A synchronous rectifier for use with a clamped-mode power converter uses in one embodiment a hybrid rectifier with a MOSFET rectifying device active in one first cyclic interval or the conduction/nonconduction sequence of the power switch and a second rectifying device embodied in one illustrative embodiment as a low voltage bipolar diode rectifying device active during an alternative interval to the first conduction/nonconduction interval. The gate drive to the MOSFET device is continuous at a constant level for substantially all of the second interval which enhances efficiency of the rectifier. The bipolar rectifier device may also be embodied as a MOSFET deice. The subject rectifier may be used in both forward and flyback power converters.
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0. 5. A switching mode power converter, comprising:
a power transformer including a magnetizing inductance requiring periodic recycling; a first power stage for converting a dc input in a periodic pulsed voltage applied to a primary winding of the transformer, including; a clamping circuit for limiting a voltage of the transformer during the periodic recycling at a substantially constant amplitude and extending the voltage duration to maintain a constant voltage for substantially an entire extent of periodic recycling; a second power stage for rectifying an output of a secondary winding of the transformer and applying it to a lead to be energized, including; a synchronous rectifier including a first rectifying device with a control gate connected to be responsive to a signal across the secondary winding such that the synchronous rectification device conducts a load current during the periodic recycling when the clamping circuit is active, and a second rectifying device connected for enabling conduction of the load current when the first rectifying device is nonconducting.
0. 1. In a power converter, comprising:
an input for accepting a dc voltage; a power transformer including a primary and secondary winding; a power switch for periodically connecting the input to the primary winding; an output for accepting a load to be energized; clamping means for limiting a voltage and extending the voltage's duration across the secondary winding at a substantially constant amplitude during substantially an entire extent of a clamping interval of a cyclic period of the power converter; a rectifier circuit connecting the secondary winding to the output; and including: a synchronous rectification device with a control terminal connected to be responsive to a signal across the secondary winding such that the synchronous rectification device conducts a load current during substantially the entire extent of the clamping interval; and a rectifying device connected for enabling conduction of the load current during a second interval other than the clamping interval. 0. 2. In a power converter, comprising
an input for accepting a dc voltage; a power transformer including a primary and secondary winding; a power switch for periodically connecting the input to the primary winding during a second interval of a cyclic period; an output for accepting a load to be energized; clamping means for limiting a voltage and extending the voltage's duration across the secondary winding at a substantially constant amplitude during substantial an entire extent of a clamping interval of a cyclic period of the power converter; a rectifier circuit connecting the secondary winding to the output; and including: a first synchronous rectification device with a control terminal connected to be responsive to a signal across the secondary winding such that the synchronous rectification deice conducts a load current during substantially the entire extent of the clamping interval, and a second synchronous rectification device with a control terminal connected to be responsive to a signal across he secondary winding such that the second synchronous rectification device conducts the load current during substantially and entire extent of the second interval other than the clomping interval. 0. 21. A method of operating a power converter, comprising:
providing a power transformer having a plurality of windings; coupling a synchronous rectification device, having a control terminal, to at least one of said plurality of windings; coupling a clamping circuit to said at least one of said plurality of windings; and limiting a voltage applied to said control terminal with said clamping circuit such that said synchronous rectification device is active for substantially all of a clamping interval.
0. 31. A method of operating a power converter, comprising:
providing a power transformer having a plurality of windings; coupling a synchronous rectification device, having a control terminal, to at least one of said plurality of windings; coupling a clamping circuit to said at least one of said plurality of windings; and limiting a voltage applied to said control terminal with said clamping circuit such that said synchronous rectification device conducts a load current for substantially all of a clamping interval.
0. 41. A method of operating a power converter, comprising:
providing a power transformer having a plurality of windings; coupling a synchronous rectification device, having a control terminal responsive to a drive signal, to at least one of said plurality of windings; coupling a clamping circuit to said at least one of said plurality of windings; and limiting said drive signal applied to said control terminal with said clamping circuit such that said drive signal is continuous for substantially all of a clamping interval.
0. 11. A method of operating a power converter, comprising:
providing a power transformer having a plurality of windings; limiting a voltage across at least one of said plurality of windings with a clamping circuit during a clamping interval of said power converter; and rectifying said voltage with a synchronous rectification device having a control terminal responsive to a signal across at least one of said plurality of windings such that said synchronous rectification device is active for substantially all of said clamping interval.
0. 51. A method of operating a power converter, comprising:
accepting a dc voltage at an input of said power converter; providing current to a load coupled to an output of said power converter; transforming a voltage from said input to said output with a power transformer having at least one primary winding and at least one secondary winding; periodically connecting said input to said at least one primary winding during a first cyclic interval of said power converter; limiting said voltage across said at least one secondary winding with a clamping circuit during a clamping interval of said power converter; and rectifying said voltage with a synchronous rectification device having a control terminal responsive to a signal across said at least one secondary winding such that said synchronous rectification device is active for substantially all of said clamping interval.
0. 3. In a power converter as claimed in
the converter connected to operate as a forward type converter.
0. 4. In a power converter as claimed in
the converter connected to operate as a flyback type converter.
0. 6. A switching mode power converter as claimed in
the second rectifying device comprises a diode.
0. 7. A switching mode power converter as claimed in
the second rectifying device comprises a rectifying device with a control gate connected to be responsive to a signal of the secondary winding.
0. 8. A switching mode power converter as claimed in
The secondary winding tapped and separated into first and second winding segments, and the first rectifying device is connected to the first winding segment and the second rectifying device is connected to the second winding segment.
