Resonant power converter

FIELD OF THE INVENTION

The invention relates to a resonant power converter and to an apparatus comprising the resonant power converter.

BACKGROUND OF THE INVENTION

Resonant power converters as such are well known. Examples of resonant power converters are the LLC, LCC, LLCC converters. Resonant converters usually comprise a transformer with a primary winding and a secondary winding, and a switching unit that is arranged between the primary winding and a DC input voltage. A load is coupled to the secondary winding via a rectifier. Usually, the switching unit is a half bridge that comprises two switches. The DC input voltage may be supplied across a series arrangement of the two switches. In the LLC converter, the junction of the main current paths of the two switches is connected to the primary winding via a series arrangement of an inductor and a capacitor, which form the resonance circuit. In an LCC converter, starting from the LLC topology, a capacitor is added which is arranged in parallel with the primary winding. In an LLCC converter, starting from the LLC topology, an inductor is added which is arranged in parallel with the primary winding.

Resonant power converters are well known in the art, for example "Principles of Power Electronics" by Kassakian, Schlecht, Verghese, Addison Wesley 1991 discloses different types of such power converters.

Resonant power converters can be very efficient if designed for a relatively small input voltage range corresponding with an optimal operating frequency range related to the resonance frequency. However, the efficiency drops severely if the input voltage range is large and the actual operating frequency has to deviate considerably from the optimal operating frequency range.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a resonant power converter that has a high efficiency over a large input voltage range. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims. In accordance with one aspect of the invention, the resonant power converter comprises a transformer that has multiple primary windings and a secondary winding, and a composed resonant circuit that may contain plural series and or parallel resonant networks. The resonant circuit has one end coupled to a DC-input voltage via a switching unit, and the other outputs coupled to the primary windings such that a frequency selectable effective turns ratio with the secondary winding is obtained. The multiple primary windings may be arranged in series. If first and second primary windings are arranged in series, a first resonant circuit is arranged in series with two windings, and a second resonant circuit is coupled to the junction of the two windings.

A controller controls a switching frequency of the switching unit such that when the DC-input voltage is within the first range, the power converter is operating in a first frequency range wherein the operation of the power converter is mainly determined by the first resonant circuit because the impedance of the second resonant circuit is so high that it has negligible influence. When the DC-input voltage is within the second range, which differs from the first range, the controller controls the power converter to operate in a second frequency range wherein the operation of the power converter is determined by the second resonant circuit because the impedance of the first resonant circuit is so high that it has negligible influence on the operation of the power converter.

Consequently, it is possible to divide the input voltage range in two smaller ranges. The power converter operates on different frequencies in the different input voltage ranges and has different resonant circuits active. To obtain the correct output voltage level, the different resonant circuits are connected to the primary winding such that different effective turns-ratios are obtained with the secondary winding. For the higher input voltage range, the effective turns ratio should be higher than for the lower input voltage range. The controller selects the switching frequency of the switching unit near the first resonant frequency or the second resonant frequency of the power converter in dependence on the input voltage level. This switching of the operating frequency may be abrupt at a particular input voltage level, or the operating frequency may slowly change with the input voltage level in a transition range. The division of the input voltage range in smaller ranges in which

the correct effective turns-ratio is present allows optimally operating the power converter in a larger input voltage range.

In an embodiment, the primary winding comprises two windings that are arranged in series. The first resonant circuit is arranged in series with the two windings and the second resonant circuit is coupled to the junction of the two windings. This embodiment has the advantage that fewer turns are required for the first winding with respect to the embodiment wherein the first winding and the second winding are arranged in parallel.

In an embodiment, the first resonant circuit comprises a first inductor and a first capacitor that are arranged in series, and the second resonant circuit comprises a second inductor and a second capacitor that are arranged in series. The impedance of such series resonant circuits has an almost zero value at its resonant frequency while its impedance steeply increases when the frequency deviates from the resonant frequency. Thus, because the resonant frequencies of the two resonant circuits are different, when the switching frequency of the power converter is near to the resonant frequency of one of the resonant circuits, this resonant circuit has a low impedance while the other resonant circuit has a high impedance. Consequently, if the switching frequency is near to the resonant frequency of the first resonant circuit, the input voltage is electrically connected via the switching unit to the first winding via the first resonant circuit which has the low impedance, while the second resonant circuit with the high impedance practically forms an open circuit and has no or a negligible influence. Thus, by changing the switching frequency of the converter it is possible to select to which primary winding the switching unit will be connected. Consequently, for different input voltage ranges different effective turns-ratios are obtained.

