Switching tube protection circuit and full-bridge power converter

Abstract

The application provides a switch tube protection circuit and a full-bridge power converter. The switch tube protection circuit comprises a logic operation unit configured to receive a primary side driving signal and a secondary side driving signal of the full-bridge power converter and output a logic operated switch signal; and a freewheeling unit, whose positive and negative poles are connected to the positive and negative poles of a secondary side rectifier bridge of the full-bridge power converter, and the on-off of the freewheeling unit is controlled by the switch signal output by the logic operation unit. The application can avoid damaging the secondary side synchronous rectification MOS tube in the full-bridge power converter.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, and in particular to a switching transistor protection circuit and a full-bridge power converter. Background Technology

[0002] A switching power supply is a power supply that uses modern power electronics technology to control the on and off time ratio of switching transistors to maintain a stable output voltage. It typically consists of a pulse-width modulation (PWM) controller, MOSFETs, a transformer, and an inductor. With the continuous development and innovation of power electronics technology, switching power supply technology is also constantly advancing.

[0003] With the rapid development of science and technology, the power consumption of electrical equipment is increasing daily. To reduce the weight, size, and cost of power supply components, it is necessary to improve the conversion efficiency of switching power supplies to achieve high efficiency, lightweight design, and miniaturization. Synchronous rectification technology is widely used to significantly improve conversion efficiency. However, synchronous rectification lacks unidirectional conduction characteristics, making it prone to reverse current, which can damage power devices in severe cases. Traditionally, synchronous rectification is only activated when the current is continuous, but this leads to a decrease in power supply performance when the current is discontinuous, especially in dynamic situations with large load slopes and large step changes. This may fail to meet the power requirements of the downstream load switching from no-load to full-load, and may even affect the normal operation of the power system. To solve the performance problem when the current is discontinuous, Forced Continuous On-Mode (FCCM) control technology has emerged. This technology maintains continuous inductor current under any load condition, thus ensuring good dynamic characteristics of the power supply. However, due to forced continuous conduction, during pre-bias startup, the primary-side drive duty cycle is small, while the secondary-side synchronous rectification drive duty cycle is large, resulting in a large reverse current and a significant drop in output voltage.

[0004] To mitigate this issue, a synchronous rectification drive soft-start is introduced during pre-bias startup. Soft-start alleviates reverse current by gradually increasing the drive duty cycle, but this can also cause the synchronous rectification drive to turn off prematurely during freewheeling. If the MOSFET is turned off when reverse current occurs, a voltage spike will be generated on the MOSFET, potentially damaging the device, especially under conditions of large dynamic load changes. This reduces the reliability of this control technique. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a switching transistor protection circuit and a full-bridge power converter that can avoid damage to the secondary-side synchronous rectifier MOS transistor in the full-bridge power converter.

[0006] To address the aforementioned technical problems, in a first aspect, the present invention provides a switching transistor protection circuit for a full-bridge power converter with secondary-side bridge synchronous rectification, comprising: a logic operation unit configured to receive the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter, and output a switching signal after logic operation; and a freewheeling unit, the positive and negative terminals of which are respectively connected to the positive and negative terminals of the secondary-side rectifier bridge of the full-bridge power converter, and the switching on and off of the freewheeling unit is controlled by the switching signal output by the logic operation unit.

[0007] Furthermore, the logic operation unit is also configured to provide an on signal for the freewheeling unit when the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter are simultaneously turned off.

[0008] Furthermore, the logic operation unit includes a first NOR gate, a second NOR gate, and an AND gate, wherein the two input terminals of the first NOR gate are used to receive two primary-side drive signals respectively, the two input terminals of the second NOR gate are used to receive two secondary-side drive signals respectively, the two input terminals of the AND gate are respectively connected to the output terminals of the first NOR gate and the second NOR gate, and the output terminal of the AND gate serves as the input terminal of the switch protection circuit.

[0009] Furthermore, the freewheeling unit includes a series branch consisting of a resistor Rs, a capacitor Cs, and a switch Qs. The two ends of the series branch are respectively connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter, and the switching on and off of the switch Qs is controlled by the switching signal output by the logic operation unit.

