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US20020003714A1 - Method and arrangement for regulating low output voltages in multile output flyback DC/DC converters - Google Patents

Method and arrangement for regulating low output voltages in multile output flyback DC/DC converters Download PDF

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US20020003714A1
US20020003714A1 US09/826,923 US82692301A US2002003714A1 US 20020003714 A1 US20020003714 A1 US 20020003714A1 US 82692301 A US82692301 A US 82692301A US 2002003714 A1 US2002003714 A1 US 2002003714A1
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voltage
output
series
rectifier
generating
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US6459595B2 (en
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Bengt Assow
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Unwired Planet LLC
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of DC power input into DC power output
    • H02M3/22—Conversion of DC power input into DC power output with intermediate conversion into AC
    • H02M3/24—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters
    • H02M3/28—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC
    • H02M3/325—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of DC power input into DC power output
    • H02M3/22—Conversion of DC power input into DC power output with intermediate conversion into AC
    • H02M3/24—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters
    • H02M3/28—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC
    • H02M3/325—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33561—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having more than one ouput with independent control
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Definitions

  • the invention relates generally to multiple output flyback DC/DC converters and more specifically to a method and an arrangement for regulating low output voltages to tight tolerances in multiple output flyback DC/DC converters.
  • the voltage drop across the diodes can be as high as 0.5 V. This means that for low output voltages, e.g. ⁇ 3.3 V, the efficiency will be low.
  • the main output voltage is regulated in a manner known per se by regulating the duty cycle of the primary switch.
  • additional output voltages can not be regulated in multiple output converters by regulating the duty cycle of the primary switch but have to be separately regulated, e.g. by means of complicated switched series-regulators.
  • the object of the invention is to bring about a method and an arrangement for controlling a MOSFET used as synchronous rectifier to regulate low output voltages to tight tolerances.
  • the output voltage can be controlled between the voltage drop in the body diode of the MOSFET and the saturation voltage of the MOSFET. This voltage difference is only about 0.3 to 0.4 V but large enough for regulating a 3.3 V output voltage to tight tolerances.
  • FIG. 1 shows an embodiment of a multiple output flyback DC/DC converter in accordance with the invention
  • FIGS. 2 A- 2 F are diagrams illustrating different signals in the embodiment in FIG. 1.
  • FIG. 1 shows an embodiment of a multiple output flyback DC/DC converter in accordance with the invention.
  • a single additional output voltage U 2 is generated in addition to a main output voltage U 1 . It is however to be understood that it is possible to generate more than one additional output voltage. In such a case, for every additional output voltage to be generated, there has to be an arrangement corresponding to the arrangement for generating the additional output voltage U 2 to be described below.
  • the converter in FIG. 1 comprises in a manner known per se, a transformer TR having a primary winding connected in series with a primary switch T 1 to a schematically illustrated DC voltage source V 1 , and a secondary winding N 1 +N 2 connected in series with a diode D 1 to an output capacitor C 1 for generating the main output DC voltage U 1 .
  • the on and off periods of the primary switch T 1 i.e. its duty cycle, is controlled by a switch duty cycle regulator or switch control unit 1 in response to output signals of an error amplifier 2 connected with its input terminals across the capacitor C 1 for generating output signals in response to differences between the actual value of the output voltage U 1 and a desired value set within the error amplifier 2 .
  • a part N 2 of the secondary winding of the transformer TR is connected in series with a synchronous rectifier in the form of a MOSFET T 2 to an output capacitor C 2 .
  • the MOSFET T 2 comprises a source S, a drain D, and a gate G as well as a body diode D 2 connected with its anode to the source S and with its cathode to the drain D of the MOSFET T 2 .
  • the drain D of the MOSFET T 2 is connected to the source S of the MOSFET T 2 via a resistor R 1 in series with a diode D 3 and a capacitor C 3 .
  • the interconnection point between the diode D 3 and the capacitor C 3 is connected to the collector of a transistor T 3 .
  • the emitter of the transistor T 3 is connected to the gate G of the MOSFET T 2 and to the emitter of a transistor T 4 whose collector is connected to the source S of the MOSFET T 2 .
  • the transistors T 3 and T 4 are two emitter-followers that quickly can charge/discharge the gate G of the MOSFET T 2 .
  • the bases of the transistors T 3 and T 4 are interconnected and connected via a resistor R 2 to the collector of the transistor T 3 .
  • the interconnected bases of the transistors T 3 and T 4 are also connected to the collector of a transistor T 5 whose emitter is connected to the source S of the MOSFET T 2 .
  • the base of the transistor T 5 is connected via resistor R 3 in series with a capacitor C 4 to the drain D of the MOSFET T 2 .
  • the interconnection point between the resistor R 3 and the capacitor C 4 is connected via a resistor R 4 to an output terminal of an error amplifier 3 .
  • the error amplifier 3 is connected with its input terminals across the capacitor C 2 for generating output signals in response to differences between the actual value of the additional output voltage U 2 and a desired value set within the error amplifier 3 to control the MOSFET T 2 .
  • the switch T 1 is the primary switch of the flyback converter.
  • the switch T 1 When the switch T 1 is on, magnetic energy is stored in the transformer TR and the voltage across the secondary winding of the transformer TR is assumed to be negative.
  • the primary switch T 1 turns off, the voltage across the secondary winding of the transformer TR goes positive and a current ID 1 will flow through the diode D 1 to charge the capacitor C 1 , and a current IT 2 will flow through the MOSFET T 2 to charge the capacitor C 2 .
  • FIGS. 2 A- 2 F are essentially theoretical, i.e. more or less idealized.
  • the primary switch T 1 is supposed to be on from the beginning and is supposed to be turned off at time t 1 and turned on again at time t 4 as illustrated in FIG. 2A.
  • FIG. 2B the source-drain voltage U S-D T 2 of the MOSFET T 2 is illustrated.
  • Base current will be supplied to the transistor T 3 via the resistor R 2 .
  • the gate G of the MOSFET T 2 will be charged causing the MOSFET T 2 to become saturated.
  • FIG. 2C illustrates the gate-source voltage U G-S T 2 of the MOSFET T 2 .
  • the error amplifier 3 senses that the output voltage U 2 is higher than the desired value and starts to discharge the capacitor C 4 via the resistor R 4 .
  • the gate charge of the gate G is supposed to have been fully discharged at time t 3 .
  • the error amplifier 2 will sense this increase of the main output voltage U 1 and in response hereto control the duty cycle regulator 1 to adjust the duty cycle of the primary switch T 1 in such a manner that the main output voltage U 1 is decreased to the desired value.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Dc-Dc Converters (AREA)

