An adaptive pre-pull-down circuit and method applied to a synchronous rectification driving chip

Abstract

The application discloses a self-adaptive pre-pull-down circuit and method applied to a synchronous rectification driving chip, samples a gate voltage VG of a synchronous rectification tube and obtains a pre-pulled comparison threshold voltage VFB by voltage reduction; samples a drain-source voltage VD of the synchronous rectification tube and compares the voltage VD with the voltage VFB, if the voltage VD is less than the voltage VFB, outputs a control signal to a VG voltage driving unit of the chip, reduces the voltage VG until the VG is pulled down to the ground when a turn-off threshold is reached. Because the voltage VG has been pulled down in advance, the voltage VG cannot be pulled back to a low value, so the voltage VG can be quickly pulled down to the ground. The application adjusts the gate potential of the synchronous rectification tube by adding the pre-pull-down circuit, thereby prolonging the conduction time of the synchronous rectification tube, reducing the conduction time of a body diode and improving the efficiency of the synchronous rectification.

Description

Technical Field

[0001] This invention relates to an analog circuit, specifically to a control circuit for the rectifier diodes in a synchronous rectification driver chip. Background Technology

[0002] In low-voltage, high-current switching power supply applications, rectifier diodes are indispensable components. Their main function is to convert AC signals into DC signals, thereby enabling the normal operation of electronic devices. However, due to the large forward voltage drop of rectifier diodes, conduction losses are easily generated. These losses not only affect system efficiency but also make it difficult to meet market and regulatory requirements, as modern electronic products increasingly demand higher energy efficiency and environmental protection standards from consumers. Therefore, synchronous rectification technology has received widespread attention and application.

[0003] Synchronous rectification technology refers to replacing rectifier diodes or Schottky diodes with MOSFETs, which have low on-resistance. A MOSFET is an active device with a very low on-resistance of only a few mΩ. This means that its on-state voltage drop is no more than 100mV, far lower than that of rectifier diodes or Schottky diodes. This low voltage drop translates to lower losses and higher efficiency. In this way, rectification losses can be significantly reduced, improving system efficiency.

[0004] Therefore, synchronous rectification technology has become an important technology in switching power supplies and will continue to be developed and innovated in the future. By using this technology, higher efficiency and lower environmental impact can be achieved, bringing more benefits to users and the environment. At the same time, the application scope of synchronous rectification technology is constantly expanding, no longer limited to switching power supplies, but also widely used in other fields such as DC-DC converters and LED lighting.

[0005] In power electronics, the flyback converter is a common converter used to convert DC power to AC power, and one of its key components is the synchronous rectifier. However, the propagation delay within the synchronous rectifier leads to reduced efficiency.

[0006] like Figure 1 As shown, in a flyback converter, the energy in the secondary-side coil supplies power to the load when the secondary side is turned on, and the secondary-side current gradually decreases. During this time, the drain-source voltage of the synchronous rectifier approaches 0V. Ideally, when the secondary-side current decreases to 0, i.e., the drain-source voltage of the synchronous rectifier is 0V, the synchronous rectifier should be turned off immediately. This maximizes the synchronous rectification drive efficiency, minimizes losses, and improves the efficiency of the flyback converter.

[0007] However, in practice, synchronous rectification technology suffers from a turn-off transmission delay. Specifically, this delay includes the comparator delay in the turn-off threshold detection module, the logic module delay, the drive delay, and the turn-off delay of the synchronous rectifier diode. During this delay, the synchronous rectifier diode cannot be turned off in time, causing the load to reverse-feed current to the secondary side coil, thereby reducing the efficiency of the flyback converter.

[0008] Therefore, in the traditional flyback converter architecture, when the drain voltage of the synchronous rectifier reaches the turn-off threshold, the synchronous rectifier is turned off. However, due to the turn-off propagation delay, the turn-off threshold is set at a potential far from 0V in the traditional design. After the synchronous rectifier is turned off, its body diode is turned on, which contradicts the introduction of synchronous rectification technology, because the voltage drop generated by the body diode is much greater than the drain-source voltage drop when the synchronous rectifier is turned on, resulting in excessive energy loss. Summary of the Invention

[0009] Purpose of the invention: To address the aforementioned prior art, an adaptive pre-pull-down circuit and method for synchronous rectification driver chips are proposed.

