Control method and control circuit of flyback converter

Abstract

The application provides a control method and a control circuit of a flyback converter. The primary inductor of the flyback converter is connected with a main power tube, and receives a feedback signal representing the output power of the flyback converter. With the increase of the feedback signal, the switching frequency of the flyback converter enters a frequency reduction working interval from a maximum working frequency. The minimum working frequency of the frequency reduction working interval is set according to the output voltage of the flyback converter. When the flyback converter works in the frequency reduction working interval, the main power tube is turned on to start the next switching cycle when the switching cycle of the main power tube reaches the switching cycle corresponding to the minimum working frequency. The application can increase the load capacity of the converter in the application of low voltage and large current.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronics, in particular to a control method and control circuit of a flyback converter. BACKGROUND

[0002] The flyback converter is widely used in mobile phone and notebook adapter due to its simplicity, reliability and high efficiency. With the emergence of universal Type-C interface and PD universal protocol, all mobile phones, notebook computers and other small power consumer electronics can share adapters. With the gradual increase of adapter power, from 20W, 40W, 65W to the current 135W, the flyback converter is the most common topology in these different power levels.

[0003] The flyback converter is generally controlled to turn on the power tube when the primary power tube voltage resonates to the trough. Here, the auxiliary winding sampling voltage is generally used to realize ZCD (inductor current zero crossing detection). When the output voltage is too low, the flyback converter works in continuous mode, the output is short-circuited or in the starting process, due to the long freewheeling time of the secondary side inductor, it may not be able to detect the zero crossing point of the primary side inductor current. The usual way is to turn on in the next cycle after a fixed time, in order to force it to work in discontinuous mode, this fixed time can be set to 100us or longer, and some schemes fix it at 40us to avoid entering the audio range. However, due to the setting of this fixed time, it limits some low-voltage and high-current applications, especially for some battery charging occasions, the power output requirements from 21V compatible to 3.3V, and the 3.3V needs to have high load capacity. These methods limit the current power application. SUMMARY

[0004] The purpose of the present application is to provide a flyback converter control method and control circuit with wide application range, which solves the problem of frequency fixation and application limitation in the prior art.

[0005] To achieve the above purpose, the present application provides a flyback converter control method, the primary side of the flyback converter includes a primary side inductor and a main power tube connected in series, an auxiliary winding is coupled with the primary side winding,

[0006] A feedback signal representing the output power of the flyback converter is received, as the feedback signal increases, the switching frequency of the flyback converter enters the frequency reduction working interval from the maximum working frequency, and the minimum working frequency of the frequency reduction working interval is set according to the output voltage;

[0007] When the flyback converter works in the frequency reduction working interval, when the switching period of the flyback converter reaches the switching period corresponding to the minimum switching frequency, the main power tube is turned on to start the next switching period.

[0008] Optionally, when the flyback converter works in the frequency reduction operation interval, the secondary side inductor current zero-crossing is detected by detecting the voltage on the auxiliary winding, or when the switching period of the flyback converter reaches the switching period corresponding to the minimum switching frequency, the main power is turned on to start the next switching period.

[0009] Optionally, the minimum operating frequency decreases with the increase of the output voltage of the flyback converter.

[0010] Optionally, when the output voltage is less than a first threshold, the minimum operating frequency decreases with the increase of the output voltage.

[0011] Optionally, the minimum operating frequency decreases linearly or stepwise with the increase of the output voltage.

[0012] Optionally, the first capacitor is charged by the first constant current and the first voltage-controlled current to obtain a first capacitor voltage;

[0013] The first capacitor voltage and the first reference voltage are compared to obtain a pulse signal representing the minimum operating frequency;

[0014] The first voltage-controlled current is inversely proportional to the output voltage.

[0015] At the turn-on time of the main power tube, the first capacitor voltage is reset to zero and then the first capacitor is charged again.

