Circuit and method for eliminating audible noise during trickle charging of charger

Abstract

The present invention relates to a circuit and method for eliminating audible noise emitted during trickle charging of a charger, wherein a switching power supply comprises an upper power tube, a lower power tube and a transformer; the transformer comprises a primary winding and a secondary winding; the common end of the upper power tube and the lower power tube is connected to the first end of the primary winding; the first end and the second end of the secondary winding are both connected in series with an output rectifier tube; the secondary winding comprises a winding tap as an output end for outputting an output voltage of the switching power supply; the circuit further comprises: a frequency compensation circuit for light load operation, a threshold ratio generation circuit, a feedback circuit and a comparison circuit. The present invention achieves that the conversion efficiency of light load conditions is consistent with the high conversion efficiency of heavy load conditions, and no operating frequency noise audible to the human ear occurs.

Description

Technical Field

[0001] The present invention relates to the field of electronic circuits, and in particular to a circuit and method for eliminating audible noise generated during trickle charging of a charger. Background Art

[0002] The entire working state machine of the charger includes two stages: fast charging under heavy load (high current) and trickle charging under light load when the battery voltage is close to the target value (or to extend the battery life). Among them, in the light load stage, the required charging current output by the charger is very small, generally about one twentieth of the normal working state. This directly leads to an increase of dozens of times in the equivalent impedance of the resonant circuit. When the physical parameters of the product are determined, the conventional solution is to adopt an intermittent working mode that reduces the number of switching times (frequency). Under no-load and light load conditions, the frequency range is reduced to the human hearing range, resulting in a whistling noise problem that can be heard by humans. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention discloses a circuit and a method for eliminating audible noise generated during trickle charging of a charger.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A circuit for eliminating audible noise generated during trickle charging of a charger, wherein the switching power supply comprises an upper power tube, a lower power tube and a transformer; the transformer comprises a primary winding and a secondary winding; a common end of the upper power tube and the lower power tube is connected to a first end of the primary winding; an output rectifier tube is connected in series to the first end and the second end of the secondary winding; the secondary winding comprises a winding tap as an output end for outputting an output voltage of the switching power supply; the circuit further comprises:

[0006] A frequency compensation circuit operating at a light load generates a gate voltage for controlling the upper power tube and the lower power tube, and outputs a first voltage signal;

[0007] A threshold ratio generating circuit, for receiving the output current of the switching power supply and outputting a second voltage signal; the second voltage signal is a ratio value, and the ratio value is the ratio of the first threshold voltage to the second threshold voltage; the first threshold voltage is a light load condition entry / exit threshold; the second threshold voltage is a burst mode entry / exit threshold;

[0008] A feedback circuit, used for sampling the output voltage and outputting a third voltage signal; the third voltage signal is correlated with the output voltage of the switching power supply;

[0009] A comparison circuit receives the first voltage signal, the second voltage signal and the third voltage signal, determines a state of the circuit, and outputs a gate voltage for controlling the upper power tube and the lower power tube according to the circuit state; the circuit state is a light load condition or a burst mode.

[0010] Its further technical solution is that the transformer also includes an auxiliary winding; the threshold ratio generating circuit includes a first voltage-dividing resistor and a second voltage-dividing resistor connected in series at both ends of the auxiliary winding; the common end of the first voltage-dividing resistor and the second voltage-dividing resistor is the output end of the threshold ratio generating circuit, which outputs the second voltage signal.

[0011] Its further technical solution is that the frequency compensation circuit for light load operation includes a first sampling capacitor, a second sampling capacitor, a first constant current source and a second constant current source; the first sampling capacitor and the second sampling capacitor are connected in series to form a first series circuit; the first constant current source and the second constant current source are connected in series to form a second series circuit; the first end of the first series circuit is connected to the second end of the primary winding, and the second end of the first series circuit is grounded; the first end of the second series circuit is connected to a first voltage source; the second end of the second series circuit is grounded; the common end of the first sampling capacitor and the second sampling capacitor, and the common end of the first constant current source and the second constant current source are connected together as the frequency compensation circuit for light load operation, and the first voltage signal is output.

