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Low loss synchronous rectifier for application to clamped-mode power converters |
| RE37889 |
Low loss synchronous rectifier for application to clamped-mode power converters
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| Patent Drawings: | |
| Inventor: |
Rozman |
| Date Issued: |
October 22, 2002 |
| Application: |
09/429,692 |
| Filed: |
October 27, 1999 |
| Inventors: |
Rozman; Allen Frank (Richardson, TX)
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| Assignee: |
Lucent Technologies Inc. (Murray Hill, NJ) |
| Primary Examiner: |
Riley; Shawn |
| Assistant Examiner: |
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| Attorney Or Agent: |
Hitt, Gaines & Boisbrun |
| U.S. Class: |
327/309; 363/20; 363/21.06; 363/21.14; 363/89 |
| Field Of Search: |
363/20; 363/21.06; 363/89; 363/97; 363/126; 363/127; 363/21.14; 327/309 |
| International Class: |
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| U.S Patent Documents: |
5066900; 5126931; 5303138; 5872705; 6081432; 6191964 |
| Foreign Patent Documents: |
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| Other References: |
Principles of Solid State Power Converstion, Tarter, Ist Ed., pp. 544-547, (1985) (no month).*.
Current-Controlled Synchronous Rectification, Acker et al., IEEE, pp. 185-191, (May 1994).*.
High Efficiency DC-DC Converter, Jitaru et al., IEEE, pp. 638-644, (May 1994).. |
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| Abstract: |
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. |
| Claim: |
I claim: .[.
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 connectingthe 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 anentire 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 toa 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 currentduring a second interval other than the clamping interval..]. .[.
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 acyclic 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 aclamping 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 signalacross 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 beresponsive 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..]. .[.
3. In a power converter as claimed in claim 1 or 2, comprising: the converter connected to operate as a forward type converter..]. .[.
4. In a power converter as claimed in claim 1 or 2, comprising: the converter connected to operate as a flyback type converter..]. .[.
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 thetransformer, 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 extentof 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 gateconnected 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 forenabling conduction of the load current when the first rectifying device is nonconducting..]. .[.
6. A switching mode power converter as claimed in claim 5, further comprising: the second rectifying device comprises a diode..]. .[.
7. A switching mode power converter as claimed in claim 5, further comprising: the second rectifying device comprises a rectifying device with a control gate connected to be responsive to a signal of the secondary winding..]. .[.
8. A switching mode power converter as claimed in claim 6 or 7, further comprising: The secondary winding tapped and separated into first and second winding segments, and the first rectifying device is connected to the first winding segment andthe second rectifying device is connected to the second winding segment..]. .[.
9. A switching mode power converter as claimed in claim 6 or 7, further comprising: the converter connected to operate as a forward type converter..]. .[.
10. A switching mode power converter as claimed in claim 6 or 7, further comprising: the converter connected to operate as a flyback type converter..]. .Iadd.
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 ofsaid 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 forsubstantially all of said clamping interval..Iaddend..Iadd.
12. The method as claimed in claim 11 wherein said clamping circuit is directly connected to said power transformer..Iaddend..Iadd.
13. The method as claimed in claim 11 wherein said clamping circuit is coupled to a primary winding of said power transformer..Iaddend..Iadd.
14. The method as claimed in claim 11 wherein said power transformer has a center-tapped secondary winding..Iaddend..Iadd.
15. The method as claimed in claim 11 further comprising connecting a primary winding of said power transformer to an input of said power converter during a first cyclic interval of said power converter..Iaddend..Iadd.
16. The method as claimed in claim 11 further comprising a further synchronous rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
17. The method as claimed in claim 11 further comprising a rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
18. The method as claimed in claim 11 wherein said clamping circuit comprises a switching device connected in series with a capacitor..Iaddend..Iadd.
19. The method as claimed in claim 18 further comprising controlling said switching device with a control circuit..Iaddend..Iadd.
