CN111342659A

CN111342659A - Light-load mode decision circuit, decision method, primary side control circuit and isolated converter

Info

Publication number: CN111342659A
Application number: CN202010300826.5A
Authority: CN
Inventors: 许超群; 林官秋; 文鹏
Original assignee: Xiamen Biyi Micro Electronic Technique Co ltd
Current assignee: Xiamen Biyi Micro Electronic Technique Co ltd
Priority date: 2020-04-16
Filing date: 2020-04-16
Publication date: 2020-06-26

Abstract

The invention provides a light load mode decision circuit, a decision method, a primary side control circuit and an isolation converter. The light load mode decision circuit comprises a comparison circuit, a statistic circuit and a decision circuit. The comparison circuit is used for comparing the state detection signal with at least one detection reference signal and outputting a comparison result. The statistic circuit is coupled with the comparison circuit and used for carrying out statistics according to the comparison result within the set time and outputting the statistic result. The decision circuit is coupled to the statistic circuit and used for comparing the statistic result with the decision reference signal so as to determine whether the isolated converter enters a light-load mode. The invention can effectively solve the problem of load depth of the isolated converter under the condition of light load, can realize the discrimination of the light load mode under the condition of load depth, can save pins of a control chip and large off-chip capacitance, and can effectively reduce the area of the control chip.

Description

Light-load mode decision circuit, decision method, primary side control circuit and isolated converter

Technical Field

The invention belongs to the electronic field, relates to the technical field of switching power supplies, and particularly relates to a light-load mode decision circuit, a decision method, a primary side control circuit and an isolation converter.

Background

The isolated converter is widely applied to the field of switching power supplies due to the advantages of strong anti-jamming capability, high safety, easiness in realizing buck-boost conversion, easiness in realizing multi-path output and the like. The isolated converter generally includes a rectifying circuit, a primary side circuit, a secondary side circuit, and the like. The isolated converter can be safely and stably output through the accurate control of the original secondary side control circuit.

Fig. 1 shows an isolated converter using primary side control in the prior art. The output voltage of the circuit board terminal of the isolated converter is V1, i.e. the voltage across the capacitor C4. The cable terminal output voltage of the isolated converter is V2, the voltage across the load resistor R2. The circuit board terminal of isolated converter is coupled through the output cable with the load, and the resistance of output cable is Rcable, therefore has the load depth. In the primary side controlled isolated converter, the output voltage V1 of the circuit board terminal can be kept constant by adjusting the pulse width or the pulse frequency of the control signal of the primary side switching tube Q1. If the board terminal output voltage V1 is set to be constant, the voltage drop across the output cable will change as the load resistance changes when different loads are used.

In order to realize the constant output voltage V2 of the cable terminal, as shown in fig. 2, the prior art adopts an output cable voltage drop compensation scheme, and by collecting a current median value or a current peak value flowing through a primary side switching tube, converting the current median value or the current peak value into a stable voltage signal or a stable current signal through a low-pass filter, and processing the signal through a voltage-current conversion circuit and outputting the signal to an FB pin of a control chip, an accurate signal representing the depth of a load can be obtained, so that the accurate control of the output voltage of the cable terminal is realized. In theory, the electrical signal output by the low-pass filter can be directly compared with the reference voltage, and the output signal of the comparator can be used as a light-load mode indicating signal. However, this solution has the disadvantage that under light load conditions, the bandwidth of the low-pass filter to be used is narrow, and therefore the filter constant of the low-pass filter is very large. Meanwhile, if a filter resistor with a large resistance value is selected, the noise interference inside the control chip is easy to be caused, which means that the capacitance value of the filter capacitor of the low-pass filter adopted by the scheme needs to be large. As shown in fig. 1, the control chip of the isolated converter is provided with a CPC pin, which is used to couple with an external filter capacitor. The technical problem in the prior art is that the area of a low-pass filter integrated on a control chip is large, so that the cost is high, and meanwhile, pins of the control chip are increased by adopting the arrangement of an on-chip capacitor.

Disclosure of Invention

In order to solve at least part of problems, the invention provides a light load mode judging circuit, a judging method, a primary side control circuit and an isolated converter, and the problem of load depth of the isolated converter under the light load condition is effectively solved.

