Power converters are used to convert an input voltage having a first value to an output voltage having a second value which is typically different than the first value. The input voltage is applied to the primary side of the power converter while the output voltage is generated on the secondary side. Power converters often provide electrical isolation between the primary side voltage and secondary voltage while converting an input voltage to an output voltage.
Transformers are often used in power converters where the output voltage is isolated from the input voltage. Transformers typically have a primary winding for receiving the input voltage, a secondary winding for generating the output voltage and a magnetic core. The primary and secondary windings are wrapped around the core such that an input voltage is magnetically coupled to the secondary winding. Since no direct connection is made between the primary and secondary windings, the windings are referred to as being electrically isolated from one another. Transformers may be used in power converters providing alternating current-to-alternating current (AC-AC) power conversion, alternating current-to-direct current (AC-DC) power conversion, and direct current-to-direct current (DC-DC) power conversion.
DC-DC power conversion is employed in a myriad of industrial and consumer products ranging from air traffic control systems to cellular telephones. DC-DC converters may employ techniques for improving such things as conversion efficiency, voltage regulation, operating speed, minimizing circuit size, reducing electromagnetic interference, minimizing the number of electrical components, and minimizing per unit manufacturing costs.
Synchronous rectification is one such technique employed for improving the performance of DC-DC converters. A “synchronous rectifier,” as used in a power converter employing a transformer, may be defined as a circuit employing two or more switches, which are typically synchronized field effect transistors (FETs) or metal oxide field effect transistors (MOSFETs), as rectifying devices along with related drive circuitry to control the on-off cycling of the MOSFETs. A synchronous rectifier operates such that the switch is in the ON-state when the rectifier is conducting current and in the OFF-state when the rectifier is blocking voltage. MOSFETs serve as good synchronous rectifiers because they contain an intrinsic diode that allows them to behave as a normal rectifier.
The operating efficiency of the power converter, to a large extent, is dependent on the control of the synchronous rectifiers. Therefore, it is important that the synchronous rectifiers are turned on and off at predictable times over the entire operating range of the converter. Prior art techniques for controlling synchronous rectifiers may couple the drive, or gate, of a synchronous rectifier to the secondary winding of the transformer. This technique is termed cross coupling. Cross coupling may reliably work over a portion of the power converter's operating range; however, this scheme may not be sufficiently reliable at other portions of the operating range such as with low range input voltages on the primary side of the transformer. What is needed is a synchronous rectifier control method and power topology that addresses these and other problems in a simple, efficient, cost effective manner.
Embodiments of the present invention improve the self-drive of synchronous rectifiers used in isolated power converters. A separate winding is added to the transformer and used to control the gate drive of the synchronous rectifiers which are MOSFETs in the exemplary embodiments used herein. This separate winding is referred to as a drive winding and it employs a turns ratio with respect to the primary winding. The drive winding is further designed to provide adequate and reliable gate voltages and/or current waveforms to the MOSFETs over substantially the entire operating range of the power converter. Embodiments of the invention not only provide better gate driving waveforms, but they also employ fewer components than is required to implement power converters having comparable output currents using common prior art implementations. Employing lower component counts allows embodiments of the invention to be fabricated in small packages, or footprints, and at lower costs as compared to typical prior art devices.
The foregoing and other features and advantages of the system and method for a self-drive for synchronous rectifiers will be apparent from the following more particular description of preferred embodiments of the system and method as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.
FIGS. 2A-D illustrate exemplary embodiments of front ends that are useful when practicing aspects of the invention;
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
Waveforms present at the primary winding 20 are magnetically coupled to the secondary winding 22 and drive winding 24 using core 19. The secondary winding is coupled to back end 26 which consists of passive and active electrical components operating cooperatively to produce a desired regulated DC output voltage 28. Drive winding 24 is coupled to at least a subset of the components making up back end 26. In particular, drive winding 24 may be connected to the gates of synchronous rectifiers utilized in back end 26. Output voltage 28 is coupled to a load 30. Load 30 may be any type of component, device or system that uses DC voltage alone or in combination with AC voltage. Examples of loads are, but are not limited to, consumer and industrial devices such as kitchen equipment, electronic games, audio and video equipment, avionic subsystems, computers, communications equipment, control systems, and the like.
