US20010024378A1 - Method and arrangement for controlling a synchronous rectifier in a DC/DC converter - Google Patents

Method and arrangement for controlling a synchronous rectifier in a DC/DC converter Download PDF

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US20010024378A1
US20010024378A1 US09/813,281 US81328101A US2001024378A1 US 20010024378 A1 US20010024378 A1 US 20010024378A1 US 81328101 A US81328101 A US 81328101A US 2001024378 A1 US2001024378 A1 US 2001024378A1
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mosfet
voltage
transistor
transients
capacitor
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US6438009B2 (en
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Bengt Assow
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Telefonaktiebolaget LM Ericsson AB
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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
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • 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
    • H02M3/33592—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 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
    • 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 DC/DC converters and more specifically to a method and an arrangement for controlling synchronous rectifiers in DC/DC converters.
  • the object of the invention is to bring about a method and an arrangement for controlling a MOSFET on the secondary side without any need of transferring control signals from the primary side of the converter.
  • FIG. 1 shows a fly-back DC/DC converter with an embodiment of a control arrangement in accordance with the invention
  • FIGS. 2 A- 2 D are diagrams illustrating different signals in the embodiment in FIG. 1.
  • FIG. 1 shows a fly-back DC/DC converter with an embodiment of a control arrangement in accordance with the invention.
  • the converter in FIG. 1 comprises a transformer TR having a primary winding connected in series with a primary switch T 1 to output terminals of a schematically illustrated DC voltage source V 1 , and a secondary winding connected in series with a MOSFET T 2 to an output capacitor C 1 for generating an output DC voltage.
  • the MOSFET T 2 is used as a synchronous rectifier and is controlled in accordance with the invention to improve the efficiency of the converter.
  • the MOSFET T 2 comprises a source S, a drain D, and a gate G as well as a body diode D 1 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 2 and a capacitor C 2 .
  • the drain D is connected to the source S via a diode D 3 in series with a capacitor C 3 .
  • the interconnection point between the diode D 2 and the capacitor C 2 is connected to the collector of a transistor T 3 whose emitter 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 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 the collector of a transistor T 6 whose emitter is connected to the interconnection point between the diode D 3 and the capacitor C 3 via a resistor R 5 and whose base is connected to the interconnection point between the diode D 2 and the capacitor C 2 .
  • the switch T 1 is the so-called primary switch of the fly-back converter.
  • the switch T 1 When the switch T 1 is on, magnetic energy is stored in the transformer TR.
  • the voltage U across the secondary winding of the transformer TR is negative.
  • the primary switch T 1 goes off, the voltage U will be positive and energy will be transferred via the MOSFET T 2 to the output capacitor C 1 .
  • 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 transistor T 5 turns the MOSFET T 2 on and off via the emitter-followers T 3 and T 4 .
  • the transistor T 5 is turned on and off by the capacitor C 4 .
  • the primary switch T 1 turns on, the MOSFET T 2 is still conducting, but the current through the MOSFET T 2 reverses its direction when the voltage U starts to fall.
  • the transistor T 5 senses this change via the capacitor C 4 and turns off tie MOSFET T 2 .
  • the time when the transistor T 5 turns on i.e. when the MOSFET T 2 turns off, can be varied.
  • the transistor T 6 senses the voltage difference between the voltage across the capacitor C 3 and the voltage across the capacitor C 2 .
  • the capacitor C 3 is charged to the peak voltage of the voltage U including any transient emanating from a non-desired turn-off current from the MOSFET T 2 , while the capacitor C 2 is charged to the voltage U excluding any transients since the resistor R 1 filters out any transients.
  • the transistor T 6 When the voltage difference between the capacitors C 3 and C 2 exceeds the base-emitter voltage of the transistor T 6 , the transistor T 6 will conduct and via the resistor R 4 set the off-time of the transistor T 5 , i.e. the on-time of the MOSFET T 2 , so that the voltage difference between the capacitors C 3 and C 2 will be smaller than a few volts after a number of switch cycles
  • FIG. 2A illustrates a couple of cycles of the voltage UT 1 across the primary switch T 1 .
  • the primary switch T 1 is supposed to be on from the beginning and is supposed to be turned off at times t 1 and t 1 ′ and turned on at times t 2 and t 2 ′.
  • 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 and 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 .
  • That transient which is illustrated in FIG. 2B at time t 2 , charges the capacitor C 3 via the diode D 3 to a voltage that is higher than the voltage across the capacitor C 2 , causing the transistor T 6 to start conducting.
  • the collector current from the transistor T 6 then sets the time when the transistor T 5 shall start to draw gate charge from the gate G of the MOSFET T 2 .
  • FIG. 2D illustrates the base-emitter voltage U B-E T 5 of the transistor T 5 .
  • the transistor T 5 will start conducting at a time tx before time t 2 ′ as indicated in FIG. 2D.
  • the transistor T 5 will begin to draw gate charge out of the gate G of the MOSFET T 2 to turn off the MOSFET T 2 earlier than time t 2 ′ as illustrated in FIG. 2C.

