Abstract:
An isolated switched mode power supply comprises a transformer, with primary and secondary windings, and a rectification network connected to the secondary winding. The rectification network and the transformer are arranged such that, during a free-wheeling period of operation of the switched mode power supply, a magnetic flux from a first portion of a secondary winding of the transformer substantially cancels a magnetic flux from a second portion of the secondary winding between the first and second portions of the secondary winding. Additionally, a secondary side circuit connected to the secondary winding comprises a switching device, which is connected to a center-tap, provided between the first and second portions of the secondary winding, and an output of the rectification network so as to conduct at least a part of a free-wheeling current flowing in the secondary side circuit during the free-wheeling period.

Description:
TECHNICAL FIELD 
     The present invention generally relates to the field of isolated switched mode power supplies (sometimes referred to as isolated switch mode power supplies or isolated switching mode power supplies) and more specifically to an isolated switched mode power supply provided with a switching device for reducing the power loss therein. 
     BACKGROUND 
     The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency, for example in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching element such as power MOSFET at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted using a feedback signal to convert an input voltage to a desired output voltage. An SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC). 
       FIG. 1  shows a background example of an isolated SMPS, i.e. an SMPS which converts an input voltage V in  to an output voltage V out  whilst isolating the input from the output through a transformer. The SMPS  100  is provided in the form of a DC-to-DC converter which has on its primary side a half-bridge arrangement comprising two transistors, Q 1  and Q 2  (which may, for example, be field-effect transistors such as MOSFETs or IGBTs) and two capacitors, C 1  and C 2 , which are connected between the power supply&#39;s inputs and to the primary winding  111  of the isolation transformer  110 , as shown. The use of only two transistors to handle currents on the primary side makes the half-bridge configuration best suited to low-power applications requiring a low parts count. Although a half-bridge configuration is employed in the present example, other well-known topologies may alternatively be used on the primary side. For example, a full-bridge configuration with four transistors may be more suitable for higher-power applications. Alternatively, a push-pull arrangement can be used. In all these configurations, the switching of the transistors is controlled by a controller circuit (not shown). 
       FIG. 1  also shows a standard topology on the secondary side of the isolated SMPS  100 , which includes a rectifying circuit and an LC filter connected to a load R. The inductor L of the LC filter is connected to the secondary winding  112  of the transformer  110 . A centre-tap  113  referenced to ground is provided between a first portion  112   a  of the secondary winding  112  having n 2  turns and a second portion  112   b  of the winding  112  also having n 2  turns. In the present example, the rectifying network employs two diodes, D 1  and D 2 , to yield full-wave rectification of the voltage induced in the secondary winding  112 . 
     Power efficiency is, of course, a key consideration in the design of switched mode power supplies and its measure generally dictates the quality of the SMPS. Much research effort has therefore been directed at improving power efficiency. For example, Schottky diodes have extremely small reverse-recovery times and are therefore often used in order to minimize power losses associated with the diode switching. Alternatively, in order to improve the efficiency of the converter shown in  FIG. 1  at higher current levels, the diodes D 1  and D 2  in the secondary side circuit in  FIG. 1  can be replaced with a synchronous rectifier circuit comprising transistors, as shown at Q 3  and Q 4  in the SMPS circuit  200  of  FIG. 2 . Each of the switching devices Q 3  and Q 4  can take any suitable or desirable form, and are preferably field-effect transistors in the form of an N-MOSFET or a P-MOSFET, or an IGBT, for example. In the example of  FIG. 2 , the switch devices Q 3  and Q 4  have an internal body drain diode, which is not shown in the switch device symbol in  FIG. 2 . The switching of these transistors is controlled by a controller circuit (not shown), which may or may not be the control circuit controlling the switching of transistors Q 1  and Q 2 . 
     The principles of operation of the SMPS shown in  FIG. 2  will be familiar to those skilled in the art, such that a detailed explanation thereof is unnecessary here. Nevertheless, some of the basics will now be reviewed, to assist understanding of the present invention. 
       FIG. 3  shows the switching cycle diagram in accordance with which the gate electrodes of switches Q 1 -Q 4  in  FIG. 2  are driven by the SMPS controller circuit so that the primary side circuit generates a series of voltage pulses to be applied to the primary winding  111  of the transformer  110 . In  FIG. 3 , “D” represents the duty cycle of the switching and “T” the switch period. The operation of the circuit during the four time periods 0 to DT, DT to T, T to (T+DT) and (T+DT) to 2T is as follows. 
