CN100495879C - Control circuit for reducing reverse current of synchronous rectifier - Google Patents

Abstract

The invention relates to a control circuit which is used for avoiding generating a reverse current in a synchronous rectifier. The control circuit of the invention comprises a prediction circuit which generates a timing signal according to the switching signal, and the timing signal is used for cutting off the synchronous rectifier so as to prevent reverse current from being generated under the conditions of light load and no load.

Description

Reduce the control circuit of the reverse current of synchronous rectifier

Technical field

The invention relates to a kind of power converter, be meant a kind of control circuit of power converter especially.

Background technology

Press, power converter is in order to erratic power supply source, is adjusted into the voltage source of rule and/or the electric current source of rule.See also Fig. 1, it is the conventional power converter with a synchronous rectifier.One switches signal S ₁, it is used to control the work period of a switch 10, to adjust the output voltage V of power converter ₀This output voltage V ₀Provide to a load 50.One charging current, it can charge to an output capacitance 40 when switch 10 conductings.See also Fig. 2, it shows that one switches signal S ₂According to cut-off state conducting one switch 20 of switch 10, and provide a low impedance current path for a discharging current IF of an inductance 30.One switches signal V _W, its when switch 10 conductings in order to this inductance 30 that charges.

(continuous current mode, CCM) in the running, these switch 10 conductings are before the energy that discharges this inductance 30 fully in the continuous current pattern.(discontinuous current mode, DCM) in the running, the energy of this inductance 30 just discharges before next switches the circulation beginning fully at discontinuous current-mode.See also Fig. 3, it is presented in the running of discontinuous current-mode, a reverse current I _RSee through switch 20 these output capacitances 40 of discharge.Under underload and non-loaded situation, the usefulness that reverse current IR will cause power loss and reduce power converter.See also Fig. 4 A and Fig. 4 B, Fig. 4 A and Fig. 4 B are respectively the waveform of continuous current pattern and discontinuous each signal of current-mode, wherein I _INBe charging current.

See also Fig. 5, it is a traditional forward type power converter, and it comprises synchronous rectifier.Power converter is used to provide output voltage V ₀To a load 55.The secondary winding of one transformer 60 produces one and switches voltage, with conducting one rectifier 16 and to an inductance 35 chargings.One electric capacity 45, it is coupled to inductance 35.In 60 off periods of transformer, switched voltage can change oppositely and rectifier 16 can be cut off and a rectifier 26 can be switched on, to discharge the energy of inductance 35. Switch 15,25 is used to reduce the power loss of rectifier 16,26 as synchronous rectifier.Switch signal S ₃, S ₄Be synchronized with switched voltage, with difference diverter switch 15,25.One switches signal V _W, it is used for when rectifier 16 conductings inductance 35 being charged.The charging current of inductance 35 and switching signal V _WVoltage and pulse bandwidth proportional.According to switching signal V _WVoltage and pulse bandwidth and output voltage V ₀, be predictable the discharge time of inductance 35, so can avoid the reverse current of synchronous rectifier to take place.

The method of the reverse current of located by prior art limits synchronization rectifier, it comprises and uses a current sensing circuit, once cutoff synchronization rectifier then when having sensed reverse current.Current sensing circuit need use a conducting resistance (R of electric crystal (synchronous rectifier) _DS-ON) or one the series connection resistance, with the detecting reverse current.Yet current sensing circuit can cause power loss and increase the complexity of system.In addition, the above-mentioned mode of commonly using can only just can be cut off synchronous rectifier after reverse current produces and detected.In view of the above, if a control circuit can not use under the current sensing circuit, can eliminate influencing this and will benefiting of reverse current to power converter.

Summary of the invention

Main purpose of the present invention is to provide a kind of control circuit, and it can prevent to produce the synchronous rectifier of reverse current in power converter, with the usefulness of avoiding power loss and reducing power converter, and the usefulness of bring to power transducer.

