Apparatus (and a method) useful in a switching power converter having a transformer and a primary switch for connecting a DC input source to a primary winding of the transformer during a portion of each of a succession of converter operating cycles, the apparatus includes a reset capacitor and a reset switch. reset circuitry cooperates with the reset switch to connect and disconnect the reset capacitor in a manner which provides for resetting the transformer and which allows a current having a non-zero average value to flow in the reset switch.
Another aspect is a method for limiting the slew rate in a switching power converter which includes a transformer and a reset circuit of the kind which non-dissipatively recycles the magnetizing energy stored in a transformer during each of a succession of converter operating cycles. The method includes sensing the magnetizing current which is flowing in said transformer, and allowing initiation of another converter operating cycle only if the magnetizing current meets a predefined criterion for safe converter operation. In embodiments of the invention the predefined criterion may be the magnetizing current passing through a predefined value, e.g., zero.
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1. A method for limiting the slew rate in a switching power converter which includes a transformer and a reset circuit of the kind which non-dissipatively recycles the magnetizing energy stored in a transformer during each of a succession of converter operating cycles, said method comprising
sensing the magnetizing current which is flowing in said transformer, and allowing initiation of another converter operating cycle only if the magnetizing current meets a predefined criterion for converter operation.
40. A power supply method comprising the steps of:
initiating energy transfer from a DC input source to a load via a transformer by connecting said DC input source to a winding of said transformer during a forward energy transfer portion of each of a succession of converter operating cycles; resetting said transformer during each of said converter cycles by providing for energy transfer between said transformer and a reset capacitor during a second portion of said converter operating cycles such that the flux in the transformer is reversed; and selectively shunting said reset capacitor to prevent polarity reversal of the voltage across said reset capacitor.
0. 134. A method for use in a switching power converter of the kind having
a transformer, a primary switch for connecting a DC input source to a winding of the transformer during a portion of each of a series of converter operating cycles, and a reset capacitor, the method comprising: providing a first unidirectional conducting device between a winding of the transformer and the reset capacitor to conduct a magnetizing current during a first portion of a magnetizing energy recycling portion of the converter operating cycles, the first unidirectional conducting device being poled to conduct the magnetizing current which flows at the time that the primary switch is opened; and sensing current flowing in the unidirectional conducting device. 0. 122. Apparatus for use in a switching power converter of the kind having
a transformer, a primary switch for connecting a DC input source to a winding of the transformer during a portion of each of a series of converter operating cycles, and a reset capacitor, the apparatus comprising: a first unidirectional conducting device connected between a winding of the transformer and the reset capacitor to conduct a magnetizing current during a first portion of a magnetizing energy recycling portion of the converter operating cycles, the first unidirectional conducting device being poled to conduct the magnetizing current which flows at the time that the primary switch is opened; and a sense circuit connected to sense the current flowing in the unidirectional conducting device. 39. Switching power converter apparatus comprising:
a DC input source; a transformer having a primary winding; a primary switch for connecting said DC input source to said primary winding during a portion of each of a succession of converter operating cycles; a reset capacitor; a series circuit connected between said reset capacitor and said transformer, said series circuit providing the only substantial path for completing a circuit comprising said capacitor leg and said transformer; said series circuit comprising a reset switch; and reset circuitry, including a reset diode connected in parallel with said reset capacitor to inhibit polarity reversal of the charge stored by said capacitor, said reset circuitry cooperating with said reset switch to connect and disconnect said reset capacitor in a manner which provides for resetting a core of said transformer, and allows a current having a non-zero average value to flow in said series circuit.
36. A reset method for use in a switching power converter of the kind having
a transformer, a primary switch for connecting a DC input source to a winding of the transformer during a portion of each of a succession of converter operating cycles, a reset capacitor, and a series circuit connected between said reset capacitor and said transformer, said series circuit providing the only substantial path for completing a circuit comprising said reset capacitor and said transformer; said series circuit comprising a reset switch which enables current flow between said reset capacitor and said transformer through said series circuit in a closed state and inhibits current flow between said reset capacitor and said transformer through said series circuit in an open state, said method comprising connecting and disconnecting said reset capacitor in a manner which provides for resetting said transformer and which allows a current having a non-zero average value to flow in said series circuit.
