CN102810984B - Switching power circuit - Google Patents

Abstract

The invention discloses a switching power circuit, which comprises a switching tube, a first resistive divider, a second resistive divider, an inductive component, a freewheel diode, a load unit, a load current detection unit, a comparator, a D latch, an analogue-to-digital converter, a digital-to-analogue converter, a pulse width modulation (PWM) operating mode gate driving voltage generation unit, a first amplifier, a PWM comparator, a pulse frequency modulation (PFM) control unit, a first either-or selector, a second either-or selector, a digital logic unit and a gate driving unit. When a PFM control mode is adopted, the voltage of the switching tube is regulated according to a load to reduce power consumption in a light load condition and improve energy transmission efficiency; and in addition, in the light load condition, the gate driving voltage and switching duty cycle of the switching tube are reduced, so that the power consumption is simultaneously reduced, and the shortcoming that output power is regulated only dependent on the duty cycle of the switching tube to narrow the variation range of the duty cycle and further retard the sudden change of the load and cause large output ripples during mode switching is overcome.

Description

A switching power supply circuit

technical field

The invention belongs to the technical field of power supplies, and in particular relates to the design of a switching power supply circuit.

Background technique

Switching power supply has the advantages of small size, light weight, high efficiency, low heat generation, stable performance, etc., and is widely used in various terminal equipment and communication equipment mainly based on electronic computers.

There are three common working modes of switching power supply: PWM (Pulse Width Modulation) mode, PFM (Pulse Frequency Modulation) mode and PWM/PFM mixed mode. PWM mode has low noise, high efficiency at full load and can work in continuous conduction mode, but due to the fixed frequency, the switching loss of its control circuit is generally large, and the efficiency decreases at light load. The frequency of PFM mode decreases as the load becomes lighter, so the efficiency is improved; and without the influence of the first amplifier, its response speed is fast, but the output ripple is large; and the PWM/PFM hybrid mode has the characteristics of both control modes. It adopts PWM mode at heavy load and PFM mode at light load. However, in the mixed mode, when the load suddenly changes and the two modes switch, a large ripple will be generated.

The existing switching power supply technology used in light load conditions is as follows:

The scheme of the synchronous rectification switching power supply circuit of the first PWM control mode is shown in Figure 1. Specifically, the output stage is used as a partition to solve the problem: when the load is heavy, most or even all of the output stages are used to increase the output current capability and reduce the output current. On-resistance; at light load, fewer output drive stages are used to reduce the switching loss of the switch tube. However, this scheme of separating the output stages increases the cost of the chip due to the large area of the output stage used.

The scheme of the switching power supply circuit of the second PFM control mode is shown in Figure 2. In this mode, as the load becomes lighter, the frequency decreases and the response speed is fast, but the output ripple is large, and the switching tube is driven at light load. There is no change in voltage, limiting further reduction in power consumption.

The scheme of the switching power supply circuit of the third PFM/PWM control mode is shown in Figure 3. When the load is heavy, the PWM control mode is used to provide stable and efficient output; when the load is light, the PFM control mode is used to reduce power consumption. However, in this solution, the voltage of the driving switch tube does not change, which limits the further reduction of power consumption; at the same time, when the load suddenly changes and the mode switches, a large ripple will be generated.

Contents of the invention

The purpose of the present invention is to solve the above-mentioned problems existing in the existing switching power supply circuit under the condition of light load, and propose a switching power supply circuit.

The technical solution of the present invention is: a switching power supply circuit, specifically comprising: a switching tube, a first voltage dividing resistor, a second voltage dividing resistor, an inductance element, a freewheeling diode, a load unit, a load current detection unit, a comparator, a D Latch, analog-to-digital converter, digital-to-analog converter, gate drive voltage generation unit in PWM mode, first amplifier, PWM comparator, PFM control unit, first two-to-one selector, second two-to-one selector , digital logic unit, gate drive unit, where,