20. The method as claimed in claim 11 wherein said power converter operates in one of: a forward mode, a flyback mode, and a forward/flyback mode..Iaddend..Iadd.
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 aclamping interval..Iaddend..Iadd.
22. The method as claimed in claim 21 wherein said clamping circuit is directly connected to said power transformer..Iaddend..Iadd.
23. The method as claimed in claim 21 wherein said clamping circuit is coupled to a primary winding of said power transformer..Iaddend..Iadd.
24. The method as claimed in claim 21 wherein said power transformer has a center-tapped secondary winding..Iaddend..Iadd.
25. The method as claimed in claim 21 further comprising connecting a primary winding of said power transformer to an input of said power converter during a first cyclic interval of said power converter..Iaddend..Iadd.
26. The method as claimed in claim 21 further comprising a further synchronous rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
27. The method as claimed in claim 21 further comprising a rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
28. The method as claimed in claim 21 wherein said clamping, circuit comprises a switching device connected in series with a capacitor..Iaddend..Iadd.
29. The method as claimed in claim 28 further comprising controlling said switching device with a control circuit..Iaddend..Iadd.
30. The method as claimed in claim 21 wherein said power converter operates in one of: a forward mode, a flyback mode, and a forward/flyback mode..Iaddend..Iadd.
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 substantiallyall of a clamping interval..Iaddend..Iadd.
32. The method as claimed in claim 31 wherein said clamping circuit is directly connected to said power transformer..Iaddend..Iadd.
33. The method as claimed in claim 31 wherein said clamping circuit is coupled to a primary winding of said power transformer..Iaddend..Iadd.
34. The method as claimed in claim 31 wherein said power transformer has a center-tapped secondary winding..Iaddend..Iadd.
35. The method as claimed in claim 31 further comprising connecting a primary winding of said power transformer to an input of said power converter during a first cyclic interval of said power converter..Iaddend..Iadd.
36. The method as claimed in claim 31 further comprising a further synchronous rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
37. The method as claimed in claim 31 further comprising a rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
38. The method as claimed in claim 31 wherein said clamping circuit comprises a switching device connected in series with a capacitor..Iaddend..Iadd.
39. The method as claimed in claim 38 further comprising controlling said switching device with a control circuit..Iaddend..Iadd.
40. The method as claimed in claim 31 wherein said power converter operates in one of: a forward mode, a flyback mode, and a forward/flyback mode..Iaddend..Iadd.
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 saidplurality 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 forsubstantially all of a clamping interval..Iaddend..Iadd.
42. The method as claimed in claim 41 wherein said clamping circuit is directly connected to said power transformer..Iaddend..Iadd.
43. The method as claimed in claim 41 wherein said clamping circuit is coupled to a primary winding of said power transformer..Iaddend..Iadd.
44. The method as claimed in claim 41 wherein said power transformer has a center-tapped secondary winding..Iaddend..Iadd.
45. The method as claimed in claim 41 further comprising connecting a primary winding of said power transformer to an input of said power converter during a first cyclic interval of said power converter..Iaddend..Iadd.
46. The method as claimed in claim 41 further comprising a further synchronous rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
47. The method as claimed in claim 41 further comprising a rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
48. The method as claimed in claim 41 wherein said clamping circuit comprises a switching device connected in series with a capacitor..Iaddend..Iadd.
49. The method as claimed in claim 48 further comprising controlling said switching device with a control circuit..Iaddend..Iadd.
50. The method as claimed in claim 41 wherein said power converter operates in one of: a forward mode, a flyback mode, and a forward/flyback mode..Iaddend..Iadd.
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 saidoutput 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 saidvoltage 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 acrosssaid at least one secondary winding such that said synchronous rectification device is active for substantially all of said clamping interval..Iaddend..Iadd.
52. The method as claimed in claim 51 wherein said clamping circuit is directly connected to said power transformer..Iaddend..Iadd.
53. The method as claimed in claim 51 wherein said clamping circuit is coupled to said at least one primary winding of said power transformer..Iaddend..Iadd.