35. Apparatus for use in a switching power converter of the kind having a transformer for transferring energy from a DC input source to a load during a forward energy transfer portion of a succession of converter operating cycles, and a primary switch for connecting said DC input source to a primary winding of said transformer during said forward energy transfer portions, the apparatus comprising:
a reset capacitor; a series circuit connected between said reset capacitor and said transformer, said series circuit providing the only substantial path for completing a circuit comprising said reset capacitor and said transformer; said series circuit comprising a reset switch; and reset circuitry which cooperates with said series circuit to connect and disconnect said reset capacitor in a manner which provides for resetting a core of said transformer, and allows a current having a non-zero average value to flow in said series circuit, said reset circuitry including a reset diode connected in parallel with said reset capacitor to inhibit polarity reversal of the voltage across said capacitor.
4. Apparatus for use in a switching power converter of the kind which delivers power from a DC input source to a load via a transformer and which has a primary switch for connecting said DC input source to a primary winding of said transformer during a portion of each of a succession of converter operating cycles, said apparatus comprising:
reset circuitry including a capacitor leg; a series circuit connected between said capacitor leg and said transformer, said series circuit providing the only substantial path for completing a circuit comprising said capacitor leg and said transformer; said series circuit comprising switch circuitry which enables current flow between said capacitor leg and said transformer through said series circuit during a closed state and inhibits current flow between said capacitor leg and said transformer through said series circuit during an open state; and additional circuitry connected to said capacitor leg which allows a current having a non-zero average value to flow in said series circuit; said reset circuitry resets said transformer by transferring energy stored in said transformer to said capacitor leg and back to said transformer without returning energy to said DC input source. 2. The method of
3. The method of
5. The apparatus of
6. The apparatus of
7. The apparatus of
8. The apparatus of
11. The apparatus of
12. The apparatus of
13. The apparatus of
15. The apparatus of
16. The apparatus of
17. The apparatus of
18. The apparatus of
19. The apparatus of
20. The apparatus of
21. The apparatus of
22. The apparatus of
24. The apparatus of
25. The apparatus of
26. The apparatus of
27. The apparatus of
28. The apparatus of
29. The apparatus of
30. The apparatus of
33. The apparatus of
34. The apparatus of
slew control circuitry for inhibiting initiation of a converter operating cycle based upon the value of a magnetizing current in said transformer.
37. The method of
38. The method of
41. The method of
42. The method of
43. The method of claims 38 or 42 wherein said finite delay accommodates charging and discharging of capacitances.
46. The method of
0. 47. The method of
0. 48. The method of
0. 49. The method of
0. 50. The method of
0. 51. The method of
0. 52. The method of
providing a source of bias current, connected through a second unidirectional conducting device to the reset capacitor side of the first unidirectional conducting device, the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and sensing the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 53. The method of
0. 54. The method of
0. 55. The method of
0. 56. The method of
0. 57. The method of
0. 58. The method of
0. 59. The method of
further comprising sensing current flowing in a primary-switch diode connected in parallel across a primary switch and poled to block current flow from a DC input source to a primary winding of the transformer; and turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged.
0. 60. The apparatus of
a sense circuit connected to the series circuit for detecting completion of a forward energy transfer portion of the converter operating cycles.
0. 61. The apparatus of
0. 62. The apparatus of
the series circuit further comprises a first unidirectional conducting device poled to conduct a magnetizing current during a first portion of a magnetizing energy recycling portion of each of the succession of the converter operating cycles; and the sense circuit sense the current flowing in the first unidirectional conducting device.
0. 63. The apparatus of
the series circuit further comprises an impedance connected in series with the first unidirectional conducting device and the sense circuit senses a voltage across the impedance induced by the magnetizing current.