The invention discloses a light load mode decision circuit for an isolated converter, which comprises:

a comparison circuit for comparing the state detection signal with at least one detection reference signal and outputting a comparison result;

the statistic circuit is coupled with the comparison circuit and used for carrying out statistics according to the comparison result within set time and outputting a statistic result; and

and the judging circuit is coupled with the counting circuit and used for comparing the counting result with a judging reference signal so as to judge whether the isolated converter enters a light load mode or not.

In an embodiment of the present invention, the statistical circuit includes: the input end of the charging and discharging circuit is coupled with the comparison circuit, the output end of the charging and discharging circuit is coupled with the judgment circuit, and the charging and discharging circuit is used for counting according to the comparison result within set time and outputting a counting result; when the comparison result is in a first comparison state, the charging and discharging circuit carries out a charging process; and when the comparison result is in a second comparison state, the charging and discharging circuit carries out a discharging process.

In an embodiment of the present invention, the at least one detection reference signal includes a first detection reference signal and a second detection reference signal; the statistical circuit further comprises a weighting circuit; when the state detection signal is greater than the first detection reference signal and less than the second detection reference signal, the statistical circuit performs weighted statistics on the comparison result by using a first weighted value; when the state detection signal is larger than a second detection reference signal, the statistical circuit carries out weighted statistics on the comparison result by a second weighted value; the second detection reference signal is greater than the first detection reference signal; the second weighted value is greater than the first weighted value.

In an embodiment of the present invention, the charging and discharging circuit includes:

the input end of the charging circuit is coupled with the comparison circuit, and the output end of the charging circuit is coupled with the judgment circuit and used for carrying out a charging process when the comparison result is in a first comparison state; and

and the input end of the discharging circuit is coupled with the comparison circuit, and the output end of the discharging circuit is coupled with the judgment circuit, and the discharging circuit is used for performing a discharging process when the comparison result is in a second comparison state.

In an embodiment of the present invention, the charging circuit includes a first current source, a first switch, and a first capacitor; the discharge circuit comprises a second current source, a second switch and a first capacitor;

a first current source for providing a charging current;

a first switch, a first end of which is coupled to the output end of the first current source, and a switch control end of which is coupled to the comparison circuit, for controlling the conducting state of the charging circuit;

a first capacitor, a first end of which is coupled to the second end of the first switch and the input end of the decision circuit, respectively, and a second end of which is coupled to ground;

a first terminal of the second switch is coupled to the first terminal of the first capacitor, a second terminal of the second switch is coupled to the second terminal of the first capacitor, and a switch control terminal of the second switch is coupled to the comparison circuit for controlling a conducting state of the discharge circuit; and

the second current source is coupled between the second switch and the first capacitor and used for providing discharge current.

In an embodiment of the invention, when the comparison result is the first comparison state, the first current source outputs the charging current positively correlated to the state detection signal.

In an embodiment of the present invention, the counting circuit includes a counting circuit for counting according to the comparison result within a set time; when the comparison result is in a first comparison state, the counting circuit performs incremental calculation for one time; when the comparison result is in a second comparison state, the counting circuit performs one-time decrement calculation.

In an embodiment of the present invention, the light load mode decision circuit further includes:

and the output end of the state detection signal acquisition circuit is coupled with the comparison circuit and is used for acquiring the state detection signal.

and the input end of the primary side switch driving circuit is coupled with the judgment circuit and is used for generating a driving signal to control the switching state of the primary side switch.

The invention discloses a primary side control circuit for an isolated converter, which is used for controlling the switching state of a primary side switch and comprises the light load mode judgment circuit.

The invention discloses an isolated converter, which comprises a primary side circuit and a secondary side circuit, wherein the primary side circuit receives input voltage and comprises a primary side winding, a primary side switch and the primary side control circuit; the secondary circuit comprises a secondary winding, and the primary winding and the secondary winding are coupled to form the transformer.

In an embodiment of the present invention, when the light-load mode decision circuit determines that the isolated converter enters the light-load mode, the primary side control circuit controls to reduce the working current of the primary side control circuit.

The invention discloses a light-load mode judgment method for an isolated converter, which comprises the following steps:

comparing the state detection signal with at least one detection reference signal and outputting a comparison result;

counting according to the comparison result within a set time, and outputting a counting result; and

and comparing the statistical result with a decision reference signal to judge whether the isolated converter enters a light load mode or not.

