GB2521509A

GB2521509A - Power Converter

Info

Publication number: GB2521509A
Application number: GB1418286.9A
Authority: GB
Inventors: Jose Santillan; Andrew John Skinner
Original assignee: TDK Lambda UK Ltd
Current assignee: TDK Lambda UK Ltd
Priority date: 2013-10-15
Filing date: 2014-10-15
Publication date: 2015-06-24
Also published as: GB201318255D0; GB201418286D0

Abstract

An AC-DC power converter having power factor correction comprises a rectifier circuit, a bulk capacitor Cl for drawing and storing power from an AC supply via the rectifier circuit, an inductive component Ll for transforming an input voltage into an output voltage, the inductive component Ll comprising a two primary windings Lla, Llc connected together in series, and a secondary winding Lib inductively coupled to the primary windings, and a switch M1 for periodically switching ON and OFF to respectively connect and disconnect the inductive component to/from an input voltage. The input voltage is supplied by the bulk capacitor Cl to the two primary windings Lla, Llc when V(c1)*N(L1c)/(N(L1c)+NL1a))>Vac, and the input voltage is supplied by one of the AC supply and the rectifier circuit to the second primary winding Llc when V(c1)*N(L1c)/(N(L1c)+NL1a))<=Vac, where V(c1) is the instantaneous bulk capacitor voltage, N(Lla) and N(Llc) are the number of turns on the first and second primary windings Lla, Llc respectively, and Vac is the magnitude of the instantaneous AC voltage. The converter further comprises one or more resistors R4 connected between the AC supply and the bulk capacitor C1 for reducing harmonic current emission from the converter.

Description

POWER CONVERTER

[0011 The present invention relates to power converters or power supplies, and more particularly to power factor improvement in oonverters rated between 75W-bOW.

[0021 European S Landard EN61000-3-2 requires LJiaL power supplies rated above 75W meet certain line frequency harmonic limits for input current. For power supplies rated above 100W, a power factor correction circuit is typically added to ensure compliance with EN61000-3-2. However, for power supplies rated between 75W and 100W, where only a small improvement in power factor is required for compliance with EN61000-3-2, lower cost solutions are known.

[0031 The simplest power factor improvement solutions involve increasing the resistance of the EMC filter and rectifier circuit to broaden the conduction angle. Although effective, this approach reduces circuit efficiency. Another known solution is to use a voltage-doubler type rectifier which reduces the capacitance of the bulk capacitor at high line voltages because two capacitors are connected in series.

This increases the ripple voiLtage and hence the conduction angle. However, this approach requires two electrolytic capacitors and additional components to reconfigure the rectification for operation in either conventional or voltage doubler modes. More efficient solutions include those specified in Us 6 088 242 and US 6 118 673 where an additional converter is also switched by switch Ml. However, these circuits require an additional inductor which increases both size and cost.

[0041 It is an object of the present invention to overcome

the drawbacks of the prior art.

[005] According to one aspect of the present invention there is provided a power converter for converting an AC voltage supplied by an AC power supply into a DC voltage, the power converter comprising:-a rectifier circuit for rectifying the AC voltage supplied by an AC power supply; a bulk capacl Lor Cl for drawing and s Luring power from the AC supply via the rectifier circuit; an inductive component Ll for transforming an input voltage into an output voltage, the inductive component Li comprising a two primary wiridings tia, tlc connected together in series, and a secondary winding Llb inductively coupled to the primary windings; and a switch Ml for periodically switching ON and OFF to respectively connect and disconnect the inductive component to/from an input voltage; wherein said input voltage is supplied by the bulk capacitor Cl to the two primary windings Lla, tic when V(cl)*N(Llc)/(N(Llc)+NLla))>Vac; and wherein said input voltage is supplied by one of the AC supply and the rectifier circuit to the second primary winding tic when V(ci)*N(Llc)/(N(tlc)tNtiafl<=Vac; where V(ci) is the instantaneous bulk capacitor voltage, N(Lla) and N(Llc) are the number of turns on the first and second primary windings Lla, tic respectively, and Vac is the magnitude of the instantaneous AC voltage; and wherein the converter further comprises one or more resistors R4 connected between the AC suppiy and the bulk capacitor Ci for reducing harmonic current emission from the converter.