0. 64. The apparatus of
0. 65. The apparatus of
0. 66. The apparatus of
0. 67. The apparatus of
a source of bias current connected through a second unidirectional conducting device to the capacitor leg side of the first unidirectional conducting device, the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and the sense circuit senses the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 68. The apparatus of
0. 69. The apparatus of
0. 70. The apparatus of
0. 71. The apparatus of
0. 72. The apparatus of
0. 73. The apparatus of
0. 74. The apparatus of
0. 75. The apparatus of
the primary switch further comprises a primary-switch diode connected in parallel across the primary switch and poled to block current flow from the DC input source to the primary winding of the transformer; and further comprising a primary switch sense circuit connected to sense current flowing in the primary-switch diode; wherein the primary switch sense circuit initiates turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged. 0. 76. The apparatus of
a sense circuit connected to the series circuit for detecting completion of the forward energy transfer portion of the converter operating cycles.
0. 77. The apparatus of
0. 78. The apparatus of
the series circuit further comprises a first unidirectional conducting device poled to conduct a magnetizing current during a first portion of a magnetizing energy recycling portion of each of the succession of the converter operating cycles; and the sense circuit senses the current flowing in the first unidirectional conducting device.
0. 79. The apparatus of
the series circuit further comprises an impedance connected in series with the first unidirectional conducting device and the sense circuit senses a voltage across the impedance induced by the magnetizing current.
0. 80. The apparatus of
0. 81. The apparatus of
0. 82. The apparatus of
0. 83. The apparatus of
a source of bias current connected through a second unidirectional conducting device to the capacitor leg side of the first unidirectional conducting device, the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and the sense circuit senses the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 84. The apparatus of
0. 85. The apparatus of
0. 86. The apparatus of
0. 87. The apparatus of
0. 88. The apparatus of
0. 89. The apparatus of
0. 90. The apparatus of
0. 91. The apparatus of
the primary switch further comprises a primary-switch diode connected in parallel across the primary switch and poled to block current flow from the DC input source to the primary winding of the transformer; and further comprising a primary switch sense circuit connected to sense current flowing in the primary-switch diode; wherein the primary switch sense circuit initiates turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged. 0. 92. The apparatus of
a sense circuit connected to the series circuit for detecting completion of a forward energy transfer portion of the converter operating cycles.
0. 93. The apparatus of
0. 94. The apparatus of
the series circuit further comprises a first unidirectional conducting device poled to conduct a magnetizing current during a first portion of a magnetizing energy recycling portion of each of the succession of the converter operating cycles; and the sense circuit senses the current flowing in the first unidirectional conducting device.
0. 95. The apparatus of
the series circuit further comprises an impedance connected in series with the first unidirectional conducting device and the sense circuit senses a voltage across the impedance induced by the magnetizing current.
0. 96. The apparatus of
0. 97. The apparatus of
0. 98. The apparatus of
0. 99. The apparatus of
a source of bias current connected through a second unidirectional conducting device to the capacitor leg side of the first unidirectional conducting device, the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and the sense circuit senses the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 100. The apparatus of
0. 101. The apparatus of
0. 102. The apparatus of
0. 103. The apparatus of
0. 104. The apparatus of
0. 105. The apparatus of
0. 106. The apparatus of
0. 107. The apparatus of
the primary switch further comprises a primary-switch diode connected in parallel across the primary switch and poled to block current flow from the DC input source to the primary winding of the transformer; and further comprising a primary switch sense circuit connected to sense current flowing in the primary-switch diode; wherein the primary switch sense circuit initiates turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged. 0. 108. The method of
sensing completion of the forward energy transfer portion of the converter operating cycles.
0. 109. The method of
0. 110. The method of
0. 111. The method of
0. 112. The method of
0. 113. The method of
0. 114. The method of
providing a source of bias current connected through a second unidirectional conducting device to the reset capacitor side of the first unidirectional conducting device, the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and sensing the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 115. The method of
0. 116. The method of
0. 117. The method of
0. 118. The method of
0. 119. The method of
0. 120. The method of
0. 121. The method of
said connecting comprises using a primary switch and further comprising providing a primary-switch diode connected in parallel across the primary switch and poled to block current flow from a DC input source to the winding of the transformer; and sensing current flowing in the primary-switch diode; turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged.
0. 123. The apparatus of
0. 124. The apparatus of
the first unidirectional conducting device comprises a diode; and the sense circuit senses the voltage across the diode.