     Time period 1 (0&lt;t&lt;DT): Switching device Q 1  is switched ON while Q 2  is OFF, allowing the input source at V in  to charge capacitors C 1  and C 2  via the primary winding  111  of the transformer  110 . During this period, switching device Q 3  is switched ON while device Q 4  is switched OFF, allowing the source to transfer energy to the load R via the secondary winding  112  of the transformer  110 . The output voltage V out =n 2 /n 1 ·V in , where n 1  is the number of turns in the primary winding. 
     The operation of the half-bridge isolated buck converter of  FIG. 2  is to be contrasted with that of a flyback converter (or a combined forward/flyback converter), where energy is stored in an air gap provided in the transformer core during this period, to be subsequently released into the secondary side circuit when the primary winding of the transformer is not being driven. No such air gap is present in the core of transformer  110  shown in  FIG. 2  or in any of the related circuits described in the following. 
     Time period 2 (DT&lt;t&lt;T): Switches Q 3  and Q 4  are both conducting and the current in the secondary side circuit therefore free-wheels through both portions of the secondary side winding in substantially equal measure, allowing the transformer flux to be balanced. In other words, the free-wheeling current generates two magnetic fluxes within the secondary winding with opposite directions in the vicinity of the centre-tap  113 , yielding a net magnetic flux equal to zero in an area between the first and second portions of the secondary winding  112 . Hence, the transformer core magnetization is balanced to zero, and the current in the primary winding during the free-wheeling period DT−T/2 is suppressed, thereby avoiding losses in the primary winding. Thus, the transformer volt-second balance is obtained over two switching periods so that a transformer reset is unnecessary. 
     Time period 3 (T&lt;t&lt;T+DT): In this interval, switching device Q 1  is switched OFF while device Q 2  is turned ON, allowing the capacitors C 1  and C 2  to discharge through the primary winding  111 , exciting it with a voltage of opposite polarity to that in the first time period described above. On the secondary side, switch Q 4  remains ON while switch Q 3  is turned OFF, allowing the EMF generated in the lower portion of the secondary winding to drive a current through the inductor L. 
     Time period 4 (T+DT&lt;t&lt;2T): The operation proceeds as in time period 2 described above. 
     In order to have the transformer magnetic flux balanced (which is necessary to guard against the magnetizing current becoming large enough to saturate the transformer), the periods for which switches Q 1  and Q 2  are turned ON should be the same in each switch period. However, where the balance is imperfect, efforts have been focused on avoiding its adverse effects, such as by connecting a capacitor in series with the transformer&#39;s primary winding so that any excess voltage is dropped across the capacitor rather than the primary winding. In order to avoid a short circuit of the source or cross-conduction on the primary side, a delay is introduced between the turn-OFF of one switching device and the turn-ON of the other. 
     An alternative SMPS topology, with an untapped secondary winding, is shown in  FIGS. 4A and 4B . The primary side of the SMPS  300 A shown in  FIG. 4A  is the same as in  FIGS. 1 and 2 , although a full-bridge, for example, may alternatively be used. However, the secondary side comprises a diode full-bridge rectifying network with diodes D 1 -D 4  connected to the load R via an LC filter. As with the example shown in  FIG. 2 , variants with semi- or full-synchronous rectification may be used in order to improve the power efficiency. An SMPS  300 B with semi-synchronous rectification is shown in  FIG. 4B . In both cases, the losses in the SMPS are mainly due to losses in the diodes. 
     The use of full- or semi-synchronous rectification on the secondary side as mentioned above is just one of the measures available to a designer seeking to improve the system efficiency. Efforts have also been directed to minimising switching and conduction losses in the transistors through the optimization of their structure, and to developing improved control architecture options (e.g. pulse skipping), as well as to reducing trace losses and other parasitics by appropriately integrating the switching devices into an IC package. Steps have also been taken to minimise losses in the passive components of the SMPS. Notably, resistive losses in the inductor windings, losses due to hysteresis and eddy currents in the transformer core, and losses in the capacitors due to their series resistance and leakage, and their dielectric losses, have all been addressed by efforts to improve the design of these components. 