In an embodiment of the present invention, the step of performing statistics according to the comparison result within a set time and outputting a statistical result includes:

performing charging and discharging processes according to the comparison result within a set time so as to perform statistics, and outputting a statistical result; when the comparison result is in a first comparison state, the charging and discharging circuit carries out a charging process; and when the comparison result is in a second comparison state, the charging and discharging circuit carries out a discharging process.

the at least one detection reference signal comprises a first detection reference signal and a second detection reference signal; when the state detection signal is greater than the first detection reference signal and less than the second detection reference signal, carrying out weighted statistics on the comparison result by using a first weighted value; when the state detection signal is greater than a second detection reference signal, carrying out weighted statistics on the comparison result by using a second weighted value; the second detection reference signal is greater than the first detection reference signal; the second weighted value is greater than the first weighted value.

when the comparison result is in a first comparison state, the counting circuit performs incremental calculation for one time; when the comparison result is in a second comparison state, the counting circuit performs one-time decrement calculation.

The invention provides a light load mode decision circuit, a decision method, a primary side control circuit and an isolation converter. The light load mode decision circuit comprises a comparison circuit, a statistic circuit and a decision circuit. The invention realizes filtering by adopting a light-load mode decision circuit, thereby effectively solving the problem of load depth of the isolated converter under the light-load condition. In addition, more specifically, the filtering is implemented in the form of discharging a small capacitor on a control chip or adopting an up-down counter. The invention can effectively save the control chip pins and the off-chip large capacitance, and the area of the control chip can be effectively reduced.

Drawings

Fig. 1 shows a circuit schematic of a prior art primary side controlled isolated converter.

Fig. 2 shows a schematic diagram of an output cable drop compensation scheme of a prior art isolated converter.

Fig. 3 shows a circuit schematic of a light load mode decision circuit according to an embodiment of the invention.

Fig. 4 shows a circuit schematic of a light load mode decision circuit according to an embodiment of the invention.

Fig. 5 shows a circuit schematic of a light load mode decision circuit according to an embodiment of the invention.

Fig. 6 shows a circuit schematic of a light load mode decision circuit according to an embodiment of the invention.

Fig. 7 shows a circuit schematic of a light load mode decision circuit according to an embodiment of the invention.

Fig. 8 is a flowchart illustrating a light load mode decision method according to an embodiment of the present invention.

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

For a further understanding of the invention, reference will now be made to the preferred embodiments of the invention by way of example, and it is to be understood that the description is intended to further illustrate features and advantages of the invention, and not to limit the scope of the claims.

The description in this section is for several exemplary embodiments only, and the present invention is not limited only to the scope of the embodiments described. It is within the scope of the present disclosure and protection that the same or similar prior art means and some features of the embodiments may be interchanged.

The term "coupled" or "connected" in this specification includes both direct and indirect connections. An indirect connection is a connection made through an intermediate medium, such as a connection made through an electrically conductive medium, which may have parasitic inductance or parasitic capacitance; indirect connections may also include connections through other active or passive devices, such as connections through switches, follower circuits, etc., that serve the same or similar functional purpose.

The primary side controlled isolated converter comprises a primary side circuit and a secondary side circuit, wherein the primary side circuit receives input voltage, and the secondary side circuit provides output voltage or output current. The primary circuit may include a primary winding, a primary switch, and a primary control circuit. The secondary circuit can comprise a secondary winding and a rectifier tube, and the primary winding and the secondary winding are coupled to form a transformer. The rectifier tube can be a diode or a synchronous rectifier tube. The switching state of the primary side switch is controlled by the primary side control circuit, so that the isolated converter can keep stable output. The output average current of the primary side control circuit in the DCM (namely discontinuous mode) satisfies the following relation:

in addition, the output average current of the primary side control circuit in the CCM mode (namely, the continuous mode) satisfies the following relation:

the method comprises the following steps of obtaining an Io _ DCM, an Io _ CCM and a Vcm, wherein the Io _ DCM is an output average current in a DCM mode, the Io _ CCM is an output average current in a CCM mode, Nps is a turn ratio of an original side and a secondary side of a transformer, Ipk is a CS peak current value in the DCM mode, and Vcs _ pk is a peak voltage corresponding to the CS peak current value in the DCM mode; imid is the CS median current value in the CCM mode, and Vcs _ mid is the median voltage corresponding to the CS median current value in the CCM mode. Tsw is the switching period of the primary side switch, Ton is the conduction time of the primary side switch, Tdem is the degaussing time, and Toff is the dead time. VREF _ TRIM1 is the first equivalent voltage, VREF _ TRIM2 is the second equivalent voltage, Rcs is the sampling resistor.

