Variable frequency soft start circuit

Abstract

The utility model discloses a kind of variable frequency slow start circuit, including PWM control circuit, switching power supply power conversion circuit and with slow start control circuit, PWM control circuit includes PWM control chip and the resistance and capacitance for controlling PWM operating frequency, PWM control circuit generates PWM drive signal and transmission to switching power supply power conversion circuit, switching power supply power conversion circuit includes push-pull power tube, transformer and rectification and output circuit, PWM drive signal controls push-pull power tube conduction and turn-off, push-pull power tube connects transformer primary side, transformer secondary side connects rectification and output circuit, slow start control circuit is connected with PWM control circuit, for circuit opening, change the equivalent resistance resistance value for controlling PWM frequency in PWM control circuit, to change PWM operating frequency. The present circuit makes ordinary PWM control chip have the function of controlling starting process by slow start control circuit.

Description

Technical Field

[0001] This utility model belongs to the field of switching power supply technology, specifically relating to a frequency conversion soft start circuit. Background Technology

[0002] Most existing PWM control chips have a soft-start setting pin, with a capacitor connected to this pin. The PWM control chip provides a charging current to this pin, and as the charging voltage increases, the maximum PWM duty limit is gradually amplified, thus limiting the inductor current. This achieves the soft-start function, but this method requires the PWM chip itself to provide a soft-start pin. Another method is to limit the PWM control command to a minimum at startup, and then gradually increase the specified limit value during startup. This method is similar to the soft-start capacitor pin approach, both limiting the maximum PWM duty. Utility Model Content

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a frequency conversion soft start circuit.

[0004] This utility model discloses a frequency conversion soft-start circuit, including a PWM control circuit, a switching power supply power conversion circuit, and a soft-start control circuit. The PWM control circuit includes a PWM control chip and resistors and capacitors connected to the PWM control chip for controlling the PWM operating frequency. The PWM control circuit generates a PWM drive signal and transmits it to the switching power supply power conversion circuit. The switching power supply power conversion circuit includes a push-pull power transistor, a transformer, and a rectification and output circuit. The PWM drive signal controls the push-pull power transistor to turn on and off. The push-pull power transistor is connected to the primary side of the transformer, and the secondary side of the transformer is connected to the rectification and output circuit. The rectification and output circuit is used to rectify and filter the voltage before outputting it. The soft-start control circuit is connected to the PWM control circuit and is used to change the equivalent resistance value of the PWM control circuit used to control the PWM frequency when the circuit is turned on, thereby changing the PWM operating frequency.

[0005] Furthermore, the PWM control circuit includes a PWM control chip and a first resistor, a second resistor, and a first capacitor connected to the PWM control chip. The first end of the first resistor is connected to a power supply, the second end of the first resistor is connected in series with the first end of the second resistor, the second end of the second resistor is connected to the first end of the first capacitor, and the second end of the first capacitor is grounded. The first resistor and the second resistor charge the first capacitor, and the first capacitor discharges through the second resistor to the PWM control chip to control the PWM operating frequency.

[0006] Furthermore, the resistance value of the first resistor is greater than the resistance value of the second resistor.

[0007] Furthermore, the PWM control chip is either a chip that does not have a soft start setting pin or has a soft start setting inside the chip but the internal soft start circuit is insufficient to adapt to the soft start of the actual circuit, and the PWM control chip is connected to an overcurrent protection detection circuit.

[0008] Furthermore, the soft-start control circuit includes a first transistor, a third resistor, a fourth resistor, and a second capacitor. The first end of the third resistor is connected to the first end of the second capacitor, and the second end of the third resistor is connected to the base of the first transistor. The second end of the second capacitor is grounded. The collector of the first transistor is connected to the first end of the fourth resistor, and the emitter of the first transistor is connected to the power supply. The second end of the fourth resistor is connected to the second end of the first resistor. When the circuit is turned on, the power supply charges the second capacitor through the first transistor and the third resistor. The charging current causes the first transistor to enter a saturation state. The fourth resistor is effectively connected in parallel with the first resistor. After the fourth resistor is connected in parallel, the equivalent resistance value is reduced, which increases the PWM operating frequency. The leakage inductance on the primary side of the transformer is used for current limiting. After the circuit completes the start-up, the PWM operating frequency returns to the normal operating frequency.