The invention provides a control circuit, to reduce the reverse current of synchronous rectifier.The present invention comprises a prediction circuit, it switches signal according to a controlling signal and and produces a sequential signal, the ON time of switching signal is represented the charging interval of the inductance of power converter, the sequential signal is used for the synchronous rectifier by power converter, to avoid producing reverse current in synchronous rectifier under underload and immunization with gD DNA vaccine.The sequential signal can increase and increase along with the voltage that switches signal.Controlling signal is associated with the output voltage of power converter.In addition, the sequential signal also can be along with the ON time of switching signal reduces and shortens.

Prediction circuit, it includes an input circuit and a timing circuit, and input circuit produces a charging signal and a discharge signal according to an input signal and controlling signal.Input signal is associated with the voltage that switches signal.Timing circuit then produces the sequential signal according to charging signal, discharge signal with the switching signal.Timing circuit elder generation foundation charging signal produces a charging voltage with switching signal, produces the sequential signal in case the switching signal ends by charging voltage and discharge signal.So synchronous rectifier can be before reverse current produces and be cut off.

The invention has the beneficial effects as follows: can prevent to produce the synchronous rectifier of reverse current, the usefulness of avoiding power loss and reducing power converter, the usefulness of bring to power transducer in power converter.

Description of drawings

Fig. 1 is the circuit diagram with conventional power converter of synchronous rectifier;

Fig. 2 is the circuit diagram of the synchronous rectifier conducting inductive discharge of conventional power converter;

Fig. 3 is conventional power converter produces reverse current under underload and immunization with gD DNA vaccine a circuit diagram;

Fig. 4 A is the oscillogram of conventional power converter running in the continuous current pattern;

Fig. 4 B is the oscillogram of conventional power converter running at discontinuous current-mode;

Fig. 5 is the circuit diagram with traditional forward type power converter of synchronous rectifier;

Fig. 6 is the circuit diagram of a preferred embodiment of power converter of the present invention;

Fig. 7 is the circuit diagram of a preferred embodiment of control circuit of the present invention;

Fig. 8 is the circuit diagram of a preferred embodiment of input circuit of the present invention;

Fig. 9 is the circuit diagram of a preferred embodiment of timing circuit of the present invention.

The figure number explanation:

10 switches, 15 switches

16 rectifiers, 20 switches

21 switches, 25 switches

26 rectifiers, 30 inductance

31 inductance, 35 inductance

40 output capacitances, 41 electric capacity

50 loads of 45 electric capacity

51 loads, 55 loads

60 transformers, 70 resistance

100 control circuits, 101 resistance

102 resistance, 105 constant current sources

110 electric capacity, 120 switches

150 prediction circuits, 160 inverters

180 with the door 200 input circuits

210 first voltages are to current converter circuit 211 operational amplifiers

212 electric crystals, 213 electric crystals

214 electric crystals, 230 second voltages are to current converter circuit

231 operational amplifiers, 232 electric crystals

233 electric crystals, 234 electric crystals

251 electric crystals, 252 electric crystals

253 electric crystals, 300 timing circuits

310 charge switchs, 320 discharge switches

350 comparators, 351 inverters

352 with door C electric capacity

I _AInput current signal I _BCurrent signal

I _CCharging signal I _DThe discharge signal

I _FDischarging current I _INCharging current

I _RReverse current I _SThe Control current signal

R _CResistance R _SResistance

S ₁Switch signal S ₂Switch signal

S _LDrive signal S _OFFThe sequential signal

T _OFFDischarge time _VCInput signal

V _CCSupply electric V _HCharging voltage

V _IInput V ₀Output voltage

V _PFormula end V _SControlling signal

V _WSwitch signal V _ZLimit voltage

Embodiment

For being had architectural feature of the present invention and the effect reached, the auditor more advances one

The understanding and the understanding in step are sincerely helped with preferred embodiment and are cooperated detailed explanation, explanation

As back:

