger EMI filter compared to the classical circuit of FIG. 1.

(23) FIG. 7 shows a known AC/DC converter in schematic form, comprising an AC input for receiving an AC input signal 12 and a DC output 70 for supplying power to a load RL. As in the circuits of FIGS. 1, 3 and 5, there is a rectifier 10 having first and second terminals 72, 74 across which the rectified signal is provided. The AC input delivers current to the rectifier through an input resistor Rin, which may comprise a fusistor (a combination of a current limiting resistor and a fuse). A bulk output capacitor 14 is connected between the first and second rectifier terminals 72, 74.

(24) The output capacitor may be a single capacitor or a network of series and/or parallel capacitors. The full rectified voltage is thus used to drive the load.

(25) The converter further comprises a constant current source circuit 76 which provides its output current to the output load RL. The constant current source is provided between the rectified signal (which is between the first and second rectifier terminals 72, 74) and the DC output 76.

(26) The capacitor 14 is thus large enough to absorb fluctuation in the input, and then enable a constant current source to provide the output current. The limitation of this circuit is that it only provides a low power factor (e.g. only slightly greater than 0.5) or, if designed for a medium power factor, has such a large voltage ripple that the driver efficiency reduces too much, or the output LED current becomes discontinuous, giving rise to undesired light flicker.

(27) FIG. 8 shows a first implementation of a circuit in accordance with the invention.

(28) Instead of providing a bulk capacitor 14 between the rectifier terminals, a smaller waveform shaping capacitor 81 is provided. It performs shaping of the input current and limits the gradient of output voltage changes in response to large surges in input voltage.

(29) A bulk capacitor must be large enough to smooth the rectified waveform and deliver a continuous current output. The waveform shaping capacitor 81 is much smaller and this enables a higher power factor to be achieved. By way of example, the waveform shaping capacitor may have a capacitance in the range 100 nF to 1 F whereas a bulk capacitor may have a capacitance in the range 1 F to 100 F. A smaller waveform shaping capacitor gives rise to a more square current waveform profile and a higher power factor, but it does not enable a continuous current to be provided to the load.

(30) This continuous current is instead ensured by providing a bulk capacitor 14 across the load itself.

(31) The circuit again comprises a current source circuit 76. As shown, the bulk capacitor 14 is only across the load and not across the series connection of the load and the current source circuit 76. The current source circuit is switched on and off with timing which is dependent on the phase of the AC input signal. In this way, power factor correction is implemented based on the timing of activation of the current source circuit 76.

(32) This converter makes use of the standard configuration of a rectifier and output capacitor, but the output capacitor is much smaller. When the current source circuit is switched on, the current is provided to the load and its parallel bulk capacitor, and when it is switched off, no current is delivered to the load and its parallel bulk capacitor, but the bulk capacitor maintains a current through the load.

(33) This circuit enables an increase in the power factor compared to a basic rectifier circuit, but without introducing excessive complexity to the circuit. The converter may be integrated into a low cost product, such as an LED bulb.

(34) The current source circuit 76 for example delivers a constant current when switched on. Thus, the output current has a square wave profile. This enables a most simple implementation.

(35) In the example shown in FIG. 8, the current source circuit 76 is implemented as a linear current source comprising a bipolar junction transistor 80 in series with the load 82. The load 82 is shown as an equivalent circuit for an LED string, comprising diode DL, voltage source VL and load resistance RL. A blocking diode 83 is also provided in series with the LED load 82, for limiting the reverse voltage that arises across the transistor 80.

(36) There is a shunt resistor 84 between the emitter and ground and a base resistor 86 coupled to a voltage source 88. The circuit 76 functions as a constant current source with a current, effectively only dependent on the voltage source 88 (minus the base-emitter voltage, typically being 0.7V), and the size of the shunt resistor 84. The base is supplied with a constant voltage from the voltage source 88. This may be of the order of a few volts.

(37) Changes in the mains input voltage result in changes in the collector-emitter (or drain-source) transistor voltage, effectively controlling the timing of switching on and off of the current source circuit. Specifically, if the voltage across the waveform shaping capacitor 81 is larger than the actual voltage across the load 82, the current source is switched on. The current source is successively switched on and off during a switching period, the switching period being a period wherein the voltage across the waveform shaping capacitor 81 is larger than the actual voltage across the load 82.

(38) This allows the duration of the on-time of the current source and the positioning of the on-time duration of the current source in time to be controlled independently. The amplitude of the current is e.g. determined by the DC voltage source, the base-emitter transistor voltage (usually around 0.7V) and the size of the shunt resistor.

(39) Another example may be that if the voltage across the waveform shaping capacitor 81 is larger than the actual voltage across the load 82, the current source is switched on, otherwise it is switched off. The amplitude of the current is e.g. determined by the DC voltage source, the base-emitter transistor voltage (usually around 0.7V) and the size of the shunt resistor. This example has the advantage that it can be more easily controlled.