0. 125. The apparatus of
a source of bias current, connected through a second unidirectional conducting device to the reset capacitor side of the first unidirectional conducting device; the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and wherein the sense circuit senses the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 126. The apparatus of
0. 127. The apparatus of
0. 128. The apparatus of
0. 129. The apparatus of
0. 130. The apparatus of
0. 131. The apparatus of
0. 132. The apparatus of
0. 133. The apparatus of
a primary-switch diode connected in parallel across the primary switch and poled to block current flow from the DC input source to the primary winding of the transformer; and a primary switch sense circuit connected to sense current flowing in the primary-switch diode; wherein the primary switch sense circuit is configured to initiate turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged.
0. 135. The method of
providing a resistance in series with the first unidirectional conducting device; and the sensing comprises sensing the voltage across the resistance.
0. 136. The method of
the first unidirectional conducting device comprises a diode; and the sensing comprises sensing the voltage across the diode.
0. 137. The method of
providing a source of bias current, connected through a second unidirectional conducting device to the reset capacitor side of the first unidirectional conducting device; the bias current and second unidirectional conducting device being poled to reverse bias the first unidirectional conducting device; and the sensing comprises sensing the voltage, Vs, across the series combination of the first and second unidirectional conducting devices.
0. 138. The method of
0. 139. The method of
0. 140. The method of
0. 141. The method of
0. 142. The method of
0. 143. The method of
0. 144. The method of
providing a primary-switch diode connected in parallel across the primary switch and poled to block current flow from a DC input source to the winding of the transformer; and sensing current flowing in the primary-switch diode; turning on the primary switch after a current flows through the primary-switch diode indicating that capacitances associated with the primary switch have discharged.
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This is a continuation of application Ser. No. 08/373,112, filed Jan. 17, 1995, now abandoned.
This invention relates to control of stored magnetic energy in power converter transformers.
In one class of power converters, called single-ended switching power converters (10, FIG. 1), a primary switching element 20 is used to repetitively connect an input source to the primary winding 329 of a power transformer 25 (
A wide variety of reset circuits (also called core reset circuits because almost all switching power converter transformers include a magnetic core), for use in single-ended forward converters, are described in the literature. These circuits differ in terms of their efficiency, cost and impact on converter power density. One such circuit 15, shown in
When used in single-ended forward converters, core reset circuits 15, 30 of the kinds shown in
In an aspect, the invention features apparatus (and a method) useful in a switching power converter having a transformer and a primary switch for connecting a DC input source to a primary winding of the transformer during a portion of each of a succession of converter operating cycles, the apparatus includes a reset capacitor and a reset switch. Reset circuitry cooperates with the reset switch to connect and disconnect the reset capacitor in a manner which provides for resetting the transformer and which allows a current having a non-zero average value to flow in the reset switch.
Embodiments may include one or more of the following features. The reset circuitry may open and close the reset switch. The reset circuitry may cause the connecting and disconnecting to occur at times based on switching of the main switch. The reset switch may be connected in series with the capacitor. The reset circuitry may include circuit elements which inhibit bidirectional energy flow between the capacitor and the transformer. The apparatus may only allow transfer of magnetizing energy between the reset capacitor and the transformer if the polarity of the voltage across the reset capacitor is of a particular polarity. The apparatus may be connected in parallel with a winding on the transformer. The winding may be the primary winding, a secondary winding, or an auxiliary winding. The reset switch may be a unidirectional switch (e.g., a MOSFET) in parallel with a unidirectional conducting element (e.g., the intrinsic body diode of the MOSFET), the unidirectional switch and the unidirectional conducting element being arranged to conduct in opposite directions. The unidirectional switch may be a MOSFET in series with a series diode, the series diode and the MOSFET being poled to conduct in the same direction. The circuit elements may include a reset diode connected in parallel with the capacitor. Bidirectional energy flow may be allowed only if the voltage across the reset capacitor is of a particular polarity. The particular polarity may be that which will result in a reversal in the polarity of transformer magnetizing current during the time that the reset switch is closed. The reset circuitry may include circuit elements which inhibit bidirectional energy flow between the capacitor and the transformer. The circuit elements may include a reset diode connected across a series circuit comprising the reset capacitor and the series diode. The switching power converter may be a forward power converter, a zero-current switching converter, or a PWM converter. The reset circuitry may open the reset switch prior to the ON period of the primary switch, may close the reset switch during the OFF period of the primary switch, and may keep the reset switch open throughout the ON period of the primary switch.