See also Fig. 6, it is the circuit diagram of a preferred embodiment of power converter of the present invention.As shown in the figure, power converter is used to provide output voltage V ₀To a load 51.One control circuit 100, it receives one and switches signal V _WAnd produce one and drive signal S _LTo control a switch 21.Switch 21 is coupled to an inductance 31 and an earth terminal, is used for when the discharging current of inductance 31 exists, and provides inductance 31 1 low impedance current path, and the effect of switch 21 is as a synchronous rectifier.One electric capacity 41, it couples inductance 31.One input V of control circuit 100 _IReceive and switch signal V _WOne formula (program) the end V of control circuit 100 _PCouple a resistance 70, to adjust a controlling signal V _S, and the discharge time of prediction inductance 31 and generation drive signal S _L, controlling signal V _SCan be according to the output voltage V of power converter ₀And adjust.Above-mentioned resistance 70 is coupled to earth terminal.

When switching signal V _WDuring conducting, a charging current can flow into inductance 31, therefore switches signal V _WON time T _ONPromptly represent the charging interval of inductance 31.Charging current is associated with switches signal V _WVoltage, output voltage V ₀, inductance 31 inductance value with switch signal V _WON time T _ONIn case switch signal V _WEnd, a discharging current will flow out from inductance 31.Output voltage V ₀, the inductance value of inductance 31 and charging current intensity be decision T discharge time _OFFIn continuous current pattern CCM running, switch signal V _WJust conducting before discharging current is discharged to zero.In discontinuous current-mode running, the discharging current of inductance 31 just has been discharged to zero before next switches the circulation beginning.The boundary condition of continuous current pattern and the running of discontinuous current-mode can be expressed as: $\frac{V_{\mathrm{IN}}-V_O}{L}\×T_{\mathrm{ON}}=\frac{V_O}{L}\×(T-T_{\mathrm{ON}})-------------------------\left(1\right)$ Wherein, V _INBe to switch signal V _WVoltage, L is the inductance value of inductance 31, T switches signal V _WSwitching time.

T discharge time of inductance 31 _OFFCan try to achieve according to equation (1), wherein T _OFF=(T-T _ON), the equation of following expression (2) and (3).

V _IN×T _ON-V _O×T _ON＝V _O×T _OFF-------------------------(2)

$T_{\mathrm{OFF}}=\frac{V_{\mathrm{IN}}-V_O}{V_O}\×T_{\mathrm{ON}}----------------------------------\left(3\right)$

From the above, discharge time T _OFFCan be by switching signal V _WVoltage V _IN, output voltage V ₀And switching signal V _WON time T _ONAnd prediction is learnt.

See also Fig. 7, it is a preferred embodiment of control circuit 100 of the present invention.As shown in the figure, a bleeder circuit, it comprises the plural resistance 101,102 of series connection.Bleeder circuit is coupled to input V _I, switch signal V to receive _WOne switch 120, it is coupled to a contact of resistance 101,102, switches signal V with sampling _WVoltage V _INTo an electric capacity 110, electric capacity 110 promptly produces an input signal V _C, and with input signal V _CBe sent to an input circuit 200.One constant current source 105, it is coupled to the formula end V of control circuit 100 _PWith a supply voltage V _CCConstant current source 105 is associated with resistance 70, to produce controlling signal V _SAnd be sent to input circuit 200.

One prediction circuit 150, it comprises an input circuit 200 and a timing circuit 300.Prediction circuit 150 is according to input signal V _C, controlling signal V _SAnd switching signal V _WProduce a sequential signal S _OFFSequential signal S _OOFRepresent the discharge time of the inductance 31 of power converter.Input signal V _CBe associated with and switch signal V _WVoltage V _INIn addition, timing circuit 300 more be coupled to one with a door input of 180, in order to transmit sequential signal S _OFFExtremely with door 180.Couple an inverter 160 to receive switching signal V with another input of door 180 _W, with the output output driving signal S of door 180 _LAnd be used for cutoff switch 21, to avoid resulting from switch 21 in underload and next reverse current of non-loaded situation.