[0010] Furthermore, the switching transistor Qs is a MOSFET or an IGBT.

[0011] Secondly, the present invention provides a full-bridge power converter, comprising a DC input power supply, a primary rectifier bridge, a transformer, a secondary rectifier bridge, and an LC filter connected in sequence, and further comprising a switching transistor protection circuit, wherein the switching transistor protection circuit adopts the switching transistor protection circuit of the first aspect, and the positive and negative terminals of the freewheeling unit in the switching transistor protection circuit are respectively connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter.

[0012] Compared with existing technologies, this invention has the following advantages: Based on this power-off protection circuit, the logic operation unit is configured to receive the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter, and output the switching signal after logic operation. The positive and negative terminals of the freewheeling unit are connected to the positive and negative terminals of the secondary-side rectifier bridge of the full-bridge power converter, respectively. When all four drives are turned off, by increasing the freewheeling path of the inductor reverse current, voltage spikes can be avoided on the secondary-side synchronous rectifier MOSFET, thereby preventing device damage. This approach retains the original pre-biased startup function while maintaining the forced continuous conduction mode across the entire load range. This mode avoids the loop stability and dynamic response problems that may arise when switching from discontinuous conduction mode (DCM) to continuous conduction mode (CCM), further enhancing the reliability of the circuit. Attached Figure Description

[0013] The accompanying drawings are included to provide a further understanding of the invention; they are incorporated into and constitute a part of this invention. The drawings illustrate embodiments of the invention and, together with this specification, serve to explain the principles of the invention. In the drawings:

[0014] Figure 1 It is a full-bridge power converter with secondary-sideband bridge synchronous rectification;

[0015] Figure 2 for Figure 1 Schematic diagram of PWM controller drive output during steady state of medium-voltage converter;

[0016] Figure 3 for Figure 1 A schematic diagram of the waveform and current path of the medium-voltage converter at its maximum duty cycle.

[0017] Figure 4 for Figure 1 Schematic diagram of the forced continuous conduction mode current path and waveform of the intermediate converter;

[0018] Figure 5 for Figure 1 A schematic diagram of the waveform and current path during the pre-biased startup of the intermediate converter;

[0019] Figure 6 This is a schematic diagram of the structure of a switching transistor protection circuit according to an embodiment of the present invention;

[0020] Figure 7 This is another schematic diagram of the switching transistor protection circuit according to an embodiment of the present invention;

[0021] Figure 8 This is a schematic diagram of the operating waveforms of a full-bridge power converter according to an embodiment of the present invention. Figure 1 ;

[0022] Figure 9This is a schematic diagram of the operating state of a full-bridge power converter according to an embodiment of the present invention. Figure 1 ;

[0023] Figure 10 This is a schematic diagram of the operating waveforms of a full-bridge power converter according to an embodiment of the present invention. Figure 2 ;

[0024] Figure 11 This is a schematic diagram of the operating state of a full-bridge power converter according to an embodiment of the present invention. Figure 2 . Detailed Implementation

[0025] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are merely some examples or embodiments of the present invention. For those skilled in the art, the present invention can be applied to other similar scenarios based on these drawings without creative effort. Unless obvious from the context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0026] It should be understood that when a component is referred to as "on another component," "connected to another component," "coupled to another component," or "in contact with another component," it can be directly on, connected to, coupled to, or in contact with that other component, or there may be an intervening component. In contrast, when a component is referred to as "directly on another component," "directly connected to," "directly coupled to," or "directly in contact with" another component, there is no intervening component. Similarly, when a first component is referred to as "electrically contacting" or "electrically coupled to" a second component, there is an electrical path between the first and second components that allows current to flow. This electrical path may include capacitors, coupled inductors, and / or other components that allow current to flow, even if there is no direct contact between the conductive components.