One end of the first voltage dividing resistor is connected to the first end of the inductance element and serves as the output end of the switching power supply circuit, and the other end of the first voltage dividing resistor is connected to the negative end of the first amplifier and the input end of the PFM control unit and passed through The second voltage dividing resistor is grounded, the positive terminal of the first amplifier is connected to the first external reference voltage, the output terminal of the first amplifier is connected to the positive terminal of the PWM comparator, and the negative terminal of the PWM comparator is connected to the external sawtooth wave, PWM The output terminal of the comparator is connected to the first input terminal of the first one-two selector; the output terminal of the PFM control unit is connected to the second input terminal of the first one-two selector;

The input terminal of the load current detection unit is connected to the output terminal of the switching power supply circuit through the load unit, and the output terminal of the load current detection unit is connected to the positive input terminal of the comparator and the input terminal of the analog-to-digital converter; the comparator The negative terminal of the comparator is connected to the second external reference voltage, the output terminal of the comparator is connected to the input terminal of the D latch, and the output terminal of the D latch is connected to the control terminal of the first two-to-one selector and the second two-to-one selector The control terminal of the analog-to-digital converter; the output terminal of the analog-to-digital converter is connected to the input terminal of the digital-to-analog converter, the output terminal of the DAC digital-to-analog converter is connected to the first input terminal of the second selection selector, and the output terminal of the gate drive voltage generating unit in PWM mode connected to the second input terminal of the second one-two selector, the output of the first one-two selector is connected to the input of the digital logic unit, the output of the digital logic unit and the output of the second one-two selector are used as the input of the gate drive unit, The output of the gate drive unit is connected to the grid of the switching tube, the source of the switching tube is connected to the second end of the inductance element, the drain is connected to the external power supply voltage, the negative pole of the freewheeling diode is connected to the source of the switching tube, and the positive pole of the freewheeling diode is grounded.

Beneficial effects of the present invention: for low-power switching power supply converters, a low-power consumption hybrid PWM/PFM switching power supply circuit is proposed, which can automatically adjust the control mode according to the weight of the load, specifically: PWM control is selected when the load is heavy Mode, PFM control mode is selected for light load. When it is in the PFM control mode, it can also adjust the voltage of the driving switch tube according to the weight of the load, further reducing the power consumption at light load and improving the efficiency of energy transmission, which is especially suitable for the long-term work of the power supply in standby or light load , and at the same time under light load conditions, the power consumption can be reduced at the same time by reducing the gate driving voltage of the switch tube and the duty cycle of the switch. Compared with the PFM/PWM hybrid control method in the background technology, it reduces The output power can be adjusted by adjusting the output power, which can reduce the variation range of the duty cycle, thereby alleviating the shortcomings of large output ripple when the load changes suddenly and the mode is switched. At the same time, compared with the PWM control method using the output stage separation technology, the chip The area is small.

Description of drawings

Figure 1 is a schematic structural diagram of a synchronous rectification switching power supply circuit using PWM control.

FIG. 2 is a schematic structural diagram of an existing switching power supply circuit using a PFM control method.

Figure 3 is a schematic diagram of the existing switching power supply circuit using PFM/PWM hybrid control mode

FIG. 4 is a schematic structural diagram of a switching power supply circuit in an embodiment of the present invention.

FIG. 5 is a schematic structural diagram of a comparator circuit in an embodiment of the present invention.

FIG. 6 is a schematic structural diagram of an ADC circuit in an embodiment of the present invention.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

Figure 4 shows that the switching power supply circuit of this example includes the first voltage dividing resistor R1, the second voltage dividing resistor R2, the inductance element L, the freewheeling diode D1, the load unit Load, the load current detection unit CS, the comparator COM, D Latch, analog-to-digital converter ADC, digital-to-analog converter DAC, PWM operation mode gate drive voltage generation unit VPWM, first amplifier EA, PWM comparator, PFM control unit, two-to-one selector S1, S2, digital logic Unit Logic, gate drive unit Driver.