[0012] Its further technical solution is that the feedback circuit includes an optocoupler and a compensation circuit; the diode of the optocoupler is connected in series to the output end and the ground end; the output end of the optocoupler is connected to the input end of the compensation circuit; the compensation circuit reverses the input voltage and outputs the third voltage signal.

[0013] Its further technical solution is that the comparison circuit includes a first comparator, a second comparator and a calculation circuit; the calculation circuit receives the first voltage signal, the second voltage signal and the third voltage signal, and outputs the first threshold voltage and the second threshold voltage; the two input ends of the first comparator respectively input the first threshold voltage and the third voltage signal; the two input ends of the second comparator respectively input the second threshold voltage and the third voltage signal; the output end of the comparison circuit outputs the gate voltage used to control the upper power tube and the lower power tube.

[0014] A further technical solution is that the calculation circuit includes a counter circuit; the counter circuit is used to calculate the number of pulses used to control the gate voltages of the upper power tube and the lower power tube.

[0015] A method for eliminating audible noise emitted by a charger during trickle charging, wherein the method for reducing audible noise emitted by a switching power supply is implemented by a circuit as described in any one of the above items; and comprises the following steps:

[0016] When the circuit is overloaded, the circuit is in a normal state, transferring energy between the switching power supply input terminal and the switching power supply output terminal;

[0017] When the circuit is lightly loaded, the frequency compensation circuit operating at light load outputs a first voltage signal as a compensation signal to control the gate voltages of the upper power tube and the lower power tube so that the switching power supply enters a burst mode, and performs soft-on and soft-off control on the circuit in the burst mode so that the first voltage signal as the compensation signal is within a first threshold voltage, and controls the output voltage of the switching power supply to be less than a set value.

[0018] A further technical solution is that at time t1, the output voltage of the switching power supply increases, so that the third voltage signal decreases to below the second threshold voltage, and the circuit enters a light-load condition; the comparison circuit controls the upper power tube and the lower power tube to stop working;

[0019] From time t1 to time t2, the output voltage of the switching power supply decreases, and the third voltage signal increases;

[0020] At time t2, the third voltage signal increases to or exceeds the first threshold voltage; the comparison circuit controls the upper power tube and the lower power tube to be turned on again, and the counter starts counting; when the value of the counter is within the first count value range, the pulse width of the gate voltage of the upper power tube and the lower power tube is increased, and the third voltage signal decreases again;

[0021] At time t3, the third voltage signal is lower than the second threshold voltage again; when the count value of the counter reaches the second count value, the circuit enters a soft-off state; in the soft-off state, the comparison circuit starts the counter to start counting, and when the count value of the counter is within the range of the third count value, the pulse width of the gate voltage of the upper power tube and the lower power tube is reduced, and the third voltage signal rises;

[0022] At time t4, the third voltage signal exceeds the second threshold voltage, and the circuit enters a soft-on state, increasing the pulse width of the gate voltage of the upper power tube and the lower power tube until re-entering a soft-off state;

[0023] At time t5, after the off period, the third voltage signal exceeds the second threshold voltage again, and before the soft turn-on is completed, the third voltage signal exceeds the first threshold voltage, the soft turn-on of the switch stops immediately, and the system exits the light load condition.

[0024] A further technical solution is that before the soft-off step is completed, if the third voltage signal is greater than the second threshold voltage, the soft-off is immediately terminated and transitioned to the soft-on.

[0025] The beneficial effects of the present invention are as follows:

[0026] The present invention achieves the same conversion efficiency under light load conditions as high conversion efficiency under heavy load conditions, and no audible operating frequency noise occurs. The specific implementation method is: through resonant cavity voltage sampling, mirror current sampling transient response line power transmission. Programming is used to set the threshold value for entering and exiting light load conditions. Different operating modes are automatically switched according to different load conditions. Under heavy load conditions, the frequency control based on the resonant cavity sinusoidal current is controlled; under light load conditions, the triangular wave frequency compensation control based on charge charging and discharging is controlled.