[0027] Figure 1 This is a full-bridge power converter with secondary-sideband bridge synchronous rectification, reference... Figure 1 As shown, the PWM controller controls the conduction time of the two power MOSFETs on the primary side, converting the input DC voltage to AC voltage. The AC voltage output from the secondary side of the transformer is rectified into DC voltage by a bridge synchronous rectifier, and then filtered by an LC filter to provide energy to the load R. The PWM controller provides eight PWM drive outputs, including four primary-side PWM drives: PA1, PA2, PB1, and PB2. PA1 and PA2 are identical, as are PB1 and PB2. The other four are secondary-side synchronous rectifier drives: SA1 and SA2 are identical and complementary to PB1 and PB2 respectively, and SB1 and SB2 are identical and complementary to PA1 and PA2 respectively. The steady-state output waveform of the PWM controller is shown below. Figure 2 As shown.

[0028] Scenario 1: Based on Figure 1 The full-bridge power converter shown above, during its operation, when the input voltage is transferred to the secondary side through the turns ratio, the voltage value ( Figure 1 When the voltage (Vs) is lower than the output voltage, it operates in open-loop mode. At this time, the primary-side drive PA1 / PA2 and PB1 / PB2 operate at their maximum duty cycle. There is a dead time for the primary-side upper and lower transistors, and the actual maximum duty cycle is slightly lower than 50%. Since the primary and secondary-side drives also have dead times, and these two dead times partially overlap, the secondary-side rectifier MOSFETs turn off simultaneously. The sudden open circuit of the inductor's continuous reverse current generates a very high voltage spike on the synchronous rectifier MOSFET, potentially damaging it. The operating waveform and current path are as follows: Figure 3 As shown.

[0029] Scenario 2: Based on Figure 1 The full-bridge power converter shown typically requires soft start to reduce device stress during startup, with the duty cycle gradually increasing. The secondary-side synchronous rectification drive, being complementary to the primary-side drive, will also gradually decrease its duty cycle. Under pre-biased startup conditions, the secondary-side synchronous rectification duty cycle is large, leading to a significant reverse current. The current path and waveform during freewheeling in the forced continuous conduction mode are also discussed. Figure 4 As shown.

[0030] To reduce current stress, the secondary-side synchronous rectification drive typically requires soft start. Because of this soft start, the synchronous rectification drive and the primary-side drive cannot complement each other, resulting in both sets of synchronous rectification drives being turned off during freewheeling. In this situation, the sudden open circuit of the reverse continuous current generates a very high voltage spike on the synchronous rectification MOSFET. This voltage spike can damage the MOSFET, leading to reduced power supply reliability. The waveform and current path of the forced continuous conduction mode during pre-biased startup are shown below. Figure 5 As shown.

[0031] One embodiment of the present invention provides a switching transistor protection circuit, see reference. Figure 6 As shown, it can be used in full-bridge power converters with secondary-side bridge synchronous rectification, but is not limited to this. It mainly includes a logic operation unit and a freewheeling unit. The logic operation unit is configured to receive the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter, and output a switching signal after logical operation. This logic operation unit performs logical operations on the primary-side switching drive signal and the secondary-side synchronous rectification drive signal to control the freewheeling circuit. The positive and negative terminals of the freewheeling unit are connected to the positive and negative terminals of the secondary-side rectifier bridge of the full-bridge power converter, respectively, and the switching on and off of the freewheeling unit is controlled by the switching signal output by the logic operation unit.

[0032] In some embodiments, when both the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter are simultaneously turned off, an on-signal for the freewheeling unit is provided. Based on this on-signal, a freewheeling unit is connected in the power circuit to provide a path for the reverse inductor current, while simultaneously using the existing power circuit to release the energy in the freewheeling unit, preparing for the next freewheeling cycle.

[0033] In some embodiments, reference Figure 7 As shown, the logic operation unit includes a first NOR gate, a second NOR gate, and an AND gate. The two inputs of the first NOR gate receive two primary-side drive signals, and the two inputs of the second NOR gate receive two secondary-side drive signals. The two inputs of the AND gate are connected to the outputs of the first and second NOR gates, respectively. The output of the AND gate serves as the input of the switching transistor protection circuit. Further, the freewheeling unit includes a series branch consisting of a resistor Rs, a capacitor Cs, and a switching transistor Qs. The two ends of the series branch are connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter, respectively. The switching on and off of the switching transistor Qs is controlled by a switching signal output from the logic operation unit. For example, the switching transistor Qs is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor).