54. The method as claimed in claim 51 wherein said at least one secondary winding has a center-tap..Iaddend..Iadd.
55. The method as claimed in claim 51 further comprising a voltage limiting device coupled to said synchronous rectification device..Iaddend..Iadd.
56. The method as claimed in claim 51 further comprising a further synchronous rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
57. The method as claimed in claim 51 further comprising rectification device, coupled to said power transformer, that is active during a first cyclic interval of said power converter..Iaddend..Iadd.
58. The method as claimed in claim 51 wherein said clamping circuit comprises a switching device connected in series with a capacitor..Iaddend..Iadd.
59. The method as claimed in claim 58 further comprising controlling said switching device with a control circuit..Iaddend..Iadd.
60. The method as claimed in claim 51 wherein said power converter operates in one of: a forward mode, a flyback mode, and a forward/flyback mode..Iaddend. |
| Description: |
FIELD OF THE INVENTION
This invention relates to switching type poser converters and in particular to forward and flyback converters having a clamp-mode topology.
BACKGROUND OF THE INVENTION
Self synchronized rectifiers refer to rectifiers using MOSFET rectifying devices having control terminals which are driven by voltages of the windings of the power transformer in order to provide the rectification of the output of thetransformer. Use of synchronous rectifiers has been limited however by the inefficiency of these rectifiers in buck derived converter topologies. Efficiency is limited due to the nature of switching of buck derived converters (i.e. buck, buck-boost,boost converters including forward and flyback topologies and due to the variability of the transformer reset voltages in the forward type converters. This variability of reset voltage limits the conduction time of one of the MOSFET rectifiers,diminishing the effectiveness and efficiency of the rectifier. This is because the rectifying devices do not conduct for the full switching period and the gate drive energy of one of the rectifiers is dissipated.
SUMMARY OF THE INVENTION
A synchronous rectifier is combined with a clamped-mode buck derived power converter. In one illustrative embodiment a hybrid rectifier includes a MOSFET rectifying device active in a first cyclic interval of the conduction/nonconductionsequence of the power switch. A second rectifying device embodied in one illustrative embodiment as a low forward voltage drop bipolar diode rectifying device is active during an alternative interval to the first conduction/nonconduction interval Thegate drive to the MOSFET device is maintained continuous at a constant level for substantially the all of the second interval by the clamping action of the clamping circuitry of the converter. This continuous drive enhances the efficiency of therectifier.
The bipolar rectifier device may also embodied as a MOSFET device in a rectifier using two MOSFET devices. The subject rectifier may be used in both forward and flyback poser converters.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic of a forward converter, of the prior are, having a synchronous rectifier;
FIG. 2 is a voltage waveform of the secondary transformer winding of the converter of FIG. 1;
FIG. 3 is a schematic of a clamped-mode forward converter with a synchronous rectifier embodying the principles of the invention;
FIG. 4 is a voltage waveform of the secondary transformer winding of the converter of FIG. 3;
FIG. 5 is a schematic of another version of a clamped-mode forward converter with a synchronous rectifier embodying the principles of the invention;
FIG. 6 is a schematic of another version of a clamper-mode forward converter with a synchronous rectifier and a center tapped secondary winding embodying the principles of the invention;
FIG. 7 is a schematic of a clamped-mode flyback converter with a synchronous rectifier embodying the principles or the invention; and
FIG. 8 is a schematic of another version of a clamped-mode forward converter with a synchronous rectifier and a center tapped secondary winding embodying the principles of the invention.
DETAILED DESCRIPTION
In the converter shown in the FIG. 1, a conventional forward topology of the prior art with an isolating power transformer is combined with a self synchronized synchronous rectifier. In such a rectifier controlled devices are used with thecontrol terminals being driven by an output winding of the power transformer.