[006] Thus, during each half cycle of the AC supply voltage, the converter is supplied by the bulk storage capacitor Cl when V(cl)*N(tic)/(N(tic)+Ntiafl>Vac, and by the AC supply when V(ci)*N(tic)/(N(Llc)+Ntia))<=Vac.

[007] The inclusion of the or each resistor R4 is important in limiting harmonic emissions, and can help to ensure compliance with EN61000-3-2. In particular, the or each resistor R4 reduces the magnitude of the l3, 15th and 17th order harmonic frequencies of the AC supply.

[008] Preferably, the total resistance of the one or more resistors R4 is non-zero over an operating temperature range of the converter. In this way, the harmonic frequencies are limited even at elevated temperatures.

[009] The total resistance of the one or more resistors R4 is at least approximately 0.4 ohms, and preferably at least approximately 1.5 ohms, over the operating temperature range of the converter. In one embodiment, the total resistance of the one or more resistors R4 is at least approximately 2.5 ohms over the operating temperature range of the converter.

The present inventors have determined that, for a 100 W converter, a minimum resistance in these ranges is sufficient to ensure compliance with ENE1000-3-2 over the whole range of operating temperatures of the converter.

[0010] The one or more resistors R4 may comprise a temperature dependent resistor. Thus, at low input voltages, where the current in R4 is higher and where EN61000-3-2 does not apply, the losses in PA are reduced.

[0011] The one or more resistors PA may comprise a temperature dependent resistor with and a further resistor with a significantly lower temperature coefficient of resistance in series. The temperature coefficient of resistance of the further resistor is sufficiently lower than that of the temperature-dependent resistor that the resistance of the further resistor varies over the nominal operating temperature range of the converter by less than 30%. The further resistor may be a fixed resistor.

[0012] Accordingly, in one embodiment, the one or more resistors R4 may comprise a temperature dependent resistor with and a fixed resistor in series. In this way, the fixed resis br caxi be selecbed so a desired minimum resls lance is maintained irrespective of the temperature of the resistor, which will fluctuate in use according to the operating conditions of the converter, and in particular due to resistive heating of the components. The temperature dependent resistor, which preferably has a negative temperature coefficient, can be selected to control inrush currents at low operating temperatures (e.g. on start-up) As the components heat up, the resistance of the temperature dependent resistor reduces to minimise losses during operation, particularly at low line voltages.

[0013] The primary windings Lia, Lic may be connected together at a point t, and the converter may further comprise a first rectifier D4 for connecting the bulk capacitor Cl to the two primary windings Lla, Llc of the transformer, and at least one second rectifier 05(D6) for connecting one of the AC supply and the rectifier circuit to point t.

[0014] 1'ith this arrangement, during each half cycle of the AC supply voltage, the converter is supplied by the bulk storage capacitor Cl (via rectifier D4) when V(cl)*N(Llc)/(N(Llc)+NLla))>Vac, and by the AC supply (via the at least one rectifier, optionally in combination with the rectifier circuit) when V(cl) *N(Llc) /(N(Llc)+NL1a) )<=Vac.

[0015] In one embodiment, the input voltage is supplied to the second primary winding Lic by the AC supply via the rectifier circuit and rectifier D5, wherein rectifier D5 is located to connect the rectifier circuit to point t.

[0016] In this embodiment, the converter comprises a third rectifier D3 for connecting the hulk capacitor Cl to the rectifier circuit, to draw and store power from the AC supply when Vac>V(cl) [0017] Thus, when Vac>V(cl), the bulk storage capacitor Cl is recharged from the AC supply via rectifier 03.

[0018] In a further embodiment, the input voltage is supplied to the second primary winding Lb by the AC supply via rectifier 05, which is located to connect one pole of the AC input to point t. In this alternative embodiment, the converter preferably comprises a further rectifier 06 for connecting the other pole of the AC supply to point t.

[0019] Thus, the converter is supplied by the AC supply via either 05 or 06, depending on which pole of the AC input is high.

[0020] In this embodiment, when Vac>V(cl), the bulk storage capacitor Cl is recharged from the AC supply via the rectifier circuit. Rectifier 05 and/or rectifier 06 is/are preferably further connected to a filter capacitor C4.