[0016] The present application also provides a control circuit of a flyback converter, the primary side of the flyback converter comprising a primary side inductor and a main power tube, the primary side inductor being connected with the main power tube, and an auxiliary winding being coupled with the primary winding, as the feedback signal representing the output power of the flyback converter increases, the switching frequency of the flyback converter enters the frequency reduction operation interval from the maximum operating frequency, and the control circuit comprises,

[0017] An adjustment circuit receives a sampling signal of the output voltage of the flyback converter, and adjusts the minimum operating frequency of the frequency reduction operation interval according to the sampling signal;

[0018] A zero-crossing detection circuit detects the auxiliary winding voltage when the main switch tube is turned off to detect the inductor current zero-crossing time;

[0019] A first control circuit controls the main power tube to be turned on to start the next switching period when the switching period of the flyback converter reaches the switching period corresponding to the minimum operating frequency, or the zero-crossing detection circuit detects the inductor current zero-crossing, when the flyback converter works in the frequency reduction operation interval.

[0020] Optionally, the minimum operating frequency decreases with the increase of the output voltage.

[0021] Optionally, the adjusting circuit comprises,

[0022] a first constant current source and a first voltage-controlled current source, the first constant current source outputs a first constant current, and the first voltage-controlled current source outputs a first voltage-controlled current; the first voltage-controlled current is inversely proportional to the output voltage;

[0023] a first capacitor and a first switch, the first switch is connected across the first capacitor, the first constant current and the first voltage-controlled current charge the first capacitor to obtain a first capacitor voltage; when the main power tube is turned on, the first switch is turned on, and after the first capacitor voltage is reset to zero, the first switch is turned off again;

[0024] a first comparator receiving the first capacitor voltage and a first reference voltage, and outputting a first pulse signal, the first pulse signal representing the minimum operating frequency.

[0025] Compared with the prior art, the present application has the following advantages: as the feedback signal increases, the switching frequency of the flyback converter enters the frequency reduction operating interval from the maximum operating frequency, and the minimum operating frequency of the frequency reduction operating interval is set according to the output voltage; when the flyback converter works in the frequency reduction operating interval, when the switching period of the flyback converter reaches the switching period corresponding to the minimum switching frequency, the main power is turned on to start the next switching period. The present application can increase the load capacity of the converter in low-voltage and high-current application scenarios; when the converter outputs high voltage, the stress on the power device can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a schematic diagram of the flyback converter of the present application;

[0027] Figure 2 is a schematic diagram of the adjusting circuit of the present application;

[0028] Figure 3 is a waveform diagram of the load capacity when the present application outputs low voltage;

[0029] Figure 4 is a waveform diagram of the switching frequency of the present application. DETAILED DESCRIPTION

[0030] The preferred embodiments of the present application are described in detail below with reference to the accompanying drawings, but the present application is not limited to only these embodiments. The present application covers any alternative, modification, equivalent method and scheme made within the spirit and scope of the present application.

[0031] In order for the public to have a thorough understanding of the present application, specific details are described in detail in the following preferred embodiments of the present application, and the present application can also be completely understood without the description of these details by those skilled in the art.

[0032] The invention is described more specifically in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the drawings are in a simplified form and use non-precise proportions, in order to facilitate and clearly illustrate the purpose of the embodiments of the invention.

[0033] like Figure 1 The diagram illustrates the schematic of the flyback converter and its control circuit of the present invention. The flyback converter includes a transformer T consisting of a primary winding Np and a secondary winding Ns, a resonant capacitor Cp, a first diode Dp, a rectifier diode D0, an output capacitor C0, and a feedback network. The resonant capacitor Np and the diode Dp are connected in parallel across the primary winding Np. The main power transistor M0 is connected to the primary winding, and the sampling resistor Rcs is connected to the main power transistor M0 to sample the current flowing through it. The rectifier diode D0 is connected to the secondary winding Ns, and the output capacitor C0 is connected across the secondary winding Ns. The feedback network is connected to the output terminal to obtain the feedback signal FB. The auxiliary winding Naux is mutually inducted with the primary winding Np. The voltage of the auxiliary winding is divided by two voltage-dividing resistors to obtain the voltage sampling signal Vs. The flyback converter operates as follows: as the feedback signal FB increases, the switching frequency of the flyback converter shifts from the maximum operating frequency to a reduced-frequency operating range. The minimum operating frequency of this reduced-frequency operating range is set based on the output voltage. The feedback signal FB represents the output power of the flyback converter, and the maximum operating frequency is not greater than the set maximum switching frequency. The minimum operating frequency decreases as the output voltage increases, and this preset value can decrease linearly or in a stepwise manner as the output voltage increases. The maximum switching frequency is the set upper limit of the system operating frequency to ensure that the system frequency is controllable.