[0027] The operating frequency of the above-mentioned triangle wave frequency compensation under light load conditions is above 20kHz, which completely eliminates the switching noise audible to the human ear. Furthermore, both working conditions can achieve a conversion efficiency of more than 95%. The present invention repeatedly executes the light load condition and the burst mode soft entry and soft disconnection algorithm, which can reduce or eliminate the audible noise of the charger under light load conditions. Furthermore, it can achieve a high conversion efficiency of more than 95% of the charger LLC converter under 10% load harsh conditions.

[0028] The circuit implementation of the present invention can also achieve soft turn-on and turn-off of power transmission under light load conditions and rapid response of exiting light load conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the circuit structure in an embodiment of the present invention.

[0030] Figure 2 4 is a control waveform diagram in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The specific implementation of the present invention will be described below in conjunction with the accompanying drawings.

[0032] Figure 1 FIG. 1 is a schematic diagram of the circuit structure of the present invention. Figure 1 As shown, the present invention includes a switching power supply and a noise adjustment circuit.

[0033] The switching power supply includes an upper power tube Q1, a lower power tube Q2 and a transformer T. The transformer T includes a primary winding and a secondary winding. The common end of the upper power tube Q1 and the lower power tube Q2 is connected to the second end of the primary winding. The first end and the second end of the secondary winding are both connected in series with an output rectifier tube. In this embodiment, the output rectifier tube is a first field effect tube Q3 and a second field effect tube Q4. The secondary winding includes a winding tap as an output end of the switching power supply, and outputs the switching power supply output voltage Vout.

[0034] The noise regulation circuit includes a frequency compensation circuit for light load operation, a threshold ratio generation circuit, a feedback circuit and a comparison circuit.

[0035] The frequency compensation circuit for light load operation receives the gate voltage for controlling the upper power tube Q1 and the lower power tube Q2, and outputs a first voltage signal VCR. The frequency compensation circuit for light load operation includes a first sampling capacitor C1, a second sampling capacitor C2, a first constant current source I1, and a second constant current source I2. The first sampling capacitor C1 and the second sampling capacitor C2 are connected in series to form a first series circuit. The first constant current source I1 and the second constant current source I2 are connected in series to form a second series circuit. The first end of the first series circuit is connected to the first end of the primary winding, and the second end of the first series circuit is grounded. The first end of the second series circuit is connected to the first voltage source. The second end of the second series circuit is grounded. The common end of the first sampling capacitor C1 and the second sampling capacitor C2, and the common end of the first constant current source I1 and the second constant current source I2 are connected together as the output end of the hybrid hysteresis control circuit, and the first voltage signal VCR is output.

[0036] In the frequency compensation circuit under light load operation, the first constant current source I1 and the second constant current source I2 are controlled by the driving signal input by the gate of the upper power tube Q1 and the lower power tube Q2. When the upper power tube Q1 is turned on, the first constant current source I1 injects a constant current into the first series circuit. When the lower power tube Q2 is turned on, the second constant current source I2 extracts the same fixed current. In such a change process, a triangular wave is superimposed on the common end of the first constant current source I1 and the second constant current source I2. Due to the characteristics of the switching power supply itself and the existence of the first sampling capacitor C1 and the second sampling capacitor C2, the first voltage signal VCR is in the shape of a sine wave superimposed on a triangular wave. And the slope of the superimposed triangular wave is variable. The switching frequency of the upper power tube Q1 and the lower power tube Q2 is defined by the voltage value of the first sampling capacitor C1 and the second sampling capacitor C2, which can accurately characterize the input current information, especially under light load and low current conditions, the control loop is equivalent to a first-order system, which can achieve excellent load and line transient response.

[0037] The frequency compensation circuit operating at light load can achieve soft turn-on, soft turn-off of power transmission under light load conditions and fast response of exiting light load conditions.