Based on the above formula, it can be known that the output average current can be accurately obtained by constructing a circuit structure or an implementation method that can satisfy the above relation. The invention aims at the problem of load depth under light load condition, and Ipk or Imid is processed by a control loop in a primary side control circuit through peak reduction current under the light load condition, so that the Ipk or Imid is controlled at a fixed low value. Meanwhile, under the condition of light load, the values of the conducting time Ton and the degaussing time Tdem of the primary side switch are very low, and the Ton and the Tdem are kept unchanged under the condition that the turn ratio Nps of the transformer is fixed. Therefore, as can be seen from the above equations (1) and (2), the light load mode decision at the load depth can be simplified to calculate the magnitude of Tsw, where Tsw is Ton + Tdem + Toff. In the light load mode, the conducting time Ton and the degaussing time Tdem of the primary side switch are far less than Toff, so that the dead time Toff can be calculated simply when the light load mode is judged.

As shown in fig. 3, an embodiment of the present invention provides a light-load mode decision circuit for an isolated converter, where the light-load mode decision circuit 12 includes a comparison circuit 121, a statistic circuit 122, and a decision circuit 123. The comparison circuit 121 is configured to compare the state detection signal with at least one detection reference signal and output a comparison result. The input terminal of the statistic circuit 122 is coupled to the output terminal of the comparison circuit 121, and the statistic circuit 122 is configured to perform statistics according to the comparison result within a set time and output the statistic result. The set time is a statistical time period selected by the statistical circuit 122 for the statistical comparison result required by the light load mode decision circuit. Specifically, the statistical circuit may be at least one of a series of circuits such as a charge/discharge circuit and a counting circuit that satisfy the above-described requirements and perform statistical calculation or a circuit equivalent to the statistical calculation. The input terminal of the decision circuit 123 is coupled to the statistic circuit 122, and the decision circuit 123 is configured to compare the statistic result with a decision reference signal to determine whether the isolated converter enters the light-load mode. The invention realizes filtering by adopting the light-load mode decision circuit, thereby effectively solving the problem of load depth of the isolated converter under the condition of light load, effectively saving pins of a control chip and large off-chip capacitance, and simultaneously effectively reducing the area of the control chip.

In an embodiment of the present invention, the state detection signal may be a switching period Tsw of the primary side switch, the state detection signal may be a dead time Toff, and the state detection signal may also be a switching frequency f of the primary side switch. When the detection reference signal is the first detection reference signal, the comparison result may be a comparison result characterization signal corresponding to two cases, that is, the state detection signal is greater than the first detection reference signal, and the state detection signal is not greater than the first detection reference signal. When the detection reference signal is the first detection reference signal and the second detection reference signal, the comparison result may be a comparison result characterization signal corresponding to three cases, that is, the state detection signal is smaller than the first detection reference signal, the state detection signal is between the first detection reference signal and the second detection reference signal, and the state detection signal is greater than the second detection reference signal, wherein the second detection reference signal is greater than the first detection reference signal. In addition, in an embodiment of the present invention, the state detection signal is a switching period Tsw or a dead time Toff of the primary switch, and when the statistical result is greater than the decision reference signal, it is determined that the isolated converter has entered the light load mode. In another embodiment of the present invention, the state detection signal is a switching frequency f of the primary side switch, and when the statistical result is smaller than the decision reference signal, it is determined that the isolated converter has entered the light load mode. Illustratively, the state detection signal is a dead time Toff, the detection reference signal is a first detection reference signal, the first detection reference signal is specifically 500us, and when Toff >500us within a set time, a charging process is performed or an incremental calculation (such as a statistical value +1 processing) is performed; when Toff is less than 500us, a discharge process is performed or a decrement calculation (e.g., statistic-1 processing) is performed. The statistic circuit 122 performs statistics according to the comparison result within a set time, and outputs the statistic result. And when the statistical result is greater than the judgment reference signal, the judgment circuit judges that the isolated converter enters a light load mode, otherwise, the isolated converter does not enter the light load mode. The invention can effectively prevent the isolated converter from entering or exiting the light-load mode by single false triggering, avoid the frequent switching between the two states of entering the light-load mode and not entering the light-load mode, and ensure the stable work of the isolated converter.

