Flyback output short-circuit protection circuit and working method thereof

Abstract

The present invention relates to a flyback output short-circuit protection circuit and its operating method. The circuit includes a control unit, a switching element, and a filtering unit. The control unit is connected to the filtering unit, which is connected to the switching element. The control unit is connected to a power supply unit. When the output of the control unit is short-circuited, the voltage of the input switching element drops, disconnecting the switching element. Under the control of the control unit, the power supply unit outputs power through the filtering unit to disconnect the switching element from the control unit, while the voltage of the input switching element gradually increases. When the voltage of the input switching element increases to a set value, the switching element is turned on and connected to the control unit. The present invention achieves the purpose of achieving thermal balance of power devices and meets the requirement of long-term output short circuit of the product.

Description

Technical Field

[0001] The present invention relates to a short-circuit protection circuit, and more particularly to a flyback output short-circuit protection circuit and a working method thereof. Background Art

[0002] Flyback power supplies commonly used in the market have two uses: one is to serve as an auxiliary power supply for the entire system, and the other is to use a primary-side sampling resistor for protection. The worst case output short-circuit conditions for a flyback circuit occur under high voltage. However, during high voltage, the charging time of the soft-start capacitor is shortened, which speeds up the switching cycle of the MOS tube and increases the heat of the MOS tube. Therefore, the flyback output is operated in a short-circuit state for a long time. This ensures that the MOS tube heats to a reasonable temperature when the output is short-circuited under high voltage conditions, while being able to start normally under low voltage conditions. However, the current usage method is not compatible with flyback power supply protection with a wide input voltage range due to the difficulty in selecting the primary-side sampling resistor, which leads to product damage if the output is short-circuited for a long time.

[0003] Therefore, it is necessary to design a new circuit to achieve the purpose of thermal balance of power devices and meet the requirements of long-term short circuit of product output. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a flyback output short-circuit protection circuit and a working method thereof.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: a flyback output short-circuit protection circuit, comprising a control unit, a switching element and a filtering unit, wherein the control unit is connected to the filtering unit, the filtering unit is connected to the switching element, and the control unit is connected to a power supply unit; when the output of the control unit is short-circuited, the voltage input to the switching element drops, and the switching element is disconnected. Under the control of the control unit, the power supply unit outputs power through the filtering unit to disconnect the switching element from the control unit, and the voltage input to the switching element gradually increases; when the voltage input to the switching element increases to a set value, the switching element is turned on, and the switching element is connected to the control unit.

[0006] A further technical solution is: the control unit includes a main control chip U3.

[0007] Its further technical solution is: the model of the main control chip U3 is UC2844AD8TR.

[0008] Its further technical solution is: the switching element includes a transistor Q2 and an optocoupler U1, the base of the transistor Q2 is connected to the optocoupler U1, the emitter of the transistor Q2 is grounded, the collector of the transistor Q2 is connected to the filter unit, and the collector of the transistor Q2 is connected to the main control chip U3.

[0009] Its further technical solution is: the filtering unit includes a filter capacitor C57 and a filter resistor R39, the filter resistor R39 is connected to the main control chip U3, the filter capacitor C57 is connected to the filter resistor R39, the other end of the filter capacitor C57 is grounded, and one end of the filter capacitor C57 is connected to the main control chip U3.

[0010] A further technical solution is as follows: the main control chip U3 is connected to a MOS tube Q1, the gate of the MOS tube Q1 is connected to the main control chip U3, and the source of the MOS tube Q1 is grounded through a resistor R15.

[0011] A further technical solution is as follows: the main control chip U3 is connected to the source of the MOSFET Q1 via a resistor R16, and a filter capacitor C20 with one end grounded is connected between the resistor R16 and the main control chip U3.

[0012] A further technical solution is as follows: the optical coupler U1 is connected to the drain of the MOSFET Q1 via a resistor R6.

[0013] A further technical solution is: the optical coupler U1 is grounded via a voltage stabilizing diode U2.

[0014] The present invention also provides a working method of a flyback output short-circuit protection circuit, comprising:

[0015] When a short circuit occurs at the output of the control unit, the voltage of the input switching element drops and the switching element is disconnected. Under the control of the control unit, the power supply unit outputs power through the filtering unit to disconnect the switching element from the control unit, and the voltage of the input switching element gradually increases. When the voltage of the input switching element increases to the set value, the switching element is turned on and connected to the control unit.

[0016] The beneficial effects of the present invention compared with the prior art are as follows: by providing a control unit, a switching element and a filtering unit, and by adjusting the time constant of the filtering unit, the present invention can extend the time for establishing a connection between the switching element and the control unit, thereby changing the hiccup time and hiccup cycle of the entire flyback power supply during short circuit, which can effectively reduce the heat generation of the MOS tube Q1 and the rectifier tube, can meet the requirements of long-term short circuit at 65°C, achieve the purpose of achieving thermal balance of the power device, and meet the requirements of long-term short circuit of product output.

