CN105048798A - Frequency jitter circuit and switching power supply - Google Patents

Abstract

The present invention relates to a frequency dithering circuit and a switching power supply, comprising a PWM controller and a waveform generating circuit, the waveform generating circuit is connected to the RT pin of the PWM controller through a series dithering capacitor Cd and a dithering resistor Rd; the PWM controller The RT pin of the PWM controller is grounded through a fixed resistor Rt; the CT pin of the PWM controller is grounded through a fixed capacitor Ct. Due to the wide applicability of the PWM controller, this dithering circuit is suitable for flyback circuits, PFC circuits and DC/DC circuits, etc., as long as the PWM control chip has RT and CT, it has wide adaptability.

Description

A frequency jitter circuit and switching power supply

technical field

The invention relates to the field of switching power converters, in particular to a frequency dithering circuit and a switching power supply.

Background technique

According to different circuit structures, power converters can be divided into two types: linear power converters and switching power converters. Among them, the circuit structure of the switching power converter is more complicated than that of the linear power converter, and the electromagnetic interference (EMI) is relatively large. However, switching power converters have high conversion efficiency, low power consumption at no-load, and light weight. Therefore, it has been widely used. However, because the switching power converter adopts the pulse width modulation (PWM) control method, its working waveform is mostly a periodic pulse series. From the spectrum analysis, it can be seen that the square wave signal contains rich high-order harmonics, and the high-order harmonics The spectrum can reach more than 1000 times of square wave frequency. As the switching frequency continues to increase, the power device will generate a large current and voltage change rate during the switching process, causing a large number of harmonic components in the high-frequency switching waveform to propagate outward through the transmission line and the space electromagnetic field. The greater the power, the electromagnetic emission The more serious it is, it will cause conduction and radiation interference problems that cannot be ignored. Therefore, various high-frequency digital circuits have more stringent requirements on the electromagnetic compatibility of switching power converters, and how to reduce electromagnetic interference has become a difficult point in the design of switching power converters.

In order to control electromagnetic interference, it meets the energy requirements of the limited amplitude of the control standard, and ensures that the normal operation of each device will not be affected by mutual interference. The traditional method of reducing electromagnetic interference is to install an electromagnetic interference filter at the input end of the power supply. Electromagnetic interference filters are generally composed of passive components such as inductors, capacitors, and resistors to achieve the purpose of filtering electromagnetic interference. However, the greater the EMI, the larger the EMI filter required. This causes an increase in the cost of the circuit, and the electromagnetic interference filter cannot deal with the radiation problem of electromagnetic interference. At present, one of the effective methods to suppress EMI is to introduce frequency jitter technology, so that the switching power supply no longer works at a certain fixed frequency, but works within a certain frequency range. In this way, the electromagnetic interference signal generated by the switching power supply will not be concentrated on a certain fixed frequency, but will be extended to a certain frequency range (that is, a wider frequency band), thereby reducing the interference of the switching power supply at a certain fixed frequency point. Electromagnetic interference, to achieve the effect of suppressing EMI.

The frequency jitter technology in the prior art, such as the patent document with the publication number CN104038046A discloses a frequency jitter circuit and a switching power supply, which generates a periodic signal with a fixed frequency by charging and discharging the capacitor C, and the oscillation unit processes the periodic signal, Output a clock signal with a fixed frequency. Add a resistance R1 between the capacitor C and the ground, and inject a periodically changing voltage signal △V at point b with a frequency of f2. The voltage signal △V changes the charging time of the capacitor C, affects the frequency of the oscillating signal formed by the charging and discharging of the capacitor C, and then obtains a clock signal with frequency jitter. However, the above-mentioned invention is mainly applicable to the flyback circuit, and its application range is relatively narrow.

Contents of the invention

The object of the present invention is to provide a frequency dithering circuit and a switching power supply to solve the problem that the existing frequency dithering technology has a narrow application range.

To achieve the above object, the solution of the present invention includes:

A frequency dithering circuit, comprising a PWM controller and a waveform generating circuit, the waveform generating circuit is connected to the RT pin of the PWM controller through a dithering capacitor Cd and a dithering resistor Rd; the RT pin of the PWM controller is passed through a fixed resistor Rt ground; the CT pin of the PWM controller is grounded through a fixed capacitor Ct.