In an embodiment of the present invention, the light load mode decision circuit further includes a state detection signal obtaining circuit, an output end of the state detection signal obtaining circuit is coupled to the comparison circuit, and the state detection signal obtaining circuit is configured to obtain the state detection signal. In one embodiment shown in fig. 3, the state detection signal obtaining circuit is specifically a dead time Toff obtaining circuit 11, an output terminal of the dead time Toff obtaining circuit 11 is coupled to an input terminal of the comparing circuit 121, and the dead time Toff obtaining circuit 11 is configured to obtain the dead time Toff.

In an embodiment of the present invention, as shown in fig. 3, the light load mode decision circuit further includes a primary side switch driving circuit 13, an input terminal of the primary side switch driving circuit 13 is coupled to an output terminal of the decision circuit 123, and the primary side switch driving circuit 13 is configured to generate a driving signal to control a switching state of the primary side switch. In an embodiment of the present invention, when the decision circuit 123 determines that the light load mode has been entered, the standby power consumption of the primary side control circuit is reduced. When entering the light load mode, the decision circuit 123 generates a decision signal. The decision signal is passed through the primary side switch drive circuit 13 to reduce the switching frequency of the primary side switch. When the load is in a light load state, the primary side switch is controlled to be turned off through the primary side switch driving circuit 13, the primary side circuit waits for the communication pulse sent by the secondary side circuit, and when the communication pulse of the secondary side circuit is not received, the primary side switch keeps in a turn-off state. When the primary side control circuit enters a sleep (light load) mode, only the communication pulse detection circuit and the most basic reference voltage and current are reserved, so that the primary side control circuit can achieve ultralow standby power consumption.

In an embodiment of the present invention, as shown in fig. 4, the light load mode decision circuit 22 includes a comparison circuit 221, a charge and discharge circuit 222, and a decision circuit 223. The comparison circuit 221 is configured to compare the state detection signal with at least one detection reference signal and output a comparison result. The input terminal of the charging and discharging circuit 222 is coupled to the output terminal of the comparing circuit 221, and the charging and discharging circuit 222 is configured to perform a charging and discharging process according to the comparison result within a set time period to perform statistics, and output a statistical result. When the comparison result is in a first comparison state, the charging and discharging circuit carries out a charging process; and when the comparison result is in a second comparison state, the charging and discharging circuit carries out a discharging process. The input terminal of the decision circuit 223 is coupled to the charging and discharging circuit 222, and the decision circuit 223 is configured to compare the statistical result with the decision reference signal to determine whether the isolated converter enters the light-load mode.

In an embodiment of the invention, when the comparison result is the first comparison state, the first current source outputs the charging current positively correlated to the state detection signal. In an embodiment, the at least one detection reference signal is a first detection reference signal, and when the state detection signal is greater than the first detection reference signal, the magnitude of the charging current output by the first current source is in a positive correlation with the magnitude of the state detection signal. In another embodiment, the at least one detection reference signal includes a first detection reference signal and a second detection reference signal, and when the state detection signal is greater than the first detection reference signal, the magnitude of the charging current output by the first current source is in a positive correlation with the magnitude of the state detection signal. Wherein the second detection reference signal is greater than the first detection reference signal.

In an embodiment of the present invention, as shown in fig. 5, the light load mode decision circuit 22 includes a comparison circuit 221, a charge and discharge circuit 222, and a decision circuit 223. The charging and discharging circuit 222 includes a charging circuit 2222 and a discharging circuit 2223. The input terminal of the charging circuit 2222 is coupled to the comparing circuit 221, the output terminal of the charging circuit 2222 is coupled to the input terminal of the determining circuit 223, and the charging circuit 2222 is configured to perform a charging process when the comparison result is the first comparison state. The input terminal of the discharging circuit 2223 is coupled to the comparing circuit 221, the output terminal of the discharging circuit 2223 is coupled to the input terminal of the determining circuit 223, and the discharging circuit 2223 is configured to perform a discharging process when the comparison result is the second comparison state.