[0038] The transformer T also includes an auxiliary winding. The threshold ratio generating circuit includes a first voltage-dividing resistor R1 and a second voltage-dividing resistor R2 connected in series at both ends of the auxiliary winding. The common end of the first voltage-dividing resistor R1 and the second voltage-dividing resistor R2 is the output end of the threshold ratio circuit, which outputs a second voltage signal BW. The threshold ratio generating circuit is used to detect the output voltage when the first field effect transistor Q3 and the second field effect transistor Q4 of the switching power supply are turned on and output the second voltage signal BW. The second voltage signal BW is a proportional value, and the proportional value is the ratio of the first threshold voltage BMTH and the second threshold voltage BMTL; the first threshold voltage BMTH is the light load condition entry / exit threshold; the second threshold voltage BMTL is the burst mode entry / exit threshold. The auxiliary winding senses the output voltage when the first field effect transistor Q3 and the second field effect transistor Q4 are turned on, so it also has an output overvoltage protection function.

[0039] The feedback circuit includes an optocoupler and a compensation circuit. The diode of the optocoupler is connected in series to the output terminal and the ground terminal of the switching power supply. The feedback circuit of the output terminal of the optocoupler is connected to the input terminal of the compensation circuit. The optocoupler samples the output voltage Vout of the switching power supply. The compensation circuit reverses the input voltage and outputs a third voltage signal FB1. The feedback circuit is used to sample the output voltage of the switching power supply, provide a reference for the transmission of the input power of the primary winding, and is part of the voltage loop control.

[0040] The comparison circuit includes a first comparator, a second comparator and a calculation circuit; the calculation circuit receives a first voltage signal VCR, a second voltage signal BM and a third voltage signal FB1, and outputs a first threshold voltage BMTH and a second threshold voltage BMTL; the two input ends of the first comparator are respectively input with the first threshold voltage BMTH and the third voltage signal FB1; the two input ends of the second comparator are respectively input with the second threshold voltage BMTL and the third voltage signal FB1; the output end of the comparison circuit outputs a gate voltage for controlling an upper power tube Q1 and a lower power tube Q2.

[0041] The feedback circuit generates a feedback value that characterizes the output voltage characteristics, and determines the circuit state by comparing it with the first threshold voltage BMTH and the second threshold voltage BMTL, and makes the circuit enter / exit the light load condition, or enter or exit the burst mode. When the output voltage Vout of the switching power supply changes from large to small to the set output low value Vo_min, the comparison circuit controls the gate voltage of the upper power tube Q1 and the lower power tube Q2 until the output voltage Vout of the switching power supply is soft-conducted and gradually rises to the set Vo_max; conversely, the comparison circuit controls the gate voltage of the upper power tube Q1 and the lower power tube Q2 until the output voltage Vout of the switching power supply decreases to Vo_min again, and the cycle is repeated until the light load condition exits. Among them, the first threshold voltage BMTH can be set as a constant, and the second threshold voltage BMTL is obtained by calculation.

[0042] The present invention also discloses a circuit and method for eliminating audible noise emitted during trickle charging of a charger, and the method is implemented by the circuit as described above; the main program is run through a computing circuit to implement a state machine and control strategy for entering and exiting a light-load condition. Figure 2 FIG. 1 is a control waveform diagram in an embodiment of the present invention. Figure 2 As shown, the following steps are included:

[0043] When the circuit is overloaded, the circuit is in a normal state, transferring energy between the switching power supply input terminal and the switching power supply output terminal;

[0044] When the circuit is lightly loaded, the frequency compensation circuit operating at light load outputs a first voltage signal VCR as a compensation signal to control the gate voltages of the upper power tube Q1 and the lower power tube Q2 so that the switching power supply enters a burst mode, and performs soft-on and soft-off control on the circuit in the burst mode so that the first voltage signal VCR as the compensation signal is within a first threshold voltage BMTH, and controls the output voltage of the switching power supply to oscillate within a set value range.

[0045] Specifically, at time t1, the output voltage Vout of the switching power supply increases, so that the third voltage signal FB1 decreases to below the second threshold voltage BMTL, and the circuit enters a light-load condition. The comparison circuit controls the upper power tube Q1 and the lower power tube Q2 to stop working; during this period, the circuit also disables some functional blocks to save power consumption.

