Synchronous rectifier circuits and method for utilizing common source inductance of the synchronous FET
First Claim
1. A synchronous rectifier circuit including a switching transistor, a synchronous transistor, and an operatively effective inductance in the current path of the synchronous transistor, wherein the operatively effective inductance is shared with the control circuit path for the transistor;
- whereinthe synchronous transistor is a MOSFET and the inductance is a common source inductance shared by the source-drain power path and a gate driver circuit loop;
wherein a gate drive for the synchronous MOSFET is provided by voltage induced across the common source inductance by current which flows through the common source inductance when the switching transistor is turned off and on; and
wherein a value of the common source inductance is set by selecting the synchronous transistor to provide the common source inductance as a parasitic inductance of the synchronous transistor such that when the switching transistor is turned ON, the common source inductance drives the synchronous MOSFET OFF at an increased rate and when the switching transistor is OFF, the common source inductance drives the synchronous MOSFET ON at an increased rate.
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Abstract
A method of improving the operation of a synchronous rectifier circuit which includes a switching transistor and synchronous transistor, by providing an operatively effective value of inductance in the current path of the synchronous transistor; which is shared by the control terminal circuit path of the transistor and by selecting a synchronous transistor having a low resistance to a control signal provided at the control terminal, as well as improved synchronous rectifier circuits designed according to the method. When the transistors are MOSFETs, the inductance provided is preferably a purely parasitic common source inductance in the range of about 2 nH to about 3 nH. The synchronous transistor exhibits a low value of gate resistance to facilitate fast energy exchange between the common source inductance and the gate-source capacitance.
49 Citations
17 Claims
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1. A synchronous rectifier circuit including a switching transistor, a synchronous transistor, and an operatively effective inductance in the current path of the synchronous transistor, wherein the operatively effective inductance is shared with the control circuit path for the transistor;
- wherein
the synchronous transistor is a MOSFET and the inductance is a common source inductance shared by the source-drain power path and a gate driver circuit loop; wherein a gate drive for the synchronous MOSFET is provided by voltage induced across the common source inductance by current which flows through the common source inductance when the switching transistor is turned off and on; and wherein a value of the common source inductance is set by selecting the synchronous transistor to provide the common source inductance as a parasitic inductance of the synchronous transistor such that when the switching transistor is turned ON, the common source inductance drives the synchronous MOSFET OFF at an increased rate and when the switching transistor is OFF, the common source inductance drives the synchronous MOSFET ON at an increased rate. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
- wherein
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14. A method of improving the operation of a synchronous rectifier circuit which includes a switching transistor and a synchronous transistor, the method comprising the steps of:
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providing an operatively effective value of inductance shared by the current path of the synchronous transistor and its control terminal circuit path; and selecting a synchronous transistor, wherein the synchronous transistor is a MOSFET and the inductance provided is purely a parasitic common source inductance wherein the step of selecting comprises selecting a MOSFET as the synchronous transistor such that the operatively effective value of inductance is set such that when the switching transistor is turned ON, the inductance drives the synchronous MOSFET OFF at an increased rate and when the switching transistor is OFF, the inductance drives the synchronous MOSFET ON at an increased rate. - View Dependent Claims (15, 16, 17)
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Specification