0. 9. A switching mode power converter as claimed in
the converter connected to operate as a forward type converter.
0. 10. A switching mode power converter as claimed in
the converter connected to operate as a flyback type converter.
0. 12. The method as claimed in
0. 13. The method as claimed in
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0. 15. The method as claimed in
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0. 17. The method as claimed in
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0. 20. The method as claimed in
a forward mode, a flyback mode, and a forward/flyback mode.
0. 22. The method as claimed in
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0. 30. The method as claimed in
a forward mode, a flyback mode, and a forward/flyback mode.
0. 32. The method as claimed in
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0. 35. The method as claimed in
0. 36. The method as claimed in
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0. 40. The method as claimed in
a forward mode, a flyback mode, and a forward/flyback mode.
0. 42. The method as claimed in
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0. 46. The method as claimed in
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0. 50. The method as claimed in
a forward mode, a flyback mode, and a forward/flyback mode.
0. 52. The method as claimed in
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0. 59. The method as claimed in
0. 60. The method as claimed in
a forward mode, a flyback mode, and a forward/flyback mode.
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In the converter shown in the
A DC voltage input Vix, at input 100, is connected to the primary winding 110 of the power transformer by a MOSFET power switch 101. The secondary winding 102 is connected to an output lead 103 through an output filter inductor 104 and a synchronous rectifier including the MOSFET rectifying devices 105 and 106. Each rectifying device includes a body diode 108 and 107, respectively.
With the power switch 101 conducting, the input voltage is applied across the primary winding 110. The secondary winding 102 is oriented in polarity to respond to the primary voltage with a current flow through the inductor 104, the load connected to output lead 103 and back through the MOSFET rectifier 106 to the secondary winding 102. Continuity of current flow in the inductor 104, when the power switch 101 is non-conducting, is maintained by the current path provided by the conduction of the MOSFET rectifier 105. An output filter capacitor 111 shunts the output of the converter.
Conductivity of the MOSFET rectifiers is controlled by the gate drive signals provided by the voltage appearing across the secondary winding 102. This voltage is shown graphically by the voltage waveform 201 in FIG. 2. During the conduction interval T1 of the power switch 101, the secondary winding voltage Vas1 charges the gate of MOSFET 106 to bias it conducting for the entire interval T1. The MOSFET 105 is biased non conducting during the T1 interval. The conducting MOSFET rectifying device 106 provides the current path allowing energy transfer to the output during the interval T1. The gate of MOSFET rectifier 106 is charged in response to the input voltage Vin. All of the gate drive energy due to this voltage is dissipated.
As the poser MOSFET switch 101 turns off, the voltage Vas1 across the secondary winding 102 reverses polarity just as the time interval T2 begins. This voltage reversal initiates a reset of the transformer magnetizing inductance, resonantly discharges the gate of MOSFET rectifier 106 and begins charging the gate of MOSFET rectifier 105. As shown by the voltage waveform of
The loss of efficiency of the synchronous rectifier limits the overall efficiency of the power converter and has an adverse effect on the possible power density attainable. Since the synchronous rectifier 105 does not continuously conduct throughout the entire switching period, a conventional rectifier diode (e.g. connected in shunt with rectifier 105) capable of carrying the load current is required in addition to MOSFET rectifier 105. This inefficiency is further aggravated by the gate drive energy dissipation associated with the MOSFET rectifier 106. This gate drive loss may exceed the conduction loss for MOSFET rectifier 106, at high switching frequency (e.g. >300 kHz).
The efficiency of a forward converter with synchronous rectification is significantly improved according to the invention by using a clamp circuit arrangement to limit the reset voltage and by using a low forward voltage drop diode in the rectifying circuitry. Such an arrangement is shown in the schematic of FIG. 3. In this forward power converter the power MOSFET device 101 is shunted by a series connection of a clamp capacitor 321 and a MOSFET switch device 322. The conducting intervals of power switch 101 and MOSFET device 322 are mutually exclusive. The duty cycle of power switch 101 is D and the duty cycle of MOSFET device 322 is 1-D. The voltage inertia of the capacitor 321 limits the amplitude of the reset voltage appearing across the magnetizing inductance during the non conducting interval of the MOSFET power switch 101.
The diode 323 of the synchronous rectifier, shown in
In the operation of the clamped mode forward converter the MOSFET switch 322 is turned off just prior to turning the MOSFET power switch on. Energy stored in the parasitic capacitances of the MOSFET switching devices 101 and 322 is commutated to the leakage inductance of the power transformer, discharging the capacitance down toward zero voltage. During the time interval T3 shown in
Control of the conductivity of the power switching devices 101 and 322 is by means of a control circuit 350, which is connected, by lead 351, to an output terminal 103 of the converter to sense the output terminal voltage. The control circuit 350 is connected, by leads 353 and 354, to the drive terminals of the power switches 101 and 322. The drive signals are controlled to regulate an the output voltage at output terminal. The exact design of a control circuit, to achieve the desired regulation, is well known in the art and hence is not disclosed in detail herein. This control circuit 350 is suitable for application to the converters of FIGS. 5,6,7 and 8.
A modified version of the circuit of
The circuit of
The converter
In some applications directs application of the gate drive signal directly from the secondary winding may result in voltage spikes exceeding the rating of the gate. A small signal MOSFET device 813 is connected to couple the gate drive to the MOSFET rectifying device 105. This device may be controlled by the control drive lead 815 to limit the peak voltage applied to the gate of rectifier 105. The MOSFET synchronous rectifier is then discharged through the body diode of the MOSFET device 813.
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