[0046] From time t1 to time t2, the output voltage Vout of the switching power supply decreases, and the third voltage signal FB1 increases;

[0047] At time t2, the third voltage signal FB1 increases to the first threshold voltage BMTH; the comparison circuit controls the upper power tube Q1 and the lower power tube Q2 to be turned on again, and the counter starts counting; when the count value of the counter is within the first count value range, the pulse width of the gate voltage of the upper power tube Q1 and the lower power tube Q2 is increased, and the third voltage signal FB1 decreases again. During this period, the circuit still works at high power transmission. In the first switch working cycle, soft start is not triggered.

[0048] At time t3, the third voltage signal FB1 is lower than the second threshold voltage BMTL again; when the count value of the counter reaches the second count value, the circuit enters the soft-off state; in the soft-off state, the counter starts to count again, and when the count value of the counter is within the range of the third count value, the pulse width of the gate voltage of the upper power tube Q1 and the lower power tube Q2 is reduced until the soft-off is completed and the third voltage signal starts to rise. Soft-on starts from the step where soft-off ends to ensure a smooth transition between soft-on and soft-off. During this period of time, if before the soft-off step is completed, when the third voltage signal FB1 is greater than the second threshold voltage BMTL, the soft-off ends immediately and directly transitions to soft-on.

[0049] At time t4, the third voltage signal FB1 exceeds the second threshold voltage BMTL, and the circuit enters the soft-on state, increasing the pulse width of the gate voltage of the upper power tube Q1 and the lower power tube Q2, and repeating the above waveform oscillation steps until it re-enters the soft-off state. Since this is not the first burst in the cycle, there is a soft switch turn-on in the first 7 switching cycles.

[0050] At time t5, after another off period, the third voltage signal FB1 exceeds the second threshold voltage BMTL again, and before the soft turn-on is completed, the third voltage signal FB1 rises to exceed the first threshold voltage BMTH at time t6, the soft turn-on of the switch stops immediately, and the system exits the light load condition.

[0051] The above description is an explanation of the present invention, not a limitation of the present invention. The scope of the present invention is defined in the claims. The present invention may be modified in any form without violating the basic structure of the present invention.

Claims

1. A circuit for eliminating audible noise during trickle charging of a charger, characterized in that: The invention comprises a switching power supply, wherein the switching power supply comprises an upper power tube, a lower power tube and a transformer; the transformer comprises a primary winding and a secondary winding; a common end of the upper power tube and the lower power tube is connected to a first end of the primary winding; an output rectifier tube is connected in series to the first end and the second end of the secondary winding; the secondary winding comprises a winding tap as an output end for outputting an output voltage of the switching power supply; the circuit further comprises: A frequency compensation circuit operating at a light load generates a gate voltage for controlling the upper power tube and the lower power tube, and outputs a first voltage signal; A threshold ratio generating circuit, for receiving the output current of the switching power supply and outputting a second voltage signal; the second voltage signal is a ratio value, and the ratio value is the ratio of the first threshold voltage to the second threshold voltage; the first threshold voltage is a light load condition entry / exit threshold; the second threshold voltage is a burst mode entry / exit threshold; A feedback circuit is used to sample the output voltage and output a third voltage signal; the third voltage signal is correlated with the output voltage of the switching power supply; a comparison circuit, receiving the first voltage signal, the second voltage signal and the third voltage signal, determining a circuit state, and outputting a gate voltage for controlling the upper power tube and the lower power tube according to the circuit state; the circuit state is a light load condition or a burst mode; The frequency compensation circuit for light load operation includes a first sampling capacitor, a second sampling capacitor, a first constant current source, and a second constant current source; the first sampling capacitor and the second sampling capacitor are connected in series to form a first series circuit; the first constant current source and the second constant current source are connected in series to form a second series circuit; the first end of the first series circuit is connected to the second end of the primary winding, and the second end of the first series circuit is grounded; the first end of the second series circuit is connected to a first voltage source; the second end of the second series circuit is grounded; the common end of the first sampling capacitor and the second sampling capacitor, and the common end of the first constant current source and the second constant current source are connected together as the frequency compensation circuit for light load operation, and the first voltage signal is output; The comparison circuit includes a first comparator, a second comparator and a calculation circuit; the calculation circuit receives the first voltage signal, the second voltage signal and the third voltage signal, and outputs the first threshold voltage and the second threshold voltage; the two input ends of the first comparator input the first threshold voltage and the third voltage signal respectively; the two input ends of the second comparator input the second threshold voltage and the third voltage signal respectively; the output end of the comparison circuit outputs the gate voltage used to control the upper power tube and the lower power tube.