[0034] The working principle of the switching transistor protection circuit in this embodiment is as follows: The primary-side drive signals (PA1, PB1) and secondary-side drive signals (SA1, SB1) are logically processed to obtain the drive signal for the switching transistor Qs. When all four drive signals PA1, PB1, SA1, and SB1 are simultaneously turned off, the switching transistor Qs is turned on, allowing the RC freewheeling path composed of resistor Rs and capacitor Cs to be connected to the power circuit. Resistor Rs has a very small resistance value and is used to discharge the current during capacitor Cs reset and to share losses; the voltage drop across resistor Rs is not considered in the subsequent analysis. When the full-bridge converter is working normally, the duty cycles of SA1 and SB1 are both greater than 50%, and there will be no situation where SA1 and SB1 are both low simultaneously. In this case, the switching transistor Qs will not be turned on, and the freewheeling unit has no impact on the circuit operation.

[0035] In this embodiment, the switching transistor protection circuit, by increasing the freewheeling path of the inductor's reverse current when all four drives of the full-bridge power converter are turned off, avoids the formation of voltage spikes on the secondary-side synchronous rectifier MOSFETs, thereby preventing device damage. This approach retains the original pre-biased startup function while maintaining the forced continuous conduction mode across the entire load range. This mode avoids loop stability and dynamic response issues that may arise when switching from discontinuous conduction mode to continuous conduction mode, further enhancing the circuit's reliability.

[0036] Another embodiment of the present invention provides a full-bridge power converter, including a DC input power supply, a primary rectifier bridge, a transformer, a secondary rectifier bridge, and an LC filter connected in sequence, and also includes a switching transistor protection circuit. The switching transistor protection circuit can be the switching transistor protection circuit as described in the previous embodiment. In the switching transistor protection circuit, the positive and negative terminals of the freewheeling unit are respectively connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter.

[0037] Scenario 3: Based on the full-bridge power converter of this embodiment, refer to... Figure 8 and Figure 9 As shown, when the input voltage drops and causes the converter to operate in open-loop mode, the converter operates at the maximum duty cycle of the primary drive. This converter can provide an inductor reverse freewheeling path. The working principle is as follows:

[0038] State 1: Inductor current flows in the forward direction. When SA1 and SB1 are both at low level, the switching transistor Qs is turned on, and the freewheeling inductor current flows in the forward direction through the secondary synchronous rectifier. The voltage Cs is clamped to approximately 0V by the body diode of the rectifier.

[0039] State 2: Inductor current flows in reverse, all secondary-side synchronous rectifier diodes are off, and the inductor current flows through the paths Rs, Cs, and Qs, increasing the Cs voltage. The larger the inductor reverse current, the higher the Cs voltage. To protect the secondary-side synchronous rectifier MOSFETs, the Cs voltage needs to be less than twice the withstand voltage of the rectifier MOSFETs.

[0040] State 3: The inductor current flows in reverse, and the secondary rectifier diodes SB1 / SB2 are turned on (SA1 / SA2 in the second half of the cycle), clamping Cs to the Vs voltage. The Vs voltage depends on the input voltage and the transformer turns ratio.

[0041] State 4: The inductor current flows in reverse, the secondary rectifier diodes conduct alternately, the VCs voltage remains unchanged, and the inductor current increases in reverse.

[0042] The process alternates between states 2 and 4 until the converter exits the maximum duty cycle operating state. When SA1 / SA2 and SB1 / SB2 are turned on simultaneously, the VCs voltage is clamped back to near 0V by the rectifier diodes, preparing for the next inductor reverse freewheeling path.