FIGS. 2A-D illustrate exemplary front ends that can be employed with a transformer 18 and back end 26.
Transformer 18 couples front end 16 to back end 26. Transformer 18 provides electrical isolation between the signals applied to the primary side 20 and signals output from the secondary side 22. Transformer 18 maybe fabricated using materials, techniques and processes known in the art. By way of example, transformer 18 may consist of primary and secondary windings made of insulated wire and wrapped around a core consisting of a material having magnetic properties. Alternatively, transformer 18 may be fabricated using planar windings consisting of thin copper plates or sheets, or foils, and separated by an insulator. A planar configuration may also include one or more interconnects passing through and in electromechanical contact with one or more copper plates. Still other exemplary embodiments of transformer 18 may consist of a combination of wire windings, plates or foils, or layers of conductive particles interspersed with insulative materials to form a hybrid transformer.
In addition to being made of many possible compositions of materials, transformer 18 may have many form factors. By way of example, transformer 18 may be fabricated as a stand alone unit and electromechanically coupled to front end 16 and back end 26 by way of leads. Alternatively, transformer 18 may be potted in materials that aid with heat dissipation, immunity to mechanical vibration, and/or protection from ambient environmental parameters such as humidity, condensed water vapor, temperature, ultraviolet radiation, and the like. In still another alternative embodiment, transformer 18 may be mounted on or be integral with an integrated circuit (IC) board, such as, for example, in an open frame configuration. Transformer 18 may be electromechanically coupled to an IC board by way of through hole mountings, press fit sockets, welding, brazing, or screw mounting.
Transformer 18 as used in embodiments of the present invention includes at least one drive winding 24 in addition to the primary winding 20 and secondary winding 22. Drive winding 24 is magnetically coupled to primary winding 20 in a preferred embodiment; however, drive winding 24 may be coupled to secondary winding 22 or, alternatively, with primary winding 20 and secondary winding 22. A turns ratio between primary winding 20 and drive winding 24 is used to establish a defined drive voltage VD over an anticipated range of voltages on the primary winding, herein referred to as primary voltage VP and primary winding waveforms.
By employing drive winding 24, the drive voltage VD is independent of the voltage on the secondary winding, herein referred to as secondary voltage VS. Drive winding 24 provides a circuit designer with the flexibility to design back end 26 in a manner that ensures synchronous rectifiers used therein will have drive voltages VD of sufficient amplitude to reliably and deterministically drive the gates on the synchronous rectifiers such that the devices operate in an optimal manner.
Secondary winding T1A is coupled to secondary winding T1B, to L1, to the drain of Q2, and to C2. Secondary winding T1B is coupled to C1 and the drain of Q1. The gate of Q2 is coupled to C 1, R3 and R1, the source of Q2 is coupled to R3, R4, the source of Q1, C3 and to ground. Drive winding TIC is coupled to R1 and R2. The gate of Q1 is coupled to C2, R4 and R2. Capacitor C3 is connected in parallel with load 30 by way of being coupled to L1 and ground.
The gates of Q1 and Q2 have capacitative characteristics that when connected to a winding having a series inductance can form an LC circuit. If such a circuit is formed and the LC response is left undamped, voltage overshoot may occur at the gate of Q1 and Q2. Embodiments of the invention employ series resistors to critically damp the signals driving the gates of Q1 and Q2. Critically damped signals present at the gates of Q1 and Q2 do not exhibit voltage overshoot or undershoot which facilitates predictable operation of power converters employing embodiments of the invention.
