Abstract:
A family of switch-mode power supplies with low current stresses on switches and capable of producing an output voltage higher than the input voltage, incorporating a multiple-tap transformer driven by at least one ground-referenced switch. When the transformer is center-tapped, it is driven by two ground-referenced switches. The current stress in each switch is equal to the difference between the source current and the load current. The converters can operate in zero current switching and zero voltage switching. Integrated magnetics can be used to reduce further component count.

Description:
FIELD OF INVENTION 
     The present invention relates generally to switch-mode power supplies, and specifically to switch-mode power supplies operating from low voltages. 
     BACKGROUND OF INVENTION 
     Power converters or power supplies are well known subjects to electronics engineers. In automotive applications where the vehicle chassis forms a ground reference, power supplies for electronics must operate from battery voltage which can get as low as 7 volts when the ambient air temperature is below freezing, even down to 3.5V when a starter is activated. Most modem electronic circuits operate on standard voltages such as +12V, +5V, and −12V. Therefore a buck-type, as depicted in FIG. 1A, or boost-type, as depicted in FIG. 1B, switch-mode power supply (SMPS) cannot be used alone in automotive applications. A flyback SMPS, as depicted in FIG. 1C, can be used, only to supply relative low power levels. As a result of low minimum battery voltages, the current level that a SMPS has to deal with can be very high, in the tens of amperes for an output power of just about 50-100 watts. Conventional approaches to SMPS, such as the forward converter taught in U.S. Pat. No. 4,870,555, or the push-pull converter taught in U.S. Pat. No. 5,140,509, or a flyback SMPS result in high losses in the MOSFET switches, and in the snubber networks, even in the so-called “lossless” snubbers or active core reset circuits. In such a lossless snubber, a current of 20 amperes through a Schottky rectifier would result in over 5 watts of heat dissipation in that rectifier. MOSFETs switching such a high current will dissipate similar heat quantities. Therefore the requirements of high efficiency, small size and low cost may not be easily met. These requirements apply even more relevantly to portable equipment operating from lower voltage battery such as portable camera and boom boxes, and home/office equipment such uninterruptable power supplies (UPS) and battery-operated appliances. 
     Thus there is a need for a better approach to power supplies for automotive, portable and mobile applications, or in general for battery-operated electronics, especially when the power demand is over 50 watts. Such power demand is common for some high power audio amplifiers, inverters, and computer-controlled systems. 
     SUMMARY OF THE INVENTION 
     The invention provides a family of high power converters operating from low voltages. This family of converters essentially replaces switches and transformer windings carrying high currents in opposing directions with switches and windings processing the difference of those high currents, an approach that results in very substantial reductions in conduction and switching losses, as well as reduced losses in auxiliary circuits such as snubber networks and core reset circuits. Some of the transformers used in the various embodiments only have a tapped winding conducting only the difference of currents, therefore they are very small compared to a conventional multiple-winding transformer processing the same power, each winding conducting much higher current. In some of the embodiments, the modulator sequences the drive signals to the switches in such a way that switches carrying heaviest currents switch in zero current switching. 
     OBJECTS AND ADVANTAGES 
     Accordingly, several objects and advantages of this invention are: 
     (a) to provide a method and apparatus to supply regulated power to battery-operated electronics 
     (b) to provide a method to minimize the size of high power SMPS operating from low voltage 
     (c) to provide a method for minimizing component count in SMPS 
     (d) to provide a method and apparatus for reducing switching losses by zero current switching. 
     Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 C show schematics of prior art SMPSs, namely a buck converter, FIG. 1A, a boost converter, FIG. 1B, and a flyback converter, FIG.  1 C. 
     FIGS. 2A to  2 E show schematics of the first embodiments of a SMPS of the present invention, where a pair of ground-refereneed push-pull switches drive a center-tapped transformer to provide a wide range of output voltages lower or higher than the input voltage, in configurations ranging from hard switching, FIG. s  2 A- 2 C, to zero current switching, FIG. 2D, and zero voltage switching, FIG.  2 E. FIG. 2F illustrates the use of MOSFETs as unidirectional devices in these first embodiments. 