     Yet despite these efforts, there still remains a need to further improve the power efficiency of the SMPS. 
     SUMMARY OF THE INVENTION 
     Since the power loss in the transformer is often so high that it makes the transformer the hot-spot that limits the thermal derating of the SMPS, the present inventors have recognized that it would be particularly desirable to reduce losses in the transformer. 
     The present inventors have found that significant losses can occur during the free-wheeling time periods of the SMPS&#39;s operation, i.e. during periods in which the transformer primary is not being driven so that energy is not being transferred from the primary side circuit to the secondary side circuit. These losses occur mainly in the transformer windings where tapped secondary side full-wave rectification is used, since the magnetic flux is constant during the free-wheeling period. These losses are a combination of DC losses and high-frequency AC losses associated with the free-wheeling current that flows in the secondary-side circuit during the free-wheeling periods. Where an untapped secondary winding with diode rectification or semi-synchronous rectification is used, the losses occur mainly in the diodes. 
     Departing from the aforementioned conventional approaches to minimising such losses, in which the presence of the free-wheeling current in the transformer windings and the rectifying network is simply accepted and the focus is on the selection or design of individual components to mitigate the losses that it causes, the present inventors have realised that the power efficiency of power supplies of the kinds described above can be improved significantly by reducing the free-wheeling current level in the highly dissipative elements of the circuit in the first place. 
     As will be explained below through embodiments of the present invention, the free-wheeling current in the transformer secondary and/or the rectifying network can be reduced or eliminated using a switching device that is arranged to conduct at least a part of the free-wheeling current flowing in the secondary side circuit during the free-wheeling period. That is, during the free-wheeling periods, the free-wheeling current can be made to flow through the switching device instead of, or in addition to, flowing through the transformer secondary and/or the rectifying network. The voltage stress over the switching device can be made half that over the switching elements of the rectifying network, making it possible to choose a switching device with a lower voltage rating, which usually has a lower ON-resistance that reduces the power loss accordingly. The reduction in the transformer current and/or the current in the rectifying network during the free-wheeling periods leads to lower losses, thus improving the thermal derating of the SMPS and allowing it to be used with less cooling. This in turn leads to an energy saving in the cooling system. 
     More specifically, the present invention provides an isolated switched mode power supply, which comprises: a transformer comprising a primary winding and a secondary winding, said secondary winding having a centre-tap provided between a first portion and a second portion thereof. The switched more power supply also includes a primary side circuit arranged to generate voltage pulses and thereby to drive the primary winding of the transformer, and further includes a secondary side circuit. The secondary side circuit comprises a rectification network connected to the secondary winding, the rectification network and the transformer being arranged such that, during a free-wheeling period of operation of the switched mode power supply in which the primary winding is not driven by the primary side circuit, a magnetic flux from the first portion of the winding substantially cancels a magnetic flux from the second portion of the winding between the first and second portions of the winding. The secondary side circuit further comprises a switching device, which is connected to the centre-tap and an output of the rectification network so as to conduct at least a part of a free-wheeling current flowing in the secondary side circuit during said free-wheeling period. 
