Dual forward power converter utilizing coupling capacitors for improved efficiency

[0002] 1. Technical Field

[0003] The present invention relates in general to power conversion and, in particular, to DC/DC power converters having improved topologies that provide high efficiency with low output impedance. More particularly, the present invention relates to a dual forward power converter and a method of operation thereof.

[0004] 2. Description of the Related Art

[0005] Regulated DC power supplies are typically needed for most analog and digital electronic systems. Two major categories of regulated DC power supplies are linear power supplies and switching power supplies. Generally, in linear power supplies, a transistor (operating in its active region) is connected in series with an input voltage source and the voltage drop across the transistor is automatically adjusted to maintain an output voltage at a desired level.

[0006] In switching power supplies, transformation of DC voltage from one level to another is accomplished typically by means of DC/DC converter circuits, such as step-down (buck) or step-up (boost) converter circuits. Solid-state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within these switching converters. Since the power devices are not required to operate in their active region, this mode of operation results in lower power dissipation. Furthermore, increasing switching speeds, higher voltage and current ratings of these power devices are some of the advantages that have increased the popularity of switching power supplies.

[0007] It is not uncommon for switching power supplies to have a switching frequencies of 100 kHz or higher. The high switching frequencies permit the components utilized in construction of the power converter, such as capacitors, inductors and transformers, to be physically small. The reduction in the overall volume (size) of the converter that results is desirable to the users of such supplies. Another important attribute of a power supply is its thermal efficiency. The higher the efficiency, the less heat that is dissipated within the supply, and the less design effort, volume, weight, and cost that must be devoted to remove this heat.

[0008] Distributed power systems are increasingly being utilized in modern systems, for example, due to different power levels requirements in a power consuming system. Furthermore, with the increasing popularity and utilization of complementary metal oxide semiconductor (CMOS) technologies in present and future projected electronic systems, highly dynamic loading conditions is a common problem encountered by power systems. This highly dynamic loading conditions necessitates a low impedance power distribution network driven by low impedance power supplies.

[0009] It is therefore an object of the invention to provide an improved low output impedance DC/DC power converter.

[0010] It is another object of the invention to provide a high efficiency dual forward power converter.

[0011] To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a dual forward power converter is disclosed. The dual forward power converter includes first and second power transformers, where each of the power transformers has first and second primary windings of opposite polarity and a secondary winding. The dual forward power converter also includes first and second switches that alternatively couple the first primary windings of the first and second power transformers, respectively, to a source of input power. The first and second switches, in an advantageous embodiment, are Field Effect Transistors (FETs) and are driven in a complementary manner with substantially 50% duty cycles. The dual forward power converter further includes first and second coupling capacitors that couple the second primary windings of the first and second power transformers to the second and first switches, respectively, and are utilized to reset the first and second power transformers with minimum losses. In a related embodiment, the dual forward power converter includes third and fourth switches that are coupled to the secondary windings of the second and first power transformers, respectively, and are driven in a substantially identical manner as the second and first switches, respectively.

[0012] The present invention discloses a novel low output impedance power converter. The present invention recognizes that the utilization of the coupling capacitors, in contrast to prior art power converters employing devices, such as diodes, improves the overall efficiency of the power converter due to their “lossless” nature. The improved efficiency is due to the substantial elimination of energy losses, i.e., thermal energy, that are a consequence when devices such as diodes are utilized. Additionally, unlike diodes, the current can flow from both directions into the coupling capacitors during their charging and discharging cycles, resulting in zero-voltage-switching (ZVS) during the turn ON of the first and second switches. Furthermore, since the coupling capacitors can charge and discharge in both directions, the currents through the first and second switches are the combination of a primary winding current and the current through a coupling capacitor. Consequently, since the coupling capacitors are also contributing to the current flow through the switches, a smaller input capacitor is required.

[0013] The foregoing description has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject matter of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.

[0014] The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

[0015] FIG. 1 illustrates an embodiment of a low output impedance DC/DC power converter; and

[0016] FIG. 2 illustrates a schematic diagram of an embodiment of a dual forward power converter (analogous to the low impedance power converter depicted in FIG. 1) according to the principles disclosed by the present invention.