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

Abstract

To control a MOSFET (T2) used as a synchronous rectifier in a fly-back DC/DC converter and connected in series with a secondary winding of a transformer (TR), the MOSFET (T2) being repetitively turned on and off in response to the turning off and on of a primary switch (T1) connected in series with a primary winding of the transformer (TR), means (C3, T5) are provided for detecting voltage transients across the MOSFET (T2) caused by reverse currents upon its turn-off, and for causing it to shorten at least its next on-period in response to the detection of such transients.

Description

    TECHNICAL FIELD
  • The invention relates generally to DC/DC converters and more specifically to a method and an arrangement for controlling synchronous rectifiers in DC/DC converters. [0001]
    • BACKGROUND OF THE INVENTION
  • To rectify high frequency AC voltages in switched DC/DC converters, synchronous rectifiers in the form of MOSFETs are often used instead of diodes. [0002]
  • Since the voltage drop across a MOSFET is lower than across a diode, the efficiency of converters with MOSFETs will be higher. [0003]
  • However, since a MOSFET cannot be turned off instantaneously, its turn-off will not occur at exactly the same moment as the turn-on of a primary switch but a little later. This will cause reverse currents through the MOSFET, causing voltage transients to appear across the MOSFET. To eliminate these voltage transients, unique control signals are needed for the MOSFET to function properly. It is known to provide these control signals from the primary side of the converter, e.g. via an additional transformer. [0004]
    • SUMMARY OF THE INVENTION
  • The object of the invention is to bring about a method and an arrangement for controlling a MOSFET on the secondary side without any need of transferring control signals from the primary side of the converter. [0005]
  • This is attained in accordance with the invention by detecting voltage transients across the MOSFET, caused by reverse currents when it is turned off, and in response hereto causing the MOSFET to shorten at least its next on-period in response to the detection of such transients. [0006]
  • Hereby, the MOSFET will be controlled more accurately and no transients will appear.[0007]
    • 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 a fly-back DC/DC converter with an embodiment of a control arrangement in accordance with the invention, and FIGS. [0008] 2A-2D are diagrams illustrating different signals in the embodiment in FIG. 1.
    • DESCRIPTION OF THE INVENTION
  • FIG. 1 shows a fly-back DC/DC converter with an embodiment of a control arrangement in accordance with the invention. [0009]
  • The converter in FIG. 1 comprises a transformer TR having a primary winding connected in series with a primary switch T[0010] 1 to output terminals of a schematically illustrated DC voltage source V1, and a secondary winding connected in series with a MOSFET T2 to an output capacitor C1 for generating an output DC voltage.
  • The MOSFET T[0011] 2 is used as a synchronous rectifier and is controlled in accordance with the invention to improve the efficiency of the converter.
  • In a manner known per se, the MOSFET T[0012] 2 comprises a source S, a drain D, and a gate G as well as a body diode D1 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[0013] 2 is connected to the source S of the MOSFET T2 via a resistor R1 in series with a diode D2 and a capacitor C2.
  • Also, the drain D is connected to the source S via a diode D[0014] 3 in series with a capacitor C3.
  • The interconnection point between the diode D[0015] 2 and the capacitor C2 is connected to the collector of a transistor T3 whose emitter 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 bases of the transistors T[0016] 3 and T4 are interconnected and connected via a resistor R2 to the collector of the transistor T3. The interconnected bases of the transistors T3 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[0017] 5 is connected via resistor R3 in series with a capacitor C4 to the drain D of the MOSFET T2. The interconnection point between the resistor R3 and the capacitor C4 is connected via a resistor R4 to the collector of a transistor T6 whose emitter is connected to the interconnection point between the diode D3 and the capacitor C3 via a resistor R5 and whose base is connected to the interconnection point between the diode D2 and the capacitor C2.
  • As mentioned above, the switch T[0018] 1 is the so-called primary switch of the fly-back converter. When the switch T1 is on, magnetic energy is stored in the transformer TR. The voltage U across the secondary winding of the transformer TR is negative. When the primary switch T1 goes off, the voltage U will be positive and energy will be transferred via the MOSFET T2 to the output capacitor C1.
  • The transistors T[0019] 3 and T4 are two emitter-followers that quickly can charge/discharge the gate G of the MOSFET T2.
  • The transistor T[0020] 5 turns the MOSFET T2 on and off via the emitter-followers T3 and T4. The transistor T5 is turned on and off by the capacitor C4. When the primary switch T1 turns on, the MOSFET T2 is still conducting, but the current through the MOSFET T2 reverses its direction when the voltage U starts to fall. The transistor T5 senses this change via the capacitor C4 and turns off tie MOSFET T2. By charging the capacitor C4 via the resistor R4, the time when the transistor T5 turns on, i.e. when the MOSFET T2 turns off, can be varied.