[0043] Scenario 4: Based on the full-bridge power converter in this embodiment, refer to... Figure 10 and Figure 11 As shown, when the secondary-side synchronous rectification drive is soft-started and the secondary-side drive is simultaneously turned off, the converter can provide an inductor reverse freewheeling path. The working principle is as follows:

[0044] State 1: The secondary-side drive and the primary-side drive are both turned off. The inductor current is not reversed. The Qs turn-on time is relatively long, resulting in the Vcs voltage being approximately equal to the output voltage at this time, and the inductor current is 0.

[0045] State 2: Primary side PB1 / PB2 is turned off (PA1 / PA2 in the second half cycle), secondary side SB1 / SB2 is turned on (SA1 / SA2 in the second half cycle), inductor current changes, Qs is not turned on, and VCs voltage remains unchanged.

[0046] State 3: Both secondary side SB1 / SB2 and SA1 / SA2 are off. At this time, Qs is on, and the inductor's negative current freewheels through Rs, Cs, and Qs. The voltage of VCs increases, and the energy release of the inductor current is complete.

[0047] State 4: Both secondary side SB1 / SB2 and SA1 / SA2 are off, Qs is continuously on, and Cs, L, and output capacitor form a π-type oscillation. When this state lasts for a long time, Cs is consistent with the output voltage.

[0048] State 2 to state 4 repeat continuously, and the duration of state 4 decreases continuously until SB1 / SB2 and SA1 / SA2 are turned on simultaneously. At this time, the VCs voltage is clamped back to near 0V by the rectifier diodes, preparing for the next inductor reverse freewheeling path.

[0049] For those skilled in the art, the above disclosure is merely illustrative and does not constitute a limitation of the invention. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements, and alterations to the invention. Such modifications, improvements, and alterations are suggested in this invention and therefore remain within the spirit and scope of the exemplary embodiments of the invention.

[0050] Although the present invention has been described with reference to specific embodiments, those skilled in the art should recognize that the above embodiments are merely illustrative of the invention, and various equivalent changes or substitutions can be made without departing from the spirit of the invention. Therefore, any changes or modifications to the above embodiments within the scope of the essential spirit of the invention will fall within the scope of the claims of the invention.

Claims

1. A switching transistor protection circuit, characterized in that, A full-bridge power converter for secondary-sideband bridge synchronous rectification includes: A logic operation unit is configured to receive the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter and output a switching signal after logic operation. The freewheeling unit has its positive and negative terminals connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter, respectively, and the switching on and off of the freewheeling unit is controlled by the switching signal output by the logic operation unit. When both the primary-side drive signal and the secondary-side drive signal of the full-bridge power converter are turned off, an on-signal is provided for the freewheeling unit.

2. The switching transistor protection circuit according to claim 1, characterized in that, The logic operation unit includes a first NOR gate, a second NOR gate, and an AND gate. The two inputs of the first NOR gate are used to receive two primary-side drive signals respectively, the two inputs of the second NOR gate are used to receive two secondary-side drive signals respectively, the two inputs of the AND gate are respectively connected to the outputs of the first NOR gate and the second NOR gate, and the output of the AND gate serves as the input of the switching transistor protection circuit.

3. The switching transistor protection circuit according to claim 2, characterized in that, The freewheeling unit includes a series branch consisting of a resistor Rs, a capacitor Cs, and a switch Qs. The two ends of the series branch are respectively connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter, and the switching on and off of the switch Qs is controlled by the switching signal output by the logic operation unit.

4. The switching transistor protection circuit according to claim 3, characterized in that, The switching transistor Qs is a MOSFET or an IGBT.

5. A full-bridge power converter, characterized in that, The device includes a DC input power supply, a primary rectifier bridge, a transformer, a secondary rectifier bridge, and an LC filter that are connected in sequence. It also includes a switching transistor protection circuit, wherein the switching transistor protection circuit adopts the switching transistor protection circuit as described in any one of claims 1-4, and the positive and negative terminals of the freewheeling unit in the switching transistor protection circuit are respectively connected to the positive and negative terminals of the secondary rectifier bridge of the full-bridge power converter.

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