Flyback power converter and control method thereof

Abstract

The present invention discloses a flyback power converter, comprising a main power switch tube, a transformer and a rectifier tube, wherein the transformer comprises a primary winding and a secondary winding, the main power switch tube is connected to the primary winding, and the rectifier tube is connected to the secondary winding; a primary control circuit is used to determine whether the output voltage of the secondary side is lower than a first threshold value, and when the output voltage of the flyback power converter is lower than the first threshold value, a reference voltage is increased to increase the switching frequency of the main power switch tube; a feedback signal representing the output voltage of the flyback power converter is subjected to error processing with a corresponding reference signal to obtain the reference voltage. The present invention improves the dynamic response of the flyback power converter.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a flyback power converter and a control method thereof. Background Art

[0002] In order to stabilize the output voltage, the existing flyback converter (switching power supply) needs to sample the output voltage on the output side for feedback, and transmit the error signal to the primary controller through the optical coupler. The primary controller adjusts the switching action of the switch tube based on the error signal to adjust the output voltage. In order to further reduce costs, in some low-power flyback transformers, the primary controller directly samples the output voltage based on the auxiliary winding on the primary side to control the output voltage by utilizing the characteristic that the voltage induced on the auxiliary winding of the transformer is proportional to the output voltage during the shutdown period of the primary switch tube. This is the primary control technology of the flyback switching power supply. In the primary control, in order to obtain the output voltage information, the primary switch must be turned on and off once before the output voltage can be sampled on the auxiliary winding. Under light load or even no-load conditions, in order to improve light load efficiency and reduce standby loss, the switching frequency of the primary switch will drop to a very low level under light load and no-load conditions, that is, the sampling frequency of the output voltage is very low.

[0003] At a very low switching frequency, if a jump occurs from no-load or light-load conditions to a heavier load, it will take a long time to increase the switching frequency of the primary main power switch tube to match the current load conditions, and the output voltage will experience a large "drop", with poor dynamic response. Summary of the invention

[0004] The object of the present invention is to provide a flyback power converter with improved dynamic response and a control method thereof, so as to solve the technical problem of poor dynamic response after load jump in the prior art.

[0005] To achieve the above-mentioned object, the present invention provides a flyback power converter of the following structure, comprising a main power switch tube, a transformer and a rectifier tube, wherein the transformer comprises a primary winding and a secondary winding, the main power switch tube is connected to the primary winding, the rectifier tube is connected to the secondary winding, the control end of the main power switch tube is connected to the primary control circuit, and the control end of the rectifier tube is connected to the secondary control circuit;

[0006] Determining whether the output voltage of the secondary side is lower than a first threshold value by the primary side control circuit, and when the output voltage of the flyback power converter is lower than the first threshold value, increasing the reference voltage to increase the switching frequency of the main power switch tube;

[0007] The feedback signal representing the output voltage of the flyback power converter is subjected to error processing with a corresponding reference signal to obtain the reference voltage.

[0008] Optionally, the flyback power converter further includes an auxiliary winding, which is coupled to the transformer, and the input voltage of the auxiliary winding is sampled to obtain the feedback signal.

[0009] Optionally, the flyback power converter further includes an error amplifier and a compensation circuit, the feedback signal and the reference signal are respectively input into two input terminals of the error amplifier, and the output terminal of the error amplifier is connected to the compensation circuit.

[0010] Optionally, the increasing of the reference voltage is achieved by increasing the gain or tail current of the error amplifier.

[0011] Optionally, the gain or tail current of the error amplifier may be increased before the output voltage of the flyback power converter is lower than a first threshold value, or after the output voltage of the flyback power converter is lower than the first threshold value.

[0012] Optionally, increasing the reference voltage is achieved by increasing the resistance value in the compensation circuit or reducing the capacitance value in the compensation circuit.

[0013] Optionally, increasing the resistance in the compensation circuit or reducing the capacitance in the compensation circuit may be performed before the output voltage of the flyback power converter is lower than a first threshold value, or after the output voltage of the flyback power converter is lower than the first threshold value.

[0014] Optionally, when the output voltage of the flyback switching power supply is lower than a first threshold value, the secondary side control circuit controls the rectifier tube to be briefly turned on; the primary side control circuit detects the negative current flowing through the main power switch tube to determine whether the rectifier tube is turned on. When the negative current reaches a second threshold value, it is determined that the output voltage of the secondary side is lower than the threshold voltage, thereby increasing the reference voltage.