[0009] Furthermore, the soft-start control circuit also includes a first diode for discharging the second capacitor when the power supply is cut off. The cathode of the first diode is connected to the power supply, and the anode of the first diode is connected to the first end of the third resistor.

[0010] The beneficial effects of this utility model are: This utility model proposes a frequency conversion soft-start circuit for simple PWM control chips that lack soft-start pins, or for PWM control chips with built-in soft-start circuits whose performance parameters cannot meet the operational requirements of the actual circuit during the startup phase. By adding a simple soft-start control circuit, it increases the PWM switching frequency during startup and utilizes the current-limiting function of the transformer's primary side leakage inductance to solve the startup cycle failure problem when starting with high output capacitors or under load. It can be implemented using ordinary PWM control chips, uses inexpensive components, does not require high precision specifications, has high component versatility, and offers high flexibility to meet circuit design requirements. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the frequency conversion soft start circuit proposed in this utility model. Detailed Implementation

[0012] The embodiments of this utility model are described in detail below. These embodiments are implemented based on the technical solution of this utility model and provide detailed implementation methods and specific operation processes. However, the protection scope of this utility model is not limited to the following embodiments.

[0013] like Figure 1 As shown, this utility model discloses a frequency conversion soft-start circuit, including a PWM control circuit, a switching power supply power conversion circuit, and a soft-start control circuit. For simple push-pull PWM control chips with overcurrent protection detection circuits but lacking soft-start pins, or PWM control chips whose internal soft-start circuits are insufficient to adapt to the actual circuit's soft-start action, this invention increases the PWM switching frequency during circuit startup by adding a simple soft-start control circuit.

[0014] The PWM control circuit includes a PWM control chip, a first resistor R98, a second resistor R101, and a first capacitor C60 connected to the PWM control chip. In this embodiment, the PWM control chip is a UCC28089DR chip. The VDD pin of the PWM control chip is connected to a +12V power supply, the GND pin of the PWM control chip is grounded, the first end of the fourth capacitor C59 is connected to the VDD pin of the PWM control chip, and the second end of the fourth capacitor C59 is connected to the GND pin of the PWM control chip. The first end of the first resistor R98 is connected to a +12V power supply, the second end of the first resistor R98 is connected in series with the first end of the second resistor R101, the second end of the first resistor R98 is connected to the DIS pin of the PWM control chip, the first end of the second resistor R101 is connected to the DIS pin of the PWM control chip, the second end of the second resistor R101 is connected to the first end of the first capacitor C60, the first end of the first capacitor C60 is connected to the CT pin of the PWM control chip, and the second end of the first capacitor C60 is grounded. The CS pin of the PWM control chip is connected to an overcurrent protection detection circuit, which includes a third capacitor C61, a fifth resistor R102, a sixth resistor R103, a seventh resistor R104, an eighth resistor R105, a ninth resistor R107, and a tenth resistor R108. The first terminal of the third capacitor C61 is connected to the CS pin of the PWM control chip, and the second terminal of the third capacitor C61 is connected to ground. The fifth resistor R102 and the sixth resistor R103 are connected in series. One end of the series-connected fifth resistor R102 and the sixth resistor R103 is connected to the first terminal of the third capacitor C61, and the other end is connected to the first terminal of the parallel circuit of the seventh resistor R104 and the eighth resistor R105. The second terminal of the parallel circuit of the seventh resistor R104 and the eighth resistor R105 is connected to the second terminal of the third capacitor C61. The first terminal of the tenth resistor R108 is connected to the second terminal of the parallel circuit of the seventh resistor R104 and the eighth resistor R105. The first terminal of the ninth resistor R107 is connected to the first terminal of the tenth resistor R108. The OUTA and OUTB pins of the PWM control chip are connected to the power conversion circuit of the switching power supply, outputting two PWM drive signals.