     The present invention also provides, as an alternative solution to the problem of reducing the aforementioned losses in an SMPS, a hard-switched, isolated switched mode power supply, comprising: a transformer comprising a primary winding and a secondary winding; a primary side circuit arranged to generate voltage pulses and thereby to drive the primary winding of the transformer; and a secondary side circuit. The secondary side circuit comprises a rectification network connected to the secondary side winding, and also includes a switching device arranged to conduct, in parallel with the rectification network, a free-wheeling current flowing in the secondary side circuit of the power supply during a free-wheeling period of operation of the power supply in which the primary winding is not driven by the primary side circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention, which have different performances in terms of power efficiency and cost, will now be explained in detail, by way of example only, with reference to the accompanying figures, in which: 
         FIG. 1  shows a background example SMPS circuit having a centre-tapped secondary side transformer winding and diode rectification; 
         FIG. 2  illustrates a variant of the SMPS shown in  FIG. 1 , which uses synchronous rectification; 
         FIG. 3  shows a timing diagram for the circuit of  FIG. 2 ; 
         FIGS. 4A and 4B  show background examples of an SMPS having full-bridge diode rectification and semi-synchronous rectification, respectively; 
         FIG. 5A  shows an SMPS according to a first embodiment of the present invention, which uses diode rectification and a free-wheeling diode; 
         FIG. 5B  shows a variant of the SMPS shown in  FIG. 5A ; 
         FIG. 6  shows an SMPS according to a second embodiment of the present invention, which uses synchronous rectification and a free-wheeling diode; 
         FIG. 7A  shows an SMPS according to a third embodiment of the present invention, which uses diode rectification and synchronous free-wheeling; 
         FIG. 7B  shows a variant of the SMPS shown in  FIG. 7A , where the ground reference is provided at the centre-tap; 
         FIG. 8  shows an SMPS according to a fourth embodiment of the present invention, which uses synchronous rectification and synchronous free-wheeling; 
         FIGS. 9A and 9B  show alternative timing diagrams according to which the SMPS shown in  FIG. 8  may operate; 
         FIG. 10  shows and SMPS according to a fifth embodiment of the present invention, which uses full-bridge diode rectification and synchronous free-wheeling; 
         FIG. 11  shows a variant of the SMPS shown in  FIG. 10 , which uses semi-synchronous rectification; 
         FIGS. 12 and 13  show plots of the power loss vs. load current for different input voltage values for an SMPS with synchronous rectification and a switching device according to an embodiment of the present invention, together with those for a conventional SMPS without such a switching device; and 
         FIG. 14  shows a thermal imaging camera picture of an SMPS with a switching device in accordance with an embodiment of the present invention placed next to an SMPS without such a switching device. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 5A  shows an isolated SMPS  400 A according to a first embodiment of the present invention, which differs from the background example shown in  FIG. 1  by having a switching device in the form of a diode, D 5 , provided in the secondary side circuit. The SMPS is otherwise the same as that described above with reference to  FIG. 1  and the description of the conventional aspects of its operation will therefore not be repeated here. 
     In the present embodiment, the centre-tap  113  and the anode of diode D 5  are earthed while the cathode of D 5  is connected between the cathodes of diodes D 1  and D 2 , and the inductor L, as shown in  FIG. 5A . Thus, the diode D 5  is connected in the secondary side circuit, between the centre-tap  113  and the output of the rectification circuit, so as to carry the free-wheeling current during the free-wheeling periods and thus reduce the free-wheeling currents in the portions  112   a  and  112   b  of the transformer&#39;s secondary winding  112 , and in diodes D 1  and D 2  of the rectifying network. In other words, switching device D 5  reduces losses in the transformer and diodes D 1  and D 2  by being arranged to provide a parallel, relatively low-resistance conduction path for the free-wheeling current. The output of this SMPS looks like that of a diode-rectified buck converter. 
     The circuit of the present embodiment has the advantage of being simple and inexpensive to manufacture, since no control circuitry is required to operate the switching device D 5 . This circuit is therefore best suited to low-current and low-cost applications, and where the resistance in the secondary side windings is sufficiently large to warrant the addition of the switching device D 5 . However, whilst the circuit of this embodiment is effective, the energy saving in the converter and the power loss reduction in the transformer will be modest in comparison with some of the alternative embodiments described below. 
     A variant of the SMPS of the first embodiment is shown in  FIG. 5B . In the SMPS  400 B of this embodiment, the polarities of diodes D 1  and D 2  are reversed, and the ground reference is provided at the anode of diode D 2  rather than being at the centre-tap  113 . 
     Embodiment 2 
       FIG. 6  shows an isolated SMPS  500  according to a second embodiment of the present invention, which differs from the background example shown in  FIG. 2  by having a switching device in the form of a diode, D 5 , provided in the secondary side circuit, and by a terminal of each of the transistors Q 3  and Q 4  (instead of the centre-tap  113 ) being earthed. This SMPS is otherwise the same as that described above with reference to  FIG. 2  and the description of the conventional parts of its operation will therefore not be repeated here. 
     In the present embodiment, a terminal of each of transistors Q 3  and Q 4 , and the anode of diode D 5 , are all earthed, while the cathode of D 5  is connected between the centre-tap  113  and the inductor L, as shown in  FIG. 6 . Thus, similarly to the first embodiment, the diode D 5  is connected in the secondary side circuit, between the centre-tap  113  and the output of the rectification circuit, so as to carry the free-wheeling current during the free-wheeling periods, thus reducing the free-wheeling currents in the portions  112   a  and  112   b  of the transformer&#39;s secondary winding  112 , and in transistors Q 3  and Q 4  of the synchronous rectification network. In other words, switching device D 5  is arranged to provide a parallel, relatively low-resistance conduction path for the free-wheeling current, thereby reducing losses in the transformer and the transistors. 