[0015] Optionally, the primary side control circuit further includes a first current limiting module, which is used to detect the current of the main power switch tube and limit the current of the main power switch tube when the current of the main power switch tube reaches a current limiting value.

[0016] Optionally, the secondary side control circuit includes a second current limiting module, and the second current limiting module is used to detect the current of the rectifier tube and limit the current of the rectifier tube when the current of the rectifier tube reaches a current limiting value.

[0017] Optionally, when the current flowing through the secondary winding drops to zero, after a first delay, detection of the negative current flowing through the main power switch tube begins.

[0018] The present invention also provides a control method for a flyback power converter, wherein the flyback power converter comprises a main power switch tube, a transformer and a rectifier tube, wherein the transformer comprises a primary winding and a secondary winding, wherein the main power switch tube is connected to the primary winding, wherein the rectifier tube is connected to the secondary winding, wherein the control end of the main power switch tube is connected to the primary control circuit, and wherein the control end of the rectifier tube is connected to the secondary control circuit;

[0019] Determining whether the output voltage of the secondary side is lower than a first threshold value by the primary side control circuit, and when the output voltage of the flyback power converter is lower than the first threshold value, increasing the reference voltage to increase the switching frequency of the main power switch tube;

[0020] The feedback signal representing the output voltage of the flyback power converter is subjected to error processing with a corresponding reference signal to obtain the reference voltage.

[0021] Compared with the prior art, the technical solution of the present invention has the following advantages: when the load becomes heavier under light load, the present invention increases the reference voltage by changing the gain of the error amplifier or the resistance or capacitance in the compensation circuit, so that the reference voltage can timely feedback the changes in the load or output voltage to increase the switching frequency and improve the dynamic response. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The schematic diagram of the circuit of the first embodiment of the flyback power converter of the present invention;

[0023] Figure 2 is a schematic diagram of the circuit structure of the error amplifier and the compensation circuit;

[0024] Figure 3 It is a waveform diagram showing the relationship between the reference voltage Vc and the switching frequency Fs of the main power switch tube;

[0025] Figure 4 It is a working waveform diagram of the flyback power converter of the present invention. DETAILED DESCRIPTION

[0026] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitution, modification, equivalent method and scheme made within the spirit and scope of the present invention.

[0027] In order to make the public have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, but those skilled in the art can fully understand the present invention without these detailed descriptions.

[0028] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all simplified and not in exact proportion, and are only used for the purpose of conveniently and clearly assisting in explaining the embodiments of the present invention.

[0029] like Figure 1 The flyback converter of the present invention includes a main power switch tube M0, a transformer and a rectifier tube. The transformer includes a primary winding Np and a secondary winding Ns. The main power switch tube M0 is connected to the primary winding Np, and the rectifier tube is connected to the secondary winding Ns. The rectifier tube adopts a transistor. Figure 1 In the embodiment shown, a field effect transistor (MOSFET) is used as the rectifier tube M. SR , also known as synchronous rectifier, can be N-type or P-type enhancement MOSFET, or other types of field effect tubes. For those skilled in the art, the above changes are common knowledge. Synchronous rectifiers can also be used as high-side rectifiers or low-side rectifiers.

[0030] This embodiment takes an N-type enhancement MOSFET as an example. When the current flows from the source to the drain, the synchronous rectifier is turned on, and when the current flows in the reverse direction, the synchronous rectifier is turned off. The normal on and off control circuit of the synchronous rectifier can be included in the secondary side control circuit or can be a separate control circuit.

[0031] The flyback power converter also includes a control circuit, which includes a primary control circuit and a secondary control circuit. The primary control circuit is connected to the control end of the main power switch tube M0 located on the primary side, and the secondary control circuit is connected to the rectifier tube M0 located on the secondary side. SR The primary control circuit and the secondary control circuit can be made into integrated circuits respectively, and the integration in the chip can be partial integration or full integration, depending on the components and the actual application. The primary control circuit is connected to an auxiliary winding Na, and the primary control circuit can be powered by the auxiliary winding Na.