The specific connection relationship is: one end of the first voltage dividing resistor R1 is connected to the first end of the inductance element L and serves as the output end of the switching power supply circuit, and the other end is grounded through the second voltage dividing resistor R2, the voltage dividing value Vfb, the first The positive terminal of the amplifier EA is connected to the external first reference voltage ref1, and the negative terminal is connected to the divided voltage Vfb. The output of the first amplifier EA is connected to the positive end of the PWM comparator, the negative end is connected to the sawtooth wave, and its output is connected to one end of the selector S1. In addition, the divided voltage Vfb is also connected to the input of the PFM module, and its output is connected to the other input terminal of the one-of-two selector S1. The load current detection circuit CS is connected to the load Load terminal, and the output of the load current detection circuit CS is connected to the positive output of the comparator COM. The negative terminal of the comparator COM is connected to the external second reference voltage ref2, the output terminal of the comparator COM is connected to the input terminal of the D latch, and the output terminal of the D latch is connected to the control terminals of the selectors S1 and S2. The output terminal of the load current detection circuit CS is also connected to the input terminal of the 3-bit ADC analog-to-digital converter, the output of the ADC is connected to the input terminal of the digital-to-analog converter DAC, and the output of the digital-to-analog converter DAC is connected to the input terminal of the selector S2. The output of the gate driving voltage generation unit VPWM in PWM working mode is connected to the other input terminal of the one-of-two selector S2. The output of the two-to-one selector S1 is connected to the digital logic unit logic, and together with the output UV, OV, OCP, OTP of the protection module and the enable signal EN, the switch tube drive signal is generated, where UV is the output of the undervoltage blockade module, and OV is the overvoltage Block the output of the module, OCP is the output of the over-current protection module, and OTP is the output of the over-temperature protection module. The output of the digital logic unit logic and the output through the one-of-two selector S2 are connected to the input of the gate driver module Driver, and the output of the Driver is connected to the gate of the switching tube. The source of the switch tube is connected to the second end of the inductance element L, and the drain is connected to the external power supply voltage.

It should be noted that the divided voltage value Vfb is determined by the rated output voltage of the switching power supply. In the PWM control mode, the frequency of the sawtooth wave determines the operating frequency of the switching tube. This point belongs to the prior art in the art. This will not be described in detail.

The function of the freewheeling diode D1 here is: when the switch tube is turned on, the external power supply provides energy to the inductor and the load through the switch tube; when the switch tube is turned off, the power supply cannot provide energy to the load, and the energy at this time It comes from the energy stored by the inductor in the previous process, but the release of this energy requires a loop, and the freewheeling diode D1 provides this loop.

As a preferred solution, in order to reduce the ripple of the output voltage, the switching power supply circuit further includes a capacitive element C1, and the capacitive element is coupled between the second end of the inductive element L and the ground potential.

The switching power supply circuit of the present invention has two control loops in total: one is a PFM/PWM control loop, and the other is a switching tube gate driving voltage determining loop. The PFM/PWM control loop includes: the first and second divider resistors, load current detection unit CS, comparator COM, first amplifier EA, PWM comparator, PFM control unit, and the two-to-one selector S1 that determines the control mode selection , logic control unit logic and gate drive unit driver. The switching tube grid driving voltage determination loop includes a load current detection unit CS, a comparator COM, a D latch, a switching tube grid driving voltage determining unit (including a PWM working mode gate driving voltage generating unit, an ADC analog-to-digital converter, a DAC digital An analog-to-analog converter, a selector S2 that determines the selection of the driving voltage, a logic control unit logic and a gate drive unit driver.

Among them, the ADC analog-to-digital converter and the DAC digital-to-analog converter form the gate drive voltage generation module of the PFM working mode, and the first amplifier EA, the two-to-one selector S1, the PWM comparator and the PFM control unit form the PWM/PFM control module .