[0010] Technical solution: An adaptive pre-pull-down circuit applied to a synchronous rectification driver chip, comprising:

[0011] The sampling module is used to sample the drain-source voltage VD of the synchronous rectifier diode;

[0012] The step-down module is used to sample the gate voltage VG of the synchronous rectifier and step it down to obtain the pre-pull-down comparison threshold voltage VFB;

[0013] The comparison module is used to compare the pre-pull-down comparison threshold voltage VFB with the voltage VD. If the voltage VD is less than VFB, a control signal is output to the VG voltage drive unit of the chip to reduce the voltage VG.

[0014] Furthermore, the step-down module includes a differential operational amplifier circuit and a resistor divider circuit; the differential operational amplifier circuit is used to compare the sampled voltage VG with the reference voltage Vref and then reduce it to voltage Vsense; the resistor divider circuit is used to divide the voltage Vsense to obtain VFB.

[0015] Furthermore, in the buck module, the resistor in the resistor divider circuit is an adjustable resistor, used to adjust the pre-pull-down comparison threshold voltage VFB.

[0016] An adaptive pre-pull-down method for a synchronous rectifier driver chip is provided, wherein the gate voltage VG of the synchronous rectifier is sampled and stepped down to obtain a comparison threshold voltage VFB for pre-pull-down; the drain-source voltage VD of the synchronous rectifier is sampled and compared with the voltage VFB; if the voltage VD is less than VFB, a control signal is output to the VG voltage driving unit of the chip to reduce the voltage VG.

[0017] Furthermore, the sampling of the voltage VG is divided into two steps: first, the sampled voltage VG is compared with the reference voltage Vref through a differential operational amplifier circuit to obtain the voltage Vsense; then, the voltage Vsense is divided by a resistor voltage divider circuit to obtain the voltage VFB.

[0018] Furthermore, the resistor divider circuit employs an adjustable resistor to adjust the pre-pull-down comparison threshold voltage VFB.

[0019] Furthermore, when the voltage VG drops to the turn-off threshold, the voltage VG is pulled down to ground.

[0020] Beneficial Effects: A key issue in synchronous rectifier design is current backflow. To address this, traditional synchronous rectifier driver chips require setting a suitable threshold to control the switching state of the synchronous rectifier. Ideally, to prevent current backflow, the threshold of the turn-off comparator should be set to 0V. However, this is impractical due to the delay in turn-off propagation. Therefore, the threshold of the turn-off comparator needs to be far from 0V to ensure that the synchronous rectifier is turned off before the current flowing to its drain drops to 0A. After the synchronous rectifier is turned off, current continues to conduct through its body diode. However, the voltage drop generated by the body diode is much greater than the voltage drop when the synchronous rectifier is on, which increases the losses in the synchronous rectifier. When the circuit operates in light-load mode, the current flowing to the drain of the synchronous rectifier drops to 0A in a very short time. At this time, the synchronous rectifier operates in the body diode conducting state most of the time. Considering the losses of the synchronous rectifier driver chip, the synchronous rectification efficiency in light-load mode is greatly reduced.

[0021] To address the above problems, this invention proposes an adaptive pre-pull-down technique for synchronous rectifier driver chips. By detecting and controlling the voltage VG of the synchronous rectifier, VG is first reduced and Vref is used to calculate Vsense. Users can flexibly adjust the Vsense value externally to obtain a reasonable voltage divider value VFB. This voltage divider value is compared with the sampling voltage VD of the synchronous rectifier to adjust the driving voltage of VG. If the value of VD is less than VFB, the VG voltage is reduced, thereby increasing the on-resistance of the synchronous rectifier and making VD relatively stable until VD becomes too small, pulling VG down to ground. This avoids setting the VG pull-down comparison threshold too high due to consideration of the synchronous rectifier's turn-off propagation delay, which would result in an excessively long body diode conduction time. This technique can shorten the conduction time of the synchronous rectifier's body diode. Although it increases the synchronous rectifier's on-resistance and increases the VDS voltage drop, it is much smaller than the body diode's on-resistance, thus reducing power consumption on the synchronous rectifier. Attached Figure Description

[0022] Figure 1 A schematic diagram of a traditional synchronous rectification driver chip architecture;

[0023] Figure 2 This is a schematic diagram of the synchronous rectification driver chip architecture of the present invention;

[0024] Figure 3 This is a schematic diagram of a pre-pull-down circuit structure. Detailed Implementation

[0025] The invention will now be further explained with reference to the accompanying drawings.

