Output voltage expansion flyback circuit based on primary side demagnetization detection

Abstract

The utility model relates to the field of flyback, and provides an output voltage expansion flyback circuit based on primary side demagnetization detection in order to realize expansion of an output voltage range on the basis of existing hardware attributes of a chip. The third resistor and the third diode are connected in series and then connected to the two ends of the original second resistor of the demagnetization detection and control circuit in parallel, the output voltage range can be expanded, an existing chip does not need to be changed, the implementation mode is simple, and the implementation cost is low.

Description

Technical Field

[0001] This utility model relates to the field of flyback circuits, specifically a flyback circuit with extended output voltage based on primary-side demagnetization detection. Background Technology

[0002] A simplified schematic diagram of the existing flyback topology is shown below. Figure 1 As shown, the flyback transformer consists of three windings: primary winding T1A, output winding T1B, and demagnetization detection winding T1C. The demagnetization detection circuit is composed of T1C, R1, and R2. The demagnetization detection winding also serves to power the flyback IC. The voltage of the demagnetization detection winding and the voltage of the output winding are in phase and satisfy the turns ratio relationship.

[0003] The flyback IC features output overvoltage and short-circuit protection. Overvoltage protection is achieved by limiting the voltage of the demagnetization detection pin FB, where the FB voltage is defined as VFB_ovp, and the corresponding output voltage as Vout_ovp. Short-circuit protection is achieved by detecting the voltage of the demagnetization detection pin FB, where the FB voltage is defined as VFB_osp, and the corresponding output voltage as Vout_osp. Given a fixed chip hardware configuration, VFB_ovp and VFB_osp are fixed values, and their ratio limits the output voltage ratio. For example, in an application with an extremely wide output voltage range (e.g., 4-52V), the output voltage ratio is 52 / 4 = 13. If this ratio is greater than the ratio of VFB_ovp to VFB_osp, the chip will be unable to handle such a wide output voltage range, triggering output short-circuit protection at low output voltages or output overvoltage protection at high output voltages. Utility Model Content

[0004] In order to extend the output voltage range based on the existing hardware attributes of the chip, this utility model provides an output voltage extension flyback circuit based on primary-side demagnetization detection.

[0005] The technical solution adopted by this utility model to solve the above problems is:

[0006] The output voltage extension flyback circuit based on primary-side demagnetization detection includes: an input circuit, an output circuit, and a demagnetization detection and control circuit. The input circuit includes input ports IN+ and IN-, a primary-side winding, a switching transistor, and a current-sensing resistor. The demagnetization detection and control circuit includes a demagnetization detection winding, a first resistor, a second resistor, a first diode, a first capacitor, and a flyback IC. The output circuit includes: output ports OUT+ and OUT-, an output winding, a second diode, and a second capacitor.

[0007] The opposite-named end of the primary winding is connected to the input port IN+, and the same-named end is connected to the drain of the switching transistor; the source of the switching transistor is connected to one end of the current sensing resistor and the current sensing pin of the flyback IC, and the gate of the switching transistor is connected to the drive pin of the flyback IC; the other end of the current sensing resistor is connected to the input port IN-.

[0008] The same-name terminal of the output winding is connected to the anode of the second diode, and the opposite-name terminal of the output winding and the negative terminal of the second capacitor are both connected to the output port OUT-; the cathode of the second diode and the positive terminal of the second capacitor are both connected to the output port OUT+.

[0009] The demagnetization detection and control circuit also includes a third resistor and a third diode.

[0010] The same-name terminal of the demagnetization detection winding is connected to one end of the first resistor and the anode of the first diode, respectively. The other end of the first resistor is connected to one end of the second resistor, one end of the third resistor, and the demagnetization detection pin of the flyback IC, respectively. The other end of the third resistor is connected to the anode of the third diode. The cathode of the first diode is connected to the positive terminal of the first capacitor and the power supply pin of the flyback IC. The opposite-name terminal of the demagnetization detection winding, the other end of the second resistor, the cathode of the third diode, the negative terminal of the first capacitor, and the GND pin of the flyback IC are all connected to the input port IN-.

[0011] The advantages of this invention compared to the prior art are: by connecting the third resistor and the third diode in series and then in parallel across the original second resistor in the demagnetization detection and control circuit, the output voltage range can be expanded without changing the existing chip. The implementation method is simple and the implementation cost is low. Attached Figure Description

[0012] Figure 1 A simplified schematic diagram of the existing flyback topology;

[0013] Figure 2 The output voltage of the flyback circuit is extended based on primary-side demagnetization detection. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this utility model clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this utility model.

[0015] like Figure 2As shown, the output voltage extension flyback circuit based on primary-side demagnetization detection includes: an input circuit, an output circuit, and a demagnetization detection and control circuit. The input circuit includes input ports IN+ and IN-, a primary-side winding T1A, a switching transistor Q3, and a current-sensing resistor RS1. The demagnetization detection and control circuit includes a demagnetization detection winding T1C, a first resistor R1, a second resistor R2, a first diode D1, a first capacitor CE1, and a flyback IC. The output circuit includes output ports OUT+ and OUT-, an output winding T1B, a second diode D2, and a second capacitor CE2.

[0016] The opposite-name terminal of the primary winding T1A is connected to the input port IN+, and the same-name terminal is connected to the drain of the switching transistor Q3; the source of the switching transistor Q3 is connected to one end of the current sensing resistor RS1 and the current sensing pin CS of the flyback IC; the gate of the switching transistor Q3 is connected to the drive pin GATE of the flyback IC; the other end of the current sensing resistor RS1 is connected to the input port IN-.