[0017] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. 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 any creative work.

[0019] Figure 1 A schematic block diagram of a flyback output short-circuit protection circuit provided in a specific embodiment of the present invention;

[0020] Figure 2 A specific circuit schematic diagram of a flyback output short-circuit protection circuit provided by a specific embodiment of the present invention;

[0021] Figure 3 This is a schematic block diagram of the internal structure of the main control chip U3 provided in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of the present invention.

[0024] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0025] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0026] In the present invention, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connect," and "fixed" should be understood broadly. For example, they may refer to connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0027] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0028] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0029] like Figures 1 to 3In the specific embodiment shown, the flyback output short-circuit protection circuit provided in this embodiment can be used when the flyback power supply is applied as an independent power supply. It can meet the characteristic of not being damaged by long-term output short circuit. In a secondary-side-based feedback method, the hiccup cycle and hiccup time during short circuit are changed by appropriately delaying the startup time to achieve the purpose of thermal balance of the power device, thereby meeting the requirement of long-term output short circuit of the product.

[0030] See also Figure 1 The above-mentioned flyback output short-circuit protection circuit includes a control unit 10, a switching element 20 and a filtering unit 30. The control unit 10 is connected to the filtering unit 30, the filtering unit 30 is connected to the switching element 20, and the control unit 10 is connected to the power supply unit 40. When the output of the control unit 10 is short-circuited, the voltage input to the switching element 20 drops and the switching element 20 is disconnected. Under the control of the control unit 10, the power supply unit 40 outputs power through the filtering unit 30 to disconnect the switching element 20 from the control unit 10, and the voltage input to the switching element 20 gradually increases. When the voltage input to the switching element 20 increases to a set value, the switching element 20 is turned on and connected to the control unit 10.

[0031] By adjusting the time constant of the filter unit 30, the time required to establish a connection between the switch element 20 and the control unit 10 can be extended, thereby changing the hiccup time and hiccup cycle of the entire flyback power supply during a short circuit. This can effectively reduce the heat generation of the MOS tube Q1 and the rectifier tube, and can meet the requirements of a long-term short circuit at 65°C.

[0032] In one embodiment, see Figure 2 The above-mentioned control unit 10 includes a main control chip U3.

[0033] In this example, see Figure 3 The model of the main control chip U3 is UC2844AD8TR. Of course, in other embodiments, the main control chip U3 can be other control chips.

[0034] In one embodiment, see Figure 2 The above-mentioned switching element 20 includes a transistor Q2 and an optical coupler U1, the base of the transistor Q2 is connected to the optical coupler U1, the emitter of the transistor Q2 is grounded, the collector of the transistor Q2 is connected to the filter unit 30, and the collector of the transistor Q2 is connected to the main control chip U3.

[0035] In this embodiment, the optocoupler U1 is a transistor receiving type optocoupler, which is a four-pin package. Specifically, the base of the transistor Q2 is connected to the emitter of the phototransistor of the optocoupler U1, the collector of the phototransistor of the optocoupler U1 is connected through the capacitor C54, and the collector of the phototransistor of the optocoupler U1 is connected to the main control chip U3.

[0036] In one embodiment, see Figure 2 The above-mentioned filtering unit 30 includes a filtering capacitor C57 and a filtering resistor R39. The filtering resistor R39 is connected to the main control chip U3. The filtering capacitor C57 is connected to the filtering resistor R39. The other end of the filtering capacitor C57 is grounded, and one end of the filtering capacitor C57 is connected to the main control chip U3.

[0037] The main control chip U3 feeds back the voltage of the power output from the power supply unit 40 to the main control chip U3 after passing it through the filter unit 30 , and the feedback voltage is used to control the connection or disconnection between the main control chip U3 and the switch element 20 .

[0038] In one embodiment, see Figure 2 The main control chip U3 is connected to the MOSFET Q1, the gate of the MOSFET Q1 is connected to the main control chip U3, and the source of the MOSFET Q1 is grounded through the resistor R15.

[0039] In one embodiment, see Figure 2 The main control chip U3 is connected to the source of the MOSFET Q1 via a resistor R16, and a filter capacitor C20 with one end grounded is connected between the resistor R16 and the main control chip U3.

[0040] In one embodiment, see Figure 2 The above-mentioned optical coupler U1 is connected to the drain of the MOSFET Q1 through the resistor R6.

[0041] In one embodiment, see Figure 2 , the above-mentioned optocoupler U1 is grounded through the voltage-stabilizing diode U2.