Input circuit 200, it is according to input signal V _CWith controlling signal V _SProduce a charging signal I _CWith a discharge signal I _D, timing circuit 300 is according to charging signal I afterwards _C, discharge signal I _DAnd switching signal V _WProduce sequential signal S _OFFAs shown in Figure 9, timing circuit 300 is according to charging signal I _CWith switching signal V _WProduce a charging voltage V _H, charging voltage V _HWith discharge signal I _DBe associated, in case switch signal V _WProduce sequential signal S when ending _OFF

See also Fig. 8, it is the circuit diagram of input circuit 200, and as shown in the figure, input circuit 200 comprises one first voltage to current converter circuit 210, and it is according to input signal V _CProduce an input current signal I _AFirst voltage comprises an operational amplifier 211, a resistance R to current converter circuit 210 _CAnd plural electric crystal 212,213,214.The positive input terminal of operational amplifier 211 receives input signal V _C, negative input end then couples the source electrode of electric crystal 212, and the output of operational amplifier 211 couples the base stage of electric crystal 212 in addition.Resistance R _C, it is coupled between the source electrode and earth terminal of electric crystal 212.The source electrode of electric crystal 213,214 connects and is coupled to a supply voltage V _CCThe base stage of electric crystal 213,214 and the drain electrode of electric crystal 213 are coupled in together, and the drain electrode of electric crystal 213 more is coupled to the drain electrode of electric crystal 212, and the drain electrode of electric crystal 214 produces this input current signal I _A

One second voltage is to current converter circuit 230, and it is according to controlling signal V _SProduce a Control current signal I _SSecond voltage comprises an operational amplifier 231, a resistance R to current converter circuit 230 _SAnd plural electric crystal 232,233,234.The positive input terminal of operational amplifier 231 receives controlling signal V _S, negative input end then couples the source electrode of electric crystal 232, and output then couples the base stage of electric crystal 232.Resistance R _S, it is coupled between the source electrode and earth terminal of electric crystal 232.The source electrode of electric crystal 233,234 is coupled to supply voltage V _CCThe base stage of electric crystal 233,234 and the drain electrode of electric crystal 233 are coupled in together, and the drain electrode of electric crystal 233 more is coupled to the drain electrode of electric crystal 232, and the drain electrode of electric crystal 234 produces Control current signal I _S

Power plural current mirror (current mirror), it comprises plural electric crystal 251,252,253.These current mirrors are according to input current signal I _AWith Control current signal I _SProduce charging signal I _CWith discharge signal I _DOne first current mirror, it comprises that electric crystal 251,252, the first current mirrors are according to Control current signal I _SProduce a current signal I _B, input current signal I _AWith current signal I _BDifference be charging signal I _CThe source electrode of electric crystal 251,252 all is coupled to earth terminal, and the base stage of electric crystal 251,252 and the drain electrode of electric crystal 251 are coupled in together, and the drain electrode of electric crystal 251 more is coupled to the drain electrode of electric crystal 234, to receive Control current signal I _SThe drain electrode of electric crystal 252 produces current signal I _B, the drain electrode of electric crystal 252 more is coupled to the drain electrode of electric crystal 214, to produce charging signal I _COne second current mirror, it comprises that electric crystal 251,253, the second current mirrors are according to Control current signal I _SProduce discharge signal I _DThe source electrode of electric crystal 253 is coupled to earth terminal, and the base stage of electric crystal 251,253 is coupled in together, and the drain electrode of electric crystal 253 produces discharge signal I _D

From the above, charging signal I _CBe by input signal V _C, controlling signal V _SAnd resistance R _CAnd R _SAnd determine, can be expressed as:

$I_C=\frac{V_C}{R_C}-\frac{V_S}{R_S}---------------------------------------\left(4\right)$

And, discharge signal I _DBe by controlling signal V _SAnd resistance R _SDecision can be expressed as:

$I_D=\frac{V_S}{R_S}-----------------------------------------\left(5\right)$

See also Fig. 9, it is the circuit diagram of timing circuit 300.As shown in the figure, timing circuit 300 comprises a capacitor C, is used to produce charging voltage V _HOne charge switch 310, it is coupled to charging signal I _CAnd between the capacitor C, with by charging signal I _CCapacitor C is charged.The conducting of charge switch 310 is by switching signal V with ending _WControl, so timing circuit 300 is according to charging signal I _CWith switching signal V _WON time produce charging voltage V _HOne discharge switch 320, it is coupled to discharge signal I _DAnd between the capacitor C, with by discharge signal I _DCapacitor C is discharged, the conducting of discharge switch 320 with by by sequential signal S _OFFDecision.