[0021] Capacitor C4, when present, increases differential mode filtering. Adding the capacitance on the DC side of the rectifier circuit allows for a smaller capacitor to be fitted. However, as an alternative, the capacitance on the AC side of the rectifier circuit could be increased.

[0022] In all embodiments, an ENC filter is preferably located between the AC voltage supply and the bridge rectifier. Where reference is made to supply of an AC voltage by the AC power suppiLy, it will be understood that the AC voltage is preferably supplied via an EtC filter.

[0023] The or each resistor R4 may be connected between the rectifier circuit and the bulk capacitor Cl. By locating the one or more resistors R4 between the rectifier circuit and Llie bulk sLorage capaciLor Cl, inrush limiLixig is only applied to the current that flows into Cl and not to current that flows via the second rectifier D5. In alternative embodiments, the or each resistor R4 is connected between the AC supply and the rectifier circuit (i.e. on the AC side of the rectifier circuit) [0024] The converter may be a flyback converter. However, the principles of the invention are not limited to a flyback converter, and could be applied to other single-ended topologies, such as (but not limited to) a forward converter topology.

[0025] Preferably, the converter comprises a snubber circuit for providing an alternative current path to protect switch Ml.

[0026] The snubber circuit is preferably an ECU snubber circuit. However, other snubber configurations could be used.

[0027] Preferably, the converter further conprises a capacitor CS connected in parallel with first rectifier D4.

[0028] As discussed in more detail below, due to the capacitance of rectifier D4, the voltage on D4 rises as the drain-source voltage Vds of Ml rings back down after current has fallen to zero. Capacitor C3 reduces this excursion and hence reduces the valley voltage for Ml, thereby reducing turn-on losses.

[0029] The turns ratio N(Lla) :N(Llc) is preferably at least 31:6, more preferably at least 33:4. The turns ratio N(Lla) :N(Llc) is preferably no more than 36:1, more preferably no more than 35:2.

[0030] The presenL inveribioti will now be described wibli reference to the accompanying drawings in which:- [0031] Figure 1 shows a schematic diagram for an AC:190 flyback power converter; [0032] Figure 2 shows a schematic diagram for an AC:DC flyback power converter with power factor improvement circuitry in accordance with a first enftodiment of the invention; [0033] Figure 3 shows approximate voltage waveforms for a converter erbodying the invention, operated in Boundary Conduction Mode; [0034] Figure 4 shows a schematic diagram for an AC:DC flyback power converter with power factor improvement circuitry in accordance with an embodiment of the present invention, which is similar to the converter of figure 2 but has an alternative placement of the snubber circuit; [0035] Figure 5 shows a schematic diagram for an AC:DC flyback power converter with power factor improvement circuitry in accordance with a second embodiment of the invention; [0036] Figure 6 shows a schematic diagram for an AC:DC flybaok power converter with power factor improvement circuitry in accordance with an embodiment of the invention, which is a variation of the converter of figure 5; [0037] Figure 7 shows a schematic diagram for an AC:DC flyback power converter with power factor improvement circuitry in accordance with an embodiment of the invention, which is a further variation of the converter of figure 5; [0038] Figure 8 shows a schematic diagram for an AC:DC flyback power converter with power factor improvement circuitry in accordance with an embodiment of the invention, which is a further variation of the converter of figure 5; [0039] Figure 9 shows a schematic diagram for a multi-output embodiment of the invention; and [0040] Figure 10 shows a schematic diagram for a forward converter with power factor improvement circuitry in accordance with an embodiment of the invention.

[0041] In the figures, elements and components common to different figures are given the same reference numerals.

[0042] In this application, the term "connect" and related terms refer to an electrical connection between components, unless stated otherwise.

[0043] In this application, the notation N(Llx) is used to refer to the number of turns associated with an inductance Lix.

[0044] Figure 1 shows a schematic diagram for a known AC:DC flyback power converter. In the circuit of figure i, an AC voltage Vac is applied to an EMC filter 6 and a bridge rectifier circuit 8 to supply a rectified voltage between a high voltage rail 10 and a lcw voltage rail 12. One side of a bulk storage capacitor Cl is connected to the high voltage rail 10 and the other side of capacitor Cl is connected to the low voltage rail 12.