[0032] When the load is light, the flyback power converter operates in the discontinuous conduction mode (DCM), and the switching frequency of the primary main power switch tube M0 is low. When the load becomes heavier, the dynamic response is slow, and the output voltage will drop. The primary control circuit is used to determine whether the output voltage of the secondary side is lower than the threshold voltage. When the output voltage of the flyback power converter is lower than the first threshold, the reference voltage Vc is increased to increase the switching frequency of the main power switch tube M0, thereby improving the dynamic response of the system; the feedback signal FB representing the output voltage of the flyback power converter is error processed with the corresponding reference signal REF to obtain the reference voltage Vc.

[0033] The auxiliary winding Na is coupled to the transformer, and the input voltage of the auxiliary winding Na is sampled to obtain the feedback signal FB.

[0034] like Figure 2 As shown, the circuit structure of the error amplifier and the compensation circuit is illustrated. The flyback power converter also includes an error amplifier A1 and a compensation circuit, the feedback signal FB and the reference signal REF are respectively input into the two input terminals of the error amplifier A1, and the output terminal of the error amplifier A1 is connected to the compensation circuit. The compensation circuit includes at least one capacitor, and can also be a combination of multiple capacitors or capacitors and resistors. In this embodiment, a structure in which a capacitor C1 and a resistor R1 are connected in series is adopted to achieve this. The increase in the reference voltage Vc is achieved by increasing the gain or tail current I1 of the error amplifier A1. The purpose of increasing the tail current I1 is ultimately to increase the gain to increase the reference voltage Vc.

[0035] In addition to changing the characteristics of the error amplifier A1, the compensation circuit can also be adjusted. The reference voltage is increased by increasing the resistance of the resistor R5 in the compensation circuit or reducing the capacitance of the capacitor C1 in the compensation circuit. Of course, adjusting the capacitance and resistance does not only refer to adjusting a single component, but also can be adjusted in the form of a capacitor group or a resistor group.

[0036] The gain or tail current of the error amplifier may be increased before the output voltage of the flyback power converter is lower than the first threshold value, or after the output voltage of the flyback power converter is lower than the first threshold value. The resistance value in the compensation circuit may be increased or the capacitance value in the compensation circuit may be decreased before the output voltage of the flyback power converter is lower than the first threshold value, or after the output voltage of the flyback power converter is lower than the first threshold value.

[0037] like Figure 3 As shown, the waveform of the relationship between the reference voltage Vc and the switching frequency Fs of the main power switch tube is illustrated. Under light load conditions, the reference voltage Vc is in the range of 0-Vc1-Vc2. In this range, the switching frequency Fs does not change with the reference voltage Vc. The purpose of increasing the quasi-voltage Vc is to make the quasi-voltage Vc enter the Vc1-Vc2 range. In this range, the switching frequency Fs increases with the increase of the reference voltage Vc. Therefore, the switching frequency can be increased by increasing the reference voltage Vc, thereby improving the dynamic response.

[0038] like Figure 4As shown, the working waveform of the present invention is illustrated. The output voltage can be detected by the voltage detection module on the secondary side. When the output voltage of the flyback switching power supply is lower than the first threshold, the rectifier tube is controlled to be turned on briefly. The present invention is generally used in the discontinuous conduction mode (DCM). Before the rectifier tube is controlled to be turned on briefly, the main power switch tube and the rectifier tube are both in the off state.

[0039] Because the rectifier tube is briefly turned on, a negative current will be generated on the primary side. Whether the rectifier tube is turned on is determined by detecting the negative current flowing through the main power switch tube. When the negative current reaches the second threshold, the negative current can be characterized by a voltage. In this embodiment, the voltage on the CS pin is used to determine whether the output voltage on the secondary side is lower than the threshold voltage, thereby increasing the reference voltage Vc.

[0040] The primary control circuit also includes a first current limiting module, which is used to detect the current of the main power switch tube and limit the current of the main power switch tube when the current reaches the current limit value, so as to prevent excessive current when the primary and secondary sides are directly connected or connected. A person of ordinary skill in the art can know the specific implementation method of the current limiting module according to the instructions of the specification, so for the sake of clarity, the specific circuit of the current limiting module is not illustrated. The secondary control circuit also includes a second current limiting module, which is used to limit the current flowing through the rectifier tube to avoid damage caused by excessive current when directly connected.

[0041] The short-term conduction is equivalent to a pulse, and the width of the pulse represents the duration of the short-term conduction. The continuous detection of the output voltage can be achieved through a secondary side control circuit, and the state of the rectifier tube is controlled according to the detection result.

