Output current protection circuit

Abstract

The utility model relates to an output current protection circuit, which comprises a voltage stabilizing circuit and an optocoupler circuit, the voltage stabilizing circuit comprises a first voltage stabilizer U1, divider resistors R5 and R4, and current limiting resistors R6 and R3, and the cathode of the first voltage stabilizer U1 and the divider resistor R5 are connected in parallel to an output cathode end BAT-; the optocoupler circuit comprises a trigger part and an optocoupler part; the trigger part comprises a second voltage stabilizer U2 and two divider resistors R7 and R8, the optocoupler part comprises an optocoupler U3 and a ninth current-limiting resistor R9, the divider resistors R5 and R8 and an anode of the second voltage stabilizer U2 are jointly connected to an output cathode end, an access point of an optocoupler circuit is a fixed voltage site, a triode of the optocoupler U3 is connected to a chip, and a triode of the optocoupler U3 is connected to the chip. And the anode of the voltage stabilizer U2 is connected to the current detection pin of the chip. As the voltage at the front end of the second voltage stabilizer U2 is stable, the second voltage stabilizer U2 can respond sensitively and accurately, overcurrent protection is realized through the two voltage stabilizers, and the overcurrent protection circuit is simple and effective in structure, low in cost and wide in application prospect.

Description

Technical Field

[0001] The utility model relates to the technical field of switch power supply circuit protection, in particular to an output current protection circuit. Background Art

[0002] A switching power supply is a high-frequency power conversion device that converts one form of energy (AC / DC) into the DC required by the customer. Its main system structure is as follows: Figure 1 As shown. Output sampling mainly involves sampling of output voltage and output current. The sampling signal is sent to the IC to adjust the output pulse width of the IC, and finally achieve the effect of stable output. Then, through voltage and current sampling and analysis, overvoltage, overcurrent and overtemperature protection are performed. Currently, there are three main types of current sampling:

[0003] 1. Resistance sampling.

[0004] The voltage across the resistor is directly taken and sent to the IC current detection pin (IS). The circuit is extremely simple, but it shares a "ground" (reference point) with the IC, that is, it can only detect the current on one side of the power input, and then convert it to the secondary side through the turns ratio of the transformer. The circuit is simple, but because it can only sample the current on one side of the input, the accuracy is not high.

[0005] 2. Resistance + op amp + optocoupler sampling

[0006] The current generates a voltage drop across the resistor, and this value is fed into the operational amplifier. The optocoupler achieves isolation, and the output pulse width of the IC is controlled by the degree of conduction of the diode in the optocoupler. The circuit is simple, and the detection circuit is isolated. However, the addition of the op amp requires a separate power supply, which is also costly.

[0007] 3. Current transformer

[0008] The input of the current transformer is connected in series in the circuit to be tested, and the voltage on the secondary side is sent to the current detection pin (IS) of the IC through the operational amplifier, and the IC adjusts the output pulse width. The circuit is also relatively simple (the operational amplifier can also be omitted), and the detection circuit is isolated. Due to the use of the current transformer, the cost is relatively high. Utility Model Content

[0009] In view of this, an output current protection circuit with simple and effective circuit structure, high sensitivity, high precision and low cost is provided.

[0010] An output current protection circuit is used to perform overcurrent protection on the output current of a switching power supply, the switching power supply having an output positive terminal BAT+ and an output negative terminal BAT-, the output current protection circuit comprising a voltage stabilizing circuit and an optocoupler circuit, the voltage stabilizing circuit comprising a first voltage stabilizer U1, two voltage-dividing resistors R5 and R4 connected in series, an access current-limiting resistor R6, and a third current-limiting resistor R3, the midpoint of the two voltage-dividing resistors R5 and R7 connected in series is connected to the reference electrode of the first voltage stabilizer U1, the parallel voltage-dividing resistor R4 and the third current-limiting resistor R3 have a common access point A and are connected to the current output end of the access current-limiting resistor R6, the current output end of the third current-limiting resistor R3 is connected to the cathode of the first voltage stabilizer U1, the cathode of the first voltage stabilizer U1 and the voltage-dividing resistor R 5 is connected in parallel to the output negative terminal BAT-; the optocoupler circuit includes a trigger part and an optocoupler part; the trigger part includes a second voltage stabilizer U2, two voltage-dividing resistors R7 and R8 connected in series, the optocoupler part includes an optocoupler U3 and a ninth current-limiting resistor R9, the midpoint of the two voltage-dividing resistors R7 and R8 connected in series is connected to the reference electrode of the second voltage stabilizer U2, the ninth current-limiting resistor R9 and the two voltage-dividing resistors R7 and R8 are connected in parallel and the parallel connection point is the common connection point A, the anode of the optocoupler U3 is connected to the current output end of the ninth current-limiting resistor R9, the cathode of the optocoupler U3 is connected to the cathode of the second voltage stabilizer U2, the triode of the optocoupler U3 is connected to the chip, and the anode of the second voltage stabilizer U2 is connected to the current detection pin of the chip.

