A low-power flyback converter with transformer primary voltage sampling

Abstract

The present invention provides a low-power flyback converter for transformer primary voltage sampling, comprising: a transformer T1, an output diode, an input capacitor, an output capacitor, a load, an N-channel MOS switch tube K1, a PNP transistor V1A, a PNP transistor V1B, a resistor R FB , resistor R REF , resistor R1, operational amplifier U1, operational amplifier U2, operational amplifier U3, boundary detection circuit, oscillator, two-input AND gate AND, RS trigger and drive circuit. The scheme of the present invention can directly collect the voltage reflected from the secondary side from the primary side, and convert it into the secondary side output voltage after processing, without the need for additional large-volume devices such as transformer winding feedback, optical isolator feedback or magnetic isolation feedback. The output voltage reflected from the transformer to the primary side is divided by two resistors and sent to the input end of the sampling and holding error amplifier for comparison with the reference value, thereby adjusting the duty cycle and frequency of the switch tube, and realizing stable power conversion and power supply isolation.

Description

Technical Field

[0001] The invention relates to the technical field of electrical engineering, and in particular to a low-power flyback converter for sampling primary voltage of a transformer. Background Art

[0002] With the development of power electronics technology, the health management of distributed systems has an increasingly broad prospect. The various parts of the system interact through high-speed information. In order to realize the information collection and status control of various parts of the system, each part is equipped with a minimum intelligent system. The minimum intelligent system has low single power, high integration, and small volume requirements. To meet this demand, high-density low-power isolated power supplies have broad application prospects.

[0003] In order to achieve stable control of the output voltage of the isolated power supply, the output voltage must be collected and then regulated according to the voltage transformation of the output voltage, so that the output voltage is stable within a specified range.

[0004] Traditional isolated low-power power supplies generally use optical isolator feedback, additional transformer winding feedback, or magnetic isolation feedback to isolate and feed back the output voltage information to the primary control end. Optical isolators need to have sufficient drive current to ensure the accuracy of the transmitted signal. For low-power isolated power supplies, this part of the current causes a waste of output current and reduces efficiency. In addition, optical isolation devices increase the overall cost and physical size of the power supply. At the same time, inherent characteristics such as limited dynamic response, nonlinearity, unit-to-unit variation, and life aging also increase the possibility of optical isolators causing system problems. Power supplies that use additional transformer windings for feedback will increase the physical size and cost of the isolation transformer because of the need to use additional windings, and the dynamic response performance of this feedback form is poor. Magnetic isolation feedback requires the setting of separate magnetic isolation devices and oscillating circuits, which are also greatly limited in physical size and cost. Summary of the invention

[0005] In view of the defects in the prior art, the present invention provides a low-power flyback converter for transformer primary voltage sampling, which realizes stable power conversion and power supply isolation and meets the power requirements of various parts of distributed system health management.

[0006] The present invention provides a low-power flyback converter for transformer primary voltage sampling, comprising: a transformer T1, an output diode, an input capacitor, an output capacitor, a load, an N-channel MOS switch tube K1, a PNP transistor V1A, a PNP transistor V1B, a resistor R FB , resistor R REF, resistor R1, operational amplifier U1, operational amplifier U2, operational amplifier U3, boundary detection circuit, oscillator, two-input AND gate AND, RS flip-flop and driving circuit;

[0007] One end of the secondary side of the transformer T1 is connected to the anode of the output diode, the cathode of the output diode is connected to one end of the output capacitor, the other end of the output capacitor is connected to the other end of the secondary side of the transformer T1, and both ends of the output capacitor are also connected to the load;