In an embodiment of the invention, as shown in fig. 5, the charging and discharging circuit 222 further includes a weighting circuit 2221, an input terminal of the weighting circuit 2221 is coupled to an output terminal of the comparing circuit 221, and output terminals of the weighting circuit 2221 are coupled to the charging circuit 2222 and the discharging circuit 2223, respectively. The at least one detection reference signal specifically includes a first detection reference signal and a second detection reference signal. When the state detection signal is greater than the first detection reference signal and less than the second detection reference signal, the weighting circuit 2221 performs weighted statistics on the comparison result by using the first weighted value; when the state detection signal is greater than the second detection reference signal, the weighting circuit 2221 performs weighted statistics on the comparison result by a second weighted value. The second detection reference signal is greater than the first detection reference signal, and the second weighting value is greater than the first weighting value. Illustratively, the state detection signal is a dead time Toff, and the at least one detection reference signal includes a first detection reference signal, specifically 500us, and a second detection reference signal, specifically 1000 us. There are three comparison results: toff >1000us, 500us < Toff <1000us and Toff <500 us. Within a set time, when Toff is greater than 1000us, performing a charging process by using a second weighted value; when 500us < Toff <1000us, performing a charging process with a first weighted value; when Toff <500us, the discharge process is performed with a first weighted value. The amount of charge during the charging process with the second weighted value is greater than the amount of charge during the charging process with the first weighted value. The weighting process can reflect the speed of entering an LL (light load) mode under different load conditions. When the load is very light load, the value of Toff is large, the current Toff is weighted, and the light load mode can be quickly judged, so that the isolated converter enters the light load mode, the working loss can be effectively reduced, and the standby power consumption of the primary side control circuit is greatly reduced.

In an embodiment of the present invention, as shown in fig. 6, the light load mode decision circuit 32 includes a comparison circuit 321, a charge and discharge circuit 322, and a decision circuit 323. The comparison circuit 321 is configured to compare the state detection signal with at least one detection reference signal and output a comparison result. The input terminal of the counting circuit 322 is coupled to the output terminal of the comparing circuit 321, and the counting circuit 322 is configured to count in a set time according to the comparison result and output the counting result. When the comparison result is in the first comparison state, the counting circuit performs one incremental calculation. When the comparison result is in a second comparison state, the counting circuit performs one-time decrement calculation. The input terminal of the decision circuit 323 is coupled to the counting circuit 322, and the decision circuit 323 is configured to compare the statistical result with a decision reference signal to determine whether the isolated converter enters the light-load mode. In another embodiment of the present invention, the light load mode decision circuit 32 further includes a Toff obtaining circuit 31 and a primary side switch driving circuit 33, the Toff obtaining circuit 31 is coupled to the input terminal of the comparing circuit 321, and the Toff obtaining circuit 31 is configured to obtain the dead time Toff. The primary side switch driving circuit 33 is coupled to the output terminal of the decision circuit 323, and the primary side switch driving circuit 33 is configured to generate a driving signal to control a switching state of the primary side switch.

In an embodiment of the present invention, the light load mode decision circuit 32 further includes a weighting circuit, the state detection signal is a dead time Toff, and the at least one detection reference signal includes a first detection reference signal and a second detection reference signal, the first detection reference signal is specifically 500us, and the second detection reference signal is specifically 1000 us. There are three comparison results: toff >1000us, 500us < Toff <1000us and Toff <500 us. Within a set time, when Toff is greater than 1000us, performing incremental calculation (such as statistics +2 processing) once by using a second weighted value; when 500us < Toff <1000us, performing an incremental calculation (such as statistics +1 processing) with a first weighting value; when Toff is less than 500us, a decrementing calculation is performed with a first weighted value (e.g., statistics-1 processing). The weighting process can reflect the speed of entering an LL (light load) mode under different load conditions. When the load is very light load, the value of Toff is large, the current Toff is weighted, and the light load mode can be quickly judged, so that the isolated converter enters a low-efficiency working state corresponding to the light load mode, and the standby power consumption of the primary side control circuit can be greatly reduced.