(40) In this circuit, a smaller sized waveform shaping capacitor 81 is used after the full bridge rectifier compared to the classical circuit of FIG. 1, for the same power requirements. The capacitor is discharged by an essentially constant block current.

(41) The phase angle at which this block current starts is actively or passively chosen such that it is smaller than 65 degrees radians. In combination with the waveform shaping capacitor 81 of the required size this results in an input current waveform that has an input power factor of greater than 0.7 whilst also meeting the special mains input current waveform requirements of IEC61000-3-2.

(42) For the mains input current to have its maximum value per mains half cycle before or at 65 degrees (according to IEC61000-3-2), the size of the waveform shaping capacitor 81 needs to be higher than a certain minimum which basically depends on the desired output power. For economic reasons and to keep the power factor high it should also not be much larger than this minimum. The maximum current is essentially at the moment the block current starts to flow.

(43) The circuit enables very simple current source control, making use simply of a DC value with no dynamic control needed. No Zener diode is also needed across the current source: a Zener diode is often only a one-time protection, under surge conditions it can easily break. The waveform shaping capacitor provides a more robust approach, especially if used in combination with disabling the current source under surge conditions.

(44) FIG. 9 shows waveforms for the circuit of FIG. 8.

(45) Plot 90 shows the mains voltage. Plot 92 shows the input current flowing from the AC input to the rectifier. Four different input current plots are shown, for four different sizes of the waveform shaping capacitor 81.

(46) Plot 94 shows the current flowing through the decoupling diode 83. It comprises a square wave current profile which is the output current of the current source circuit 76. This current flows to the combined parallel circuit of the LED load and bulk capacitor. The bulk capacitor maintains a current through the load when the current source circuit is off.

(47) Plot 96 shows the timing of operation of the current source circuit 76.

(48) The block current (plot 94) is synchronized with twice the mains frequency (corresponding to the frequency of the rectified signal), and the phase angle at which it starts conducting can be directly or indirectly controlled. The start is before 60 degrees. With the peak of the mains current at or before 65 degrees, this helps to guarantee that the special waveform of IEC61000-3-2 is met.

(49) The block current is preferably actively or passively controlled such that it does not cease before or at /2 radians (90 degrees).

(50) For meeting standards, it should at that phase angle have an amplitude of at least 5% of its peak.

(51) In the example shown, the current source circuit is turned on from 3 ms to 7 ms during a mains frequency half cycle of 10 ms in this example. The current through the input resistor Rin, which is the mains input current, nicely shows the desired behavior for fulfilling the special waveform requirements. The influence of the size of the waveform shaping capacitor can also be seen. It should neither be too small thus not peaking at the beginning, nor too large for the given output power (the mains current pulse becomes too small). Thus, the two extreme examples of the plot 92 should be avoided.

(52) The triangular waveform is for a large capacitor 81for example equivalent to a conventional bulk capacitor. The flatter waveforms are for progressively smaller capacitor 81.

(53) In another example, shown in FIG. 10, the current source circuit comprises a switch mode power converter, in particular a buck converter 100. This enables a higher efficiency to be achieved over a wider input voltage range. A buck-boost converter may be used.

(54) The converter includes a blocking diode so that the blocking diode 83 of FIG. 8 is not shown.

(55) The buck converter may be a constant current converter that is only enabled when the block current needs to flow. The buck converter has an additional input pin 104 where it can be enabled or disabled depending on the required timing. The timing is for example controlled using a microprocessor or other timing circuitry.

(56) FIG. 11 shows an AC/DC conversion method, comprising:

(57) in step 110 receiving an AC input signal;

(58) in step 112 rectifying the AC input signal;

(59) in step 114 shaping the rectifier signal using a waveform shaping capacitor; and

(60) in step 116 providing an output current to an output load with a bulk capacitor in parallel with the output load, the output current being delivered from the rectifier and waveform shaping capacitor using a current source circuit, wherein step 116 comprises switching on and off the current source circuit with timing which is dependent on the phase of the AC input signal.

(61) The current source switching is entirely passive in the example above and follows automatically from the input voltage waveform and the load voltage. Active switching may instead be carried out.

(62) As mentioned above, one requirement that may need to be met is the special current waveform shape defined in IEC62000-3-2. For completeness, the requirements are explained with reference to FIG. 12.

(63) The requirements are that the third harmonic current, expressed as a percentage of the fundamental current, shall not exceed 86% and the fifth harmonic current shall not exceed 61%.

(64) Also, the waveform of the input current shall be such that it reaches the 5% current threshold before or at 60 degrees (3.33 ms for 50 Hz), has its peak value before or at 65 degrees (3.611 ms) and does not fall below the 5% current threshold before 90 degrees (5 ms), referenced to any zero crossing of the fundamental supply voltage. The current threshold is 5% of the highest absolute peak value that occurs in the measurement window, and the phase angle measurements are made on the cycle that includes this absolute peak value. Components of current with frequencies above 9 kHz shall not influence this evaluation.

(65) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.