[0011] Preferably, the voltage stabilizing circuit also includes a filter capacitor C1, a first end of the filter capacitor C1 is connected to the midpoint of the two voltage-dividing resistors R5 and R7, and a second end is connected to the midpoint of the current output end of the third current-limiting resistor R3 and the cathode of the first voltage stabilizer U1, and the filter capacitor C1 is used to make the voltage stabilizing circuit self-recover when the first voltage stabilizer U1 is turned on until the first voltage stabilizer U1 is turned off.

[0012] Preferably, one end of the voltage-dividing resistor R8 is connected to the reference electrode of the second voltage stabilizer U2, and the other end is connected to the output negative terminal BAT-.

[0013] Furthermore, the current detection pin of the chip is grounded.

[0014] Furthermore, the collector of the transistor at the secondary side of the optocoupler U3 is connected to the compensation terminal COMP of the chip, and the emitter is connected to the ground terminal of the chip.

[0015] Furthermore, the first voltage regulator U1 and the second voltage regulator U2 both use TL431 voltage regulators. Furthermore, the chip is a UC3845 chip. The TL431 and EL816 chips are preferably used in combination. These two products are easy to purchase and inexpensive. Moreover, the actual 48V / 2A case verifies that they are stable and reliable in operation.

[0016] Furthermore, the two voltage-dividing resistors R5 and R4 have the same resistance value, and the connected current-limiting resistor R6 has the same resistance value as the third current-limiting resistor R3.

[0017] Furthermore, the resistance of the seventh voltage-dividing resistor R7 is greater than the resistance of the eighth voltage-dividing resistor R8, and the sum of the resistances of the two voltage-dividing resistors R7 and R8 is equal to or slightly greater than the sum of the resistances of the other two voltage-dividing resistors R4 and R5.

[0018] Furthermore, when the output current increases, the potential of the reference electrode of the second regulator U2 increases until the second regulator U2 is turned on, the primary side U3-B of the optocoupler U3 is turned on, the secondary side U3-A is turned on, the compensation terminal COMP of the chip is pulled low, the chip does not work, and overcurrent protection is achieved.

[0019] The above-mentioned output current protection circuit adopts two voltage stabilizers, which are divided into two paths. The first voltage stabilizer U1 outputs the positive terminal voltage stably, that is, the access terminal of the second voltage stabilizer U2 is first allowed to maintain a relatively fixed voltage, so as to facilitate the second voltage stabilizer U2 to work more stably. The cathode front end of the second voltage stabilizer U2 is connected to the optocoupler, and the anode of the second voltage stabilizer U2 is connected to the current detection pin. When the output current increases, the voltage across the reference pole changes. After reaching the conduction voltage difference, the optocoupler is turned on, the chip compensation terminal level is pulled down, the chip does not work, and the system realizes overcurrent protection. Since the front-end voltage of the second voltage stabilizer U2 is stable, slight current changes will affect the second voltage stabilizer U2, greatly improving the sensitivity and accuracy, and the circuit protection is safe and reliable. The overall circuit structure is simple and effective, the response is fast, and the cost is low, which is conducive to mass production and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the framework structure of the switching power supply.

[0021] Figure 2 It is a structural schematic diagram of an output current protection circuit provided by an embodiment of the utility model.

[0022] Figure 3 It is a schematic diagram of the simplified structure of the sampling circuit. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the utility model will be clearly and completely described below in conjunction with the drawings in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, rather than all of the embodiments; based on the embodiments of the utility model, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the utility model.