One comparator 350, it is coupled to capacitor C, to produce sequential signal S through one with door 352 _OFF, sequential signal S _OFFThe discharge time of the inductance 31 of expression power converter.The positive input terminal of comparator 350 is coupled to capacitor C to receive charging voltage V _H, the negative input end of comparator 350 receives a limit voltage V _ZThe output of comparator 350 is coupled to and a door input of 352, then sees through an inverter 351 with another input of door 352 and is coupled to and switches signal V _WInverter 351, it is coupled to and switches signal V _WAnd and the input of door 352 between.Produce sequential signal S with the output of door 352 _OFFTherefore, sequential signal S _OFFAccording to switching signal V _WCut-off state and conducting.In addition, comparator 350 compares charging voltage V _HWith limit voltage V _ZAnd by sequential signal S _OFFCharging voltage V _HCan be expressed as:

$V_H=\frac{I_C}{C}\×T_{\mathrm{ON}}=\frac{\frac{V_C}{R_C}-\frac{V_S}{R_S}}{C}\×T_{\mathrm{ON}}------------------------\left(6\right)$

If resistance R _CAnd resistance R _SResistance value be R, then equation (6) can be rewritten as follows:

$V_H=\frac{V_C-V_S}{R\×C}\×T_{\mathrm{ON}}----------------------------------\left(7\right)$

And, T discharge time of capacitor C _OFFCan be expressed as:

$T_{\mathrm{OFF}}=\frac{C\×V_H}{I_D}=\frac{C\×V_H}{\frac{V_S}{R}}-------------------------------\left(8\right)$

According to equation (7) and (8), T discharge time of capacitor C _OFFCan be designed to T discharge time as inductance 31 _OFF, shown in the following equation:

$T_{\mathrm{OFF}}=\frac{V_C-V_S}{V_S}\×T_{\mathrm{ON}}--------------------------------\left(9\right)$

If VC equals α * V _IN, V _SEqual β * V ₀, and α equals β, then equation (9) can be expressed as:

$T_{\mathrm{OFF}}=\frac{\mathrm{\α}\×V_{\mathrm{IN}}-\mathrm{\β}\×V_O}{\mathrm{\β}\×V_O}\×T_{\mathrm{ON}}=\frac{V_{\mathrm{IN}}-V_O}{V_O}\×T_{\mathrm{ON}}----------------\left(10\right)$

Wherein, α is a constant and being determined by the ratio of resistance 101,102, and β also is a constant and being determined by the current mirror ratio of electric crystal 251,252.

Because of sequential signal S _OFFSo the discharge time of the inductance 31 of expression power converter is sequential signal S _OFFCan be shown in equation (10), along with switching signal V _WVoltage V _INIncrease and increase, and can be along with switching signal V _WON time T _ONReduce and shorten.Controlling signal V _SCan be set, just controlling signal V _SBe the signal of programmed (programmable), it can be set as according to output voltage V ₀, with T discharge time of prediction inductance 31 _OFF, so switch 21 can be cut off, and produces to prevent reverse current.

The above, it only is a preferred embodiment of the present invention, be not to be used for limiting scope of the invention process, all equalizations of doing according to the described shape of claim of the present invention, structure, feature and spirit change and modify, and all should be included in the interest field of the present invention.