  • The transistor T[0021] 6 senses the voltage difference between the voltage across the capacitor C3 and the voltage across the capacitor C2. The capacitor C3 is charged to the peak voltage of the voltage U including any transient emanating from a non-desired turn-off current from the MOSFET T2, while the capacitor C2 is charged to the voltage U excluding any transients since the resistor R1 filters out any transients.
  • When the voltage difference between the capacitors C[0022] 3 and C2 exceeds the base-emitter voltage of the transistor T6, the transistor T6 will conduct and via the resistor R4 set the off-time of the transistor T5, i.e. the on-time of the MOSFET T2, so that the voltage difference between the capacitors C3 and C2 will be smaller than a few volts after a number of switch cycles
  • With reference to FIGS. [0023] 2A-2D, the operation of the converter illustrated in FIG. 1 will now be described in more detail.
  • FIG. 2A illustrates a couple of cycles of the voltage UT[0024] 1 across the primary switch T1. The primary switch T1 is supposed to be on from the beginning and is supposed to be turned off at times t1 and t1′ and turned on at times t2 and t2′.
  • When the primary switch T[0025] 1 is on, the capacitor C2 is charged via the resistor R1 in series with the diode D2, and the capacitor C3 is charged via the diode D3.
  • At time t[0026] 1 when the primary switch T1 turns off, the body diode D1 of the MOSFET T2 begins to conduct to charge the output capacitor C1.
  • If FIG. 2B, the source-drain voltage U[0027] S-DT2 of the MOSFET T2 is illustrated.
  • Base current will be supplied to the transistor T[0028] 3 via the resistor R2 and 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[0029] G-ST2 of the MOSFET T2.
  • At time t[0030] 2, the primary switch T1 turns on again.
  • When the voltage across the MOSFET T[0031] 2 changes polarity at time t2 as illustrated in FIG. 2B, the transistor T5 becomes saturated via the capacitor C4 and the resistor R3, and the MOSFET T2 goes off.
  • However, due to the fact that it takes some time to discharge the gate of the MOSFET T[0032] 2 after that the transistor T5 is turned on, the MOSFET T2 will turn off a little late causing a reverse current to flow from the output capacitor C1 back into the transformer TR. This reverse current causes a voltage transient across the MOSFET T2 when the MOSFET T2 is off.
  • That transient, which is illustrated in FIG. 2B at time t[0033] 2, charges the capacitor C3 via the diode D3 to a voltage that is higher than the voltage across the capacitor C2, causing the transistor T6 to start conducting.
  • The collector current from the transistor T[0034] 6 then sets the time when the transistor T5 shall start to draw gate charge from the gate G of the MOSFET T2.
  • Towards the end of the next off-period of the primary switch T[0035] 1, i.e. towards time t2′, almost all gate charge has been drawn from the gate G of the MOSFET T2 before the primary switch T1 goes on again at time t2′ as illustrated in FIG. 2C.
  • When the primary switch T[0036] 1 turns on at time t2′, the MOSFET T2 is prepared, i.e. most of the gate charge has been drawn off and the MOSFET T2 is not fully conducting. Thus, no reverse current spike will appear across the MOSFET T2 at time t2′ as apparent from FIG. 2B.
  • FIG. 2D illustrates the base-emitter voltage U[0037] B-ET5 of the transistor T5.
  • At time t[0038] 1, the voltage change across the secondary winding of the transformer TR turns off the transistor T5 via the capacitor C4 and the resistor R3. Hereby, the gate G of the MOSFET T2 is charged via the resistor R2 and the transistor T3.
  • Between times t[0039] 1 and t2, the base-emitter voltage UB-ET5 of the transistor T5 is reversed and, consequently, the transistor T5 is not conducting.
  • When the primary switch T[0040] 1 is turned on at time t2, the reverse voltage across the MOSFET T2 turns on the transistor T5 and turns off the MOSFET T2.
  • Between times t[0041] 2 and t1′, the base of the transistor T5 is forward biased. Therefore, the gate G of the MOSFET T2 is low and the MOSFET T2 is not conducting as apparent from FIG. 2C.
  • For the next period, i.e. the time between t[0042] 1′ and t2′, the voltage difference between the capacitors C2 and C3 caused by to the transient appearing at time t2, turns on the transistor T6 causing the capacitor C4 to be discharged via the resistor R4.
  • As a result, the transistor T[0043] 5 will start conducting at a time tx before time t2′ as indicated in FIG. 2D.
  • Also at time tx, via the transistor T[0044] 4, the transistor T5 will begin to draw gate charge out of the gate G of the MOSFET T2 to turn off the MOSFET T2 earlier than time t2′ as illustrated in FIG. 2C.
  • Thus, due to the earlier turn-off of the MOSFET T[0045] 2, the transient or current spike that appeared at time t2 will not appear at time t2′ as illustrated in FIG. 2B.
  • However, should a current spike be present also at time t[0046] 2′, the MOSFET T2 will be turned off earlier towards the end of its next on-period.
  • As should be apparent from the above, no control signals have to be transferred from the primary side of the converter to control the MOSFET on the secondary side. [0047]