2. The circuit for eliminating audible noise during trickle charging of a charger according to claim 1, characterized in that: The transformer also includes an auxiliary winding; the threshold ratio generating circuit includes a first voltage-dividing resistor and a second voltage-dividing resistor connected in series at both ends of the auxiliary winding; the common end of the first voltage-dividing resistor and the second voltage-dividing resistor is the output end of the threshold ratio generating circuit, which outputs the second voltage signal.

3. The circuit for eliminating audible noise during trickle charging of a charger according to claim 1, characterized in that: The feedback circuit includes an optocoupler and a compensation circuit; the diode of the optocoupler is connected in series to the output end and the ground end; the output end of the optocoupler is connected to the input end of the compensation circuit; the compensation circuit reverses the input voltage and outputs the third voltage signal.

4. The circuit for eliminating audible noise during trickle charging of a charger according to claim 1, characterized in that: The calculation circuit includes a counter circuit; the counter circuit is used to calculate the number of pulses used to control the gate voltages of the upper power tube and the lower power tube.

5. A method for eliminating audible noise during trickle charging of a charger, characterized in that: The method is implemented by the circuit for eliminating audible noise emitted during trickle charging of a charger according to any one of claims 1 to 4; and comprises the following steps: When the circuit is overloaded, the circuit is in a normal state, transferring energy between the switching power supply input terminal and the switching power supply output terminal; When the circuit is lightly loaded, the frequency compensation circuit operating at light load outputs a first voltage signal as a compensation signal to control the gate voltages of the upper power tube and the lower power tube so that the switching power supply enters a burst mode, and performs soft-on and soft-off control on the circuit in the burst mode so that the first voltage signal as the compensation signal is within a first threshold voltage, and controls the output voltage of the switching power supply to be less than a set value.

6. The method for eliminating audible noise during trickle charging of a charger according to claim 5, characterized in that: At time t1, the output voltage of the switching power supply increases, causing the third voltage signal to decrease to below the second threshold voltage, and the circuit enters a light-load condition; the comparison circuit controls the upper power tube and the lower power tube to stop working; From time t1 to time t2, the output voltage of the switching power supply decreases, and the third voltage signal increases; At time t2, the third voltage signal increases to be greater than the first threshold voltage; the comparison circuit controls the upper power tube and the lower power tube to be turned on again, and the counter starts counting; when the count value of the counter is within the first count value range, the pulse width of the gate voltage of the upper power tube and the lower power tube is increased, and the third voltage signal decreases again; At time t3, the third voltage signal is lower than the second threshold voltage again; when the count value of the counter reaches the second count value, the circuit enters a soft-off state; in the soft-off state, the comparison circuit starts the counter to start counting, and when the count value of the counter is within the range of the third count value, the pulse width of the gate voltage of the upper power tube and the lower power tube is reduced, and the third voltage signal rises; At time t4, the third voltage signal exceeds the second threshold voltage, and the circuit enters a soft-on state, increasing the pulse width of the gate voltage of the upper power tube and the lower power tube until re-entering a soft-off state; At time t5, after the off period, the third voltage signal exceeds the second threshold voltage again, and before the soft turn-on is completed, the third voltage signal exceeds the first threshold voltage, the soft turn-on of the switch stops immediately, and the system exits the light load condition.

7. The method for eliminating audible noise during trickle charging of a charger according to claim 6, characterized in that: Before the soft-off step is completed, if the third voltage signal is greater than the second threshold voltage, the soft-off is immediately terminated and transitions to the soft-on.

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