Capacitors C1 and C2 may be employed to counteract the internal capacitance of Q1 and Q2 which is referred to as the Miller capacitance associated with the gate and drain of the MOSFETs Q1 and Q2. The Miller capacitance inhibits the switching speed of Q1 and Q2 if not compensated for using, for example, C1 and C2. In a preferred embodiment, values of C1 and C2 are selected such that
where Qgd is gate-drain charge and can be found in a manufacturer's data sheet for a particular MOSFET used for Q1 or Q2, and 2*V is the input voltage to back end 26A which is obtained from secondary winding T1. Values for C1 and C2 that vary from the value obtained by equation 1 may not adversely impact operation of 26A so long as the variation is not large. Series resistors may be used with C1 and C2 if desired. If used, the series resistors should have small resistance values.
R1 and R2 are used in series with the gates of Q2 and Q1, respectively, to dampen resonances that may occur as a result of leakage inductances associated with drive winding T1C and the gate capacitances of Q1 and Q2. In a preferred embodiment, the values of R1 and R2 are selected such that
A value for R1 or R2 that varies from results obtained using equation 2 may not adversely impact the operation of back end 26A. In an alternative embodiment of back end 26A, R1 does not have to equal R2. For example, R1 can be replaced with a value of zero ohms and R2 can be made twice as large.
R3 and R4 are employed to ensure that the average values of the gates of Q2 and Q1, respectively, are zero volts. In addition, R3 and R4 prevent Q2 and Q1 from turning on if back end 26A is powered by an external voltage across C3. In an alternative embodiment of back end 26A, R3 and R4 can be coupled to the other side of R1 and R2, respectively, and ground.
Inductor L1 operating in conjunction with C3 filters the rectified waveform at “A” (see
Back end 26A is designed to ensure that where “B” (see
Back end 26A facilitates the use of optimized waveforms for driving the gates of Q1 and Q2 because the gate drive waveforms are not cross coupled to the power windings of transformer 18 as is done in certain prior art implications of self-drive techniques for synchronous rectifiers.
Back end 26C operates in a manner similar to that of back end 26A; however, some differences between the two embodiments are present. For example, the gate drive waveforms are rectified and are driven from 0 volts to V*NG/NS, where NG is the number of turns associated with drive winding T2C, and NS is the number of turns associated with secondary windings T2A, T2B as opposed to the gate drive signals for Q1 and Q2 which were driven from
The embodiment of
This gate voltage is sufficient to allow the MOSFETs to be conducting current in a low loss manner, which improves the efficiency of back end 26C as compared to prior art implementations. The embodiment of
Embodiments of the invention described hereinabove avoid the use of clamping diodes in conjunction with the gate drives of Q1, Q2, Q3 and Q4, respectively, in order to avoid potentially detrimental impacts on the performance of the respective gate drive signals. Embodiments of the invention further eliminate the potentially detrimental effects on circuit performance which can occur when zener diodes are employed. For example, in certain circumstances, the use of zener diodes may facilitate clamping of the secondary winding with a zener voltage that is less than the open circuit voltage of the winding. This situation can lead to the destruction of the zener diode and subsequent malfunction of the circuit.
After step 96 the method loops back to the input of step 94. The method may include a decision block that determines if back end 26 is coupled to an external power supply (per step 100). If back end 26 is coupled to an external supply, the method may terminate in that it no longer has to utilize drive waveforms or secondary waveforms for supplying power to a load. If back end 26 is not coupled to an external power supply, the flow may loop back to step 90.
Embodiments of the invention may be constructed to fit particular form factors such as industry standard ⅛, ¼ brick or ½ brick formats. In addition, components used in power converters such as those illustrated in
As seen by the embodiments described hereinabove, the invention is not limited to a particular size, component layout, or by way of the types of components used.
The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.
The present application claims the benefit of U.S. Provisional Application No. 60/583,142, filed Jun. 25, 2004. The entire contents of the above application is incorporated herein by reference in its entirety.
Number | Date | Country | |
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60583142 | Jun 2004 | US |