Abstract

To generate, in addition to a main output DC voltage (U1), at least one additional output DC voltage (U2) in a flyback DC/DC converter having a transformer (TR) with a primary winding connected in series with a primary switch (T1), a first secondary winding (N1+N2) connected in series with a first rectifier (D1) to an first output capacitor (C1) for generating said main output DC voltage (U1), and a second secondary winding (N2) connected in series with a a synchronous rectifier (T2) to a second output capacitor (C2) for generating said additional output DC voltage (U2), an error amplifier (3) is provided to compare the actual value of said additional output DC voltage (U2) with a desired value, and a control circuit (C4, R4, R3, T5) is interconnected between the error amplifier (3) and the synchronous rectifier (T2) to adjust the turn-off time of the synchronous rectifier (T2) in response to the ratio between the actual value of said additional output DC voltage (U2) and the desired value to adjust the actual value to the desired value.

Description

    TECHNICAL FIELD
  • The invention relates generally to multiple output flyback DC/DC converters and more specifically to a method and an arrangement for regulating low output voltages to tight tolerances in multiple output flyback DC/DC converters. [0001]
    • BACKGROUND OF THE INVENTION
  • There is a demand for converters supplying in addition to a main output DC voltage, at least one additional output DC voltage lower than the main output voltage. [0002]
  • If diodes are used for generating the additional output DC voltages, the voltage drop across the diodes can be as high as 0.5 V. This means that for low output voltages, e.g. ≦3.3 V, the efficiency will be low. [0003]
  • When high efficiency is needed, synchronized rectification has to be used since the voltage drop across synchronous rectifiers, i.e. MOSFETs, is much lower than across diodes. [0004]
  • In multiple output converters, the main output voltage is regulated in a manner known per se by regulating the duty cycle of the primary switch. However, additional output voltages can not be regulated in multiple output converters by regulating the duty cycle of the primary switch but have to be separately regulated, e.g. by means of complicated switched series-regulators. [0005]
    • SUMMARY OF THE INVENTION
  • The object of the invention is to bring about a method and an arrangement for controlling a MOSFET used as synchronous rectifier to regulate low output voltages to tight tolerances. [0006]
  • This is attained in accordance with the invention by controlling the switch-off time of the MOSFET to vary its on-period. [0007]
  • Hereby, the output voltage can be controlled between the voltage drop in the body diode of the MOSFET and the saturation voltage of the MOSFET. This voltage difference is only about 0.3 to 0.4 V but large enough for regulating a 3.3 V output voltage to tight tolerances.[0008]
    • BRIEF DESCRIPTION OF THE DRAWING
  • The invention will be described more in detail below with reference to the appended drawing on which FIG. 1 shows an embodiment of a multiple output flyback DC/DC converter in accordance with the invention, and FIGS. [0009] 2A-2F are diagrams illustrating different signals in the embodiment in FIG. 1.
    • DESCRIPTION OF THE INVENTION
  • FIG. 1 shows an embodiment of a multiple output flyback DC/DC converter in accordance with the invention. [0010]
  • In the embodiment in FIG. 1, in addition to a main output voltage U[0011] 1, a single additional output voltage U2 is generated. It is however to be understood that it is possible to generate more than one additional output voltage. In such a case, for every additional output voltage to be generated, there has to be an arrangement corresponding to the arrangement for generating the additional output voltage U2 to be described below.
  • The converter in FIG. 1 comprises in a manner known per se, a transformer TR having a primary winding connected in series with a primary switch T[0012] 1 to a schematically illustrated DC voltage source V1, and a secondary winding N1+N2 connected in series with a diode D1 to an output capacitor C1 for generating the main output DC voltage U1.
  • The on and off periods of the primary switch T[0013] 1, i.e. its duty cycle, is controlled by a switch duty cycle regulator or switch control unit 1 in response to output signals of an error amplifier 2 connected with its input terminals across the capacitor C1 for generating output signals in response to differences between the actual value of the output voltage U1 and a desired value set within the error amplifier 2.
  • To generate the additional output DC voltage U[0014] 2 in accordance with the invention, in the embodiment in FIG. 1, a part N2 of the secondary winding of the transformer TR is connected in series with a synchronous rectifier in the form of a MOSFET T2 to an output capacitor C2.
  • Instead of a common secondary winding N[0015] 1+N2, it is to be understood that separate secondary windings (not shown) can be used for generating the main and the additional output DC voltages U1 and U2, respectively.
  • In a manner known per se, the MOSFET T[0016] 2 comprises a source S, a drain D, and a gate G as well as a body diode D2 connected with its anode to the source S and with its cathode to the drain D of the MOSFET T2.