[0015] The switching power supply power conversion circuit includes push-pull power transistors, transformer T1, and rectification and output circuits. The OUTA pin of the PWM control chip is connected to the first terminal of the eleventh resistor R99; the second terminal of the tenth resistor R108 is connected to the second terminal of the eleventh resistor R99; the second terminal of the eleventh resistor R99 is connected to the gate of the first push-pull power transistor Q25; the source of the first push-pull power transistor Q25 is connected to the first terminal of the parallel circuit of the seventh resistor R104 and the eighth resistor R105; the OUTB pin of the PWM control chip is connected to the first terminal of the twelfth resistor R100; the second terminal of the ninth resistor R107 is connected to the second terminal of the twelfth resistor R100; the second terminal of the twelfth resistor R100 is connected to the gate of the second push-pull power transistor Q27; the source of the second push-pull power transistor Q27 is connected to the first terminal of the parallel circuit of the seventh resistor R104 and the eighth resistor R105; the first terminal of the fourth capacitor C59, the drain of the first push-pull power transistor Q25, and the drain of the second push-pull power transistor Q27 are connected to the first side of transformer T1; the second side of transformer T1 is connected to the rectification and output circuits. The rectification and output circuit is used to rectify and filter the voltage before outputting it, and includes a first rectifier diode D11, a second rectifier diode D12, and a filter capacitor C56.

[0016] The soft-start control circuit is connected to the PWM control circuit and is used to change the equivalent resistance value of the PWM frequency control circuit when the circuit is turned on, thereby changing the PWM operating frequency. The soft-start control circuit includes a first transistor Q28, a third resistor R109, a fourth resistor R110, and a second capacitor C62. The first terminal of the third resistor R109 is connected to the first terminal of the second capacitor C62, and the second terminal of the third resistor R109 is connected to the base of the first transistor Q28. The second terminal of the second capacitor C62 is grounded. The collector of the first transistor Q28 is connected to the first terminal of the fourth resistor R110, and the emitter of the first transistor Q28 is connected to a +12V power supply. The second terminal of the fourth resistor R110 is connected to the second terminal of the first resistor R98. The soft-start control circuit also includes a first diode D13 disposed between the first end of the third resistor R109 and the +12V power supply, which is used to discharge the second capacitor C62 when the power supply is cut off. The cathode of the first diode D13 is connected to the +12V power supply, and the anode of the first diode D13 is connected to the first end of the third resistor R109.

[0017] The UCC28089DR chip lacks a soft-start setting pin and is connected to an overcurrent protection detection circuit. If the PWM control chip detects multiple overcurrent signals consecutively, it will enter a shutdown state for tens of milliseconds. This will result in a continuous cycle of startup, current limiting, shutdown, and output voltage drop when the circuit has a high output capacitance or is starting under load, preventing normal startup. This application adds a soft-start control circuit to increase the PWM switching frequency during circuit startup and utilizes the leakage inductance of the transformer primary side for current limiting. After the circuit completes startup, the PWM switching frequency returns to the normal operating frequency.

[0018] The PWM control chip IC uses a first resistor R98, a second resistor R101, and a first capacitor C60 to control the PWM operating frequency. The PWM control circuit uses the first resistor R98 and the second resistor R101 to control the charging of the first capacitor C60, which is the turn-on time. Once the charging level is reached, the first capacitor C60 discharges through the second resistor R101 to the DIS pin of the PWM control chip, which is the turn-off time. The formula for calculating the PWM frequency is: .

[0019] The resistance of the first resistor R98 is much greater than that of the second resistor R101, which can be understood as being more than 10 times greater. Therefore, in this circuit, the resistance of the first resistor R98 can be more than 10 times that of the second resistor R101. In the soft-start control circuit, when the circuit is first turned on, the +12V power supply charges the second capacitor C62 through the first transistor Q28 and the third resistor R109. At this time, the charging current causes the first transistor Q28 to enter a saturated state. The fourth resistor R110 is effectively connected in parallel with the first resistor R98. Since the resistance of the first resistor R98 is much larger than that of the second resistor R101, the PWM operating frequency is largely determined by the first resistor R98. When the first resistor R98 is connected in parallel with the fourth resistor R110, the equivalent resistance decreases significantly, thus increasing the PWM operating frequency. For example, the equivalent resistance of the fourth resistor R110 is reduced by approximately 11 times, resulting in an 11-fold increase in the PWM operating frequency. The original design PWM operating frequency was approximately 80kHz, but now it rises to approximately 880kHz. Although the duty cycle remains unchanged, the significantly increased frequency shortens the cycle time, reducing the turn-on time by 11 times. Therefore, the PWM signal output is turned off before the current reaches the current limit, achieving a soft-start function. During circuit startup, as the voltage of the second capacitor C62 gradually increases, the first transistor Q28 gradually enters the linear region. The impedance connected in parallel to the first resistor R98 gradually increases, and the PWM operating frequency gradually decreases. Eventually, the first transistor Q28 is turned off. At this point, the first resistor R98 is no longer affected by the fourth resistor R110, and the PWM operating frequency returns to its normal operating frequency. When the +12V power supply is cut off, the second capacitor C62 discharges through the first diode D13 to meet the charging requirements during the next startup.

