Single-phase self-driven full-bridge synchronous rectification

#The present invention relates to methods and circuits for single-phase self-driven full-bridge synchronous rectification, and in particular though not exclusively to the application of such methods and circuits in the secondary circuits of inductively powered systems.

#The conduction loss of a diode rectifier contributes significantly to the overall power loss in a power supply, especially in low out-voltage applications. The rectifier conduction loss is the product of its forward-voltage drop, VF, and the forward conduction current IF. FIG. 1(a) shows one kind of rectifier circuit known as a ‘current doubler’. Even when a low forward-voltage drop Schottky diode is used, the voltage drop across D1 or D2 (normally 0.3-0.4V) is still significant compared to the low output voltage (e.g. equal to or less than 5V). If the current through the diode is 1 A, the power loss from diodes is about 0.3 W-0.4 W, which is considerable, compared to the output power, e.g. 5 W.

#One solution known in the prior art is ‘synchronous rectification’ (SR), i.e. using a low conduction loss active switch, such as a MOSFET, operating in the III quadrant to replace the diode. A n-channel (n-type) quadrant III MOSFET means that the source terminal is connected to a higher voltage than the drain terminal and current flows from source to drain. A p-channel (p-type) quadrant III MOSFET means that the drain terminal is connected to a higher voltage than the source terminal and current flows from drain to source. The internal resistance of a MOSFET during conduction is normally very low, which consequently reduces the rectifier conduction loss. FIG. 1(b) is a simple schematic of self-driven SR applied to current doubler. The gate drive scheme of the MOSFET is to cross-couple the drive to the input AC voltage.

#Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

#FIG. 1(a) is a circuit diagram of a current doubler rectifier according to the prior art,

#FIG. 1(b) shows the application of self-driven SR to the current doubler of FIG. 1(a),

#To solve the defect in the circuit shown in FIG. 3, a new principle is proposed herein as described below.

#In any current loop (like the one formed by M1 and M4 in FIG. 3(b)), there cannot exist two voltage controlled self-driven (VCSD) MOSFETs. At least one of the switches in each current loop must be either a diode or an active switch with a similar property to a diode in that the active switch will be turned off when its current is reversed. In FIG. 4, for a clear view, take only one current loop formed by S1 and S4 as an example. Suppose S1 is a VCSD active switch, then S4 must be either a diode or an active switch that has the capability of blocking current flow when the current is in reverse direction. In FIG. 4, the implementation of the switches is also illustrated. S1 is a VCSD MOSFET and is driven by the complementary gate drive circuit formed by Q1 and Q2, and the input of this gate drive circuit is cross-connected to one input voltage terminal (Point B in this example). S4 can be a diode or an active switch. If it is an active switch, it must be controlled by sensing the current. Such an active switch may be termed a current-controlled self-driven (CCSD) active switch. As shown in FIG. 4, a sensing resistor Rsen is used to detect the current flow direction of S4. Comparator U1 can produce the driving pulses according to the current flow direction detected by Rsen. A positive current through S4 (‘positive’ is defined as current flow from ground to point B) will cause the output of comparator U1 to be high. The high voltage level of the comparator output, in turn, will drive the complementary gate drive circuit formed by Q7 and Q8. S4 is therefore turned ON according to its positive current flow direction and turned OFF in opposite manner. Both the VCSD and the CCSD active switches are self-driven in the sense that they do not require external control circuitry.

#A detailed description of the above principle will be given with the following examples.