Method for limiting maximum duty ratio and realizing soft start for DC / DC converter

Abstract

The invention discloses a method for limiting the maximum duty ratio and achieving soft start for a DC / DC converter, and particularly relates to the technical field of aerospace power supplies. Therefore, the voltage stress of each device in the forward DC / DC converter is controlled, the starting of the converter under a special working condition is controlled, and the reliability of the DC / DC converter in a complex whole machine application process can be effectively improved. Through the newly added discharge loop, a rapid and reliable discharge channel independent of Vo voltage is provided for the soft start capacitor C1, so that the problem that the soft start circuit is not controlled when the converter is restarted within a short time due to slow output power failure of the DC / DC converter is effectively solved. Even under the extreme condition that the converter is repeatedly started and stopped in a short time due to input under-voltage jitter or forbidding signal jitter, each time of starting is controlled soft starting, and the electricity utilization safety of aerospace equipment is improved.

Description

Technical Field

[0001] This invention relates to the field of aerospace power technology, and more specifically, to a method for limiting the maximum duty cycle and achieving soft start for a DC / DC converter. Background Technology

[0002] Modern aerospace power systems place extremely stringent requirements on DC / DC converters, demanding high reliability, adaptability, and power density. However, in the extremely complex environments of aerospace applications, the power supply bus voltage may be unstable (e.g., fluctuating continuously near the undervoltage protection point), and control signals (e.g., disable signals) may also fluctuate. Furthermore, due to load characteristics, the converter output voltage may drop slowly. These conventional and unconventional operating conditions pose continuous and severe challenges to the voltage stress on the converter's internal components, the reliability of the switching process, and the control of the start / stop logic.

[0003] Existing DC / DC converters often lack sufficient protection mechanisms and adaptability when dealing with complex operating conditions involving multiple intertwined stresses, mainly due to the following two problems: 1. Risk of primary-side duty cycle runaway: When the input voltage fluctuates in the undervoltage region or the disable signal fluctuates, the primary control chip may output an abnormally increased duty cycle signal, resulting in a severely insufficient core reset time for the converter. This causes the main power MOSFET to experience a voltage spike far exceeding the design value at the moment of turn-off. This voltage spike can easily exceed the withstand voltage limit of the switching device, causing overvoltage breakdown and permanent damage to the converter.

[0004] 2. Risk of Secondary Side Soft-Start Failure: Under conditions of slow output power loss, the capacitor charge in the secondary soft-start circuit cannot be released in a timely and complete manner. When the converter restarts quickly, the residual charge will cause the soft-start function to fail, resulting in a huge inrush current and uncontrollable device stress during the startup process. This "hard start" can severely damage internal components and may even trigger secondary faults.

[0005] The aforementioned problems, whether individually or in combination, significantly reduce the adaptability and lifespan of DC / DC converters in high-reliability applications such as aerospace. Therefore, this invention proposes a method for limiting the maximum duty cycle and implementing soft-start for DC / DC converters as a further improvement. Summary of the Invention

[0006] The purpose of this invention is to provide a method for limiting the maximum duty cycle and achieving soft start in a DC / DC converter, in order to solve the problem that in a single-ended forward DC / DC converter, the slow output power loss during input voltage jitter or inhibiting voltage jitter causes excessive stress on the internal components of the converter and soft start failure, which in turn leads to failure and damage.

[0007] To achieve the above objectives, the present invention provides a method for limiting the maximum duty cycle and implementing soft start for a DC / DC converter, thereby solving the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for limiting the maximum duty cycle and implementing soft start in a DC / DC converter, comprising the following steps: On the primary side, by configuring the peripheral R of the main control chip T / C T The timing element parameters limit the maximum duty cycle of its output while maintaining a constant switching frequency, so as to control the voltage stress of the power devices inside the converter. On the secondary side, by adding a discharge path to the output ground in the soft-start circuit, together with the original discharge path through the positive output, a dual-loop discharge structure is formed to ensure that the soft-start capacitor can quickly release the charge after the converter is turned off, thereby ensuring that the soft-start function is effective during the next startup.

[0009] Furthermore, in the primary side, the step of limiting the maximum duty cycle includes: using a reduced resistance value R. T With the increased capacitance value C T The parameter combination limits the maximum duty cycle output by the main control chip from a first preset value to a lower second preset value.

[0010] Furthermore, the range of the second preset value is set to 75%-85%.

[0011] Furthermore, in the secondary side, an additional discharge path to the output ground is provided, which includes: connecting a diode V2 in series between the soft-start capacitor C1 and the VCCS node, and grounding the VCCS node through a resistor R2; the anode of the diode V2 is connected to the soft-start capacitor C1, and the cathode of the diode V2 is connected to the VCCS node.