In an aspect, the invention features a method for limiting the slew rate in a switching power converter which includes a transformer and a reset circuit of the kind which non-dissipatively recycles the magnetizing energy stored in a transformer during each of a succession of converter operating cycles. The method includes sensing the magnetizing current which is flowing in said transformer, and allowing initiation of another converter operating cycle only if the magnetizing current meets a predefined criterion for safe converter operation. In embodiments of the invention the predefined criterion may be the magnetizing current passing through a predefined value, e.g., zero. Among the advantages are the following. Resonance between the transformer and the reset capacitor is inhibited. Very low dissipation is achieved. The selection of Cr can be made based solely on the operating requirements of the converter.
Other aspects, features, and advantages will become apparent from what follows.
Waveforms for the core reset circuit 30 of
Core reset circuits of the kind shown in
Because the core reset circuit of
In converters in which operating frequency varies (e.g., in zero-current switching (ZCS) converters of the kind described in Vinciarelli, "Forward Converter Switching at Zero Current," U.S. Pat. No. 4,415,959, incorporated by reference) additional problems may arise, especially as the converter operating frequency drops to values comparable to, or below, the frequency fr. In such cases, the waveforms of
Another characteristic of the circuit of
One way to minimize the effects of resonance in the reset circuit of
Effective damping has been achieved by setting the value of the damper capacitor, Cd, to approximately two times the value of Cr, and setting the damper resistor value, Rd, so that the damper frequency, fd=1/Rd*Cd, is equal to about one-half the undamped characteristic frequency fr. To avoid excessive dissipation in the damper resistor, fr is chosen to be well below (e.g., approximately 10% of) the maximum operating frequency of the converter since this puts the "normal" operating frequency range of the converter well above fr and provides for minimum damper dissipation in this range. Thus, the characteristic frequency, fr, of a ZCS converter operating at frequencies up to 1 MHz (or of a constant frequency PWM converter operating at a similar frequency) would be set to be near 100 KHz. This provides effective damping with relatively little power loss at converter operating frequencies well above fr.
In practice, however, use of the damped reset circuit requires balancing several sometimes conflicting requirements. On the one hand, use of a low value for Cr is generally desirable as it provides for a higher allowable converter slew rates and increases the value of fr above the crossover frequency of the converter. On the other hand, low values of Cr result in increased dissipation in the damper resistor. For example, if the damped reset circuit 230 of
The beneficial effects of the diode in preventing resonance may be seen, in part, by comparing the waveforms in
While selection of the value of the reset capacitor in the damped reset circuit (
A fundamental and important difference between the prior art reset circuit topology 30 of FIG. 3 and the improved topology 330 of
As shown in
In practice the reset switch 332 may, as shown in
Several embodiments of an improved reset circuit 330a, 330b, 330c are shown in
As a practical matter the diode 344 (
The breakdown voltage rating, Vav, of the series diode 344 should not be made too low, however. As the drain-source voltage, Vs, across the MOSFET (
Controlling the reset switch 332 (
Ways to sense the state of the primary switch are shown in
A preferred method for sensing the state of the primary switch as a means of safely enabling the reset switch is shown in FIG. 18B. In the Figure a diode 410, a large value resistor 412 and bias source 414, of voltage Vb<Vin, are used as sensing elements. Prior to opening the primary switch (e.g., prior to time t=tm in
With reference to
One way of sensing the magnetizing current is to measure it directly, as is done in the circuit of
It is also to be noted that, if the magnetizing current is scaled properly, operation of the reset circuit may be exploited to allow the primary switch to be switched on at zero-voltage. Assume, for example, that the circuits of
When the PSE pulse ends the output of gate 514 goes low, turning off primary switch 20. When the switch becomes non-conductive at time t=tm the signal Vs 519 (discussed above with reference to
Other embodiments are within the scope of the following claims. For example, the topologies and circuits described above may be adapted for use in other than single-ended converters (e.g.; full bridge or half-bridge converters).
Vinciarelli, Patrizio, Prager, Jay
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