[0042] The primary control circuit is connected to an auxiliary winding Na, which is used to power the primary control circuit and is connected to the power supply pin VCC of the primary control circuit. The auxiliary winding Na is coupled to the transformer, and a voltage detection module for detecting the voltage across the auxiliary winding Na is connected to the auxiliary winding Na. The voltage detection module is composed of voltage-dividing resistors R1 and R2 connected in series, and the common end of the voltage-dividing resistors R1 and R2 is connected to the VS pin of the primary control circuit. Since the auxiliary winding Na is coupled to the output winding, there is a fixed turns ratio relationship. When the main power switch tube M0 is turned on, the transformer stores energy. When the main power switch tube M0 is turned on, the secondary rectifier tube M SR The voltage induced at both ends of the Na winding is proportional to the output voltage. The primary control circuit can obtain the output voltage information by sampling the VS pin voltage information. It should be pointed out that the primary control circuit can only detect the output voltage when the secondary rectifier is turned on.

[0043] Resistor R3 samples the current flowing into the main power switch tube M0, and one end of R3 is connected to the primary ground (zero potential). When the voltage on R3 is a positive voltage, it indicates that the current is positive. When the voltage on R3 is a negative voltage, it indicates that the current is negative. If the voltage detection module detects that the input voltage of the auxiliary winding is less than a smaller set threshold value (low threshold value), the low threshold value can be zero or a value close to zero, indicating that the rectifier tube is in the off state. At this time, if the negative current detected by the primary control circuit reaches the second threshold value, the main power switch tube M0 is controlled to be turned on.

[0044] The detection of negative current is often characterized by negative voltage, such as Figure 1 As shown, the first end of the main power switch tube M0 is connected to the primary winding Np, the second end of the main power switch tube M0 is grounded via a resistor R3, the common end of the second end of the main power switch tube M0 and the resistor R3 is connected to a resistor R4, the other end of the resistor R4 is connected to the detection pin CS of the primary control circuit, and the voltage of the CS pin is related to the voltage of the sampling resistor R3. The polarity and magnitude of the voltage of the CS pin characterize the polarity and magnitude of the sampled current. The voltage at the pin CS is used to characterize the negative current and compare it with the voltage characterizing the second threshold value, so as to determine whether the negative current reaches the second threshold value. In addition, the primary control circuit also includes a drive pin DRV, which is connected to the gate of the main power switch tube M0.

[0045] In the secondary winding Ns and the rectifier tube M SR The common end of the secondary side control circuit is connected to the pin SW. The secondary side control circuit is further provided with an output voltage pin Vo, and the voltage between the pin Vo and the ground pin GND represents the output voltage.

[0046] Combination Figure 1 The technical scheme and working process of the present invention are further described. After the secondary current passes through zero, the excitation inductor and parasitic capacitor will generate an oscillating current of parasitic oscillation, generally in about 10 cycles. In order to prevent the oscillating current from being mistakenly detected as the negative current, a delay first time t1 is set after the secondary current passes through zero or the rectifier tube is turned off, such as Figure 4 As shown, the first time t1 can be set to 30us, and the negative current flowing through the main power switch tube M0 is detected, that is, the negative current detection threshold (that is, the second threshold) can be around -10 to 20mV. If the voltage detection module detects that the input voltage of the auxiliary winding is zero, that is, VS = 0, it means that the rectifier tube M SR In the off state, if the negative current detected by the primary control circuit reaches the second threshold, the rectifier tube M is controlled SRIf no negative current is detected, the primary power switch is turned on again in response to the secondary voltage drop; if no negative current is detected, the primary power switch is turned on according to the preset control. In addition, within the first time t1, if the primary power switch has been turned on again, it means that the switching frequency is still relatively high, which means that there is no dynamic response problem.

[0047] Regarding the secondary side control circuit, in the rectifier tube M SR After shutting down according to normal logic, the output voltage detection function is enabled. When the output voltage is less than the set value (i.e., the first threshold) (e.g., 4.75V), the rectifier tube M SR After the shutdown, the second time t2 is delayed, and the rectifier tube M of the secondary control circuit is briefly turned on. SR , in order to "transfer" negative current to the primary side to notify the primary side to increase the reference voltage Vc. At the same time, the detection of the output voltage can also be enabled all the time.

[0048] The second time t2 can be set to about 50us. The consideration for setting the time of 50us is that the primary side needs to be able to detect negative current after 30us. Considering a certain tolerance or margin, the secondary side time is appropriately longer. Generally speaking, the rectifier tube is turned off when the secondary current passes through zero, so the time when the secondary current passes through zero and the time when the rectifier tube is turned off are approximately the same time, but in practice they are not strictly the same time.