[0008] One end of the primary side of the transformer T1 is connected to the input power supply Vin, the input capacitor and the emitter of the PNP transistor V1A, and the other end of the primary side of the transformer T1 is connected to the drain of the N-channel MOS switch tube K1, the resistor R REF One end of the resistor R REF The other end of the transistor is connected to the emitter of the PNP transistor V1B, the base of the PNP transistor V1A is connected to the base of the PNP transistor V1B, the collector of the PNP transistor V1A is connected between the base of the PNP transistor V1A and the base of the PNP transistor V1B, the source of the N-channel MOS switch tube K1 is simultaneously connected to one end of the resistor R1 and the positive input end of the operational amplifier U1, and the other end of the resistor R1 is simultaneously connected to the reverse input end of the operational amplifier U1, the resistor R FB One end of the resistor R FB The other end is connected to the collector of the PNP transistor V1B, the output end of the operational amplifier U1 is connected to the positive input end of the operational amplifier U3, the reverse input end of the operational amplifier U2 is connected to the collector of the PNP transistor V1B, the positive input end is connected to the reference source 1, the output end of the operational amplifier U2 is connected to the reverse input end of the operational amplifier U3, the output end of the operational amplifier U3 is connected to the R input end of the RS trigger, the input end of the boundary detection circuit is connected to the collector of the PNP transistor V1B, the output end of the boundary detection circuit is connected to the input end of the two-input AND gate AND, the output end of the oscillator is connected to the other input end of the two-input AND gate AND, the output end of the two-input AND gate AND is connected to the S input end of the RS trigger, the Q output end of the RS trigger is connected to the input end of the drive circuit, and the output end of the drive circuit is connected to the gate of the N-channel MOS switch tube K1.

[0009] Compared with the prior art, the present invention has the following beneficial effects:

[0010] The circuit design adopted in the present invention can directly collect the voltage reflected from the secondary side from the primary side, and convert it into the secondary side output voltage after processing, without the need for additional large-volume devices such as transformer winding feedback, optical isolator feedback or magnetic isolation feedback. The output voltage reflected from the transformer to the primary side is divided by two resistors and sent to the input end of the sampling and holding error amplifier for comparison with the reference value, thereby adjusting the duty cycle and frequency of the switching tube, achieving stable power conversion and power supply isolation, and meeting the power requirements of various parts of the distributed system health management. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0012] Figure 1 A schematic diagram of the structure of a low-power flyback converter for sampling the primary voltage of a transformer provided in an embodiment of the present disclosure;

[0013] Figure 2 This is the startup waveform of the prototype power supply;

[0014] Figure 3 This is the load step waveform of the prototype power supply;

[0015] Figure 4 This is the output ripple diagram of the prototype power supply. DETAILED DESCRIPTION

[0016] Embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although certain embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as being limited to the embodiments described herein, which are instead provided for a more thorough and complete understanding of the present invention. It should be understood that the drawings and embodiments of the present invention are only for exemplary purposes and are not intended to limit the scope of protection of the present invention.

[0017] To achieve stable power conversion and power supply isolation and meet the power demand of various parts of the distributed system health management, the key issue is how to transfer the isolated output voltage information from the secondary side of the transformer to the primary side of the transformer for regulation. When the switch tube of the flyback converter is turned off, the energy stored in the transformer is released on the secondary side. In this process, the voltage at the output end of the secondary side will be reflected to the primary side of the transformer. The pulse waveform reflected from the secondary side to the primary side can be used to collect the secondary side voltage of the isolated output. In order to achieve this function, in addition to conventional reference, voltage regulator, oscillator, current amplifier, current comparator, driver and other circuits, the power supply system also requires a reflected pulse voltage detection circuit and a sample-and-hold error amplifier. The reflected pulse voltage detection circuit is used to send the voltage to the input end of the sample-and-hold error amplifier when the secondary voltage is reflected to the primary side. The sample-and-hold error amplifier samples the voltage when the secondary current is zero.

[0018] The technical solution of the present invention is described in detail with specific embodiments below. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0019] Figure 1 A low-power flyback converter for sampling the primary voltage of a transformer is provided in an embodiment of the present disclosure, such as Figure 1 As shown, it includes: transformer T1, output diode, input capacitor, output capacitor, load, N-channel MOS switch tube K1, PNP transistor V1A, PNP transistor V1B, resistor R FB , resistor R REF , resistor R1, operational amplifier U1, operational amplifier U2, operational amplifier U3, boundary detection circuit, oscillator, two-input AND gate AND, RS flip-flop and driving circuit;

[0020] One end of the secondary side of transformer T1 is connected to the positive electrode of the output diode, the negative electrode of the output diode is connected to one end of the output capacitor, the other end of the output capacitor is connected to the other end of the secondary side of transformer T1, and the two ends of the output capacitor are also connected to the load;