     FIGS. 3A to  3 F show schematics of the second embodiments of single-switch SMPS of the present invention, in configurations ranging from hard switching, FIGS. 3A-3B, to zero current switching, FIG. 3C-2D, zero voltage switching, FIG. 2E, and with integrated magnetics, FIG.  3 F. FIG. 3G shows a modified configuration of FIG. 3B for putting out an output voltage that can be lower than the input voltage. FIG. 3H shows its ZCS derivation, while FIG. 3I shows its integrated magnetic derivation and FIG. 3J its ZCS integrated magnetic configuration. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invented arrangement in a first embodiment, FIG. 2A, comprises a center-tapped transformer T 1  operated in push-pull mode. The transformer T 1  is driven by a pair of push-pull switches Q 5  and Q 6 , which are preferably MOSFETs, but which can be any other sort of electronic switch known to those skilled in the art, such as a bipolar transistor, an IGBT. However its similarity to a push-pull SMPS stops there. The two end taps E 1  and E 2  of the center-tapped transformer T 1  now are connected to rectifiers D 1  and D 2  driving a buck converter  22 , which is driven synchronously by the same modulator  26  which drives the push-pull switches Q 5 -Q 6 . The conventional buck converter  22  comprises a switch Q 1 , a rectifier D 3 , and a current smoothing L 1 , and optionally a capacitor C 1 . The modulator  26  can be a pulse-width modulator (PWM), a pulse-frequency modulator (PFM), a hysteretic modulator etc . . . . This invented circuit arrangement has a voltage gain of 2 times the duty ratio D of the push-pull stage, which can be varied from 0 to about 98%. Thus, this converter  20  can supply any output voltage VOUT from zero to almost twice its input voltage VIN, including the very popular 12-volt if the battery voltage varies between 7 and 15 volts. Additionally, a cursory analysis of the circuit arrangement shows that the currents switched by the push-pull switches Q 5 -Q 6  are substantially the same as the output current IR, and are approximately half of the input current IBT. Thus the conduction losses in the switches are only one-fourth of the conduction losses of a conventional push-pull SMPS, being proportional to the square of the switch current (P=RI 2 ). Similarly, losses in auxilliary circuits such as snubbers, if used, which are also proportional to the square of the current, would be only one-fourth compared to a conventional push-pull converter. 
     In a more general embodiment, the center-tapped transformer T 1  is tapped for a 1:n turn ratio between input turns and output turns, FIG.  2 B. The gain of this circuit arrangement now is (n+1) * D. This is different from a gain of n * D of a conventional push-pull SMPS. The current processed by the switches Q 5 -Q 6  is the difference between the battery current IBT and the rectifier current IR. Additionally, the demands on snubbers, if they are used, are much lower. When the output voltage VOUT is to be higher than the input voltage VIN, the MOSFET Q 1  and rectifier D 3  are not needed, as shown in the embodiment of FIG.  2 C. On the other hand, by adding a resonant capacitor CR across the rectifier D 3  and constructing the transformer T 1  with a controlled leakage inductance LR, FIG. 2D, the resulting circuit arrangement can operate in zero current switching (ZCS). When resonant capacitors CR 1 -CR 2  are added across the switches Q 5 -Q 6 , FIG. 2E, the circuit arrangement can operate in zero voltage switching (ZVS). In addition to smaller size of components due to lower current stresses, one major advantage of these embodiments of FIGS. 2A-2E is the inherent transformer core reset due to the symmetry of the transformer T 1 . Resetting the transformer core has been a major issue and ensuing litigation for high efficiency power converters, as discussed in U.S. Pat. Nos. and 6,141,224 and other related patents. It is interesting to note that the “optimum” transformer reset scheme of U.S. Pat. Nos. 4,441,146 or 6,141,224 requires an additional switch, for a total of two active switches, the same number of switches as in the embodiments of FIG. 2C-2E of the present invention. FIG. 2F shows another variation of the embodiment of FIG. 2A where two P-channel MOSFET Q 1 -Q 2  take the position of the rectifiers D 1 -D 2  but act as part of the buck converter  22 , while a center-tapped rectifier D 1  takes the place of the free-wheeling rectifier D 3  of FIG.  2 A. This embodiment is shown for completeness, because it may not be cost-effective in some cases. 