In an embodiment, the first resonant circuit comprises a first inductor and a first capacitor that are arranged in parallel, and the second resonant circuit comprises a second inductor and a second capacitor that are arranged in parallel. The impedance of such parallel resonant circuits has a very high value at its resonant frequency while its impedance steeply decreases when the frequency deviates from the resonant frequency. Consequently if the switching frequency of the converter is near to the resonant frequency of one of the parallel resonant circuits, this parallel circuit forms an open circuit while the low impedance of the other resonant circuit creates a connection with the primary winding. Again, by changing the switching frequency of the converter it is possible to select whether the first or the second resonant circuit is connected to the primary winding with the corresponding effective turns-ratio difference.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Fig. 1 schematically shows a prior art LLC converter,

Fig. 2 schematically shows an LLC converter with two series resonant circuits in accordance with an embodiment of the invention,

Figs. 3A and 3B show electrical replacement circuits for the two modes of the LLC converter shown in Fig. 2,

Fig. 4 schematically shows a transfer function of the LLC converter to elucidates its two operating modes,

Fig. 5 shows an embodiment of the switching frequency of the LLC converter and the DC input voltage, Fig. 6 shows an alternative embodiment of the LLC converter of Fig. 2,

Fig. 7 shows an LLC converter with two parallel resonant circuits, Fig. 8 shows an LCC converter with two series resonant circuits, Fig. 9 shows an LLLC converter with two parallel resonant circuits, and Fig. 10 shows an apparatus comprising the resonant converter in accordance with the invention.

It should be noted that items that have the same reference numbers in different

Figures, have the same structural features and the same functions, or are the same signals.

Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically shows a prior art LLC converter. The LLC converter comprises a transformer TR with a primary winding PW and a secondary winding SW. A switching unit Ml, M2 is arranged between the primary winding PW and a DC input voltage Vi. A load ZL is coupled to the secondary winding SW via a rectifier DB. A smoothing capacitor Co is connected in parallel with the load ZL. The output voltage Vo is present across the capacitor Co. Usually, the switching unit Ml, M2 is a half bridge that comprises two switches Ml and M2. The two switches Ml, M2 are arranged in series and the DC input voltage is supplied across this series arrangement. In the LLC converter, the junction of the

main current paths of the two switches Ml, M2 is connected to the primary winding PW via a series arrangement of an inductor L and a capacitor C that form a series resonance circuit. The magnetizing inductance of the transformer TR is represented by the inductor Lm2 in parallel with the primary winding. Usually, the switches Ml, M2 each comprise a FET and a diode (normally present in a power MOSFET) arranged in parallel with the FET. Further, to minimize EMC, often a small capacitor (not shown) is arranged in parallel with the diode. A controller CO supplies switch control signals to the control inputs of the FET's Ml and M2. Always only one of the switches Ml or M2 is conductive while the other is non-conductive. The switching frequency of switches Ml and M2 is identical and selected to be near to the correct frequency setting of the power converter.

The operation of this resonant power converter is not elucidated in detail because it is well known in the art, see for example "Principles of Power Electronics" by Kassakian, Schlecht, Verghese, Addison Wesley 1991. The issues relevant to understanding the present invention will be elucidated with respect to Figs. 3A, 3B and 4.

Fig. 2 schematically shows an LLC converter with two series resonant circuits in accordance with an embodiment of the invention. The LLC converter shown in Fig. 2 is based on the LLC converter shown in Fig. 1. The same components are indicated by the same references and are not elucidated again. Now, the primary winding comprises two windings PWl, PW2 which are arranged in series. The magnetizing inductance of the winding PWl is indicated by the inductor LmI, the magnetizing inductance of the winding PW2 is indicated by the inductor Lm2. The first resonant circuit comprises the series arrangement of the first capacitor Cl and the first inductor Ll. This series arrangement is arranged between the node Nl, which is the junction of the main current paths of the switches Ml and M2, and the end of the winding PWl that is not connected to the winding PW2 at the node N2. The second resonant circuit comprises the series arrangement of the second capacitor C2 and the second inductor L2. This series arrangement is arranged between the node Nl and the node N2.

At a first switching frequency at which the first resonant circuit has a relatively low impedance such that it actually act as a through connection, the second resonant circuit has a relatively high impedance and actually acts as a open circuit. At a second switching frequency at which the second resonant circuit has a relatively low impedance such that it actually act as a through connection, the first resonant circuit has a relatively high impedance and actually acts as a open circuit. Now, the effective turns-ratio between the primary winding and the secondary winding is lower because the winding PW2

has fewer turns than the series arrangement of the windings PWl and PW2. Thus, this second switching frequency should be selected for an input voltage range lower than for the first switching frequency. It has to be noted that the resonant power converter operates exactly the same as the prior art resonant power converter for each of the switching frequencies wherein one of the series resonant circuit has a low impedance while the other series resonant circuit has a high impedance. This is further elucidated with respect to Figs. 3A, 3B and 4.