A DC voltage input V.sub.ix, 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 asynchronous 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 loadconnected 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 theconduction 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 conductioninterval T.sub.1 of the power switch 101, the secondary winding voltage V.sub.as1 charges the gate of MOSFET 106 to bias it conducting for the entire interval T.sub.1. The MOSFET 105 is biased non conducting during the T.sub.1 interval. The conductingMOSFET rectifying device 106 provides the current path allowing energy transfer to the output during the interval T.sub.1. The gate of MOSFET rectifier 106 is charged in response to the input voltage V.sub.in. All of the gate drive energy due to thisvoltage is dissipated.
As the poser MOSFET switch 101 turns off, the voltage V.sub.as1 across the secondary winding 102 reverses polarity just as the time interval T.sub.2 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 FIG. 2, the voltage across the secondary winding 102 is not a constant value, but is rather a variablevoltage that collapses to zero in the subsequent time interval T.sub.3, which occurs prior to the subsequent conduction interval of the power switch 101. This voltage is operative to actually drive the rectifier 105 conducting over only a portion of thetime interval T.sub.2 which is indicated by the cross hatched area 202 associated with the waveform 201 n FIG. 2. This substantially diminishes the performance of the rectifier 105 as a low loss rectifier device. This is aggravated by the fact that thebody diode 108 of the rectifier 105 has a large forward voltage drop which is too large to efficiently carry the load current.
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 conductthroughout 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 gatedrive 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 therectifying 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 conductingintervals 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 voltageappearing across the magnetizing inductance during the non conducting interval of the MOSFET power switch 101.
The diode 323 of the synchronous rectifier, shown in FIG. 3, has been substituted for the MOSFET device 106 shown in the FIG. 1. Due to the dissipation of gate drive energy the overall contribution of the MOSFET rectifier 106 in FIG. 1 islimited. The clamping action of the clamping circuitry results in the constant voltage level 402 shown in the voltage waveform 401, across the secondary winding 102, in the time period T.sub.2. This constant voltage applied to the gate drive of theMOSFET rectifier 105 drives it into conduction for the entire T.sub.2 reset interval. In this arrangement there is no need for a bipolar or a body diode shunting the MOSFET rectifier 105. An advantage in the clamped mode converter is that the peakinverse voltage applied to the diode 323 is much less than that applied to the similarly positioned MOSFET device in FIG. 1. Accordingly the diode 323 may be a very efficient low voltage diode which may be embodied by a low voltage diode normallyconsidered unsuitable for rectification purposes.
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 tothe leakage inductance of the power transformer, discharging the capacitance down toward zero voltage. During the time interval T.sub.3 shown in FIG. 4, voltage across the primary winding is supported by the leakage inductance. The voltage across thesecondary winding 102 drops to zero value as shown in the FIG. 4. With this zero voltage level of the secondary winding, the output inductor resonantly discharges the gate capacitance of the MOSFET rectifying device 105 and eventually forward biases thethe bipolar diode 323. The delay time T.sub.3 is a fixed design parameter and is a factor in the control of the power switches 101 and 322, which may be switched to accommodate soft waveforms. This synchronous rectification circuit of FIG. 3 providesthe desired efficiencies lacking in the arrangement of the circuit shown in FIG. 1
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 circuit350 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 desiredregulation, 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 FIG. 3 is shown in the circuit schematic of the FIG. 5. The converter of FIG. 5 is a clamped mode forward converter having two gated synchronous rectifying devices 105 and 106. In this embodiment of thesynchronous rectifier the synchronized rectifying device 106 can be used without adversely affecting the converter efficiency at lower operating frequencies.
The circuit of FIG. 6 is a clamped mode forward converter having a rectifier analogous to that of FIG. 3 in using one bipolar rectifying diode. The secondary winding is tapped creating two secondary winding segments 603 and 602.
The converter FIG. 7 operates in a flyback mode. The bipolar and synchronous rectifier device are in a reversed connection from the connection of FIG. 3 to accommodate the flyback operation.
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 theMOSFET 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 device813.
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