     An extension of this topology is a non-isolated SMPS with transformer T 1  and a ground-referenced switch Q 1 , as shown in FIG.  3 A. This topology is new in many aspects. The first novelty is that the switch Q 1  is ground-referenced, which makes it very easy and economical to drive. The second novelty is the voltage at the common node  44  of the two rectifiers D 1 -D 2  swing between VIN * n and VIN. The third novelty is the fact the output voltage VOUT can be of any magnitude and polarity, positive or negative, depending on the transformer turn ratio and the direction of the rectifiers. Yet another variation of this topology is the converter of FIG. 3B, where now one end of the rectifier D 3  is grounded. These two similar topologies enjoy a further improvement, FIG. 3F, when the transformer T 1  is combined with the inductor L 1  into a single piece of magnetics IM 1  with three windings, two of which are in series. It can be demonstrated by the skilled-in-the-art that with a proper turn ratio between the L 1  inductance winding and the transformer T 1  windings, namely a turn ratio proportional to the ratio of their respective voltages, the ripple current in the inductance winding can be made very small, due to the ripple steering effect of integrated magnetics IM 1 , FIG. 3F, when the integrated magnetics IM 1  is properly designed. It is essential to emphasize that these topologies represent unequaled improvements over the well-known buck converter in many areas, the first being the ground-reference switch Q 1  which is simple and economical to drive, the second being its capability of providing an output voltage VOUT of any magnitude and polarity. The third improvement is the lower current stress on the switch  1  in case of positive output voltage VOUT. The fourth improvement is the possible use of integrated magnetics IM 1 , not only to reduce size and cost but also to reduce the ripple current in the output capacitor C 1 . Of course synchronous rectifiers can be used in lieu of regular rectifiers D 1 -D 2  to further improve the efficiency of this family of converters. 
     A further improvement on the circuit arrangement of the embodiment of FIG. 3A and 3B is the addition of resonant components, the inductor LR and the capacitor CR, FIGS.  3 C- 3 D- 3 E. A skilled-in-the-art will recognize that the two circuits of FIGS. 3C and 3D are equivalent, and that they can operate in zero current switching (ZCS), while the circuit arrangement of FIG. 3E can operate in zero voltage switching (ZVS), when their modulator  26  is a pulse-frequency modulator supplying appropriate pulse width and pulse frequency. The inductor LR can be just a leakage inductance of the transformer T 1 , or it can be a separate component. Again, integrated magnetics combining the transformer T 1  and the inductor L 1  can bring about cost and size savings in addition to reduced ripple current in the output capacitor C 1 , therefore the ripple voltage of the output voltage VOUT. 
     Another improvement on the circuit arrangement of the embodiment of FIG. 3A is the addition of the switch Q 2  in series with the tap E 1  of the transformer T 1 . In a typical implementation, FIG. 3G, the switch Q 2  is a P-channel MOSFET driven by another tap E 3  of the transformer T 1 , which can be the tap connected to the battery BT 1 . A center-tapped rectifier D 1  can be conveniently used to reduce package count. This converter can supply an output voltage VOUT ranging from zero to a voltage higher than the input voltage VIN. The driving of the switches Q 1 -Q 2  is very simple and economical due to the locations of the switches Q 1 -Q 2 . Its version of ZCS is shown FIG. 3H, where the resonant capacitor CR is added across the bottom part of rectifier D 1 . Its practical implementation using integrated magnetics IM 1  is shown in FIG. 3J, where the best mode construction using two windings in series but in two different legs a core to obtain definite value of a leakage inductance LR shown explicitly in FIG. 3H but not practical to show in FIG.  3 J. Of course the converter of FIG. 3G can also exist in integrated magnetics form as shown in FIG.  3 I. 
     From the description above, a number of advantages of the invented circuit arrangements become evident: 
     (a) the current stress on the switches is significantly lower, 
     (b) the transformer can be significantly smaller than a conventional push-pull transformer, or a forward transformer, or a flyback transformer, 
     (c) the energy efficiency is much higher due reduced conduction losses in all components, and in many cases, virtually no switching losses, 
     (d) the switches are either ground-referenced or transformer-referenced, therefore very easy to drive. 
     SUMMARY, RAMIFICATION AND SCOPE 
     Accordingly the reader would see that the invented circuit arrangements are significantly advantageous in automotive or battery-operated electronics when compared to well known SMPS topologies such as forward, flyback, push-pull topologies. 
     Even though embodiments with positive an output voltage were shown, a skilled in the art can recognize that negative output voltages can also be supplied, as well as multiple output voltages by adding appropriate taps on the transformer and additional rectifiers. 
     While the preferred embodiment of the present invention has been shown and described herein, it will be obvious that such embodiment is provided by way of example only. Numerous variations, changes, and substitutions will occur to those of skill in the art without departing from the invention herein. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the example given.