It has to be noted that no hardware switches are required to switch between the two modes of operation, it suffices to select the switching frequency of the resonant converter in the correct range.

Figs. 3A and 3B show electrical replacement circuits for the two modes of the LLC converter shown in Fig. 2. The block indicated by Ml, M2 indicates the two series arranged switches Ml, M2 and the control circuit CO of Fig. 2.

Fig. 3A shows the situation when the switching frequency of the power converter is selected such that the second resonant circuit L2, C2 has a low impedance. Therefore, the second resonant circuit L2, C2 is replaced by a short circuit. Usually, this switching frequency at which the second resonant circuit L2, C2 has a low impedance is lower than the switching frequency at which the first resonant circuit Ll, Cl has a low impedance. At this relatively low switching frequency, the first resonant circuit Ll, Cl acts as a capacitor Cl which at the relatively low switching frequency a high impedance.

Fig. 3 B shows the situation when the switching frequency of the power converter is selected such that the first resonant circuit Ll, Cl has a low impedance. Therefore, the first resonant circuit Ll, Cl is replaced by a short circuit. At this relatively high switching frequency, the second resonant circuit L2, C2 acts as an inductor L2 with high impedance.

Fig. 4 schematically shows a transfer function of the LLC converter to elucidate its two operating modes. The transfer function is determined based on the circuit shown in Fig. 2. The magnetizing inductances LMl and LM2 are combined in a single inductance LM that is arranged in parallel with the winding PWl . The voltage Vg is the voltage at the node Nl and the voltage Vr is the voltage across the primary winding, thus across the series arrangement of the windings PWl and PW2. The turns-ratio between the winding PWl and the secondary winding SW is nl, and the turns-ratio between the winding PW2 and the secondary winding SW is n2. The voltage across the winding PWl is (1-α) Vx,

and the voltage across the winding PW2 is α Vx, wherein α = n2 / (nl+n2) and Vx is (nl + n2) Vo.

The switching frequency fr is indicated along the horizontal axis, the output- input voltage ratio M = Vr/Vg (further referred to as the voltage ratio) is indicated along the vertical axis. From Fig. 2 and using the Kirchhoff laws it can easily be found that the voltage ratio is equal to:

Zl

1 + cc

M = ^ = Zl

Vs _{1 2} Z\ , _λ , ₂ Zl Zl ⁵ 1 + oc — + (l -α) — + — Z2 Zm ZO

wherein Zl = jωLl + 1/jωCl, Z2 = jωL2 + l/jωC2, Zm = jωLM, and ZO = (nl + n2) ² Zout, wherein Zout is the parallel arrangement of Co and ZL.

If the switching frequency fr of the power converter is near to the resonance frequency FrI of capacitor Cl and the inductor Ll, the impedance Zl is relatively low. At this resonance frequency FrI relatively far away from the resonance frequency Fr2 of the capacitor C2 and the inductor L2, the impedance Z2 is many orders larger than the impedance Zl, and consequently M ~ 1. If the switching frequency fr of the power converter is near to the resonance frequency Fr2 of C2 and L2, the impedance Z2 is relatively low. At this resonance frequency Fr2 relatively far away from the resonance frequency FrI, the impedance Zl is many orders larger than the impedance Z2, and consequently M ~ 1/α. Fig. 4 shows an example wherein α = ¹ A and wherein the capacitors Cl and C2 and the inductors Ll, L2 are selected to obtain FrI / Fr2 = 2. In this example it is possible to select two different effective turns-ratios: M = 1 or M ~ 2 by selecting the switching frequency near to FrI or Fr2, respectively.

If the input voltage Vi is relatively low, the effective turns-ratio between the primary winding and the secondary winding SW should also be relatively low and thus the input voltage should be connected via the switching unit or circuit Ml, M2 to the second winding PW2 as shown in Fig. 3A. If the input voltage Vi is relatively high, the effective turns-ratio should be relatively high and thus the input voltage should be connected via the switching circuit Ml, M2 to the first winding PWl as shown in Fig. 3B. It has to be noted that the resonance frequency FrI may be lower than Fr2.

Fig. 5 shows an embodiment of the switching frequency of the LLC converter and the DC input voltage. The input voltage Vi is indicated along the horizontal axis and the switching frequency is indicated along the vertical axis. In the embodiment elucidated with respect to Fig. 5, for relatively high input voltages Vi in the voltage range VrI the switching frequency fr of the LLC converter is the relatively high value fsh at which the first winding PWl together with the second winding PW2 determine the effective turns-ratio with the secondary winding SW. For relatively low input voltages Vi in the voltage range Vr2, the switching frequency fr has the relatively low value fsl at which only the second winding PW2 and the secondary winding SW determine the effective turns-ratio. It has to be noted that the Fig. 5 is a very stylistic representation of the actual behavior of the LLC converter. Usually, the switching frequency fr in the voltage ranges VrI and Vr2 varies with the value of the input voltage Vi and with variations of the load ZL. Although is shown that the two voltage ranges VrI and Vr2 meet each other at the input voltage level Vs, these two voltage ranges VrI and Vr2 need not be adjacent. For example, in a practical implementation it may suffice that one of the voltage ranges covers the Japanese 90V and the USA 110V mains range while the other voltage range covers the European 230V mains range. The switching frequency fr need not jump at the voltage Vs, it may gradually change from one level to the other.