[0045] A transformer or coupled inductor Ill has a primary winding Lla which Is inductively coupled to a secondary winding fib. A firs b end of primary winding Lia is conxiecLed to the high voltage rail 10 at a node a in the circuit, and a second end of primary winding Lla is connected to a node b in the circuit.

[0046] The drain terminal of a switch Ml is connected to node b, and the source terminal of switch Ml is connected to one end of a resistor Ri, the other end of which is connected to the low voltage rail 12. The gate terminal of switch Ml is connected to a controller, not shown. switch Ml is typically a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) [0047] An ROD snubber circuit 14 comprises a capacitor 02, a diode Dl and two resistors R2 and P2. One side of capacitor 02 is connected to node a. The other side of capacitor 02 is connected to the cathode of diode Dl. The anode of diode Dl is connected to one end of resistor R2.

The other end of resistor R2 is connected to node b.

Resistor RB is connected in parallel with capacitor 02.

[0048] One end of secondary winding fib is connected to the anode of a diode D2. The cathode of diode D2 is connected to one side of an output capacitor 03. The other side of capacitor C3 is connected to the other end of secondary winding fib. Secondary winding Llb is oriented relative to the primary winding Lla and diode D2 such that the voltage induced by current flow in Lie when switch Ml is conducting reverse biases diode D2, and such that the voltage induced -1_C -when switch Ml is not conducting forward biases diode Dl.

[0049] Output capacitor 03 supplies a DC voltage.

[0050] The bulk storage capacitor Ci draws and stores energy from the AC supply via the ENC filter and the bridge recLifier. When swiLcli Mt is ON, Llie primary winding of Llie coupled inductor Li is connected to capacitor Cl which supplies an input voltage to the coupled inductor. In this state, the primary current and magnetic flux in the coupled inductor increases, inducing a negative voltage in the secondary winding, such that diode D2 is reverse biased. In this state, current cannot flow in the secondary winding and energy is stored in the coupled inductor. When switch Ml is subsequently switched OFF, the primary current and magnetic flux in the coupled inductor drops, inducing a positive voltage in the secondary winding, such that diode D2 is forward biased. In this state current can flow in the secondary winding, and energy stored in the coupled inductor is transferred to the output capacitor 03 to be transferred to the load.

[0051] Figure 2 shows a schematic diagram for an AC:OO flyback power converter, with power factor improvement circuitry in accordance with a first embodiment of the present invention. The circuit is similar to that illustrated in figure 1, and the above description of the elements common to both circuits applies here also.

[0052] However, with the present invention, tte primary winding is tapped at a point t, so that the converter can be powered directly from the AC supply (via rectifiers) at voltages lower than that required to recharge the bulk capacitor Cl. In the embodiment illustrated in figure 2, this is achieved by means of three additional rectifiers, in this -1_i -case, diodes D3, D4 and D5.

[0053] The transformer tap located at point t separates the primary winding into two smaller primary windings Lia and tic, connected to one another in series at point t.

[0054] The anode of diode D3 is ooxixiecbed Lo Llie high voltage rail 10 and the cathode of diode 03 is connected to one end of a resistor PA. The other end of resistor PA is connected to the anode of diode D4. The cathode of diode D4 is connected to node a.

[0055] The anode of diode 05 is connected to the high voltage rail tO and the cathode of diode 05 is connected to the transformer tap at point u. Thus, diode 05 connects from the bridge rectifier to the transformer tap.

[0056] Resistor R4 is added so that inrush limiting is only applied to the current that flows into Cl and rot to the current that flows via diode D5. Although illustrated in the accompanying drawings as a single fixed resistor, resistor R4 can be embodied as one or more resistors that may comprise for example a fixed resistor, a temperature dependent resistor, or a fixed resistor and temperature dependent resistor in series, as will be explained in more detail below.

[0057] Operation of the circuit of figure 2 is similar to that of figure 1 as described above. However, the additional rectifiers allow current to be drawn directly from the AC supply (ie bypassing the bulk storage capacitor) for some time during each half cycle of the AC supply voltage when the instantaneous AC voltage is lower than that of the bulk capacitor.

--

[0058] There are three operating modes of interest.

[0059] In a first operating mode, AC is low and diodes D3 arid D5 are reverse biased.

[0060] The circuit is operating in this mode if V(cl)*N(Llc)/N(Llc)+N[Lla))>Vac (1) where Vac is the magnitude of the instantaneous value of the AC supply voltage.