[0024] See also Figure 1 and Figure 2 , Figure 1 The schematic diagram of the switching power supply frame is shown. Figure 2 An output current protection circuit provided by an embodiment of the utility model is shown, which is used for overcurrent protection of the output current of a switching power supply, wherein the switching power supply has an output positive terminal BAT+ and an output negative terminal BAT-, and the output current protection circuit includes a voltage stabilizing circuit and an optocoupler circuit, wherein the voltage stabilizing circuit includes a first voltage stabilizer U1, two voltage-dividing resistors R5 and R4 connected in series, an access current limiting resistor R6, and a third current limiting resistor R3, wherein the midpoint of the two voltage-dividing resistors R5 and R7 connected in series is connected to the reference electrode of the first voltage stabilizer U1, the voltage-dividing resistor R4 connected in parallel has a common access point A with the third current limiting resistor R3 and is connected to the current output end of the access current limiting resistor R6, the current output end of the third current limiting resistor R3 is connected to the cathode of the first voltage stabilizer U1, and the cathode of the first voltage stabilizer U1 is connected to the cathode of the first voltage stabilizer U1. The electrode and the voltage-dividing resistor R5 are connected in parallel to the output negative terminal BAT-; the optocoupler circuit includes a trigger part and an optocoupler part; the trigger part includes a second voltage stabilizer U2, two voltage-dividing resistors R7 and R8 connected in series, the optocoupler part includes an optocoupler U3 and a ninth current-limiting resistor R9, the midpoint of the two voltage-dividing resistors R7 and R8 connected in series is connected to the reference electrode of the second voltage stabilizer U2, the ninth current-limiting resistor R9 and the two voltage-dividing resistors R7 and R8 are connected in parallel and the parallel access point is the common access point A, the anode of the optocoupler U3 is connected to the current output end of the ninth current-limiting resistor R9, the cathode of the optocoupler U3 is connected to the cathode of the second voltage stabilizer U2, the triode of the optocoupler U3 is connected to the chip, and the anode of the second voltage stabilizer U2 is connected to the current detection pin of the chip.

[0025] Preferably, the voltage stabilizing circuit further includes a filter capacitor C1, a first end of the filter capacitor C1 is connected to the midpoint of the two voltage-dividing resistors R5 and R7, a second end is connected to the midpoint of the current output end of the third current-limiting resistor R3 and the cathode of the first voltage stabilizer U1, and the filter capacitor C1 is used to make the voltage stabilizing circuit self-recovery when the first voltage stabilizer U1 is turned on until the first voltage stabilizer U1 is turned off. Therefore, the filter capacitor C1 can respond to the change when the voltage changes slightly, compensate for the current or voltage, and promptly fix the common access point A at a predetermined voltage, so that the circuit is more stable, responsive, accurate and reliable.

[0026] Preferably, one end of the voltage-dividing resistor R8 is connected to the reference electrode of the second voltage regulator U2, and the other end is connected to the output negative terminal BAT-. The voltage difference between the reference electrode and the anode of the second voltage regulator U2 is controlled by the voltage-dividing resistor R8, and the voltage-dividing resistor R8, the voltage-dividing resistor R5 and the anode of the first voltage regulator U1 are connected to the output negative terminal BAT-, and the two voltage regulators are interconnected and respond at the same time, further improving the accuracy.

[0027] Furthermore, the current detection pin of the chip is grounded. Furthermore, the collector of the transistor at the secondary side of the optocoupler U3 is connected to the compensation terminal COMP of the chip, and the emitter is connected to the ground terminal of the chip.

[0028] Furthermore, the first voltage regulator U1 and the second voltage regulator U2 both use TL431 voltage regulators. Furthermore, the chip is a UC3845 chip. The TL431 and EL816 chips are preferably used in combination. These two products are easy to purchase and inexpensive. Moreover, the actual 48V / 2A case verifies that they are stable and reliable in operation.