[0017] The same-name terminal of the output winding T1B is connected to the anode of the second diode D2, and the opposite-name terminal of the output winding T1B and the negative terminal of the second capacitor CE2 are both connected to the output port OUT-; the cathode of the second diode D2 and the positive terminal of the second capacitor CE2 are both connected to the output port OUT+.

[0018] The demagnetization detection and control circuit also includes a third resistor R3 and a third diode D3A.

[0019] The same-name terminal of the demagnetization detection winding T1C is connected to one end of the first resistor R1 and the anode of the first diode D1, respectively. The other end of the first resistor R1 is connected to one end of the second resistor R2, one end of the third resistor R3, and the demagnetization detection pin FB of the flyback IC, respectively. The other end of the third resistor R3 is connected to the anode of the third diode D3A. The cathode of the first diode D1 is connected to the positive terminal of the first capacitor CE1 and the power supply pin VCC of the flyback IC. The opposite-name terminal of the demagnetization detection winding T1C, the other end of the second resistor R2, the cathode of the third diode D3A, the negative terminal of the first capacitor CE1, and the GND pin of the flyback IC are all connected to the input port IN-.

[0020] Let the voltage difference between OUT+ and OUT- be Vout. For constant current drive, its output voltage usually has a rated working range. Define the minimum working voltage Vmin, the maximum working voltage Vmax, the output short-circuit protection voltage Vout_osp, and the output open-circuit protection voltage Vout_ovp. The design needs to ensure that Vmin > Vout_osp and Vmax < Vout_ovp. The flyback IC has the function of output overvoltage protection, which realizes output overvoltage protection by limiting the voltage of the demagnetization detection pin FB. The FB voltage here is defined as VFB_ovp. The flyback IC has the function of output short-circuit protection, which realizes output short-circuit protection by detecting the voltage of the demagnetization detection pin FB. The FB voltage here is defined as VFB_osp. Let the number of turns of the demagnetization detection winding T1C and the output winding T1B be Naux and Ns respectively, the conduction voltage drop of the second diode D2 be VF2, and the conduction voltage drop of the third diode D3A be VF3. Usually, VFB_osp < VF3, so D3A does not conduct during output short-circuit protection. According to the turn ratio and connection relationship, there are the following formulas:

[0021] Output short-circuit protection voltage:

[0022] ;

[0023] Output open-circuit protection voltage: ;

[0024] Chip FB parameter ratio: ;

[0025] Actual output voltage ratio: ;

[0026] The introduction of the third resistor R3 and the third diode D3A results in K2 > K1 being established, that is, the output voltage range is expanded.

[0027] Proved by an actual engineering design case, the output working voltage range of the product is [5V, 52V]. The design parameters need to ensure that the output short-circuit protection voltage is lower than the lowest output voltage of the product, and the output open-circuit protection voltage is higher than the highest output voltage of the product. The design parameters are as follows:

[0028] Ns = 17TS, Naux = 8TS, R1 = 30.9K, R2 = 5.1K, R3 = 3.16K, VF2 = 1V, VF3 = 0.6V, the chip's inherent parameter VFB_osp = 0.2V, VFB_ovp = 2V. Substituting into the formula, we get:

[0029] Output short-circuit protection voltage: ,

[0030] Output open-circuit protection voltage:

[0031] ,

[0032] Chip FB parameter ratio: ,

[0033] Actual output voltage ratio: ,

[0034] That is, the actual output voltage ratio can be greater than the FB parameter ratio, and the output voltage expansion circuit is effective.

Claims

1. An output voltage extension flyback circuit based on primary-side demagnetization detection, comprising: The circuit includes an input circuit, an output circuit, a demagnetization detection and control circuit, and an input circuit including input ports IN+ and IN-, a primary winding, a switching transistor, and a current sensing resistor. The demagnetization detection and control circuit includes a demagnetization detection winding, a first resistor, a second resistor, a first diode, a first capacitor, and a flyback IC; the output circuit includes: output ports OUT+ and OUT-, an output winding, a second diode, and a second capacitor; The opposite-named end of the primary winding is connected to the input port IN+, and the same-named end is connected to the drain of the switching transistor; the source of the switching transistor is connected to one end of the current sensing resistor and the current sensing pin of the flyback IC, and the gate of the switching transistor is connected to the drive pin of the flyback IC; the other end of the current sensing resistor is connected to the input port IN-. The same-name terminal of the output winding is connected to the anode of the second diode, and the opposite-name terminal of the output winding and the negative terminal of the second capacitor are both connected to the output port OUT-; the cathode of the second diode and the positive terminal of the second capacitor are both connected to the output port OUT+. The demagnetization detection and control circuit is characterized by further including a third resistor and a third diode. The same-name terminal of the demagnetization detection winding is connected to one end of the first resistor and the anode of the first diode, respectively. The other end of the first resistor is connected to one end of the second resistor, one end of the third resistor, and the demagnetization detection pin of the flyback IC, respectively. The other end of the third resistor is connected to the anode of the third diode. The cathode of the first diode is connected to the positive terminal of the first capacitor and the power supply pin of the flyback IC. The opposite-name terminal of the demagnetization detection winding, the other end of the second resistor, the cathode of the third diode, the negative terminal of the first capacitor, and the GND pin of the flyback IC are all connected to the input port IN-.

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