Claims (3)

1. A method of controlling, in a fly-back DC/DC converter, a MOSFET (T2) used as a synchronous rectifier and connected in series with a secondary winding of a transformer (TR) for generating an output DC voltage, comprising repetitively turning the MOSFET (T2) on and off in response to the turning off and on of a primary switch (T1) connected in series with a primary winding of the transformer (TR) to an input DC voltage source (V1), characterized by detecting voltage transients across the MOSFET (T2) caused by reverse currents upon its turn-off, and, in response hereto, causing it to shorten at least its next on-period in response to the detection of such transients.
2. An arrangement for controlling, in a fly-back DC/DC converter, a MOSFET (T2) used as a synchronous rectifier and connected in series with a secondary winding of a transformer (TR) for generating an output DC voltage, the MOSFET (T2) being repetitively turned on and off in response to the turning off and on of a primary switch (T1) connected in series with a primary winding of the transformer (TR) to an input DC voltage source (V1), characterized by means (C3) for detecting voltage transients across the MOSFET (T2) caused by reverse currents upon its turn-off, and means (T5) for causing it to shorten at least its next on-period in response to the detection of such transients.
3. The arrangement according to
claim 2
, characterized in that said means for detecting comprises a capacitor (C3) that is adapted to be charged by the transients, and that said means for causing comprises a transistor (T5) that is adapted to sense the charging of the capacitor (C3) by the transients and in response hereto draw current from the gate (G) of the MOSFET (T2) before the primary switch (T1) turns on.
US09/813,281 2000-03-24 2001-03-21 Reducing voltage transients across a MOSFET in a synchronous rectifier in a DC/DC converter Expired - Fee Related US6438009B2 (en)

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SE0001055-3 2000-03-24
SE0001055A SE517685C2 (en) 2000-03-24 2000-03-24 Method and apparatus for controlling a synchronous rectifier in a DC converter
SE0001055 2000-03-24

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WO2001073930A1 (en) 2001-10-04
US6438009B2 (en) 2002-08-20
EP1269615A1 (en) 2003-01-02
SE0001055D0 (en) 2000-03-24
AU2001242975A1 (en) 2001-10-08
SE517685C2 (en) 2002-07-02

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