[0021] One end of the primary side of transformer T1 is connected to the input power supply Vin, input capacitor and emitter of PNP transistor V1A, and the other end of the primary side of transformer T1 is connected to the drain of N-channel MOS switch tube K1, resistor R REF One end of the resistor R REFThe other end of the resistor R1 is connected to the emitter of the PNP transistor V1B, the base of the PNP transistor V1A is connected to the base of the PNP transistor V1B, the collector of the PNP transistor V1A is connected between the base of the PNP transistor V1A and the base of the PNP transistor V1B, the source of the N-channel MOS switch tube K1 is connected to one end of the resistor R1 and the positive input end of the operational amplifier U1, and the other end of the resistor R1 is connected to the reverse input end of the operational amplifier U1, the resistor R FB One end of the resistor R FB The other end is connected to the collector of PNP transistor V1B, the output of operational amplifier U1 is connected to the positive input of operational amplifier U3, the reverse input of operational amplifier U2 is connected to the collector of PNP transistor V1B, the positive input is connected to reference source 1, the output of operational amplifier U2 is connected to the reverse input of operational amplifier U3, the output of operational amplifier U3 is connected to the R input of RS trigger, the input of boundary detection circuit is connected to the collector of PNP transistor V1B, the output of boundary detection circuit is connected to the input of two-input AND gate AND, the output of oscillator is connected to the other input of two-input AND gate AND, the output of two-input AND gate AND is connected to the S input of RS trigger, the Q output of RS trigger is connected to the input of driving circuit, and the output of driving circuit is connected to the gate of N-channel MOS switch tube K1.

[0022] In this embodiment, PNP transistors V1A and V1B form a pair of transistors. When the emitter voltage of PNP transistor V1B is greater than the emitter voltage of PNP transistor V1A, PNP transistor V1B is turned on to collect the voltage reflected from the secondary side. The drive circuit is a circuit that increases the current driving capability, and a totem pole or a special analog drive chip can be used. The operational amplifier U1 is used to collect the primary current for primary overcurrent protection. When an overcurrent fault occurs in the system, the N-channel MOS switch tube K1 can be turned off to avoid the spread of the fault caused by large current. The operational amplifier U2 is a sampling and holding error amplifier. The operational amplifier U2 and the operational amplifier U3 are used to realize the comparison of the collected value with the reference 1 and the error amplification. When the collected value is greater than the reference 1, a control signal for closing the duty cycle is generated.

[0023] When the N-channel MOS switch tube K1 is turned on, the PNP transistor V1B is turned off. At this time, the voltage at terminal a will not be fed back to the operational amplifier U1, and the transformer T1 is in the energy storage stage; when the N-channel MOS switch tube K1 is turned off, the secondary output voltage is transmitted to the primary terminal a through the transformer T1, making the voltage at terminal a higher than that at terminal d. The voltage difference between terminal a and terminal d is V FLBK =(V out +V F )×N (where VF is the output diode voltage difference, V outis the output voltage), and at the same time, the PNP transistor V1B is turned on, V FLBK Through the resistor R FB and R REF Converted to current I RFB , I RFB With R REF A voltage based on the ground line is generated. This voltage is the sampled voltage and is fed back to the inverting input terminal of the operational amplifier U1 to be compared with reference 1.

[0024] The circuit samples the output voltage from the voltage at point a. To improve the sampling accuracy, the operational amplifier U1 generally takes 0.3us-0.4us to stabilize and sample the voltage. At this time, the secondary side of the transformer T1 conducts current for at least 0.3us-0.4us. Therefore, the output end of the power supply needs to be pre-loaded according to the actual output voltage. For example, a resistor can be directly added to the output end to ensure that there is always a resistive load on the output end.

[0025] From the analysis, we can see that the voltage between point a and point d in the circuit is crucial to the accuracy of voltage acquisition, so we need to minimize the loop area of ​​the above four points to reduce the impact of EMI and power switching noise. The loop area can be reduced by compactly arranging components, shortening and thickening the routing distance between devices, and avoiding cross-layer wiring.

[0026] The oscillator is used to provide a high-level input to the RS trigger at a fixed time, so that the RS trigger has the possibility of outputting a high level to turn on the switch. The circuit can be implemented in many forms, the most common of which are 555 timer or RC relaxation oscillator.