Fig. 6 shows an alternative embodiment of the LLC converter of Fig. 2. This embodiment differs from the embodiment shown in Fig. 2 in that now the first winding PWl and the second winding PW2 are not arranged in series but in parallel. The number of turns nl of the first winding PWl is larger than the number of turns n2 of the second winding PW2. The operation of this embodiment is identical to the operation shown in Fig. 2 and thus is not further elucidated.

Fig. 7 shows an LLC converter with two parallel resonant circuits. The difference with the LLC converter of Fig. 2 is that the first series resonant circuit is replaced by a parallel resonant circuit which comprises the inductor Ll and the capacitor Cl, the second series resonant circuit is replaced by a parallel resonant circuit which comprises the inductor L2 and the capacitor C2. Further, a capacitor C3 is added in series with the primary winding PWl, PW2 to block a DC-current through the inductors Ll, L2 and the primary windings PWl, PW2.

The impedance of a parallel resonant circuit has a very high value at its resonant frequency while its impedance steeply decreases when the frequency deviates from the resonant frequency. Consequently if the switching frequency of the converter is near to the resonant frequency of one of the parallel resonant circuits, this parallel circuit forms an open circuit while the low impedance of the other resonant circuit creates a connection with the primary winding. Again, by changing the switching frequency of the converter it is possible to select whether the first or the second resonant circuit is connected to the primary winding causing the corresponding effective turns-ratio difference.

Fig. 8 shows an LCC converter with two series resonant circuits. The LCC converter is based on the same circuit diagram as the LLC converter shown in Fig. 2 to which the capacitors C3 and C4 are added. The capacitor C3 is arranged in parallel with the first winding PWl, and the capacitor C4 is arranged in parallel with the winding PW2. The magnetizing inductances LmI and Lm2 do not have a substantial influence on the operation of the LCC converter.

The operation of a LCC converter with a single series resonant circuit is well known and not further elucidated. The principle operation of having two series resonant circuits connected to two different taps of the primary winding (or to two primary windings which have a different number of turns) is the same as for the LLC converter. Dependent on the switching frequency fr of the LCC converter either the first resonance circuit Ll, Cl has a low impedance while the second resonance circuit L2, C2 has a high impedance, or the other way around. Thus, again, it is possible to select the effective turns-ratio between the primary winding and the secondary winding of the transformer TR dependent on the level of the input voltage Vi by selecting the switching frequency fr in the correct range. It has to be noted that in the LCC converter instead of the series resonant circuits also parallel resonant circuits can be used as is shown in Fig. 7 for the LLC converter.

Fig. 9 shows an LLLC converter with two parallel resonant circuits. The LLLC converter shown is based on the LLC converter of Fig. 7 to which the capacitor C3 has been added in parallel with the first winding PWl, and the capacitor C4 has been added in parallel with the second winding PW2. The operation of an LLLC converter with a single parallel resonant circuit is well known and not further elucidated. The principle operation of having two series resonant circuits connected to two different taps of the primary winding (or

to two primary windings which have a different number of turns) is the same as for the LLC converter. Dependent on the switching frequency fr of the LLLC converter either the first resonance circuit Ll, Cl has a low impedance while the second resonance circuit L2, C2 has a high impedance, or the other way around. Thus, again, it is possible to select the effective turns-ratio between the primary winding and the secondary winding of the transformer TR dependent on the level of the input voltage Vi by selecting the switching frequency fr in the correct range.

Fig. 10 shows an apparatus comprising the resonant converter in accordance with the invention. The apparatus optionally comprises an input rectifier circuit 1 for rectifying the input mains voltage Vm to obtain the DC-input voltage Vi. The resonant power converter 2 receives the input voltage Vi and supplies the output voltage Vo to a circuit 3 of the apparatus. For example, the apparatus may be any consumer apparatus, for example a television receiver, a computer monitor, an audio receiver and/or amplifier, a set-top box, or a DVD(R) player.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. Although the embodiments are directed to LLC, LCC and LLLC power converters, the invention is relevant to any resonant power supply which should operate at high efficiency in its whole input voltage range. It might be required to use more than two different resonant circuits to be able to select more than two different effective turns-ratios, by selecting more than two switching frequency ranges. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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.