[0017] Highly dynamic loads present unique problems to a supply power system. This is especially true with load systems, such as a computer system, utilizing complementary metal oxide semiconductor (CMOS) technologies. In these systems, it is not uncommon for portions of the load system to switch from ten percent to full load in a few microseconds. This load behavior makes it difficult to maintain the power system bus voltages in regulation. One approach to resolve the widely varying output load swings is to utilize large decoupling capacitors at the load. This approach provides some load regulation relief, however at low bus voltages, the amount of decoupling required becomes unmanageable. To illustrate, at a given power level, four times as much decoupling is required to maintain a 2.5V bus within a specified percentage tolerance as for a 5V bus.

[0018] Since electrolytic capacitor impedance is primarily dominated by equivalent series resistance and inductance at high frequencies, and these parameters are determined mostly by case size (independent of the voltage rating of the capacitor), four times as much space must be allocated to decoupling at the lower voltage level as at the higher voltage level. This problem may be alleviated by placing most of the decoupling at an intermediate, higher voltage level and coupling it to a load through a DC/DC power converter with very low input to output impedance. For example, a 2.5V to 20V conversion ratio would represent a 64 fold reduction in decoupling capacitor volume, which is a significant improvement.

[0019] Furthermore, to achieve the best possible coupling between the intermediate capacitors and the load, a DC/DC power converter should be operated at nearly full duty cycle and have minimum inductance in the high current paths. However, a full duty cycle DC/DC power converter cannot provide voltage regulation since it is essentially a DC transformer, a DC/DC preregulator is required to sense the load voltage and regulate the voltage level on an intermediate capacitor to compensate for variations in input voltage, component drift and long term load changes. A block diagram of an embodiment of a low output impedance DC/DC power converter 100 is depicted in FIG. 1.

[0020] As illustrated, power converter 100 includes a preregulator 110 coupled to a source of input power that provides an intermediate voltage across a intermediate capacitor Cint to a low impedance power converter 120. A regulation controller 130, such as a conventional pulse-width-modulation (PWM), is utilized to sense the DC output voltage of low impedance power converter 120 and provide control signals to maintain the voltage across intermediate capacitor Cint at a predetermined level. While the design of preregulator 110 is relatively straightforward, existing conventional switch mode power converter topologies may be advantageously utilized, the design of low impedance power converter 120 presents some challenges. Employing full wave single transformer topologies, such as push-pull, half or full-bridge, require almost perfect volt time balance between the halves of the operating cycle to keep the magnetic flux in the transformer centered. If balance is not maintained, the transformer core can saturate, resulting in the failure of the power converter. Current mode control, which is the commonly utilized method of transformer core balancing in a push-pull or full-bridge power converter, is undesirable because it requires current sensing, feedback control and dead time adjustment, all of which indirectly increases the impedance of the power converter. In the case of the half-bridge topology, which does not require external transformer's magnetic flux balancing, the capacitor divider circuit at its input adds impedance and greatly increases the size of the power converter. For single-ended converter designs utilizing a single power train, long dead times are required to reset the transformer core necessitating substantial output filtering that, in turn, results in higher impedance. The construction and operation of a low impedance power converter topology that mitigates the above-described deficiencies in prior-art converter topologies is described hereinafter in greater detail with respect to FIG. 2.

[0021] Referring now to FIG. 2, there is illustrated a schematic diagram of an embodiment of a dual forward power converter 110 (analogous to low impedance power converter 120 depicted in FIG. 1) according to the principles disclosed by the present invention. As shown in the illustrated embodiment, dual forward power converter 200 includes first and second power transformers T1, T2 coupled to an input capacitor Cin. First power transformer T1 includes first and second primary windings L1, L11 that are of opposite polarities and a secondary winding L12. Second power transformer T2 also includes first and second primary windings L2, L21, which are also of opposite polarities, and secondary winding L22. The power transformers should be tightly coupled to reduce leakage inductance and, in an advantageous embodiment, have equal turn ratios in their primary windings, i.e., L1=L11=L2=L21. Additionally, the turn ratios of first and second power transformers' T1, T2 secondary windings are also equal, i.e., L12=L22.