[0061] In this first operating mode, the circuit operates as a conventional flyback converter, with the exception that diode D4 conducts whilst switch Mi is conducting and whilst current flows in snubber diode Di after NIL switches off.

[0062] Although efficiency is reduced in this mode due to the conduction of diode D4, the overall efficiency and volume are improved compared to other solutions.

[0063] In a second operating mode, diode D5 is forward biased and diode D3 is reverse biased.

[0064] If V(cl)*N(Llc)/N(Llc)+N[Llafl<=Vac (2) and Vac<V(Cl) (3) then diode D5 will be forward biased when Ml is conducting.

This forces diode 04 to become reverse biased. The flyback converter now draws power directly from the AC supply when switch Ml is conducting, using only winding Llc. When switch Mi turns OFF, diode D4 will become forward biased so that -1_3 -current can flow through the snubber circuit. This forces diode 05 into reverse bias, when switch Ml subsequently turns ON at the start of the next switching cycle, D5 will again become forward biased.

[0065] In a third operating mode, diodes DO and D3 are forward biased.

[0066] when Vac>\J(Cl) (4) diode D3 becomes forward biased and recharges the bulk capacitor Cl. Since condition (2) must also hold here, operation of the flyback converter is as described for the second operating mode.

[0067] In practice, there will be a small/negligible voltage drop across the bridge rectifier and a small/negligible error introduced by the EMC filter due to phase shifts created by the filter ripple voltages that exist on its output. Thus, Vac in conditions (1)-(4), should be interpreted as an approximation of the precise instantaneous AC voltage magnitude to account for these factors.

[0068] Figure 3 shows approximate waveforms for the drain-source voltage Vds of switch Ml and the cathode voltage Vcathode of diode D4, for a converter errbodying the present invention operated in Boundary Conduction Mode (SCM) [0069] It can be seen from figure 3 that Vcathode for D4 rises as Vds of switch Ml rings back down after current has fallen to zero. This is due to the capacitance of diode D4.

Adding a small capacitor across D4 can reduce this excursion and hence reduce the valley voltage for Ml. This reduces turn-on losses. For a 100W converter this capacitor is preferably in the range lOOpF-lnF, and is preferably a -:I_4 -surface mount ceramic component.

[0070] As noted above, the one or more resistors PA may include a resistor with a negative temperature coefficient of resistance (NTC) . Thus, at low input voltages, where the current in R4 is higher, and where standard EN61000-3-2 does b apply, Llie losses iii R4 are reduced.

[0071] Table 1 lists the current limits for each harmonic under EN61000-3-2, together with the measured current (Imeas) in a representative circuit according to figure 2 at an operating temperature of approximately 125°C. The components used are as listed in table 2 below, except in that R4 is a ohm NTC resistor with no fixed resistor in series. As can be seen, the harmonic reguirements of FN61000-3-2 are not met for the 15th harmonic, and are only just met for the 13Th and 17Th harmonics.

Table 1

Harmonic Order Current Limit Imeas Margin Pass/Fail (n) (A) (A) (A) _____________ ________________ Odd Harmonics _______ ______________ 3 2.3000 0.3525 1.9475 Pass 1.4000 0.2890 1.1110 Pass 7 0.7700 0.2298 0.5402 Pass 9 0.4000 0.1923 0.2077 Pass 11 0.3300 0.1761 0.1539 Pass 13 0.2100 0.1676 0.0424 Pass 0.1500 0.1512 -0.0012 Fail 17 0.1324 0.1209 0.0115 Pass 19 0.1184 0.0838 0.0347 Pass 21 0.1071 0.0507 0.0565 Pass 23 0.0978 0.0289 0.0689 Pass 0.0900 0.0197 0.0703 Pass 27 0.0833 0.0156 0.0678 Pass 29 0.0776 0.0082 0.0694 Pass 31 0.0726 0.0051 0.0675 Pass 33 0.0682 0.0145 0.0537 Pass 0.0643 0.0174 0.0469 Pass 37 0.0608 0.0133 0.0476 Pass 39 0.0577 0.0066 0.0511 Pass -1_5 -Harmonic Order Current Limit Imeas Margin Pass/Fail (n) (A) (A) (A) _____________ Even Harmonics 2 1.0800 0.0041 1.0739 Pass 4 0.4300 0.0039 0.4261 Pass 6 0.3000 0.0035 0.2965 Pass 8 0.2300 0.0032 0.2268 Pass 0.1840 0.0030 0.1810 Pass 12 0.1533 0.0025 0.1508 Pass 14 0.1314 0.0021 0.1294 Pass 16 0.1150 0.0015 0.1135 Pass 18 0.1022 0.0014 0.1008 Pass 0.0920 0.0010 0.0910 Pass 22 0.0836 0.0007 0.0829 Pass [0072] Whilst the performance illustrated in Table 1 is acceptable under certain conditions, in one variant, the present invention provides for improved harmonic suppression.