[0012] Furthermore, the diode V2 is configured to: conduct in the forward direction when the voltage of the VCCS node is lower than the voltage on the soft-start capacitor C1, so as to provide a fast discharge path for the soft-start capacitor C1; and be reverse cut off when the voltage of the VCCS node is higher than the voltage on the soft-start capacitor C1, so as to avoid affecting the normal operation of the converter.

[0013] A DC / DC converter that achieves primary-side maximum duty cycle limitation and secondary-side soft-start through the method described above.

[0014] The technical effects and advantages of this invention are as follows: This invention improves the reliability of DC / DC converters in complex applications by controlling the voltage stress of individual components within the forward DC / DC converter and ensuring controlled startup under special operating conditions through primary limiting of the main control chip's duty cycle and the addition of a soft-start discharge circuit in the secondary winding. The added discharge circuit provides a fast and reliable discharge path for the soft-start capacitor C1, independent of the Vo voltage, effectively resolving the issue of uncontrolled soft-start circuitry when the DC / DC converter's output power drops slowly, leading to a short-term restart. This ensures that even in extreme cases where input undervoltage jitter or inhibit signal jitter causes repeated short-term start-stop cycles, each startup is a controlled soft-start, enhancing the electrical safety of aerospace equipment. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the method of the present invention.

[0016] Figure 2 This is a block diagram illustrating the principle of limiting the maximum duty cycle and achieving soft start in this invention.

[0017] Figure 3 A schematic diagram showing how the maximum duty cycle of the primary side of the DC / DC converter of the present invention is limited.

[0018] Figure 4 This is a circuit diagram of adding a soft-start discharge circuit to the secondary side of the DC / DC converter of the present invention.

[0019] Figure reference numerals: U1, main control chip; T1, main transformer; D1, rectifier; LC, filter; Vo, output voltage; N1, error amplifier; T2, magnetic reversing transformer; R1, Resistor; R2, Resistor; R3, Resistor; R4, Resistor; R5, Resistor; R6, Resistor; C1, Soft-start capacitor; V1, Diode; V2, Diode; Detailed Implementation To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0020] Technical Background and System Overview: The DC / DC converter involved in this invention is structurally divided into an electrically isolated primary side and a secondary side by a transformer. The primary side, also known as the secondary winding, is connected to the input power supply and includes an input filter, a main control chip, a drive circuit, and a main power MOSFET Q1. The secondary side, also known as the secondary winding, is connected to the output load and includes an output rectifier, an output filter, an output voltage feedback circuit, and a soft-start circuit. The duty cycle is generated by the main control chip on the primary side and is a PWM signal used to control the on and off times of the main power MOSFET Q1 on the primary side; it is a key control parameter within the primary side.

[0021] This invention provides a method for limiting the maximum duty cycle and achieving soft start in a DC / DC converter, with the following embodiment: Example 1: Primary Side Maximum Duty Cycle Limitation and Closed-Loop Control: On the primary side, by configuring the peripheral R of the main control chip T / C T The timing element parameters limit the maximum duty cycle of its output while maintaining a constant switching frequency, so as to control the voltage stress of the power devices inside the converter. Specifically, such as Figure 2 As shown: The DC / DC converter of this invention is a single-ended forward excitation isolated feedback circuit, and the main control chip is HS1843; The primary-side main control chip U1 outputs a square wave, driving the main power MOSFET Q1 for high-frequency switching. Transformer T1, working in conjunction with Q1, couples the pulsating primary-side electrical energy to the secondary side via an alternating electromagnetic field. The secondary side, after rectification by rectifier D1 and filtering by filter LC, yields a stable DC output voltage Vo. Error amplifier N1 samples this output voltage Vo and compares it with a highly stable reference voltage within N1, generating an error signal. This error amplifier amplifies the voltage difference (error) and outputs a corresponding analog control signal (this signal directly reflects whether the output voltage is too high (requiring a reduced duty cycle) or too low (requiring a increased duty cycle)). The analog control signal output by error amplifier N1 is transmitted to the main control chip U1 via magnetic transformer T2. Based on this control signal, the main control chip U1 adjusts the pulse width (duty cycle) of its output PWM drive signal in real time, thereby controlling the conduction time of the main power MOSFET Q1, ultimately achieving precise and stable control of the output voltage Vo.

[0022] Thus, a complete closed-loop negative feedback control circuit is formed: the fluctuation of the output voltage Vo is sensed by the error amplifier N1 on the secondary side and converted into an error signal. This signal is isolated and transmitted to the main control chip U1 on the primary side through the magnetic feedback transformer T2. The main control chip U1 adjusts the duty cycle of the output PWM in real time and automatically, and compensates for the fluctuation of the output voltage Vo by controlling the conduction time of the main power MOSFET Q1, so as to stabilize it at the preset value.