Claims (12)

1. A control circuit for reducing the reverse current of a synchronous rectifier, characterized in As, it contains: a prediction circuit, generating a timing signal according to a switching signal and a control signal, and shutting down the synchronous rectifier of a power converter according to the timing signal, the timing signal indicating the discharge time of an inductor of the power converter; Wherein, the control signal is related to an output voltage of the power converter, and the conduction time of the switching signal is related to the charging time of the inductor. 2. The control circuit according to claim 1, wherein the on-time of the timing signal is shortened as the on-time of the switching signal decreases, and increases as the voltage of the switching signal increases . 3. The control circuit according to claim 1, wherein the predictive circuit comprises: An input circuit for generating a charging signal and a discharging signal according to an input signal and the control signal, the input signal being related to the voltage of the switching signal; a timing circuit, coupled to the input circuit and generating the timing signal according to the conduction time of the charging signal, the discharging signal and the switching signal; Wherein, the timing circuit generates a charging voltage according to the conduction time of the charging signal and the switching signal, and generates the timing signal according to the charging voltage and the discharging signal when the switching signal is off. 4. The control circuit according to claim 3, wherein the input circuit comprises: a first voltage-to-current conversion circuit, generating an input current signal according to the input signal; a second voltage-to-current conversion circuit, generating a control current signal according to the control signal; A plurality of current mirrors are coupled to the first voltage-to-current conversion circuit and the second voltage-to-current conversion circuit, and generate the charging signal and the discharging signal according to the input current signal and the control current signal. 5. The control circuit according to claim 3, wherein the timing circuit comprises: a capacitor for generating the charging voltage; A charging switch, coupled between the charging signal and the capacitor, the charging signal charges the capacitor, the charging switch is turned on and off controlled by the switching signal; A discharge switch, coupled between the discharge signal and the capacitor, the discharge signal discharges the capacitor, and the discharge switch is turned on and off controlled by the timing signal; A comparator is coupled to the capacitor to generate the timing signal, and the comparator compares the charging voltage with a critical value to cut off the timing signal. 6. The control circuit as claimed in claim 1, wherein the control signal is a programmable signal. 7. A control circuit for reducing the reverse current of a synchronous rectifier, characterized in that it comprises: a predictive circuit generating a timing signal to turn off the synchronous rectifier of a power converter according to a switching signal and a control signal related to the output voltage; Wherein, the conduction time of the switching signal is related to the charging time of an inductor of the power converter. 8. The control circuit as claimed in claim 7, wherein the on-time of the timing signal is shortened as the on-time of the switching signal decreases, and increases as the voltage of the switching signal increases. 9. The control circuit according to claim 7, characterized in that, the prediction circuit comprises: An input circuit generates a charging signal and a discharging signal according to an input signal and a control signal, the input signal is related to the voltage of the switching signal, and the control signal is related to an output voltage of the power converter; a timing circuit, coupled to the input circuit and generating the timing signal according to the charging signal, the discharging signal and the switching signal; Wherein, the timing circuit generates a charging voltage according to the conduction time of the charging signal and the switching signal, and generates the timing signal according to the charging voltage and the discharging signal when the switching signal is cut off. 10. The control circuit as claimed in claim 9, wherein the control signal is a programmable signal. 11. The control circuit according to claim 9, wherein the input circuit comprises: a first voltage-to-current conversion circuit, generating an input current signal according to the input signal; a second voltage-to-current conversion circuit, generating a control current signal according to the control signal; A plurality of current mirrors are coupled to the first voltage-to-current conversion circuit and the second voltage-to-current conversion circuit, and generate the charging signal and the discharging signal according to the input current signal and the control current signal. 12. The control circuit according to claim 9, wherein the timing circuit comprises: a capacitor for generating the charging voltage; a charging switch, coupled between the charging signal and the capacitor, the charging signal charges the capacitor, and the charging switch is turned on and off controlled by the switching signal; A discharge switch, coupled between the discharge signal and the capacitor, the discharge signal discharges the capacitor, and the discharge switch is turned on and off controlled by the timing signal; A comparator is coupled to the capacitor to generate the timing signal, and the comparator compares the charging voltage with a critical value to cut off the timing signal.

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