[0073] In this variant, R4 is a pair of resistors, comprising an NTC resistor connected in series with an additional resistor with a significantly lower temperature coefficient of resistance, such as a fixed resistor.

[0074] This arrangement effectively sets a minimum value for the total resistance of the resistor pair R4, such that a minimum resistance value is maintained regardless of the operating temperature. In this way, a minimum charging time-constant for capacitor Cl can be set at a pre-determined value. This prevents the rectifiers from operating at a narrow conduction angle, and the present inventors have determined that this characteristic serves to reduce harmonic resonance in the circuit at the 13, l5l and l7l harmonics of the supply frequency.

[0075] The inventors have determined that, when the combined resistance of the resistor pair R4 (i.e. the sum of the resistance of the NTC resistor and the fixed resistor) exceeds between approximately 0.4 ohms and approximately -t6 - 2.5 ohms over the operating temperature range of the device, the current limits for all of the harmonics listed in Table 1 are met for a 100 N converter.

[0076] Advantageously, in this variant, the temperature-dependent NTC resistor and the fixed resistor of the resistor pair R4 caxi be selec bed ixidependexi bly. The NTC resis Lor can therefore be selected to provide optimum limitation of in-rush currents, whilst the fixed resistor can be chosen to provide the desired control of harmonics, particularly at elevated temperatures.

[0077] Figure 4 shows a variation of the embodiment illustrated in figure 2, with an alternative placement for the snubber circuit. Here, the snubber circuit 14' is connected to the anode side of diode D4 rather than the cathode side. That is to say, capacitor 02 is connected to the anode side of diode P4. Connection of the snubber circuit to other nodes in the circuit, such as zero-volts is also possible. The configuration illustrated in figure 4 results in a slight increase (5-10%) of the rms (root mean square) current in D4. However, this configuration also reduces the number of components that exhibit a voltage with respect to zero volts when 04 recovers. This can be of benefit in terms of EMO.

[0078] Figure 5 illustrates a second embodiment of the present invention, which results in improved efficiency as compared with the first embodiment.

[0079] The circuit shown in figure 5 is similar to the circuits shown in figures 2 and 4. However, instead of connecting the bridge rectifier circuit to the transformer tap at point t, diode D5 connects one pole of the AC supply to point t, whilst an additional diode D6 connects the other -1_7 -pole of the AC supply to point t.

[0080] Figure 6 illustrates a variation of the oircuit shown in figure 5. In the circuit shown in figure 6, diodes D5 and DO are connected to one side of an additional capacitor 04 as well as to point t. The other side of capacitor 04 is conxiec bed Ic She low vol Sage rail 12.

[0081] Capacitor 04 is added to increase differential mode filtering. Adding this capacitance on the DC side of the bridge rectifier allows for a smaller capacitor to be used.

Alternatively however, the capacitance on the AC side of the bridge reotifier could be increased.

[0082] In the circuit shown in figure 6, the snubber circuit is configured as in figure 4, except that a capacitor 05 is connected across D4 for the purpose of reducing turn-on losses as discussed in more detail above. In this embodiment, diodes D5 and D6 connect from the AC supply to the transformer tap and capacitor 04.

[0083] In figures 5 and 6, resistor(s) R4 is shown located between the bridge rectifier and the anode side of diode D4.

However, as with the other embodiments, the one or more resistors BA may be placed on the AC side of the bridge rectifier.

[0084] By way of example, figure 7 illustrates another variant of the circuit of figure 5. In this variant, the resistor R4 is connected between the anode side of diode DO and the bridge rectifier 8, so that the AC input current flows through the resistor R4 before reaching the bridge rectifier 8.