The frequency dithering circuit further includes an operational amplifier, the waveform generating circuit is connected to the dithering capacitor Cd through the operational amplifier, and the operational amplifier is used to amplify the signal of the waveform generator.

A switching power supply, comprising a PWM controller and a waveform generating circuit, the waveform generating circuit is connected to the RT pin of the PWM controller through a jitter capacitor Cd and a jitter resistor Rd; the RT pin of the PWM controller is grounded through a fixed resistor Rt ; The CT pin of the PWM controller is grounded through a fixed capacitor Ct.

The frequency dithering circuit further includes an operational amplifier, the waveform generating circuit is connected to the dithering capacitor Cd through the operational amplifier, and the operational amplifier is used to amplify the signal of the waveform generator.

The beneficial effects of the present invention are: the fixed resistance Rt and the fixed capacitance Ct are respectively connected with the internal pins CT and RT corresponding to the PWM controller to generate a clock signal with a fixed frequency; and the waveform generation circuit is connected to the PWM controller The RT pin implements frequency dithering. At the same time, the range of the jitter frequency around the fixed switching frequency is changed by changing the resistance value of the jitter resistor and the capacitance value of the jitter capacitor. Due to the wide applicability of the PWM controller, this dithering circuit is suitable for flyback circuits, PFC circuits and DC/DC circuits, etc., as long as the PWM control chip has RT and CT, it has wide applicability.

The present invention mainly changes the charge and discharge current of RT inside the PWM chip by jointly changing the jitter capacitor, the jitter resistance and the waveform generating circuit. Compared with the reference document: (1) The waveform generation circuit does not directly act on the fixed capacitor, but is connected to the fixed resistor through the jitter resistor and the jitter capacitor; (2) It is not necessary to connect a fixed reference voltage Vref to the fixed resistor, and there is no Vref can work normally, that is, the first power supply is not needed; (3) the jitter resistance increased by the present invention can change the frequency jitter range, which is not negligible; (4) the present invention also increases the jitter capacitor, which also has a corresponding Function; (5) The present invention adopts a waveform generating circuit instead of a second power supply; (6) The scope of application of the present invention is wider, different from the example of the flyback main circuit given by the above patent, this dithering circuit is suitable for the flyback circuit , PFC circuit and DC/DC circuit, etc., as long as the PWM control chip has all circuits of RT and CT.

Description of drawings

Fig. 1 is the schematic diagram of frequency dithering circuit embodiment;

Fig. 2 is a structural diagram of an embodiment of a frequency dithering circuit.

Detailed ways

Frequency dithering circuit embodiment

As shown in Figure 1, the frequency dithering circuit includes a waveform generating circuit, and the waveform generating circuit is connected to the RT pin of the PWM controller through a series dithering capacitor Cd and a dithering resistor Rd; the RT pin of the PWM controller is grounded through a fixed resistor Rt ; The CT pin of the PWM controller is grounded through a fixed capacitor Ct.

Wherein, the waveform generating circuit is used to generate the fundamental wave, and may be a sine wave generating circuit, a triangular wave generating circuit, a sawtooth wave generating circuit, and the like. The fixed resistor Rt and the fixed capacitor Ct are respectively connected to the corresponding internal pins CT and RT of the PWM controller for generating a clock signal with a fixed frequency. The waveform generation circuit is connected to the jitter capacitor Cd and the jitter resistor Rd in turn, and the jitter resistor Rd is connected in parallel with the fixed resistor Rt to the RT pin of the PWM controller. The charging current Icharge of the pin and the discharge circuit Idischarge, and then a clock signal with frequency jitter is obtained. Changing the resistance value of the jitter resistor can change the jitter frequency range around the fixed switching frequency. The larger the resistance, the smaller the jitter range. When it is close to infinity, it is equal to the fixed switching frequency. Changing the capacitance of the jitter capacitor can also change the jitter frequency range around the fixed switching frequency. The larger the capacitance, the smaller the jitter range. When it is close to infinity, it is equal to the fixed switching frequency.

