Electric power conversion systems – Current conversion – Using semiconductor-type converter
Utility Patent
1999-10-07
2001-01-02
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S053000, C363S089000
Utility Patent
active
06169683
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to DC-to-DC converters, and to a resonant gate drive for synchronous rectifiers using an external driving circuit. More particularly, the invention relates to an externally driven synchronous rectifier circuit for a DC-to-DC power converter having an energy recovery circuit configured for storing energy associated with charging and discharging the input capacitance of MOSFET type synchronous rectifier devices.
BACKGROUND OF THE INVENTION
As logic integrated circuits (ICs) have migrated to lower working voltages in search for higher operating frequencies, and as overall system sizes have continued to decrease, power supply designs with smaller and higher efficiency power modules are in demand. In an effort to improve efficiencies and increase power densities, synchronous rectification has become necessary for these type of applications. Synchronous rectification has gained great popularity in the last ten years as low voltage semiconductor devices have advanced to make this a viable technology. However, as the frequency of operation increases, switching losses become important.
For applications with synchronous rectification, the energy dissipated by charging and discharging the input capacitance of the rectifiers can be significant. In order to obtain the full benefit from synchronous rectification, components with low drain to source resistance have to be selected. However, low drain to source resistance usually results in devices with a relatively large die and a large input capacitance. Furthermore, the input capacitance needs to be charged and discharged in nano-seconds. This means that as the frequency of operation increases the losses associated with the gate-drive circuitry become significant.
Topologies have been suggested to minimize switching losses due to current and voltage overlap and to minimize switching losses due to the output capacitance of typical semiconductor devices. In “A MOS gate drive with resonant transitions”, IEEE PESC 91 Conference Proceedings, PP. 527-532, D. Maksimovic presented a resonant gate drive based on the quasi-square-wave power conversion. This solution provides a means for charging and discharging the input capacitance of a MOS type device in a loss-less fashion but at the expense of large amounts of circulating energy. A similar idea based on the zero-voltage-switched (ZVS) quasi-resonant converter (QRC) was proposed in “Novel High Efficiency Base Drive Using Zero Voltage Switching Converter”, IEEE PESC 91 Conference Proceedins, pp. 545-550 by H. S. Kim et al. and B. S. Jacobson, in “High Frequency Resonant Gate Driver With Partial Energy Recovery”, High Frequency Power Conversion Conference Proceedings 1993, pp. 133-141, proposed a third solution where a fraction of the charging and discharging energy is recovered.
The first two (2) prior art solutions do not embrace a level of efficiency where most of the charging and discharging energy is recovered. The third solution is better suited for switching frequencies in the megahertz range because of its basic operation thus limiting its use in synchronous rectifier circuits. What is needed is a resonant gate drive for an externally-driven synchronous rectification circuit which does not lose large amounts of circulating energy and which can be efficiently used with switching frequencies outside the megahertz range.
SUMMARY OF THE INVENTION
This present invention is a new gate drive configuration that can be used in applications where an external driving circuit is needed to drive the synchronous rectifiers in non self-driven applications.
In one embodiment, disclosed is an energy recovery mechanism for an externally driven synchronous rectifier circuit having a primary transformer, first and second synchronous rectifiers, an output terminal, and an external driving circuit configured to provide the timing signals for driving said first and second synchronous rectifiers. The energy recovery mechanism comprises first and second resonant inductors each having first and second terminals. Corresponding first and second recovery switches are coupled to the first terminals of the first and second resonant inductors. A first set of diodes is arranged to direct current into the second terminal of the first resonant inductor and a second set of diodes arranged to direct current into the second terminal of the second resonant inductor. The first and second recovery switches are further coupled to the external driving circuit for causing current to enter the first and second resonant inductors for recovering energy used in charging and discharging the input capacitance associated with the first and second synchronous rectifiers.
Also disclosed is a resonant gate drive for an externally-driven synchronous rectifier suitable for use in a DC-to-DC power converter. The synchronous rectifier circuit comprises a primary transformer having a primary and secondary winding with the secondary winding having a first terminal and a second terminal. A first synchronous rectifier is operably coupled to the first terminal of the secondary winding and a second synchronous rectifier is operably coupled to the second terminal of the secondary winding. An external drive circuit is used and operably coupled to the first and second synchronous rectifiers to provide the drive timing for the first and second synchronous rectifiers. An energy recovery circuit is coupled to the first and second synchronous rectifiers and configured for storing energy associated with charging and discharging their input capacitance. An output voltage terminal is coupled to said energy recovery circuit for receiving rectified voltage waveforms.
In another embodiment, the energy recovery circuit for each rectifier includes an additional resistor coupled to the first synchronous rectifier diode of the recovery circuit. The resistor ensures that the energy recovery circuit will operate correctly in situations where current flows back through the first synchronous diode thus re-charging the input capacitance of the synchronous rectifiers which will turn back on at an incorrect time. By adding the resistor, the voltage across the gate of the synchronous rectifiers is allowed to swing below zero to account for the back current.
In yet another embodiment, the circuit for energy recovery will utilize N-type MOSFETS to limit the voltage across the synchronous rectifiers to a different value than that of the input voltage of the synchronous rectifiers.
In another embodiment, the circuit for energy recovery will utilize an auxiliary winding to charge the gate capacitance of the synchronous rectifiers in a manner where the energy to charge the gate capacitance is recovered. The auxiliary winding will facilitate the correct charging pulse for the synchronous rectifiers.
Further disclosed is a method of recovering energy of an externally-driven synchronous rectifier circuit. The method includes the steps of capturing energy from the input capacitance of the synchronous rectifiers in at least one inductor and transferring the energy from the storage inductors to the output when the recovery switches turn off. The method also includes the step of recovering the energy needed for charging and discharging the synchronous rectifiers.
A technical advantage of the invention is the use of a resonant gate drive utilizing an externally driven synchronous rectification scheme.
Still another advantage is the achievement of a loss-less drive that can be used with both full-wave and half-wave rectifier configurations.
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Ericsson Inc.
Navarro Arthur I.
Vu Bao Q.
Wong Peter S.
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