Self-coupled driver used in dual-switch forward power converter

A dual-switch forward power converter, and a method of operating the same, employs a self-coupled driver to achieve among other advantages higher efficiency, lower part count and component cost. In one aspect of the present invention, a power converter comprises a transformer and two switching transistors, and said transformer has two serially-connected primary windings with the first winding connected to a first switching transistor which is biased by a pulse controller, and the second winding couples the voltage across said first winding to bias the second switching transistor. In addition, the circuit on the primary side of said transformer further comprises means of dissipating magnetization current and the circuit on the secondary side comprises a rectifier and a low-pass filter.

The present application is a national stage entry of PCT/SG2007/000056 filed Feb. 27, 2007.

FIELD OF THE INVENTION

The present invention relates to a switch mode power supply, and more particularly to a dual-switch forward converter employing a self-coupled driver to achieve among other advantages higher efficiency, lower part count and component cost.

BACKGROUND OF THE INVENTION

Switch mode power supplies have been widely used in a great variety of applications and appliances which require light and compact regulated power sources of high efficiency. In addition, said power supplies have high reliability and low power loss, and they can easily be configured to step up and down supply voltages in accordance with design requirements.

In a switch mode power supply, power regulation is accomplished by applying pulse-width modulation to the switching transistors, in particular, the control of the on-time to off-time ratio of said transistors which operate at frequencies up to hundreds of kilo-hertz. The output voltage depends on the duty cycle of said control pulses and the input voltage, and it is essentially load independent. Moreover, changing the switching pulse width accordingly results in constant output voltage even when the input voltage varies.

A plurality of power converter topologies has been devised to address the different design issues such as power level, output voltage and input-output isolation. Flyback converters, for instance, require relatively large transformer cores and switching transistors, and they are suitable for applications which require low part count and low power levels. In the popular forward topology, energy is supplied to the output capacitor while the switching transistor is conducting, and it achieves significantly better transformer utilization than the flyback design. However, forward converters employing single switching transistor suffer from the same shortcoming as the flyback design, namely the voltage across said transistor is inherently unconstrained. This results in higher voltage rating requirement for the switching transistor, and large voltage transients which must be clamped by snubber circuit or additional reset winding. The undesirable power loss in said snubber circuit results in lower power conversion efficiency.

As illustrated inFIG. 1, a two-switch forward converter typically employs two switching transistors406&407on the primary side of transformer102. Magnetizing current builds up during the conducting periods. When said transistors turn off and interrupt the current path, the magnetizing inductance acts as a voltage source. Reverse voltage from this inductive source forward biases and turns on the two diodes403&404to maintain current flow. In sufficient time, the magnetizing inductance is depleted by this voltage until the stored energy is returned to input source401. Its clamped transformer voltage operation, with a maximum duty cycle of 50%, allows easy reset of transformer core. Thus, the voltage across said switching transistors is constrained to the input voltage and this allows lower-voltage and less expensive switching transistors to be used.

The conventional two-switch forward configuration has one known disadvantage, namely an isolated driver circuit101is required to couple pulses from controller402to transistor407. Said drive circuit is commonly implemented with additional driver transformer or active semiconductor isolation device, typically together with external components, at the expense of increased bill-of-material cost, power consumption, part count, and overall circuit board estate.

Accordingly, there is an imperative need for innovative gate drive circuit designs which could meet the increasing demand for high-efficiency, low-cost and compact switch mode power supplies. The power converter of the present invention satisfies the need. Other advantages of this invention are apparent with reference to the detailed description provided herewith.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, a power converter comprises a transformer and two switching transistors. Said transformer has two serially-connected primary windings with the first winding connected to a first switching transistor which is biased by a pulse controller, and the second winding couples the voltage across said first winding to bias the second switching transistor. Thus, the power converter of the present invention does not require a stand-alone isolation driver typically used to couple pulses generated by said controller to one of the switching transistors. In particular, said transformer couples input energy to an output capacitor when both of said switches are conducting. In addition, the circuit on the primary side of said transformer further comprises means of dissipating magnetization current and the circuit on the secondary side comprises a rectifier and a low-pass filter.

In another aspect, the present invention provides a method of operating a power converter comprising the steps of coupling an input voltage to said first primary winding and periodically biasing said switching transistors by said controller and a self-coupled driver implemented with said second primary winding of said transformer; and deriving an output from an output capacitor of said low-pass filter in said circuit on the secondary side of said transformer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to the following detailed description of certain embodiments of the invention.

One solution to said challenge of designing cost-effective forward converters is to use transformer-based gate driver circuit201&202as shown inFIG. 2instead of using the relatively costly semiconductor isolation driver circuit101inFIG. 1. Upper switching transistor407is connected between the high voltage bus of input source401and the primary winding of transformer102, whereas its gate terminal is coupled to the secondary winding of a second transformer201which has the first end of its primary winding coupled by capacitor202to the output of controller402. The second end of said primary winding is connected to the 0V or ground return bus of input source401. The voltage level (with respect to 0V) at the source of transistor407is not constant. When transistor407is conducting, said voltage level is almost equal to the input voltage401. On the other hand when transistor407is not conducting, said voltage will drop to a level that almost equals to 0V or the voltage of the ground return bus of input source401, and this voltage starts to rise when the energy in the core is fully dissipated. As a result, said transformer201serves as an inductive coupling device between controller402and gate-to-source input of transistor407which have different voltage references.