The working process of the switching power supply circuit of the present invention is as follows: the whole circuit first detects the load current through the load current detection unit CS. The load current detection unit CS has an integral function, that is, it can collect the average value of the load current, and the CS converts the detected current into a voltage signal and sends it to the comparator COM, and the comparator COM compares the detection signal with the set external first The two reference voltages ref2 are compared, and the comparison output will determine the control mode adopted by the control mode loop and the gate drive voltage adopted by the switch tube gate drive voltage loop. When the detection signal is higher than ref2, its output is high, and at this time, the PWM control mode and the gate drive voltage generating unit VPWM of the PWM working mode will be adopted. When the detection signal is lower than ref2, its output is low, and at this time, the PFM control mode and PFM working mode gate drive voltage generation module will be adopted. At the same time, the output voltage adopting circuit will sample the output voltage, and the sampled voltage will be sent to the first amplifier EA and the PFM control unit. If the circuit selects the PWM control mode, the circuit compares the sampling voltage input to the first amplifier EA with the external first reference voltage ref1 to generate an error voltage and send it to the PWM comparator for comparison with the same sawtooth wave, thereby generating different duty cycles The square wave is used to adjust the duty cycle of the switching tube according to the change of the output voltage, and then achieve the predetermined purpose of stabilizing the output voltage. The gate driving voltage of the switching tube comes from the constant voltage generated by the gate driving voltage generating unit in PWM mode. On the contrary, if the PFM control mode is selected, the circuit adjusts the output frequency of the PFM control unit according to the detected output voltage; The voltage driven by the gate of the switch tube can be adjusted according to the output load current. The output of the two-to-one selector S1 is sent to the logic control unit logic, and the output OTP of the over-temperature protection module of the logic control unit logic, the output UV of the under-voltage protection module, the output OV of the over-voltage protection module, and the output of the over-voltage protection module The output OCP of the current protection module and the output EN of the enable module jointly generate the switch tube drive signal and send them to the gate drive unit driver to drive the switch tube together with the output of the gate drive voltage generation loop to realize the energy transmission from the power supply to the load.

It should be noted that: ref1 is determined by the rated output voltage of the switching power supply circuit; ref2 is determined by the voltage signal corresponding to the output current at the switching point of the control mode in the switching power supply circuit; for the entire circuit, the output voltage is kept constant through loop control , the output current changes according to the weight of the load. The function of the switching power supply circuit is to keep the voltage constant while keeping the output current changing with the load.

FIG. 5 shows a schematic diagram of the circuit structure of the comparator COM in the embodiment of the present invention. The comparator adopts a positive feedback loop to realize the hysteresis function.

As a preferred solution, the analog-to-digital converter is specifically a 3-bit analog-to-digital converter; the input 3-bit digital signals correspond to 8 different levels, and the input 3-bit digital signals are input by the analog-to-digital converter Converted to the corresponding analog signal.

Figure 6 shows a schematic diagram of the circuit structure of the 3-bit analog-to-digital converter in the embodiment of the present invention. The circuit uses a voltage-dividing resistor to divide the reference voltage, and each voltage-dividing terminal is sent to the positive terminal of the corresponding comparator, and the analog signal into the negative terminal of each comparator. All comparator output terminals are sent to the encoding circuit for encoding, and 3-bit digital signals are output.

The ADC in the present invention is a 3-bit analog-to-digital converter. By comparing the output voltage of the load current detection module with the subsection standard voltage, it is designated as one of the eight voltage levels and the conversion result is output to The next stage circuit DAC. The DAC is a 3-digit digital-to-analog converter, which converts the output of the upper-stage ADC into a corresponding output voltage. Through the ADC and DAC modules, the current output by the load unit can be accurately converted into the voltage driven by the gate of the switch tube.

The load current detection unit CS detects the current output by the load unit in real time, and converts the detected current signal into a voltage signal and sends it to a comparator for comparison and output. The output of the comparator is also connected with a D latch, whose clock frequency is the sawtooth wave frequency at the input of the PWM comparator. The output of the comparator is collected at each rising edge of the clock, and the 2-to-1 selector S1, S2 determines the output of the comparator. The selected control mode and switch gate drive voltage in the clock cycle. In this way, the control mode is determined by the output current detection in each cycle, and the load change can be responded to in time through the loop, and the D latch is used to prevent the frequent change of the working mode selection in a single cycle, which will cause the output voltage to fluctuate. Stablize.