[0029] Furthermore, the resistance values ​​of the two voltage-dividing resistors R5 and R4 are the same, and the resistance value of the connected current-limiting resistor R6 is the same as the resistance value of the third current-limiting resistor R3. Figure 2 As shown, the resistance values ​​of the two voltage-dividing resistors R5 and R4 and the resistance values ​​of the two current-limiting resistors R6 and R3 are the same, both of which are 7.5k Ω. Furthermore, the resistance value of the seventh voltage-dividing resistor R7 is greater than the resistance value of the eighth voltage-dividing resistor R8, and the sum of the resistance values ​​of the two voltage-dividing resistors R7 and R8 is equal to or slightly greater than the sum of the resistance values ​​of the other two voltage-dividing resistors R4 and R5. In a specific embodiment, as Figure 2 As shown, the resistance of the eighth voltage-dividing resistor R8 is 3.0kΩ, and the resistance of the seventh voltage-dividing resistor R7 is 4.7kΩ. The resistance of the ninth current-limiting resistor R9 is 1.0kΩ. Of course, the specific resistance of each resistor is not limited to the above examples, and the actual resistance is to make the current of the circuit meet the performance requirements of the voltage regulator.

[0030] Furthermore, when the output current increases, the potential of the reference electrode of the second regulator U2 increases until the second regulator U2 is turned on, the primary side U3-B of the optocoupler U3 is turned on, the secondary side U3-A is turned on, the compensation terminal COMP of the chip is pulled low, the chip does not work, and overcurrent protection is achieved.

[0031] The working process of the above output current protection circuit is as follows:

[0032] When no-load, that is, there is no current in the circuit, the voltage divider resistor R5 reaches and exceeds 2.5V in the series connection of R6, R4 and R5, which is the reference voltage inside the first voltage regulator U1. Therefore, the first voltage regulator U1 works normally. Taking the voltage of the reference electrode of the first voltage regulator U1 as 2.5V as an example (assuming that "BAT-" is ground), and the resistance values ​​of R4 and R5 are the same, then the voltage at point A (that is, the intersection of R3 and R6) is 5V (taking "BAT-" as ground). Then the voltage of the reference electrode of the second voltage regulator U2 (that is, the intersection of R7 and R8) is 1.95V, which is lower than the reference voltage of the second voltage regulator U2 (such as 2.5V). Applied to Figure 3 When the output is unloaded, due to the circuit protection Figure 3The current flowing through R2 is approximately zero, so there is no voltage drop across R2, and it can be approximated that the levels of the two positions "IS" and "BAT-" (or "CHG-") are the same. At this time, the second regulator U2 is not working and is not conducting. The optocoupler U3 is also conducting and not working. Therefore, the signal of the chip's compensation terminal COMP is set high, and the overcurrent control does not take effect. At this time, the UC3845 chip maintains normal output, that is, the output pin OUTPUT drives the output.

[0033] When there is loading, until the current limiting point is reached, this embodiment takes 2A as an example.

[0034] Depend on Figure 3 It can be seen that when the output current increases, that is, the current flowing through R2 increases, a voltage is generated at its two ends, and the level of the "IS" point (relative to) "CHG-" (BAT-) increases. At this time, the two points cannot be equivalent to the same point.

[0035] Depend on Figure 2 It can be seen that when the output current increases, for example, it reaches above the current limit point, at this time, it is equivalent to Figure 3 The current in increases, through Figure 3 The current of the sampling resistor R2 in the circuit increases. Figure 1 The level of point A in the circuit has not changed and remains at 5V, while the voltage of the reference electrode of the second voltage regulator U2 changes from the initial 1.95V. Due to the voltage difference across the sampling resistor R2, the potential between the IS position and CHG- (BAT-) is different. The second voltage regulator U2 uses "IS" as the "ground" (zero reference point), so "CHG-" (BAT-) is a negative value relative to the IS position (zero reference point of U2). Then the potential of the reference electrode of the second voltage regulator U2 is raised. When the level here rises to about 2.5V, the second voltage regulator U2 works, the optocoupler U3-B is turned on, the optocoupler U3-A is turned on, the compensation terminal COMP of the UC3845 chip is pulled low, UC3845 does not work, and the system achieves overcurrent protection. It can be seen that the front-end voltage of the second regulator U2 is fixed, and slight current changes will affect the second regulator U2, which greatly improves the sensitivity and accuracy, and the circuit protection is safe and reliable. The overall circuit structure is simple and effective, the response is fast, and the cost is low, which is conducive to mass production and use.