[0026] like Figure 2 , Figure 3 As shown, an adaptive pre-pull-down circuit and method applied to a synchronous rectification driver chip includes:

[0027] The sampling module is used to sample the drain-source voltage VD of the synchronous rectifier diode.

[0028] The buck module samples the gate voltage VG of the synchronous rectifier and steps it down to obtain the pre-pull-down comparison threshold voltage VFB. Specifically, the buck module includes a differential operational amplifier circuit and a resistor divider circuit. The differential operational amplifier circuit compares the sampled voltage VG with the reference voltage Vref and then reduces it to voltage Vsense.

[0029]

[0030] A resistor divider circuit is used to divide the voltage Vsense to obtain VFB:

[0031]

[0032] R1 and R2 are the two resistors in the resistor divider circuit.

[0033] The comparison module compares the pre-pull-down comparison threshold voltage VFB with the voltage VD. If VD is greater than VFB, the pre-pull-down is not enabled; if VD is less than VFB, the pre-pull-down is enabled, i.e., a control signal is output to the chip's VG voltage drive unit to reduce VG. By reducing VG, the on-resistance RDS of the synchronous rectifier diode is increased, thereby reducing the change in voltage VD.

[0034] Therefore, voltage VG will decrease as voltage VD decreases. When the turn-off threshold is reached, because voltage VG has already been pulled down in advance, the voltage VG has dropped to a lower value and can no longer pull VD back up, so voltage VG can be quickly pulled down to ground. For CCM mode, a fast response can also be obtained when the secondary side is quickly turned off.

[0035] Furthermore, in the buck module, the resistor in the resistor divider circuit is an adjustable resistor, allowing the user to adjust the pre-pull-down comparison threshold voltage VFB. By using a pre-pull-down scheme and setting an appropriate resistor divider range and accuracy, power consumption of the circuit can be saved to some extent.

[0036] The present invention improves the efficiency of synchronous rectification by adding an adaptive pre-pull-down circuit to extend the conduction time of the synchronous rectifier and reduce the conduction time of the body diode.

[0037] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An adaptive pre-pull-down circuit applied to a synchronous rectification driver chip, characterized in that, include: The sampling module is used to sample the drain-source voltage VD of the synchronous rectifier diode; The step-down module is used to sample the gate voltage VG of the synchronous rectifier and step it down to obtain the pre-pull-down comparison threshold voltage VFB; The comparison module is used to compare the pre-pull-down comparison threshold voltage VFB with the voltage VD. If the voltage VD is less than VFB, a control signal is output to the VG voltage driving unit of the chip to reduce the voltage VG. The step-down module includes a differential operational amplifier circuit and a resistor divider circuit; the differential operational amplifier circuit is used to compare the sampled voltage VG with the reference voltage Vref and then reduce it to voltage Vsense; the resistor divider circuit is used to divide the voltage Vsense to obtain VFB; In the buck module, the resistor in the resistor divider circuit is an adjustable resistor, used to adjust the pre-pull-down comparison threshold voltage VFB.

2. An adaptive pre-pull-down method applied to synchronous rectification driver chips, characterized in that, The gate voltage VG of the synchronous rectifier is sampled and stepped down to obtain the pre-pull-down comparison threshold voltage VFB; the drain-source voltage VD of the synchronous rectifier is sampled and compared with the voltage VFB. If the voltage VD is less than VFB, a control signal is output to the VG voltage drive unit of the chip to reduce the voltage VG. Sampling the voltage VG involves two steps: first, the sampled voltage VG is compared with the reference voltage Vref using a differential operational amplifier circuit to obtain the voltage Vsense; then, the voltage Vsense is divided using a resistor voltage divider circuit to obtain the voltage VFB. The resistor divider circuit uses an adjustable resistor to adjust the pre-pull-down comparison threshold voltage VFB; When the voltage VG drops to the turn-off threshold, the voltage VG is pulled down to ground.

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