In the above embodiment, the frequency dithering circuit further includes an operational amplifier, the waveform generating circuit is connected to the dithering capacitor Cd through the operational amplifier, and the operational amplifier is used to amplify the signal of the waveform generator.

The present invention will be further elaborated below in conjunction with FIG. 2 . As shown in Figure 2, the power supply VCC is connected to the cathode of the thyristor V1 through the current limiting resistor R3, and the anode of the thyristor V1 is grounded, and its output voltage is filtered by the resistor R4, resistor R5 and capacitor C2 to obtain the required stable voltage value for comparison Voltage source Vref for voltage reference. Vref is divided by resistors R6 and R7 to obtain a comparison voltage Vh, which enters the non-inverting input terminal of the operational amplifier U1A. The power supply VCC charges the capacitor C1 through the resistor R1 and the resistor R2; the capacitor C1 discharges through the resistor R2; C1 is connected to the inverting input terminal of the operational amplifier U1A. First, the power supply charges C1. The charging time is related to the resistance value of resistor R1 and resistor R2 and the capacitance of capacitor C1. The charging amplitude is related to the voltage value of the non-inverting input terminal. When the charging voltage is greater than the non-inverting input voltage value, the op amp’s When the output is zero, the capacitor C1 is discharged through the resistor R2, the diode D1 and the operational amplifier U1A. After the discharge is completed, the charging process is carried out, and so on to form a waveform generating circuit.

The waveform generation circuit performs the same-phase ratio amplification through U1B, resistors R9 and R10, and its amplification ratio is related to the resistance values of R9 and R10. Resistor R11 is a jitter resistor, and capacitor C3 is a jitter capacitor. When the jitter capacitor C3 is constant, changing the jitter resistor R11 can change the charging current of the built-in oscillator of the PWM controller, thereby changing the frequency of the output jitter square wave. When the jitter resistance R11 is constant, change the capacitance of the jitter capacitor C3, change the high and low voltage thresholds when the jitter capacitor C3 is charged and discharged, and then change the charging time of the built-in oscillator of the PWM controller, thereby changing the frequency of the output jitter square wave.

The PWM controller is UCC3895, its fixed resistor is R12, and its fixed capacitor is C4, so its fixed frequency is

${\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}{\mathrm{<span\; class="notranslate">\; 48</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 120</span>}\mathrm{<span\; class="notranslate">\; no</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}$

Among them: R _T is the fixed resistor R12, C _T is the fixed capacitor C4.

The output OUTA ~ OUTD is a jittering square wave. Among them, OUTA and OUTB are completely complementary, and the working duty cycle can reach 50%. The jittering square waves output by OUTA and OUTC have a certain phase shift, and the jittering square waves output by OUTB and OUTD also have a certain phase shift.

Example of switching power supply

A switching power supply includes a frequency dithering circuit. The frequency dithering circuit is the same as the frequency dithering circuit in the above embodiment, so it will not be repeated here.

Claims (4)

1. a frequency jitter circuit, comprises PWM controller, it is characterized in that: also comprise wave generator circuit, and the shake electric capacity Cd that described wave generator circuit passes through to connect is connected the RT pin of PWM controller with shake resistance Rd; The RT pin of described PWM controller is by fixed resistance Rt ground connection; The CT pin of described PWM controller is by fixed capacity Ct ground connection.

2. frequency jitter circuit according to claim 1, it is characterized in that: described frequency jitter circuit also comprises operational amplifier, described wave generator circuit connects shake electric capacity Cd by operational amplifier, and described operational amplifier is used for amplifying the signal of waveform generator.

3. a Switching Power Supply, is characterized in that: comprise PWM controller and wave generator circuit, and the shake electric capacity Cd that described wave generator circuit passes through to connect is connected the RT pin of PWM controller with shake resistance Rd; The RT pin of described PWM controller is by fixed resistance Rt ground connection; The CT pin of described PWM controller is by fixed capacity Ct ground connection.

4. Switching Power Supply according to claim 3, it is characterized in that: described frequency jitter circuit also comprises operational amplifier, described wave generator circuit connects shake electric capacity Cd by operational amplifier, and described operational amplifier is used for amplifying the signal of waveform generator.