[0034] The control circuit includes an adjustment circuit U01, a zero-crossing detection circuit U02, and a first control circuit U03. The adjustment circuit U01 receives the sampling signal Vs of the output voltage to adjust the minimum operating frequency. When the flyback converter is in the frequency reduction operating range, if the switching cycle of the flyback converter reaches the switching cycle corresponding to the minimum operating frequency, or if the zero-crossing detection circuit U02 detects that the secondary inductor current crosses zero, the first control circuit U03 controls the main power transistor to turn on to start the next operating cycle.

[0035] like Figure 2The diagram illustrates the frequency adjustment circuit of the present invention, including a fixed current source i0, a voltage-controlled current source i1, a first capacitor Cf, a first switch k1, and a first comparator U101. The first switch k1 is connected in parallel across the first capacitor Cf. When the main power transistor is turned on, a short pulse signal is applied to control the first switch k1 to turn on. After the voltage of the first capacitor Cf discharges to zero, the first switch k1 turns off again. The output currents of the fixed current source i0 and the voltage-controlled current source i1 charge the first capacitor Cf to obtain a first capacitor voltage Vc1. The first comparator U101 compares the first capacitor voltage Vc1 with a first reference voltage VREF to obtain a comparison signal f. QR_min When the voltage of the first capacitor Vc1 rises to the first reference voltage VREF, the comparison signal f QR_min From valid to invalid, compare signal f QR_min The pulse frequency corresponding to the high level represents the minimum operating frequency. The output current of the voltage-controlled current source i1 is controlled by the voltage sampling signal Vs and is inversely proportional to the voltage sampling signal Vs.

[0036] Based on the above analysis, when the high voltage output is 20V, the switching frequency of the main switch transistor M0 can be designed to be around 20kHz, which can effectively prevent the converter from operating in CCM mode; when the low voltage output is 3.3V, the switching frequency of the main switch transistor can be designed to be 40-50kHz. Although the flyback converter operates in CCM mode at this time, it can improve the load-carrying capacity at low voltage output.

[0037] like Figure 3 The diagram illustrates the load-carrying capacity waveform of the flyback converter at low voltage output. Taking an output voltage of 3.3V as an example, the higher the switching frequency, the higher the feedback voltage FB, the higher the output power Po, and the stronger the load-carrying capacity.

[0038] like Figure 4 The diagram illustrates the switching frequency waveform of the flyback converter of the present invention. Taking an output voltage of 20V as an example, when the feedback voltage FB increases to V1, the switching frequency of the flyback converter is the maximum operating frequency of the flyback converter. When the feedback voltage FB is greater than V1, the flyback converter enters the frequency reduction operating range. As the feedback voltage FB further increases, the minimum operating frequency of the frequency reduction operating range decreases with the increase of the output voltage. Preferably, the minimum operating frequency can decrease linearly or stepwise with the increase of the output voltage. The stepwise decrease specifically means that as the output voltage increases, the minimum operating frequency decreases in stages, while the minimum operating frequency remains unchanged in the corresponding stages. For example, as the output voltage increases, when the output voltage is between VO1 and VO2, Fs is k1; when the output voltage is between VO2 and VO3, Fs is k2 and less than k1, and so on.

[0039] Although the above embodiments are described and illustrated separately, the technologies involved in the parts in common can be replaced and integrated between the embodiments, and the content not explicitly recorded in one embodiment can be referred to the other recorded embodiment.