  • In the embodiment shown in FIG. 1, the drain D of the MOSFET T[0017] 2 is connected to the source S of the MOSFET T2 via a resistor R1 in series with a diode D3 and a capacitor C3.
  • The interconnection point between the diode D[0018] 3 and the capacitor C3 is connected to the collector of a transistor T3. The emitter of the transistor T3 is connected to the gate G of the MOSFET T2 and to the emitter of a transistor T4 whose collector is connected to the source S of the MOSFET T2.
  • The transistors T[0019] 3 and T4 are two emitter-followers that quickly can charge/discharge the gate G of the MOSFET T2.
  • The bases of the transistors T[0020] 3 and T4 are interconnected and connected via a resistor R2 to the collector of the transistor T3.
  • The interconnected bases of the transistors T[0021] 3 and T4 are also connected to the collector of a transistor T5 whose emitter is connected to the source S of the MOSFET T2.
  • The base of the transistor T[0022] 5 is connected via resistor R3 in series with a capacitor C4 to the drain D of the MOSFET T2.
  • In accordance with the invention, the interconnection point between the resistor R[0023] 3 and the capacitor C4 is connected via a resistor R4 to an output terminal of an error amplifier 3. The error amplifier 3 is connected with its input terminals across the capacitor C2 for generating output signals in response to differences between the actual value of the additional output voltage U2 and a desired value set within the error amplifier 3 to control the MOSFET T2.
  • As mentioned above, the switch T[0024] 1 is the primary switch of the flyback converter. When the switch T1 is on, magnetic energy is stored in the transformer TR and the voltage across the secondary winding of the transformer TR is assumed to be negative. When the primary switch T1 turns off, the voltage across the secondary winding of the transformer TR goes positive and a current ID1 will flow through the diode D1 to charge the capacitor C1, and a current IT2 will flow through the MOSFET T2 to charge the capacitor C2.
  • With reference to the diagrams in FIGS. [0025] 2A-2F, the operation of the converter illustrated in FIG. 1 during one off-period of the primary switch T1 will now be described in more detail.
  • It should be pointed out that the diagrams in FIGS. [0026] 2A-2F are essentially theoretical, i.e. more or less idealized.
  • The primary switch T[0027] 1 is supposed to be on from the beginning and is supposed to be turned off at time t1 and turned on again at time t4 as illustrated in FIG. 2A.
  • When the primary switch T[0028] 1 is on, the voltage across the winding N2 is negative and the capacitor C3 is charged via the resistor R1 in series with the diode D3.
  • At time t[0029] 1 when the primary switch T1 turns off, the body diode D2 of the MOSFET T2 begins to conduct to charge the output capacitor C2.
  • In FIG. 2B, the source-drain voltage U[0030] S-DT2 of the MOSFET T2 is illustrated.
  • Base current will be supplied to the transistor T[0031] 3 via the resistor R2. The gate G of the MOSFET T2 will be charged causing the MOSFET T2 to become saturated.
  • FIG. 2C illustrates the gate-source voltage U[0032] G-ST2 of the MOSFET T2.
  • In the following description, it is assumed that, at time t[0033] 1, the output voltage U2 is higher than the desired value set within the error amplifier 3.
  • The error amplifier [0034] 3 senses that the output voltage U2 is higher than the desired value and starts to discharge the capacitor C4 via the resistor R4.
  • This causes the transistor T[0035] 5 to start to conduct at time t2 as apparent from FIG. 2D that illustrates the base-emitter voltage UB-BT5 of the transistor T5.
  • When the transistor T[0036] 5 starts to conduct at time t2, the gate charge of the gate G of the MOSFET T2 will begin to be discharged via the transistor T4 as illustrated in FIG. 2C.
  • The gate charge of the gate G is supposed to have been fully discharged at time t[0037] 3.
  • Thus, at time t[0038] 3, the MOSFET T2 turns off.
  • When the MOSFET T[0039] 2 turns off at time t3, the current IT2 through it will drop as illustrated in FIG. 2E. However, due to imperfections of the transformer TR and the existence of the body diode D2 in the MOSFET T2, the current IT2 will not drop to zero.
  • The drop of the current IT[0040] 2 will cause a corresponding jump of the current ID1 through the diode D1 as illustrated in FIG. 2F.
  • This jump of the current ID[0041] 1 tends to increase the main output voltage U1.
  • The error amplifier [0042] 2 will sense this increase of the main output voltage U1 and in response hereto control the duty cycle regulator 1 to adjust the duty cycle of the primary switch T1 in such a manner that the main output voltage U1 is decreased to the desired value.
  • Between time t[0043] 3 when the MOSFET T2 turns off, and time t4 when the primary switch T1 turns on again, the currents IT2 and ID1 decrease due to the fact that the energy stored in the transformer TR decreases.
  • By controlling the MOSFET T[0044] 2 in this manner, i.e. by adjusting its turn-off time in response to that the actual value of the additional voltage U2 exceeds the desired value, it is possible to regulate low additional output DC voltages in the interval between the voltage drop of the body diode D2 and the saturation voltage of the MOSFET T2 to tight tolerances.