[0036] It should be noted that the present invention is not limited to the above-mentioned embodiments. According to the creative spirit of the present invention, technical personnel in this field can also make other changes. These changes made according to the creative spirit of the present invention should be included in the scope of protection required by the present invention.

Claims

1. An output current protection circuit for overcurrent protection of an output current of a switching power supply, wherein the switching power supply has an output positive terminal BAT+ and an output negative terminal BAT-, characterized in that: The output current protection circuit includes a voltage stabilizing circuit and an optocoupler circuit; The voltage stabilizing circuit includes a first voltage stabilizer U1, two voltage-dividing resistors R5 and R4 connected in series, an access current-limiting resistor R6, and a third current-limiting resistor R3, wherein the midpoint of the two voltage-dividing resistors R5 and R7 connected in series is connected to the reference electrode of the first voltage stabilizer U1, the voltage-dividing resistor R4 connected in parallel and the third current-limiting resistor R3 have a common access point A and are connected to the current output end of the access current-limiting resistor R6, the current output end of the third current-limiting resistor R3 is connected to the cathode of the first voltage stabilizer U1, and the cathode of the first voltage stabilizer U1 and the voltage-dividing resistor R5 are connected in parallel to the output negative terminal BAT-; The optocoupler circuit includes a trigger part and an optocoupler part; the trigger part includes a second voltage stabilizer U2, two voltage-dividing resistors R7 and R8 connected in series, the optocoupler part includes an optocoupler U3 and a ninth current-limiting resistor R9, the midpoint of the two voltage-dividing resistors R7 and R8 connected in series is connected to the reference electrode of the second voltage stabilizer U2, the ninth current-limiting resistor R9 and the two voltage-dividing resistors R7 and R8 are connected in parallel and the parallel access point is the common access point A, the anode of the optocoupler U3 is connected to the current output end of the ninth current-limiting resistor R9, the cathode of the optocoupler U3 is connected to the cathode of the second voltage stabilizer U2, the triode of the optocoupler U3 is connected to the chip, and the anode of the second voltage stabilizer U2 is connected to the current detection pin of the chip.

2. The output current protection circuit according to claim 1, characterized in that: The voltage stabilizing circuit also includes a filter capacitor C1, a first end of the filter capacitor C1 is connected to the midpoint of the two voltage-dividing resistors R5 and R7, and a second end is connected to the midpoint of the current output end of the third current-limiting resistor R3 and the cathode of the first voltage stabilizer U1. The filter capacitor C1 is used to make the voltage stabilizing circuit self-recover when the first voltage stabilizer U1 is turned on until the first voltage stabilizer U1 is turned off.

3. The output current protection circuit according to claim 1, characterized in that: One end of the voltage-dividing resistor R8 is connected to the reference electrode of the second voltage stabilizer U2 , and the other end is connected to the output negative terminal BAT−.

4. The output current protection circuit according to claim 1, characterized in that: The current detection pin of the chip is grounded.

5. The output current protection circuit according to claim 1, characterized in that: The collector of the transistor at the secondary side of the optocoupler U3 is connected to the compensation terminal COMP of the chip, and the emitter is connected to the ground terminal of the chip.

6. The output current protection circuit according to claim 1, characterized in that: The first voltage regulator U1 and the second voltage regulator U2 both adopt TL431 voltage regulator.

7. The output current protection circuit according to claim 1, characterized in that: The two voltage-dividing resistors R5 and R4 have the same resistance value, and the connected current-limiting resistor R6 has the same resistance value as the third current-limiting resistor R3.

8. The output current protection circuit according to claim 1, characterized in that: The resistance of the seventh voltage-dividing resistor R7 is greater than the resistance of the eighth voltage-dividing resistor R8, and the sum of the resistances of the two voltage-dividing resistors R7 and R8 is equal to or slightly greater than the sum of the resistances of the other two voltage-dividing resistors R4 and R5.

9. The output current protection circuit according to claim 1, characterized in that: When the output current increases, the potential of the reference electrode of the second regulator U2 increases until the second regulator U2 is turned on, the primary side U3-B of the optocoupler U3 is turned on, the secondary side U3-A is turned on, the compensation terminal COMP of the chip is pulled low, the chip does not work, and overcurrent protection is achieved.

10. The output current protection circuit according to claim 1, characterized in that: The chip is a UC3845 chip.

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