[0022] A first coupling capacitor C1 connects a terminal of first power transformer's T1 second primary winding L11 to second power transformer's T2 first primary winding L2. A second coupling capacitor C2 is similarly utilized to couple first power transformer's T1 first primary winding L1 to second power transformer's T2 second primary winding L21. Dual forward power converter 200 also includes first and second switches Q1, Q2 that are coupled to first and second power transformers' T1, T2 first primary windings L1, L2 and are operationally utilized to alternatively couple first and second power transformers T1, T2 to an input DC waveform, i.e. voltage across intermediate capacitor Cin shown in FIG. 1. Thus, in an advantageous embodiment, first and second switches are operated complementarily with constant frequency rectangular waveforms with duty cycles of about 50%. Also shown in the illustrated embodiment are first and second antiparallel diodes D1, D2 that are coupled across first and second switches Q1, Q2, respectively. First and second switches Q1, Q2, in an advantageous embodiment, are field effect transistors (FETs), such as metal oxide semiconductors FETs (MOSFETs) with their corresponding internal body diodes, i.e., first and second antiparallel diodes D1, D2. In another advantageous embodiment, other switching devices, such as insulated gate bipolar transistors (IGBTs), may also be advantageously employed.

[0023] Third and fourth switches Q3, Q4 are also depicted coupled to the secondary windings of first and second power transformers T1, T2. It should readily apparent to those skilled in the art that, alternatively, diodes may also be utilized in place of third and fourth switches Q3, Q4 to couple the secondary windings of first and second transformers T1, T2 to an output capacitor Cout. Third and fourth switches Q3, Q4 are similarly driven as with second and first switches Q2, Q1, respectively. Thus when first switch Q1 is turned ON, i.e., conducting, fourth switch Q4 is also turned ON to deliver the energy in first transformer T1 to an output load (not shown). Similarly, when second switch Q2 is conducting, third switch Q3 is also turned ON to couple second transformer T2 to the output load. In this manner, energy is transferred to the load during each half cycle from either the first or second power transformer T1, T2, thereby providing continuous access to energy stored in input capacitor Cin coupled to a bulk input voltage Vin.

[0024] When first switch Q1 is turned ON, i.e., conducting, input voltage Vin, i.e., voltage across input capacitor Cin, is impressed across first primary winding L1 of first power transformer T1. The magnetizing inductance in first power transformer T1 controls the slope of the magnetizing current flowing in the primary windings and of the magnetizing energy that accumulates in the transformer core. At this time, second primary winding L11 is inoperative since second switch Q2 is not conducting, i.e., turned OFF. The drain current through first switch Q1 is equal to the current through first primary winding L1 and the current through second coupling capacitor C2 which is being charged. Since fourth switch Q4 is also been turned ON, the magnetizing energy in first power transformer T1 is transferred to secondary winding L12 to the output load. It should be noted that before first switch Q1 turns ON, second coupling capacitor C2 is discharged and the drain voltage across first switch Q1 is substantially zero, thus minimizing switching losses when first switch Q1 is turned ON, i.e., zero voltage switching (ZVS). When first switch Q1 is turned OFF, second primary winding L21 of second power transformer T2 is utilized to reset the magnetic flux in first power transformer T1 to zero by recycling the remaining magnetizing energy present in first power transformer in second coupling capacitor C2 without generating any significant thermal losses.

[0025] Similarly, when second switch Q2 is turned ON, the drain current through second switch Q2 is equal to the current through first primary winding L2 of second power transformer T2 and the current through first coupling capacitor C1. As with first switch Q1, before second switch Q2 is turned ON, first coupling capacitor C1 is discharged and the drain voltage across second switch Q2 is substantially zero, resulting in ZVS. The magnetizing energy in second power transformer T2 is transferred to the output load via third switch Q3, which is also driven in a similar fashion as second switch Q2. Second primary winding L11 of first power transformer T1 is utilized to reset second power transformer T2 core by recycling the magnetizing energy through first coupling capacitor C1. It should be noted that utilizing first and second coupling capacitors C1, C2 to charge and discharge the magnetizing currents of first and second power transformers T1, T2 results in a more efficient power converter since the magnetizing energy is being “saved” in the capacitors instead of being dissipated as heat.

[0026] While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.