[0085] Figure 8 illustrates a further variant of the circuit -t8 -of figure 5. In this case, the resistor R4 is connected in series between the EMP filter 6 and the anode side of diode 56. The bridge rectifier is aiso connected to the anode side of diode D6. Again, in this variant, the AC input current flows through the resistor R4 before reaching the bridge rectifier 8.

[0086] Conditions (l)-(4) described above in relation figures 2 and 4 also apply for figures 5 to 8.

[0087] That is to say, whilst condition (f) is satisfied, diode D4 is forward biased and current is drawn from bulk capacitor Cl, with diodes D5 and 56 both reverse biased.

[0088] Whilst conditions (2) and (3) are satisfied, then either diode D5 or diode D6 is forward biased, depending on which AC input is high, diode D4 becomes reverse biased, and current is drawn directly from the AC supply.

[0089] Whilst condition (4) is satisfied, the bridge rectifier diodes become forward biased and recharge the capacitor Cl through resistor R4.

[0090] For the purpose of illustration, a list of suitable components for the circuits shown in figure 2 and figure 5 is given in table 2. However, it will be appreciated that alternative components may be selected.

-1_9 -

TABLE 2

Capacitors Cl FPB22W225K 2U2, 450\J 02 4N7, 630V 1206 03 25ZL}-I1000MEFCCE1OX23 l000uF, 25V Resistors Ri 2 x R47 2 x 1W in parallel R2 47k 0.5W R3 39K 3W R4 10k (NTC) + R47 (2W) NIC + resistor in series Diodes Dl NURS16OT3G 1A, 600V D2 STPS2OH1000T 20A, 100V D3 FES3J 3A, 600V D4 FES3J 3A, 600V D5 FES3J 3A, 600V DO FES3J 3A, 600V lET Ml NDF13N65BTR N-Channel P4OSE'ET 650V, 14A, 0.460 Transformer parameters Nia 31 Nb 251 Nlb 31 [0091] Test results on a iCOW flyback converter operating at 23OVac showed an improvement in power-factor from approximately 0.43 to 0.54 with tic having 35 turns and Lla 2 turns. This increases to 0.57 with a turns ratio of 33:4 for Llc:Lla. However, the increased number of turns for Lla results in a higher l5 harmonic, requiring that R4 has a larger value to achieve compliance with EN61000-3-2. Where R4 is a pair of resistors comprising an NTC resistor connected in series with a fixed resistance resistor, for a 100W converter, the resistance of R4 in steady-state is -20 -preferably in the range 0.4-2.5 ohms, to ensure compliance with EN61000-3-2.

[0092] It will he understood that the embodiments illustrated above show an application of the invention only for the purposes of illustration. In practice the invention rray be applied bo many differeriL config nra Lions, Llie de bailed embodiments being straightforward for those skilled in the art to implement.

[0093] The converters illustrated in the figures employ an ROD snubber circuit. however, other snubber circuits are known, and would be straightforward for a person skilled in the art to implement.

[0094] The converters illustrated in the figures employ diodes as rectifiers. However, components other than diodes may be suitable for this purpose.

[0095] The schematic diagrams of figures 1, 2, 4, 5, 6, 7 and 8 show a single output. However, it will be appreciated that the circuits may have additional outputs, as illustrated in figure 9. Each additional output, represented by outputa in figure 9, comprises a secondary winding Llb, which is inductively coupled to the primary winding Lla, a diode D2a and an output capacitor 03a. Phe additional output(s) may be floating, their output voltage being determined by the number of turns on winding Elba.

[0096] The schematic diagrams of figures 2 and 4 to 9 show the power factor improvement circuit of the present invention applied to a flyback converter. However, the power factor improvement circuit may be applied to other topologies. For example, figure 10 shows the power factor improvement circuit applied to a forward converter.

-21 - [0097] The configuration is similar to that illustrated in figure 2, except that secondary winding Llb is oriented the other way round, so that current flows in this winding when switch Ml is closed, and primary circuit currents are dependent on the secondary circuit loading.

[0098] Except that the main magnetic component is essentially used as a transformer, operation on the primary side of the converter is identical to that described in relation to the flyback embodiment of figure 2. In particular, whilst condition (2) is satisfied, the primary circuit current is drawn directly from the AC supply.