[0023] Example 2: Secondary-side soft start and dual-circuit discharge: On the secondary side, by adding a discharge path to the output ground in the soft-start circuit, together with the original discharge path through the positive output, a dual-loop discharge structure is formed. This ensures that the soft-start capacitor can quickly release its charge after the converter is turned off, thus guaranteeing the effectiveness of the soft-start function during the next startup. In other words, by adding a discharge loop to the soft-start circuit on the secondary side, the voltage stress on all components inside the DC / DC converter is kept within a controllable range, while the converter can still achieve soft-start functionality under special operating conditions. Specifically, such as Figure 4 As shown: The secondary-side improvement of the present invention lies in the soft-start circuit and its discharge circuit.

[0024] The main control chip U1 is controlled by an external timing resistor R. T and timing capacitor C T With the switching frequency set to a constant 400kHz, the maximum duty cycle of the main control chip U1 is limited from 90% to 75%. At this point, the stress on the internal components of the DC / DC converter is under control, and none exceed their rated withstand voltage. Figure 4 As shown, when the DC / DC converter starts up, the VCCS voltage is established, and the soft-start capacitor C1 is charged through resistors R1 and R4. Due to the integral characteristic of the capacitor, the voltage on the soft-start capacitor C1 slowly rises to the reference voltage of the error amplifier N1. Since this rising voltage serves as the reference voltage input of the error amplifier N1, the reference voltage of N1 does not reach the target value instantaneously, but rises slowly, thereby causing the PWM duty cycle to increase slowly, achieving soft start. Thus, the original discharge path is: C1 → R1 → R4 → V1 → Vo (relying on Vo for rapid discharge).

[0025] This invention proposes a secondary-side soft-start circuit that adds a discharge loop. The charge on the soft-start capacitor C1 is connected to the VCCS via diode V2, and the VCCS is grounded through resistor R2. This creates a new discharge path: C1 → V2 → VCCS → R2 → GND. Thus, the charge in the soft-start capacitor C1 is released through two discharge loops, Vo and GND. When the DC / DC converter output power loss is slow, causing the original path to fail, the newly added path to GND provides the soft-start capacitor C1 with a fast and reliable discharge channel independent of the Vo voltage. This effectively solves the problem of uncontrolled soft-start circuit restarts when the DC / DC converter output power loss is slow, leading to repeated short-term start-stops. This ensures that even in extreme cases where input undervoltage jitter or prohibition signal jitter causes repeated start-stops within a short period, each startup is a controlled soft-start, improving the electrical safety of aerospace equipment.

[0026] In a preferred embodiment, as shown in the appendix Figure 3 As shown, in the primary side, the step of limiting the maximum duty cycle includes: using a reduced resistance value R T With the increased capacitance value C T The parameter combination limits the maximum duty cycle output by the main control chip from a first preset value to a lower second preset value.

[0027] Among them, Big R T Little C T The corresponding main control chip has a large maximum duty cycle, and small R T Big C T The maximum duty cycle of the main control chip output is relatively small. While maintaining the converter's operating frequency, the maximum duty cycle of the main control chip output is limited by adjusting the peripheral parameters R of the main control chip U1. T C T The value of R will be large. T Little C T Change to Little R T Big C T For example, according to the formula f=1 / (K*R) T *C T ); where K is a constant determined by the chip design.

[0028] If K is 1.4, then f = 1 / (1.4 × R) T ×C T ), using large R T Little C T At that time, R T =7.78kΩ, C T =470pF, at which point the duty cycle is 95%; using small R T Big C T At that time, RT =1.6kΩ, C T =2200pF, at which point the duty cycle is 75%, meaning the duty cycle has been successfully changed from 95% to 75%. After the change, the DC / DC converter's three-temperature full performance indicators are normal.

[0029] Among them, such as Figure 3 As shown, the R of the main control chip U1 T and C T The pins correspond to respectively Figure 3 The middle 4 feet and the outer R T C T This terminal is the frequency setting terminal for the internal oscillator, connected via an external resistor R. T and capacitor C T The oscillation frequency of the PWM signal is determined, and it also participates in the duty cycle control logic. The core mechanism of the control logic is that the oscillator of the main control chip U1 uses charging and discharging timing logic, combined with an internal comparator, to achieve frequency and duty cycle regulation. The specific process is as follows: 1. During the capacitor charging stage, the internal current source charges C... T Charging, at this time C T Voltage V t It increases linearly with time. This stage corresponds to the high level of the PWM output. 2. Capacitor discharge stage, when V t When the voltage rises to the threshold voltage of the internal comparator, the internal switch turns on, and C... T Through R T Rapid discharge, V t It decreases linearly over time. This stage corresponds to the low level of the PWM output. 3. The duty cycle control logic is shown in the waveform diagram. When R... T Big, C T Hours, C T Slow charging, fast discharging, and a large duty cycle output; when R T Small, C T When large, C T It charges quickly, discharges slowly, and has a small duty cycle.