     The circuit of the present embodiment is preferable when using highly resistive, small switching devices Q 3  and Q 4 , or when the secondary winding  112  has a large resistance due to it having many turns and/or thin wires, as compared with the resistance and voltage drop over the free-wheeling diode D 5 . The circuit is also simple and cost-effective to manufacture since there is no need for any signaling beyond that used in existing circuits of the kind shown in  FIG. 2 . 
     The earthing of a terminal of each of the switching devices Q 3  and Q 4  in the present embodiment makes it easier and cheaper to drive these switches when using N-MOSFETs. This arrangement is preferable to providing the ground reference at the centre-tap, which requires high-side drivers with boot-strap circuitry. 
     Embodiment 3 
       FIG. 7A  shows an isolated SMPS  600 A according to a third embodiment of the present invention, which differs from the variant of the first embodiment described above with reference to  FIG. 5B  by having a switching device in the form of a transistor Q 5  (which may, for example, be a field-effect transistor such as a MOSFET or an IGBT) provided in the secondary side circuit, in place of diode D 5 . This SMPS is otherwise the same as that shown in  FIG. 5B  and the description of its operation will therefore not be repeated here. 
     Similarly to the above-described variant of the first embodiment, the transistor Q 5  is connected in the secondary side circuit, between the centre-tap  113  and the output of the rectifying network, so as to carry the free-wheeling current during the free-wheeling periods, thus reducing the free-wheeling currents in the portions  112   a  and  112   b  of the transformer&#39;s secondary winding  112 , and in diodes  01  and  02  of the rectification network. In other words, switching device Q 5  reduces losses in the transformer and diodes  01  and  02  by providing a parallel, relatively low-resistance conduction path for the free-wheeling current during the free-wheeling periods. The switch Q 5  is turned ON and OFF in accordance with control signals generated by a pulse width modulation (PWM) controller. In one embodiment the controller to minimize drawing complexity) is arranged to enable the switching device Q 5  to switch only when the input voltage measured by an input voltage measuring unit exceeds a voltage threshold value, or the output current measured by an output current measuring unit exceeds a current threshold value, or both the measured input voltage exceeds the voltage threshold value and the measured output current exceeds the current threshold value. To minimize drawing complexity, the controller, input voltage measuring unit, and output voltage measure unit are shown only in  FIG. 7A , but can also be used in the other depicted embodiments. 
     Replacing the free-wheeling diode D 5  in  FIG. 5B  with the transistor Q 5  yields synchronous free-wheeling. The circuit of the present embodiment is better suited to handling larger currents, and especially when the free-wheeling time periods (DT&lt;t&lt;T) and (T+DT&lt;t&lt;2T) are large, hence when the duty cycle D is small. The control and driving of the switching device Q 5  is also simple since it has ground as reference, so that no boot-strap circuitry is required. This makes the circuit of the present embodiment suitable for primary-side control, with only one signal needing to be transferred over the isolation barrier. The circuit is therefore most suitable for applications which require low cost, wide input voltage ranges, high output voltages and modest output currents. Furthermore, configuring the switching device Q 5  to be self-driven would avoid the need to pass control signals over the isolation barrier, thereby reducing costs further. 
       FIG. 7B  shows a variant of the third embodiment, in which the polarities of diodes D 1  and D 2  are reversed and the ground reference is provided at the centre-tap  113 . 
     Embodiment 4 
       FIG. 8  shows an isolated SMPS  700  according to a fourth embodiment of the present invention, which differs from the background example shown in  FIG. 2  by having a switching device in the form of a transistor, Q 5 , connected to the centre-tap  113  and the output of the rectification network, and by a terminal of each of the transistors Q 3  and Q 4  (instead of the centre-tap  113 ) being earthed. Such earthing of Q 3  and Q 4  is preferable since N-MOSFETs can then be used without high-side drivers with boot-strap circuitry, in contrast with the topology in  FIG. 2 , where the switches Q 3  and Q 4  are floating. The SMPS  700  of the present embodiment is otherwise the same as that of the background example described above with reference to  FIG. 2 , and the description of the conventional aspects of its operation will therefore not be repeated here. As with the embodiments and variants thereof described above, the SMPS of the present embodiment is preferably hard-switched. 