[0099] Although figure 10 shows a single output, it will be appreciated that the circuit may have multiple outputs.

Claims

-22 -CLAIMS1. A power converter for converting an AC voltage supplied by an AC power supply into a DC voltage, the power converter comprising:-a rectifier circuit for rectifying the AC voltage supplied by cxi AC power supply; a bulk capacitor Cl for drawing and storing power from the AC supply via the rectifier circuit; an inductive component Li for transforming an input voltage into an output voltage, the inductive component Li comprising a two primary wiridings Lla, Llc connected together in series, and a secondary winding Llb inductively coupled to the primary windings; and a switch Ml for periodically switching ON and OFF to respectively connect and disconnect the inductive component to/from an input voltage; wherein said input voltage is supplied by the bulk capacitor Cl to the two primary windings Lla, Llc when V(ci)*N(Lic)/(N(Llc)+NLia))>Vac; wherein said input voltage is supplied by one of the AC supply and the rectifier circuit to the second primary winding Lic when V(ci)*N(Llc)/(N(Llc)+NLia))<=Vac; where V(ci) is the instantaneous bulk capacitor voltage, N(Lla) and N(Llc) are the number of turns on the first and second primary windings Lla, Llc respectively, and Vac is the magnitude of the instantaneous AC voltage; and wherein the converter further comprises one or more resistors P4 connected between the AC supply and the bulk capacitor Cl for reducing harmonic current emission from the converter.
2. A converter as claimed in claim 1, wherein the total resistance of the or each resistor P4 is non-zero over an operating temperature range of the converter.

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3. A converter as claimed in claim 2, wherein the total resistance of the or each resistor PA is at least approximately 1.5 ohms over the operating temperature range of the converter.
4. A converter as claimed in claim 2, wherein the total resis Lance of Lhe or each resis Lor R4 is a L leas L approximately 2.5 ohms over the operating temperature range of the converter.
5. A converter as claimed in any preceding claim, wherein the or each resistor R4 is connected between the rectifier circuit and the bulk capacitor Ci.
6. A converter as claimed in any of claims 1 to 4, wherein the or each resistor PA is connected between the AC supply and the rectifier circuit.
7. A converter as claimed in any preceding claim, wherein the or each resistor R4 reduces the 130, 15u and 170.order harmonic currents of the AC supply.
8. A converter as claimed in any preceding claim, wherein the one or more resistors R4 comprises a fixed resistor and a temperature dependent resistor in series.
9. A converter as claimed in claim 8 wherein said temperature dependent resistor has a negative temperature coefficient.
10. A converter as claimed in any preceding claim, wherein the primary windings lie, Llc are connected together at a point t, and wherein the converter further comprises: e first rectifier D4 for connecting the bulk capacitor Cl to the two primary windings Ela, Lb of the -24 -transformer; and at least one second rectifier D5(D6) for connecting one of the AC supply and the rectifier circuit to point t.
11. A converter as claimed in claim 10 further comprising a third rectifier D3 for connecting the bulk capacitor Cl to Llie recLifier circui L, Lu draw and s Lore power front Llie AC supply when Vac>V(cl)
12. A converter as claimed in claim 10 further comprising a rectifier D6 for connecting the AC supply to point t.
13. A converter as claimed in claim 10 or claim 12 wherein the second rectifier D5 is further connected to a filter capacitor C4.
14. A converter as claimed in claim 13 when dependent on claim 12, wherein the rectifier D6 is further connected to the filter capacitor C4.
15. A converter as claimed in any preceding claim, configured as a flyback converter.
16. A converter as claimed in claim 15 comprising a snubber circuit to protect switch Ml.
17. A converter as claimed in claim 15 or 16 comprising an RCD snubber circuit.
18. A converter as claimed in any preceding claim further comprising a capacitor CS connected in parallel with first rectifier D4.
19. A converter as claimed in any preceding claims wherein the turns ratio N(Lla) :N(Llc) is in the range 36:1 -25 -to 31:6.
20. A converter as claimed In any preceding claims wherein the turns ratio N(Lla) :N(Llc) Is in the range 35:2 to 33:4.
21. A conver Ler subs Lan Lially as liereinbefore described with reference to figures 2 to 10 of the accompanying drawings.