[0030] In a preferred embodiment, as shown in the appendix Figure 3 As shown, the range of the second preset value is set to 75%-85%. Preferably, it is 75%, which ensures that the transformer T1 has at least 25% magnetic reset time per cycle. The measured turn-off voltage spike of the main power MOSFET Q1 is significantly reduced, the three-temperature full performance test is normal, and the voltage stress is always controllable.

[0031] Example: On the primary side, the peripheral parameters R of the main control chip are adjusted. T / C TThis results in a maximum duty cycle of 75% for the pulse signal output by the main control chip to drive the main power MOSFET Q1. At this time, the maximum turn-on time of the main power MOSFET Q1 is 75%, and the maximum turn-off time is 25%. For a forward DC / DC converter, when the main power MOSFET Q1 is turned on, energy from the primary side is transferred to the secondary side through the transformer; when the main power MOSFET Q1 is turned off, the main transformer T1 performs a magnetic reset, generating a reset voltage waveform between the drain and source of the main power MOSFET Q1. The amplitude of this reset voltage is closely related to the turn-off time: the shorter the turn-off time, the higher its amplitude. An excessively short turn-off time will prevent the transformer core from resetting properly, causing the voltage spike experienced by the main power MOSFET Q1 when it is turned off to far exceed its rated withstand voltage, resulting in device breakdown and failure. If this state continues for several switching cycles, it will further lead to magnetic saturation of the main transformer T1, causing catastrophic damage to the DC / DC converter. Therefore, by limiting the maximum duty cycle of the main control chip output, sufficient turn-off time can be ensured, making the voltage stress on the main power MOSFET Q1 controllable and effectively preventing the saturation of the main transformer T1.

[0032] In a preferred embodiment, as shown in the appendix Figure 4 As shown, in the secondary side, the addition of a discharge path to the output ground includes: connecting a diode V2 in series between the soft-start capacitor C1 and the VCCS node, and grounding the VCCS node through a resistor R2; the anode of the diode V2 is connected to the soft-start capacitor C1, and the cathode of the diode V2 is connected to the VCCS node.

[0033] In a preferred embodiment, as shown in the appendix Figure 4 As shown, the diode V2 is configured to: conduct in the forward direction when the voltage of the VCCS node is lower than the voltage on the soft-start capacitor C1, so as to provide a fast discharge path for the soft-start capacitor C1; and be reverse cut off when the voltage of the VCCS node is higher than the voltage on the soft-start capacitor C1, so as to avoid affecting the normal operation of the converter.

[0034] As attached Figure 1-4 The DC / DC converter shown above achieves maximum duty cycle limitation on the primary side and soft start on the secondary side through the above method, which significantly improves the reliability, adaptability and safety in harsh application environments such as aerospace.

[0035] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0036] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for limiting maximum duty ratio and implementing soft start for a DC / DC converter, characterized in that, The method comprises the following steps: Primary side, through the configuration of the master chip peripheral R T / C T Timing element parameters, under the premise of maintaining constant switching frequency, limit the maximum duty cycle of its output, so that the voltage stress of the internal power device of the converter is controlled; The secondary side is provided with a discharge path to the output ground, and a discharge path through the output positive is provided to form a double-loop discharge structure, so that the soft-start capacitor can quickly release the electric charge after the converter is turned off, and the soft-start function is ensured to be effective in the next start.

2. The method of claim 1, wherein: In the primary side, the step of limiting the maximum duty cycle comprises: using a reduced resistance value R T in combination with an increased capacitance value C T to limit the maximum duty cycle output by the master chip from a first preset value to a second preset value that is lower.

3. The method of claim 2, wherein: The second preset value is set to be 75%-85%.

4. The method of claim 1, wherein: The secondary side is provided with a discharge path to the output ground, and a discharge path through the output positive is provided to form a double-loop discharge structure, so that the soft-start capacitor can quickly release the electric charge after the converter is turned off, and the soft-start function is ensured to be effective in the next start.

5. The method of claim 4, wherein: The anode of the diode V2 is connected to the soft-start capacitor C1, and the cathode of the diode V2 is connected to the VCCS node.

6. A DC / DC converter, characterized by The diode V2 is configured to be forward-biased when the voltage of the VCCS node is lower than the voltage on the soft-start capacitor C1, so as to provide a fast discharge path for the soft-start capacitor C1; and the diode V2 is reverse-biased when the voltage of the VCCS node is higher than the voltage on the soft-start capacitor C1, so as to avoid affecting the normal operation of the converter. The DC / DC converter is realized by the method according to any one of claims 1 to 5.

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