[0040] The above-described embodiments do not constitute a limitation on the protection scope of the technical solutions. Any modifications, equivalent replacements, improvements, and the like made within the spirit and principles of the above-described embodiments shall be included in the protection scope of the technical solutions.

Claims

1.A control method of a flyback converter, the primary side of the flyback converter comprising a primary winding and a main power transistor connected in series, and an auxiliary winding coupled to the primary winding, the method comprising: receiving a feedback signal indicative of an output power of the flyback converter, and as the feedback signal increases, the switching frequency of the flyback converter enters a frequency reduction region from a maximum operating frequency, and the minimum operating frequency of the frequency reduction region is set according to an output voltage of the flyback converter; when the flyback converter operates in the frequency reduction region, the main power transistor is turned on to start a next switching cycle when a switching period of the flyback converter reaches a switching period corresponding to the minimum operating frequency. when the flyback converter operates in the frequency reduction region, the secondary winding current zero-crossing is detected by detecting a voltage on the auxiliary winding, or the main power transistor is turned on to start a next switching cycle when a switching period of the flyback converter reaches a switching period corresponding to the minimum operating frequency. the minimum operating frequency decreases as the output voltage of the flyback converter increases. when the output voltage is less than a first threshold, the minimum operating frequency decreases as the output voltage increases. the minimum operating frequency decreases linearly or stepwise as the output voltage increases. a first constant current and a first voltage-controlled current are used to charge a first capacitor to obtain a first capacitor voltage; the first capacitor voltage and a first reference voltage are compared to obtain a pulse signal indicative of the minimum operating frequency; the first voltage-controlled current is inversely proportional to the output voltage; at the turn-on time of the main power transistor, the first capacitor voltage is reset to zero, and the first capacitor is charged again. as the feedback signal indicative of the output power of the flyback converter increases, the switching frequency of the flyback converter enters a frequency reduction region from a maximum operating frequency, and the control circuit comprises: an adjustment circuit receiving a sampling signal of an output voltage of the flyback converter, and adjusting the minimum operating frequency of the frequency reduction region according to the sampling signal; a zero-crossing detection circuit detecting the auxiliary winding voltage when the main switch is off to detect the inductor current zero-crossing time; a first control circuit, when the flyback converter operates in the frequency reduction region, the main power transistor is turned on to start a next switching cycle when a switching period of the flyback converter reaches a switching period corresponding to the minimum operating frequency, or the zero-crossing detection circuit detects the inductor current zero-crossing. the minimum operating frequency decreases as the output voltage increases. the minimum operating frequency decreases linearly or stepwise as the output voltage increases. the adjustment circuit comprises: a first constant current source and a first voltage-controlled current source, the first constant current source outputs a first constant current, and the first voltage-controlled current source outputs a first voltage-controlled current; the first voltage-controlled current is inversely proportional to the output voltage; a first capacitor and a first switch connected across the first capacitor, the first constant current and the first voltage-controlled current charge the first capacitor to obtain a first capacitor voltage; at the turn-on time of the main power transistor, the first switch is turned on, the first capacitor voltage is reset to zero, and the first switch is turned off again. ​ ​ 2. The control method of a flyback converter according to claim 1, characterized in that: ​ 3. The control method of a flyback converter according to claim 1, characterized in that: ​ 4. The control method of a flyback converter according to claim 3, characterized in that: ​ 5. The control method of a flyback converter according to claim 3 or 4, characterized in that: ​ 6. The control method of a flyback converter according to claim 1, characterized in that: ​ ​ ​ ​ 7. A control circuit of a flyback converter, a primary side part of the flyback converter comprising a primary winding and a main power tube, the primary winding being connected with the main power tube, an auxiliary winding being coupled with the primary winding, characterized in that: ​ ​ ​ ​ 8. The control circuit of a flyback converter according to claim 7, characterized in that: ​ 9. The control circuit of a flyback converter according to claim 7, characterized in that: ​ 10. The control circuit of a flyback converter according to claim 7, characterized in that: ​ ​ ​ A first comparator receives the first capacitor voltage and a first reference voltage, and outputs a first pulse signal, the first pulse signal representing the minimum operating frequency.

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