In an embodiment of the present invention, as shown in fig. 7, the light load mode decision circuit includes a comparison circuit, a charge and discharge circuit, and a decision circuit. The charging and discharging circuit comprises a charging circuit and a discharging circuit. The charging circuit comprises a first current source, a first switch K1 and a first capacitor Cjudge. The discharge circuit comprises a second current source, a second switch K2 and a first capacitor Cjudge. The first current source is used for providing a charging current Ichg. The first terminal of the first switch K1 is coupled to the output terminal of the first current source, the switch control terminal of the first switch K1 is coupled to the output terminal of the comparison circuit, and the first switch K1 is used for controlling the conducting state of the charging circuit. The first terminal of the first capacitor Cjudge is coupled to the second terminal of the first switch K1 and the input terminal of the decision circuit, respectively, and the second terminal of the first capacitor Cjudge is coupled to ground. A first terminal of the second switch K2 is coupled to the first terminal of the first capacitor Cjudge, a second terminal of the second switch K2 is coupled to the second terminal of the first capacitor Cjudge, a switch control terminal of the second switch K2 is coupled to the output terminal of the comparison circuit, and the second switch K2 is used for controlling the conducting state of the discharge circuit. A second current source is coupled between the second switch K2 and the first capacitor Cjudge for providing a discharge current Idischg. In an embodiment of the invention, the first detection reference signal may be set to 500us, and when the dead time Toff is greater than 500us, the charging and discharging circuit controls to turn on the first switch K1, so that the first current source charges the first capacitor. When the dead time Toff is less than 500us, the charging and discharging circuit controls to turn on the second switch K2, so that the second current source discharges the first capacitor. Preferably, when the dead time Toff is greater than 500us, the magnitude of the charging current output by the first current source is in a positive correlation with the magnitude of the dead time Toff. In addition, the judgment circuit compares the voltage Vjudge at the first end of the first capacitor Cjudge with a judgment reference signal Vref1, and when the voltage Vjudge is greater than the judgment reference signal Vref1, the light-load mode judgment circuit judges that the isolated converter enters a light-load mode, so that the primary side control circuit controls the switching state of the primary side switch, and the isolated converter enters a low-efficiency working state corresponding to the light-load mode. The capacitance value of the first capacitor Cjudge used by the invention is very small, and the primary side control circuit (namely the primary side chip) only occupies a very small area.

In an embodiment of the present invention, a primary side control circuit for an isolated converter is disclosed, which includes the light-load mode decision circuit as described above, and the primary side control circuit is configured to control a switching state of a primary side switch.

In an embodiment of the present invention, an isolated converter is disclosed, which includes a primary circuit and a secondary circuit, wherein the primary circuit receives an input voltage, and the primary circuit includes a primary winding, a primary switch, and the primary control circuit. The secondary circuit comprises a secondary winding, and the primary winding and the secondary winding are coupled to form the transformer.

In an embodiment of the present invention, when the light-load mode decision circuit determines that the isolated converter enters the light-load mode, the primary side control circuit controls to reduce the working current of the primary side control circuit, so that the isolated converter enters a low-efficiency working state corresponding to the light-load mode.

As shown in fig. 8, an embodiment of the present invention discloses a light-load mode decision method for an isolated converter, which includes the steps of:

s100, comparing the state detection signal with at least one detection reference signal and outputting a comparison result;

s200, counting according to the comparison result within a set time, and outputting a counting result; and

and S300, comparing the statistical result with the judgment reference signal to judge whether the isolated converter enters a light-load mode.

In an embodiment of the present invention, the step of performing statistics according to the comparison result within a set time and outputting a statistical result includes: performing a charging and discharging process according to the comparison result within a set time so as to perform statistics, and outputting a statistical result; when the comparison result is in a first comparison state, the charging and discharging circuit carries out a charging process; and when the comparison result is in a second comparison state, the charging and discharging circuit carries out a discharging process.

In an embodiment of the present invention, the step of performing statistics according to the comparison result within a set time and outputting a statistical result includes: the at least one detection reference signal comprises a first detection reference signal and a second detection reference signal; when the state detection signal is greater than the first detection reference signal and less than the second detection reference signal, carrying out weighted statistics on the comparison result by using a first weighted value; and when the state detection signal is greater than a second detection reference signal, carrying out weighted statistics on the comparison result by using a second weighted value. Wherein the second detection reference signal is greater than the first detection reference signal; the second weighting value is greater than the first weighting value.