[0042] In this embodiment, the anode of the diode of the optocoupler U1 is connected to the resistor R6, the cathode of the diode of the optocoupler U1 is grounded via the Zener diode U2, and the Zener diode U2 is grounded via the resistor R36. The Zener diode U2 is connected to the MOSFET Q1 via the resistor R32, the capacitor C43, and the resistor R37. The cathode of the diode of the optocoupler U1 is connected to the anode of the diode of the optocoupler U1 via the resistor R22.

[0043] In this embodiment, the filter resistor R39 is connected to the main control chip U3 via the resistor R35 , and two ends of the resistor R35 are respectively connected to a capacitor C52 and a capacitor C53 .

[0044] See also Figure 2 and Figure 3 , when the entire flyback power supply starts normally, the working sequence is that when the soft-start capacitor is charged to the threshold value of the main control chip U3, the main control chip U3 starts to work, then the power supply starts to work, and the VCC terminal of the main control chip U3 works, and the VREF terminal also has 5V. Then at this time, the VREF terminal charges the VFB terminal through the filter resistor R39 and the filter capacitor C57. At the same time, the chip also starts to send PWM waves, and the circuit starts to work. However, at this time, the COMP terminal has not yet established a connection with the optocoupler U1. Before the voltage of the VFB terminal is charged to 2.5V, the COMP terminal must be established. That is to say, the time constants of the filter resistor R39 and the filter capacitor C57 must be adjusted appropriately. Otherwise, the entire main control chip U3 will hiccup repeatedly and cannot be turned on. When the output voltage, VDD5V, slowly rises, optocoupler U1 turns on, and the current flowing through optocoupler U1 turns on transistor Q2. At this point, the VFB pin, which is the collector of transistor Q2, is pulled to GND by the emitter. At this point, the COMP pin begins to build up. Then, if any output path of main control chip U3 is short-circuited, the induced electromotive force will be transmitted to the feedback path, which is the ISNS pin. VDD5V will also drop, causing optocoupler U1 to be unable to provide sufficient base conduction current to Q2. When transistor Q2 is turned off, the VREF pin immediately charges the VFB pin through filter resistor R39 and filter capacitor C57. When the voltage is charged to above 2.5V, the COMP pin is pulled low, and main control chip U3 enters protection mode. At this point, the output of main control chip U3 has no voltage, and the short circuit is removed. VDD5V then slowly rises, and optocoupler U1 continues to conduct transistor Q2, reestablishing the COMP pin's voltage normally. Because the output short circuit still exists, main control chip U3 enters protection mode again. This enters cyclic hiccup protection mode. Therefore, by properly adjusting the values ​​of filter resistor R39 and filter capacitor C57, extending the COMP pin's voltage establishment time and thus varying the hiccup duration and period during a short circuit, the heat generation of MOS tube Q1 and the rectifier can be effectively reduced. This can meet the requirements of a long-term short circuit at 65°C.

[0045] The entire circuit was tested, and the specific experimental data are shown in Table 1 below. At room temperature of 25°C, the MOS tube temperature is 58°C.

[0046] Table 1. Experimental data

[0047] No. Input voltage Filter capacitor C57 Filter resistor R39 Time constant capacitor R15 Ton T Short circuit MOS tube Q1 temperature Low voltage 90VAC start

[0048] 1 290VAC 10n 100K 1ms 0.47R 0.8ms 280ms Blank ×

[0049] 2 290VAC 1U 100K 100ms 0.47R 50ms 430ms Blank OK

[0050] 3 290VAC 0.47U 100K 47ms 0.47R 38ms 360ms Blank OK

[0051] 4 290VAC 0.22U 100K 22ms 0.47R 26ms 330ms Blank OK

[0052] 5 290VAC 0.1U 100K 10ms 0.47R 18ms 310ms 82℃ OK

[0053] 6 290VAC 10n 1M 10ms 0.47R 8ms 310ms Blank OK

[0054] 7 290VAC 4.7n 1M 4.7ms 0.47R 3.5ms 280ms Blank ×

[0055] 8 290VAC 2.2n 1M 2.2ms 0.47R 1.5ms 280ms Blank ×

[0056] 9 290VAC 1n 2.2M 2.2ms 0.47R 1.5ms 280ms Blank ×

[0057] 10 290VAC 4.7n 2.2M 10.34ms 0.47R 7.3ms 300ms Blank OK

[0058] 11 290VAC 4.7n 3M 14.1ms 0.47R 9.8ms 318ms Blank ×

[0059] 12 290VAC 4.7n 910K 4.277ms 0.47R 3ms 288ms Blank ×

[0060] 13 290VAC 10n 910K 9.1ms 0.47R 6.65ms 300ms Blank OK

[0061] 14 290VAC 10n 820K 8.2ms 0.47R 6ms 300ms Blank OK

[0062] 15 290VAC 10n 690K 6.9ms 0.47R 4.85ms 297ms 58℃ OK

[0063] 16 290VAC 10n 540K 5.4ms 0.47R 3.9ms 293ms Empty OK

[0064] 17 290VAC 10n 500k 5ms 0.47R 3.8ms 290ms Empty×

[0065] 18 290VAC 10n 690K 6.9ms 2R / / 1R Empty Empty ×

[0066] 19 290VAC 10n 1M 10ms 2R / / 1R 7.1ms 290ms 63℃ OK

[0067] The flyback power supply with wide input range and multiple outputs can be used as an independent power supply. Even if any output line is accidentally short-circuited, the product can protect itself from damage.