[0049] For the rectifier tube M SR The short-term conduction time can be limited to within or around 1us, and the specific setting needs to be based on actual parameters. In addition to using time to limit the short-term conduction time, this time can also be determined based on the secondary current.

[0050] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0051] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. A flyback power converter, comprising a main power switch tube, a transformer, a rectifier tube and an auxiliary winding, wherein the auxiliary winding is coupled to the transformer, the transformer comprises a primary winding and a secondary winding, the main power switch tube is connected to the primary winding, the rectifier tube is connected to the secondary winding, the control end of the main power switch tube is connected to the primary control circuit, and the control end of the rectifier tube is connected to the secondary control circuit; When the output voltage of the flyback power converter is lower than the first threshold, the secondary control circuit controls the rectifier tube to be turned on briefly; the primary control circuit detects the negative current flowing through the main power switch tube to determine whether the rectifier tube is turned on, and when the negative current reaches the second threshold, it is determined that the output voltage of the secondary side is lower than the first threshold voltage; when the output voltage of the flyback power converter is lower than the first threshold, the primary control circuit increases the reference voltage to increase the switching frequency of the main power switch tube; in, The feedback signal representing the output voltage of the flyback power converter is subjected to error processing with a corresponding reference signal to obtain the reference voltage; and the input voltage of the auxiliary winding is sampled to obtain the feedback signal.

2. The flyback power converter according to claim 1, characterized in that: The flyback power converter further includes an error amplifier and a compensation circuit. The feedback signal and the reference signal are respectively input into two input terminals of the error amplifier, and the output terminal of the error amplifier is connected to the compensation circuit.

3. The flyback power converter according to claim 2, characterized in that: The raising of the reference voltage is achieved by increasing the gain or tail current of the error amplifier.

4. The flyback power converter according to claim 3, characterized in that: The gain or tail current of the error amplifier may be increased before the output voltage of the flyback power converter is lower than the first threshold, or after the output voltage of the flyback power converter is lower than the first threshold.

5. The flyback power converter according to claim 2, characterized in that: The raising of the reference voltage is achieved by increasing the resistance value in the compensation circuit or reducing the capacitance value in the compensation circuit.

6. The flyback power converter according to claim 5, characterized in that: The increasing of the resistance value in the compensation circuit or the decreasing of the capacitance value in the compensation circuit may be performed before the output voltage of the flyback power converter is lower than the first threshold value, or after the output voltage of the flyback power converter is lower than the first threshold value.

7. The flyback power converter according to claim 1, characterized in that: The primary side control circuit further includes a first current limiting module, which is used to detect the current of the main power switch tube and limit the current of the main power switch tube when the current of the main power switch tube reaches a current limiting value.

8. The flyback power converter according to claim 1, characterized in that: The secondary side control circuit includes a second current limiting module, which is used to detect the current of the rectifier tube and limit the current of the rectifier tube when the current reaches a current limiting value.

9. The flyback power converter according to claim 1, characterized in that: When the current flowing through the secondary winding drops to zero, after a first delay, the negative current flowing through the main power switch tube begins to be detected.

10. A control method for a flyback power converter, the flyback power converter comprising a main power switch tube, a transformer, a rectifier tube and an auxiliary winding, the auxiliary winding is coupled to the transformer, the transformer comprises a primary winding and a secondary winding, the main power switch tube is connected to the primary winding, the rectifier tube is connected to the secondary winding, the control end of the main power switch tube is connected to the primary control circuit, and the control end of the rectifier tube is connected to the secondary control circuit; characterized in that: When the output voltage of the flyback power converter is lower than the first threshold, the secondary control circuit controls the rectifier tube to be turned on briefly; the primary control circuit detects the negative current flowing through the main power switch tube to determine whether the rectifier tube is turned on, and when the negative current reaches the second threshold, it is determined that the output voltage of the secondary side is lower than the first threshold voltage; when the output voltage of the flyback power converter is lower than the first threshold, the primary control circuit increases the reference voltage to increase the switching frequency of the main power switch tube; The feedback signal representing the output voltage of the flyback power converter is subjected to error processing with a corresponding reference signal to obtain the reference voltage; and the input voltage of the auxiliary winding is sampled to obtain the feedback signal.

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