Claims (2)

1. A method of generating, in addition to a main output DC voltage, at least one additional output DC voltage in a flyback DC/DC converter having a transformer with a primary winding connected in series with a primary switch to an input DC voltage source, a first secondary winding connected in series with a first rectifier to a first output capacitor for generating said main output DC voltage across said first output capacitor, and a second secondary winding connected in series with a second rectifier to a second output capacitor for generating said additional output DC voltage across said second output capacitor, characterized by
selecting a synchronous rectifier as said second rectifier,
comparing the actual value of said additional output DC voltage with a desired value, and
adjusting the turn-off time of the synchronous rectifier in response to the ratio between the actual value and the desired value to adjust the actual value to the desired value.
2. An arrangement for generating, in addition to a main output DC voltage (U1), at least one additional output DC voltage (U2) in a flyback DC/DC converter having a transformer (TR) with a primary winding connected in series with a primary switch (T1) to an input DC voltage source (V1), a first secondary winding (N1+N2) connected in series with a first rectifier (D1) to an first output capacitor (C1) for generating said main output DC voltage (U1) across said first output capacitor (C1), and a second secondary winding (N2) connected in series with a second rectifier (T2) to a second output capacitor (C2) for generating said additional output DC voltage (U2) across said second output capacitor (C2), characterized in
that said second rectifier (T2) is a synchronous rectifier,
that an error amplifier (3) is provided to compare the actual value of said additional output DC voltage (U2) with a desired value, and
that a control circuit (C4, R4, R3, T5) is interconnected between the error amplifier (3) and the synchronous rectifier (T2) to adjust the turnoff time of the synchronous rectifier (T2) in response to the ratio between the actual value of said additional output DC voltage (U2) and the desired value to adjust the actual value to the desired value
US09/826,923 2000-04-07 2001-04-06 Method and arrangement for regulating low output voltages in multiple output flyback DC/DC converters Expired - Lifetime US6459595B2 (en)

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SE0001300 2000-04-07
SE0001300A SE519148C2 (en) 2000-04-07 2000-04-07 Method and device for regulating low voltages in flyback type DC converters with multiple outputs
SE0001300-3 2000-04-07

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EP (1) EP1282933A1 (en)
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US6459595B2 (en) 2002-10-01
SE519148C2 (en) 2003-01-21
EP1282933A1 (en) 2003-02-12
SE0001300L (en) 2001-10-08
WO2001078223A1 (en) 2001-10-18
AU2001247015A1 (en) 2001-10-23

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