For daily electrical appliances, the detection circuit often works in standby for a long time, which means a light load for the electrical power supply. For the switching power supply circuit, the total power consumption of the circuit is composed of the static power consumption of the switching power supply circuit, the conduction loss of the switching tube and the switching loss. Compared with the switching loss of the switching tube, the switching loss of other gate circuits can be ignored. The switching loss of the switching tube is specifically: P _gate =C _gate *V _swing ² *f _sw , where C _gate is the gate capacitance of the switching tube, V _swing is the gate voltage swing of the switching tube, and f _sw is the switching frequency of the switching tube. It can be seen from the above formula that reducing V _swing and f _sw can reduce switching loss.

Due to the large load current during heavy load, the conduction loss of the switch tube plays a major role in the overall power consumption. While the load current is very small at light load, the switching loss of the switching tube plays a major role. In the PFM control of the background art, the frequency reduction is used to reduce the switching loss at light load, but the gate voltage of the switch tube does not change. In the present invention, the switch frequency is reduced at light load and the grid drive voltage of the switch tube is reduced, the power consumption of the circuit is further reduced, and the efficiency is improved, which is equivalent to prolonging the standby time for the portable power supply.

Those skilled in the art will appreciate that the embodiments described here are to help readers understand the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical revelations disclosed in the present invention without departing from the essence of the present invention, and these modifications and combinations are still within the protection scope of the present invention.

Claims (3)

1. A switching power supply circuit, specifically comprising: a switching tube, a first voltage dividing resistor, a second voltage dividing resistor, an inductive element, a freewheeling diode, a load unit, a load current detection unit, a comparator, a D latch, the first An amplifier, a PWM comparator, a PFM control unit, a first one-two selector, a digital logic unit and a gate drive unit, and it is characterized in that it also includes: a switch tube gate drive voltage determination unit, which is provided with a second two a selector, an analog-to-digital converter, a digital-to-analog converter and a gate drive voltage generating unit in PWM operation mode, in, One end of the first voltage dividing resistor is connected to the first end of the inductance element and serves as the output end of the switching power supply circuit, and the other end of the first voltage dividing resistor is connected to the negative end of the first amplifier and the input end of the PFM control unit and passed through The second voltage dividing resistor is grounded, the positive terminal of the first amplifier is connected to the first external reference voltage, the output terminal of the first amplifier is connected to the positive terminal of the PWM comparator, and the negative terminal of the PWM comparator is connected to the external sawtooth wave, PWM The output terminal of the comparator is connected to the first input terminal of the first one-two selector; the output terminal of the PFM control unit is connected to the second input terminal of the first one-two selector; The input terminal of the load current detection unit is connected to the output terminal of the switching power supply circuit through the load unit, and the output terminal of the load current detection unit is connected to the positive input terminal of the comparator and the input terminal of the analog-to-digital converter; the comparator The negative terminal of the comparator is connected to the second external reference voltage, the output terminal of the comparator is connected to the input terminal of the D latch, and the output terminal of the D latch is connected to the control terminal of the first two-to-one selector and the second two-to-one selector The control terminal of the analog-to-digital converter; the output terminal of the analog-to-digital converter is connected to the input terminal of the digital-to-analog converter, the output terminal of the digital-to-analog converter is connected to the first input terminal of the second two-to-one selector, and the output terminal of the gate drive voltage generation unit in PWM operation mode connected to the second input terminal of the second one-two selector, the output of the first one-two selector is connected to the input of the digital logic unit, the output of the digital logic unit and the output of the second one-two selector are used as the input of the gate drive unit, The output of the gate drive unit is connected to the grid of the switching tube, the source of the switching tube is connected to the second end of the inductance element, the drain is connected to the external power supply voltage, the negative pole of the freewheeling diode is connected to the source of the switching tube, and the positive pole of the freewheeling diode is grounded. 2. The switching power supply circuit according to claim 1, characterized in that, the switching power supply circuit further comprises a capacitive element, and the capacitive element is coupled between the second terminal of the inductive element and the ground potential. 3. The switching power supply circuit according to claim 1 or 2, wherein the analog-to-digital converter is specifically a 3-bit analog-to-digital converter.