[0020] This circuit does not have an output filter inductor; only the primary side of the transformer has a small amount of leakage inductance. The soft-start method involves increasing the PWM switching frequency during circuit startup. The soft-start time is controlled by the third resistor R109 through the second capacitor C62, and the maximum soft-start frequency is controlled by the fourth resistor R110. Current limiting is achieved using the leakage inductance on the primary side of the transformer. After the circuit has started, the PWM switching frequency returns to the normal operating frequency. This circuit offers high flexibility and meets circuit design requirements, enabling ordinary PWM control chips to control the startup process through the soft-start control circuit.

[0021] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A frequency conversion soft-start circuit, characterized in that: The system includes a PWM control circuit, a switching power supply power conversion circuit, and a soft-start control circuit. The PWM control circuit includes a PWM control chip and resistors and capacitors connected to the PWM control chip for controlling the PWM operating frequency. The PWM control circuit generates a PWM drive signal and transmits it to the switching power supply power conversion circuit. The switching power supply power conversion circuit includes a push-pull power transistor, a transformer, and a rectification and output circuit. The PWM drive signal controls the push-pull power transistor to turn on and off. The push-pull power transistor is connected to the primary side of the transformer, and the secondary side of the transformer is connected to the rectification and output circuit. The rectification and output circuit is used to rectify and filter the voltage before outputting it. The soft-start control circuit is connected to the PWM control circuit and is used to change the equivalent resistance value of the PWM control circuit used to control the PWM frequency when the circuit is turned on, thereby changing the PWM operating frequency.

2. The frequency conversion soft-start circuit according to claim 1, characterized in that: The PWM control circuit includes a PWM control chip and a first resistor R98, a second resistor R101, and a first capacitor C60 connected to the PWM control chip. The first terminal of the first resistor R98 is connected to a power supply, the second terminal of the first resistor R98 is connected in series with the first terminal of the second resistor R101, the second terminal of the second resistor R101 is connected to the first terminal of the first capacitor C60, and the second terminal of the first capacitor C60 is grounded. The first resistor R98 and the second resistor R101 charge the first capacitor C60, and the first capacitor C60 discharges through the second resistor R101 to the PWM control chip to control the PWM operating frequency.

3. The frequency conversion soft-start circuit according to claim 2, characterized in that: The resistance of the first resistor R98 is greater than that of the second resistor R101.

4. The frequency conversion soft-start circuit according to claim 1, characterized in that: The PWM control chip is either without a soft-start setting pin or has an internal soft-start setting but the internal soft-start circuit is insufficient to adapt to the actual circuit's soft-start, and the PWM control chip is connected to an overcurrent protection detection circuit.

5. The frequency conversion soft-start circuit according to claim 3, characterized in that: The soft-start control circuit includes a first transistor Q28, a third resistor R109, a fourth resistor R110, and a second capacitor C62. The first terminal of the third resistor R109 is connected to the first terminal of the second capacitor C62, and the second terminal of the third resistor R109 is connected to the base of the first transistor Q28. The second terminal of the second capacitor C62 is grounded. The collector of the first transistor Q28 is connected to the first terminal of the fourth resistor R110, and the emitter of the first transistor Q28 is connected to the power supply. The second terminal of the fourth resistor R110 is connected to the second terminal of the first resistor R98. When the circuit is turned on, the power supply charges the second capacitor C62 through the first transistor Q28 and the third resistor R109. The charging current causes the first transistor Q28 to enter a saturation state. The fourth resistor R110 is effectively connected in parallel with the first resistor R98. After the fourth resistor R110 is connected in parallel, the equivalent resistance value is reduced, which increases the PWM operating frequency. The leakage inductance on the primary side of the transformer is used for current limiting.

6. The frequency conversion soft-start circuit according to claim 5, characterized in that: The soft-start control circuit also includes a first diode D13, which is used to discharge the second capacitor C62 when the power supply is cut off. The cathode of the first diode D13 is connected to the power supply, and the anode of the first diode D13 is connected to the first end of the third resistor R109.

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