In spite of its advantage, the above transformer-based approach may not be suitable for applications with tight constraints on component footprint due to the considerable size of transformer201. In addition, gate driver transformer201couples the effective load of the switching transistors and other components to the output of controller402, thus drawing additional current from said controller. Finally, the amplitude of the gate pulses coupled to transistor407is inversely proportional to the duty cycle of the control pulses generated by controller402. This relationship, which is illustrated in the waveform diagrams inFIG. 3, poses another limitation to the maximum duty cycle of the pulses which controller402can generate without excessively driving down the amplitude of the gate pulses delivered to transistor407, which fails to turn on when said pulse amplitude is lower than the cut-off threshold of said transistor. Coupling capacitor202must have large enough capacitance, otherwise it would round-off pulses generated by controller402and those coupled to the gate of transistor407, which further limits the duration of the turn-on time or the maximum duty cycle of control pulses.

The present invention provides a dual-switch forward power converter using an efficient yet simple self-coupled driver configuration in the primary side of the converter circuitry. Said self-coupled driver does not require any semi-conductor device or stand-alone transformer as the isolation driver circuit101,201&202(refer toFIGS. 1 & 2).FIG. 4illustrates the schematic diagram of said dual-switch forward converter having self-coupled driver. Controller402generates periodic pulses alternating between a predetermined high voltage and a predetermined low voltage at a duty cycle which is predetermined or variable in accordance with the predetermined or detected magnitudes of input voltage401and/or the output voltage across capacitor414, and said alternating voltage levels are capable of turning on and off the switches used in the forward converter. Although the description of the switches used in the present invention is referenced to a type of field effect transistors, it is apparent to those skilled in the art that said switches can be implemented with a variety of devices including but not restricted to field-effect transistors (FETs), metal-Oxide semiconductor field-effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs) and bipolar junction transistors (BJTs).

InFIG. 4, controller402is connected to the gate terminal of first switching transistor406. The source of said transistor is tied to the 0V return bus of input source401. Second switching transistor407has its drain terminal connected to the high voltage power bus of said input source401. Transformer415comprises two sets of primary winding408and409having Np1and Np2number of turns respectively. First set of primary winding408has two of its ends connected respectively to the drain of said first transistor406and the source of said second transistor. Second primary winding409has two of its ends connected respectively to the gate and source terminals of said second transistor407. A first magnetization current path is formed by first diode403with its cathode and anode connected respectively to the high power bus of said input source401and the drain of said first transistor406, and a second current path is formed by second diode404with its cathode and anode connected respectively to the source of said second transistor407and said 0V power bus of said input source401. A resistor405is connected across the drain and source terminals of said second transistor407. Finally, secondary winding410of transformer415having Nsnumber of turns is connected to a rectifying circuit comprising a third diode411in series with inductor413and capacitor414. Second end of said secondary winding410is connected to the anode of a fourth diode412whose cathode is tied together with the cathode of said third diode411. An output voltage source is derived across said capacitor414.

Referring again toFIG. 4, controller402outputs a continuous stream of pulses of pre-determined amplitude and duty cycle to turn on and off first switching transistor406alternately. No isolation driver circuit is required between said controller402and second switching transistor407. When said first transistor406is conducting, resistor405provides a path for a very small amount of current to first transformer primary winding408. The voltage across said first primary winding (VNp1) almost equals to the voltage Vinof input source401: VNp1≈Vin. The voltage across second primary winding409is proportional to that across said first primary winding408: VNp2=nVNp1with n denoting the turn ratio Np2/Np1. Turn ratio is chosen such that VNp2is sufficiently large to turn on said second transistor407(when said first transistor406is conducting), which has, at this stage, a much lower drain-source resistance than resistor405. Thus, said second primary winding409draws almost all its current flow through said second switching transistor407. When said first switching transistor406is not conducting, the magnetization current of transformer415flows through first and second diodes403&404and back to said input source401. At this stage, the voltage across said first primary winding408is very close to that of input source401, but with the polarity reversed: VNp1≈−Vin. The voltage across said second primary winding409is therefore VNp2=−nVNp1. Thus, said second switching transistor407is reverse-biased and it stops conducting. Accordingly, both switching transistors406&407in the dual-switch forward power converter of the present invention are capable of being turned on and off simultaneously. The voltage across fourth diode412is a pulse-width-modulated voltage which jumps between nVNp1and a level close to 0V. The low-pass filter formed by inductor413and output capacitor414produces an average value from said voltage across said fourth diode412. The output voltage depends on the duty cycle of the control pulses and the input voltage, and it is essentially load independent.

The choice of components affects the overall conversion efficiency of any switch mode power supplies including said dual-switch forward converter of the present invention. Switching transistors must have low resistance during conducting cycles and should meet the required bandwidth, voltage and current ratings with safety margin. Suitable transistor types include but not restricted to FET, MOSFET, IGBT and BJT. When bipolar transistors are used to replace FETs as the switches, the base, emitter and collector terminals of the bipolar transistors replace respectively the gate, source and drain terminals of the corresponding FETs. Diodes should preferably have low on-resistance, low forward voltage drop and they should meet the required forward and breakdown voltage and current requirements. Higher switching frequency allows smaller size of the inductor to be used, at the expense of higher switching losses of the switching transistors.

It is evident to those skilled in the art that the dual-switch forward converter of the present invention provides many advantages over the widely used designs employing isolation driver circuit. Firstly, the additional second primary winding409is inexpensive and secondly it does not take up additional circuit board estate; thirdly, the power dissipated in said winding and resistor405is insignificant compared to the typical power consumption of a semiconductor isolator chip; and that an isolation driver circuit, whether it is active or passive, adds additional load to said pulse controller. Finally, compared to forward converters employing transformer-based drive circuits, the self-coupled driver of the present invention does not pose additional limitation on the maximum duty cycle of the control pulses.

While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited. Alternative embodiments of the present invention will become apparent to those having ordinary skill in the art to which the present invention pertains. Such alternate embodiments are considered to be encompassed within the spirit and scope of the present invention. Accordingly, the scope of the present invention is described by the appended claims and is supported by the foregoing description.