     Similarly to the third embodiment, the transistor Q 5  is connected in the secondary side circuit, between the centre-tap  113  and the output of the synchronous rectification network. More specifically, a terminal of each of transistors Q 3 , Q 4  and Q 5  is earthed, while the remaining current-carrying terminal of Q 5  is connected between the centre-tap  113  and the inductor L, as shown in  FIG. 8 . 
     Accordingly, the transistor Q 5  is connected so as to carry the free-wheeling current during the free-wheeling periods, thus reducing the free-wheeling currents in the portions  112   a  and  112   b  of the transformer&#39;s secondary winding  112 , and in transistors Q 3  and Q 4  of the rectification network. In the present embodiment, switching device Q 5  is arranged to provide a parallel, relatively low-resistance conduction path for the free-wheeling current, thereby reducing losses in the transformer and in the transistors Q 3  and Q 4 . The switch Q 5  is turned ON and OFF in accordance with control signals generated by a PWM controller (not shown). 
     Using synchronous rectification and synchronous free-wheeling makes the circuit suitable for higher current levels. The control of the switch devices is preferably performed on the secondary side but primary side control is also possible. The switching in the present embodiment may be controlled in two different ways, namely to provide free-wheeling via: 
     1. both the transformer secondary  112  and switching device Q 5 , or 
     2. the switching device Q 5  only. 
     These alternative ways of controlling the switching of transistors Q 1 -Q 5  in the fourth embodiment are illustrated in the timing diagrams of  FIGS. 9A and 9B . 
       FIG. 9A  shows the timing diagram in accordance with which free-wheeling is allowed to take place in both the secondary winding  112  of the transformer  110  and the switching device Q 5 . This is made possible by switching ON transistors Q 3 , Q 4  and Q 5  during the free-wheeling periods (DT&lt;t&lt;T) and (T+DT&lt;t&lt;2T). Free-wheeling in both the transformer secondary  112  and the switching device Q 5  yields the lowest possible resistance for the free-wheeling current and hence the best possible power efficiency. This timing diagram requires less accurate timing with dead-times between the switching of the synchronous rectification switching devices Q 3  and Q 4 , and the free-wheeling switching device Q 5 . 
     However, if the transformer  110  is the hot-spot in the SMPS, it may be preferable to implement free-wheeling via Q 5  only, using the timing diagram shown in  FIG. 9B . In this timing sequence, Q 3  and Q 4  are both switched OFF during the free-wheeling periods, while Q 5  is switched ON. Since the free-wheeling current is required to flow through Q 5  (and not through Q 3  and Q 4 ) in the scheme of  FIG. 9B , the timing sequence shown requires more accurate handling of the dead times in order not to decrease the power efficiency of the SMPS. The term “dead time” as used herein refers to the (usually very short) time interval (not shown) between Q 3  switching OFF, for example at t=DT, and Q 5  switching ON shortly thereafter, which is necessary to prevent cross-conduction in the secondary side circuit. 
     Embodiment 5 
       FIG. 10  shows an isolated SMPS  800  according to a fifth embodiment of the present invention, which differs from the background example shown in  FIG. 4A  by having a switching device in the form of a transistor, Q 5 , provided in the secondary side circuit. The transistor Q 5  may, for example, be a field-effect transistor in the form of a P-MOSFET or an N-MOSFET, or an IGBT, and is connected across the outputs of the rectifying network comprising diodes D 1 ′ to D 4 ′. The SMPS is otherwise the same as that in the background example which has been described above with reference to  FIG. 4A , and the description of the conventional aspects of its operation will therefore not be repeated here. 
     It is noted that the SMPS of the present embodiment is hard-switched. In other words, in contrast to zero-voltage switching (ZVS) and zero-current switching (ZCS), the switching time instants in each of the switching devices in the present embodiment occur regardless of the current in the device or the voltage over it. 