[0027] The boundary detection circuit includes an operational amplifier, the reverse input end of which is connected to the collector of the PNP transistor V1B, and the forward input end is connected to the reference source 2, which is used to detect the voltage of the primary side voltage drop process of the transformer T1, and compare the voltage collected at the end of the detection with the reference 2 to determine the speed of the load end voltage drop in the cycle. The result is used to determine the end state of the critical conduction mode, so that the power supply works in the PWM+PFM mode, thereby reducing the switching loss at light load and improving the power conversion efficiency at light load. The specific working process is that when the secondary side load is small, the output voltage drops slowly. At the beginning of the next cycle of the oscillator, the output voltage is still higher than the output voltage set by the output, and the voltage reflected to the primary side is higher than the reference 2. At this time, the output of the two-input AND gate AND of the boundary detection circuit and the oscillator is low, the RS trigger output is low, and the N-channel MOS switch tube K1 is not turned on, thereby realizing the function of reducing the frequency at light load, reducing the switching loss of the N-channel MOS switch tube K1, and thus improving the conversion efficiency at light load.

[0028] According to the above method, a prototype power supply with a power of 10W was built, with a power input of 28V and an output of 15V. The quality of the power supply was tested, and the results are as follows:

[0029] like Figure 2 As shown in the figure, from the startup waveform, there is no output overshoot at the output end of the power supply, which proves that the loop feedback speed of the circuit is fast and meets the requirements for general power supply use;

[0030] like Figure 3 As shown in the figure, from the step waveform, the load step of the power supply is no more than 1.3%, which proves that the dynamic response capability of the circuit is good and meets the requirements for general power supply use;

[0031] like Figure 4 As shown in the figure, from the output waveform, the output ripple of the power supply is no more than 0.35%, which proves that the ripple characteristic of the circuit is good and meets the requirements for general power supply use.

Claims

1. A low-power flyback converter for transformer primary voltage sampling, characterized in that: include: Transformer T1, output diode, input capacitor, output capacitor, load, N-channel MOS switch tube K1, PNP transistor V1A, PNP transistor V1B, resistor R FB , resistor R REF , resistor R1, operational amplifier U1, operational amplifier U2, operational amplifier U3, boundary detection circuit, oscillator, two-input AND gate AND, RS flip-flop and driving circuit; One end of the secondary side of the transformer T1 is connected to the anode of the output diode, the cathode of the output diode is connected to one end of the output capacitor, the other end of the output capacitor is connected to the other end of the secondary side of the transformer T1, and both ends of the output capacitor are also connected to the load; One end of the primary side of the transformer T1 is connected to the input power supply Vin, the input capacitor and the emitter of the PNP transistor V1A, and the other end of the primary side of the transformer T1 is connected to the drain of the N-channel MOS switch tube K1, the resistor R REF One end of the resistor R REF The other end of the transistor is connected to the emitter of the PNP transistor V1B, the base of the PNP transistor V1A is connected to the base of the PNP transistor V1B, the collector of the PNP transistor V1A is connected between the base of the PNP transistor V1A and the base of the PNP transistor V1B, the source of the N-channel MOS switch tube K1 is simultaneously connected to one end of the resistor R1 and the positive input end of the operational amplifier U1, and the other end of the resistor R1 is simultaneously connected to the reverse input end of the operational amplifier U1, the resistor R FB One end of the resistor R FB The other end is connected to the collector of the PNP transistor V1B, the output end of the operational amplifier U1 is connected to the positive input end of the operational amplifier U3, the reverse input end of the operational amplifier U2 is connected to the collector of the PNP transistor V1B, the positive input end is connected to the reference source 1, the output end of the operational amplifier U2 is connected to the reverse input end of the operational amplifier U3, the output end of the operational amplifier U3 is connected to the R input end of the RS trigger, the input end of the boundary detection circuit is connected to the collector of the PNP transistor V1B, the output end of the boundary detection circuit is connected to the input end of the two-input AND gate AND, the output end of the oscillator is connected to the other input end of the two-input AND gate AND, the output end of the two-input AND gate AND is connected to the S input end of the RS trigger, the Q output end of the RS trigger is connected to the input end of the drive circuit, and the output end of the drive circuit is connected to the gate of the N-channel MOS switch tube K1.

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