[0068] The above-mentioned flyback output short-circuit protection circuit is provided with a control unit 10, a switching element 20 and a filtering unit 30. By adjusting the time constant of the filtering unit 30, the time for the switching element 20 to establish a connection with the control unit 10 can be extended, thereby changing the hiccup time and hiccup cycle of the entire flyback power supply during short circuit. It can effectively reduce the heat generation of the MOS tube Q1 and the rectifier tube, can meet the requirements of long-term short circuit at 65°C, achieve the purpose of achieving thermal balance of power devices, and meet the requirements of long-term short circuit of product output.

[0069] 10. A method for operating a flyback output short-circuit protection circuit, comprising:

[0070] When a short circuit occurs at the output of the control unit 10, the voltage of the input switching element 20 drops and the switching element 20 is disconnected. Under the control of the control unit 10, the power supply unit 40 outputs power through the filtering unit 30 to disconnect the switching element 20 from the control unit 10, and the voltage of the input switching element 20 gradually increases; when the voltage of the input switching element 20 increases to the set value, the switching element 20 is turned on and connected to the control unit 10.

[0071] It should be noted that technicians in the relevant field can clearly understand that the specific implementation process of the working method of the above-mentioned flyback output short-circuit protection circuit can refer to the corresponding description in the aforementioned flyback output short-circuit protection circuit embodiment. For the convenience and brevity of description, it will not be repeated here.

[0072] The above examples are merely provided to further illustrate the technical content of the present invention for easier understanding by the reader, but do not limit the embodiments of the present invention to these examples. Any extension or re-creation of the technology based on the present invention is protected by the present invention. The scope of protection of the present invention shall be determined by the claims.

Claims

1. The flyback output short-circuit protection circuit is characterized in that: The invention comprises a control unit, a switch element and a filter unit, wherein the control unit is connected to the filter unit, the filter unit is connected to the switch element, and the control unit is connected to a power supply unit; when the output of the control unit is short-circuited, the voltage input to the switch element drops, the switch element is disconnected, and under the control of the control unit, the power supply unit outputs power through the filter unit to disconnect the switch element from the control unit, and the voltage input to the switch element gradually increases; when the voltage input to the switch element increases to a set value, the switch element is turned on, and the switch element is connected to the control unit; the control unit comprises a main control chip (U3), the filter unit comprises a filter capacitor (C57) and a filter resistor (R39), the filter resistor (R39) is connected to the main control chip (U3), the filter capacitor (C57) is connected to the filter resistor (R39), the other end of the filter capacitor (C57) is grounded, and one end of the filter capacitor (C57) is grounded. The switching element comprises a transistor (Q2) and an optical coupler (U1), the base of the transistor (Q2) is connected to the optical coupler (U1), the emitter of the transistor (Q2) is grounded, the collector of the transistor (Q2) is connected to the filter unit, and the collector of the transistor (Q2) is connected to the main control chip (U3); the main control chip (U3) is connected to a MOSFET (Q1), and the gate of the MOSFET (Q1) is connected to the The main control chip (U3) is connected, and the source of the MOSFET (Q1) is grounded through a resistor (R15); the main control chip (U3) is connected to the source of the MOSFET (Q1) through a resistor (R16), and a filter capacitor (C20) with one end grounded is connected between the resistor (R16) and the main control chip (U3); the optical coupler (U1) is connected to the drain of the MOSFET (Q1) through a resistor (R6); and the optical coupler (U1) is grounded through a voltage stabilizing diode (U2).

2. The flyback output short-circuit protection circuit according to claim 1, wherein: The model of the main control chip (U3) is UC2844AD8TR.

3. The operating method of the flyback output short-circuit protection circuit according to claim 1 or 2, characterized in that: include: When a short circuit occurs at the output of the control unit, the voltage of the input switching element drops and the switching element is disconnected. Under the control of the control unit, the power supply unit outputs power through the filtering unit to disconnect the switching element from the control unit, and the voltage of the input switching element gradually increases. When the voltage of the input switching element increases to the set value, the switching element is turned on and connected to the control unit.

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