     The transistor Q 5  is connected in the secondary side circuit, between the ground reference and the output of the rectification network, so as to carry the free-wheeling current during the free-wheeling periods, thus reducing the free-wheeling current in the rectifying network (and, to a lesser extent, in the transformer secondary winding  312 ). In other words, switching device Q 5  reduces losses primarily in the rectifying circuit by providing a parallel, relatively low-resistance conduction path for the free-wheeling current during the free-wheeling periods. 
       FIG. 11  shows a more efficient variant of the fifth embodiment, in which two of the diodes (D 2 ′ and D 4 ′) in the full-wave rectification bridge are replaced with transistor switches (Q 6  and Q 7 ). Each of the transistors Q 6  and Q 7  may be a field-effect transistor such as a P-MOSFET or an N-MOSFET, or an IGBT. The good pre-bias immunity is not destroyed, as the two remaining diodes, D 1 ′ and D 3 ′, prevent the SMPS from sink current to ground when Q 5  is turned OFF during start-up. Using semi-synchronous rectification avoids problems with pre-bias start and costs due to high-side switch device drivers, which are required in full synchronous rectification. 
     Experimental Results 
       FIG. 12  shows plots of the power loss vs. load current for different input voltage values for an SMPS with a centre-tapped secondary winding, which uses synchronous rectification and a switching device according to an embodiment of the present invention. Corresponding plots for a conventional SMPS not having the switching device are also shown, for comparison. 
     More specifically, a 400 W full-bridge SMPS with centre-tapped secondary side transformer with synchronous rectification is used as the reference. The converter has an input voltage range of 36 to 75 V and an output voltage of 12 V. The free-wheeling transistor is switched in accordance with the timing shown in  9 A. That is, free-wheeling is allowed to occur both in the switching device Q 5  and the transformer&#39;s secondary winding  112 . 
     In  FIG. 12 , the power losses are compared using input voltages of 36 V and 48 V. At an input voltage of 36 V, the circuit with the free-wheeling switch device shows a small increase in power loss at light loads but at larger loads the losses are very similar. 
     At an input voltage of 48 V, the power loss shows the same behavior at light loads but at loads greater than 25 A, the free-wheeling device reduces the power loss. Thus, the plots demonstrate that while the switching device Q 5  has little effect when the SMPS input voltage is 36 V, it does decrease the power loss in the SMPS for an input voltage of 48 V, particularly where the output current is above about 25 A. 
       FIG. 13  shows similar plots as  FIG. 12 , but here the power losses are compared for input voltages of 53 V and 75 V. At an input voltage of 53 V, the power loss shows the same behavior as for the input voltage of 48 V at light loads, but at load currents greater than 22 A, the switching device Q 5  has the effect of reducing the power loss. Hence, the load current value at which the efficiency gains due to the switching device Q 5  become apparent decreases with increasing input voltage. At an input voltage of 75 V, the reduction in power loss is already apparent at a load of 7 A, and the power loss reduction is observed to increase with increasing load. 
     To put these figures into practical context, reference is now made to  FIG. 14 , which shows a thermo-camera picture of two DC/DC converters; one with, and one without, the free-wheeling switching device Q 5 . In both cases, the input voltage was set at 75 V and load current at 10 A.  FIG. 14  reveals that the transformer (A) of the power supply with a switching device Q 5  has a hot-spot (at 114.6° C.) which is over 5° C. cooler than the hot-spot (at 120.1° C.) on the transformer (B) of the conventional power supply. A difference of this size in the operating temperature leads to significant energy savings in the power supply&#39;s cooling system. 
     MODIFICATIONS AND VARIATIONS 
     Many modifications and variations can be made to the embodiments described above. 
     For example, the switching device Q 5  could be self-driven instead of being driven directly by a PWM controller, as described above. 
     Although the above-described embodiments employ a half-bridge configuration on the primary side, other well-known topologies may alternatively be used. For example, a full-bridge configuration with four transistors may be more suitable for higher-power applications. Alternatively, a push-pull arrangement can be used. 
     In light of the experimental results shown in  FIGS. 12 and 13 , it would be preferable to control the switching device Q 5  by a controller so that the device is used only in circumstances where it will reduce the power loss: namely, when the input voltage of the SMPS measured by an input voltage measuring device is above a certain threshold and/or when the load current measured by an output current measuring device is above a certain threshold. The threshold value(s) would, of course, need to be determined for the particular SMPS of interest, using standard power-loss measurement techniques.