The invention provides a light load mode decision circuit, a decision method, a primary side control circuit and an isolation converter. The light load mode decision circuit comprises a comparison circuit, a statistic circuit and a decision circuit. The invention realizes filtering by adopting a light-load mode decision circuit, thereby effectively solving the problem of load depth of the isolated converter under the light-load condition. Specifically, the filtering is realized by controlling the discharge of a small capacitor on a chip or by adopting an up-down counter mode. The invention can effectively save the control chip pins and the off-chip large capacitance, and the area of the control chip can be effectively reduced.

The above description and applications of the invention herein are illustrative and are not intended to limit the scope of the invention to the above described embodiments. The descriptions related to the effects or advantages mentioned in the embodiments may not be reflected in the experimental examples due to the uncertainty of the specific condition parameters, and are not used for limiting the embodiments. Variations and modifications of the embodiments disclosed herein are possible, and alternative and equivalent various components of the embodiments will be apparent to those skilled in the art. It will be clear to those skilled in the art that the present invention may be embodied in other forms, structures, arrangements, proportions, and with other components, materials, and parts, without departing from the spirit or essential characteristics thereof. Other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.

Claims

1. A light-load mode decision circuit for an isolated converter, the light-load mode decision circuit comprising:

2. The light load mode decision circuit of claim 1, wherein the statistics circuit comprises:

the input end of the charging and discharging circuit is coupled with the comparison circuit, the output end of the charging and discharging circuit is coupled with the judgment circuit, and the charging and discharging circuit is used for counting according to the comparison result within set time and outputting a counting result; when the comparison result is in a first comparison state, the charging and discharging circuit carries out a charging process; and when the comparison result is in a second comparison state, the charging and discharging circuit carries out a discharging process.

3. The light load mode decision circuit of claim 1, wherein the at least one detection reference signal comprises a first detection reference signal and a second detection reference signal; the statistical circuit further comprises a weighting circuit; when the state detection signal is greater than the first detection reference signal and less than the second detection reference signal, the statistical circuit performs weighted statistics on the comparison result by using a first weighted value; when the state detection signal is larger than a second detection reference signal, the statistical circuit carries out weighted statistics on the comparison result by a second weighted value; the second detection reference signal is greater than the first detection reference signal; the second weighted value is greater than the first weighted value.

4. The light load mode decision circuit of claim 2, wherein the charge and discharge circuit comprises:

5. The light-load mode decision circuit of claim 4, wherein the charging circuit comprises a first current source, a first switch, and a first capacitor; the discharge circuit comprises a second current source, a second switch and a first capacitor;

a first current source for providing a charging current;

6. The light-load mode decision circuit of claim 5, wherein the first current source outputs a charging current positively correlated to a state detection signal when the comparison result is a first comparison state.

7. The light load mode decision circuit of claim 1, wherein the counting circuit includes a counting circuit for counting statistically based on the comparison result within a set time; when the comparison result is in a first comparison state, the counting circuit performs incremental calculation for one time; when the comparison result is in a second comparison state, the counting circuit performs one-time decrement calculation.

8. The light load mode decision circuit of claim 1, wherein the light load mode decision circuit further comprises:

9. The light load mode decision circuit of claim 1, wherein the light load mode decision circuit further comprises:

10. A primary side control circuit for an isolated converter, said primary side control circuit for controlling the switching state of a primary side switch, comprising a light load mode decision circuit as claimed in any one of claims 1 to 9.

11. An isolated converter, comprising a primary circuit and a secondary circuit, the primary circuit receiving an input voltage, the primary circuit comprising a primary winding, a primary switch, and the primary control circuit of claim 10; the secondary circuit comprises a secondary winding, and the primary winding and the secondary winding are coupled to form the transformer.

12. The isolated converter of claim 11 wherein the primary control circuit controls the reduction of the operating current of the primary control circuit when the light mode decision circuit determines that the isolated converter enters the light mode.

13. A light-load mode decision method for an isolated converter is characterized by comprising the following steps:

14. The light load mode decision method according to claim 13, wherein the step of performing statistics according to the comparison result within a set time and outputting a statistical result comprises:

15. The light load mode decision method according to claim 13, wherein the step of performing statistics according to the comparison result within a set time and outputting a statistical result comprises:

16. The light load mode decision method according to claim 13, wherein the step of performing statistics according to the comparison result within a set time and outputting a statistical result comprises: