A converter and a control method thereof

Abstract

The application relates to the technical field of switching power supplies, and provides a converter and a control method thereof. Compared with the prior art, the converter of the application adds a capacitor Ca, an inductor La, a switch tube Q3 and diodes D1 and D2. In the application environment with large output current, the converter can effectively reduce the loss caused by hard turn-off of a main switch tube Q1, and can effectively relieve the problem of device selection difficulty and poor EMI caused by the stress peak generated by hard turn-off of the main switch tube Q1.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, and in particular to a converter and its control method. Background Technology

[0002] High efficiency and high power density are the main trends in the development of current switching power supply technology. Hard switching technology, due to its large switching losses and poor EMI performance, is gradually being replaced by soft switching technology. Currently, the most widely used soft switching technology is zero-voltage turn-on technology, which achieves zero voltage across the switching transistor when it is turned on.

[0003] For example, Chinese patent application CN202011039117.2 proposes a control method for soft-turning on the main switch Q1 of a BUCK converter. The circuit diagram of the BUCK converter is shown in Figure 1. The BUCK converter achieves ZVS (zero-voltage turn-on) by raising the midpoint voltage Vs of the BUCK converter to near the input voltage Vin before turning on the main switch Q1, and then turning on the main switch Q1. This control method primarily relies on detecting whether the current in the inductor L crosses zero.

[0004] However, the above control method has the following drawbacks: Although the main switch Q1 is turned on at zero voltage, it is still hard-turned off, and the maximum current flowing through the main switch Q1 is greater than twice the output current. In applications with large output current, the above-mentioned BUCK converter will experience large turn-off peak stress and high turn-off losses in the main switch Q1, leading to difficulties in component selection, poor EMI, and low overall converter efficiency. In contrast, using a traditional capacitor snubber circuit (such as...) Figure 2 As shown in the figure, this will lead to a significant decrease in efficiency. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a converter and its control method, which can effectively reduce the turn-off stress of the converter's main switch, improve the converter's applicability, device reliability, and EMI performance, and reduce the difficulty of device selection. Simultaneously, it can achieve zero-voltage turn-off of the converter's main switch, improving the converter's efficiency.

[0006] The technical solution of the present invention is as follows:

[0007] A converter includes switching transistors Q1 and Q2, an inductor L, and a capacitor Co. The drain of switching transistor Q1 is connected to the input power supply, and the source of switching transistor Q1 is connected to both one end of inductor L and the drain of switching transistor Q2. The other end of inductor L is connected to one end of capacitor Co. The source of switching transistor Q2 is connected to ground, and the other end of capacitor Co is connected to ground.

[0008] The converter also includes an auxiliary circuit, which has a switching transistor Q3, an inductor La, a capacitor Ca, a diode D1, and a diode D2. One end of the switching transistor Q3 is connected to the input power supply, and the other end of the switching transistor Q3 is connected to one end of the inductor La and the cathode of the diode D2. The other end of the inductor La is connected to one end of the capacitor Ca and the anode of the diode D1. The cathode of the diode D1 is connected to the drain of the switching transistor Q2, and the anode of the diode D2 and the other end of the capacitor Ca are connected to ground.

[0009] Preferably, capacitor Ca is a collection of multiple capacitors connected in series or in parallel, and switching transistor Q3 is a collection of multiple switching transistors connected in series or in parallel.

[0010] The control methods for the above-mentioned converter include:

[0011] When the control switch Q1 is turned on, the input power supply is supplied to the capacitor Co and the load of the converter through the switch Q1 and the inductor L.

[0012] During the conduction of switch Q1, switch Q3 is controlled to conduct for a fixed time. During the conduction of switch Q3, the input power supply charges capacitor Ca through switch Q3 and inductor La, so that the voltage of capacitor Ca is charged to the input voltage. At this time, switch Q2, diode D1, and diode D2 are all cut off.

[0013] When the control switches Q1 and Q3 are turned off, since there is a voltage on capacitor Ca that is the same as the input voltage, the voltage across the main switch Q1 is clamped to zero when it is turned off, achieving zero-voltage turn-off. After switch Q1 is turned off, capacitor Ca supplies power to capacitor Co and the load through diode D1 and inductor L until the voltage on capacitor Ca is completely discharged, and then control switch Q2 is turned on.

[0014] It should be noted that the required conduction time T of the switching transistor Q3 within one switching cycle is calculated as follows:

[0015]

[0016] Where Vin is the input voltage of the converter, u la For the voltage across the auxiliary inductor La, u ca This is the voltage across the auxiliary capacitor Ca.

[0017] Solving the above equations simultaneously, we can obtain u ca =Vin,

[0018] It can be seen that the conduction time T is only related to the selection of inductor La and capacitor Ca, and is not related to the input voltage Vin.

[0019] Preferably, the conduction time of switch Q3 is less than the conduction time of switch Q1, and the start time of switch Q3 conduction is not earlier than the start time of switch Q1 conduction, and the turn-off time of switch Q3 is not later than the turn-off time of switch Q1.

[0020] Preferably, when the switch Q3 is turned off, the voltage across the capacitor Ca is equal to the voltage of the input power supply, and the turn-off time of the switch Q3 is synchronized with the turn-off time of the switch Q1.

[0021] Preferably, when the output load current of the converter is less than a certain value, the switch Q3 does not operate.

[0022] The present invention also provides a converter for connection to a load. The converter includes a switching transistor Q1, a switching transistor Q2, an inductor L, and a capacitor Co. The drain of the switching transistor Q1 is connected to the input power supply, the source of the switching transistor Q1 is connected to one end of the inductor L and the drain of the switching transistor Q2, the other end of the inductor L is connected to one end of the capacitor Co, the source of the switching transistor Q2 is connected to ground, and the other end of the capacitor Co is connected to ground. The converter is characterized in that it further includes an auxiliary circuit for storing energy during the period when the switching transistor Q2 is off and the switching transistor Q1 is on, and for making the voltage across the switching transistor Q1 zero when the switching transistor Q1 is off, so as to achieve zero-voltage turn-off of the switching transistor Q1.

[0023] The detailed working principle of this invention will be analyzed and explained in conjunction with specific embodiments, and will not be elaborated here. The beneficial effects of this invention are as follows:

[0024] 1. When the converter's switching transistor Q1 is turned off, the voltage across capacitor Ca is the input voltage, and the voltage across Q1 is zero, achieving zero-voltage turn-off. This significantly reduces turn-off stress and losses, solves component selection and EMI issues, and improves the overall efficiency of the converter. Furthermore, the energy stored in capacitor Ca is completely transferred to the output terminal without any energy loss.

[0025] 2. Traditional technology addresses the stress problem of the main switch when it is turned off by directly connecting an auxiliary capacitor in parallel with the freewheeling diode of the converter. However, since the auxiliary capacitor is always engaged, the negative inductor current required to achieve ZVS (zero voltage turn-on) for the main switch increases, which leads to a decrease in the overall efficiency of the converter. In contrast, this invention adds a diode D1 so that the capacitor Ca is not engaged during the conduction of the switch Q2 (freewheeling diode), thus avoiding the increase in the negative current required to achieve ZVS turn-on of the switch Q1 (main switch). Attached Figure Description

[0026] Figure 1 Schematic diagram of an existing synchronous rectifier BCUK converter;

[0027] Figure 2 The schematic diagram of an existing synchronous rectifier BCUK converter (with freewheeling diode and auxiliary capacitor in parallel);

[0028] Figure 3 Schematic diagram of the synchronous rectification BUCK converter of this invention;

[0029] Figure 4 The control timing and waveforms of the synchronous rectifier BUCK converter of this invention. Detailed Implementation

[0030] 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.

[0031] First Embodiment

[0032] Figure 3 This is a schematic diagram of a synchronous rectified BCUK converter according to the first embodiment of the present invention. The synchronous rectified BCUK converter includes: a switch Q1 (main switch Q1), a switch Q2 (freewheeling switch Q2), an inductor L, an output capacitor Co, a switch Q3, an inductor La, a diode D1, a diode D2, and a capacitor Ca.

[0033] The drain of the main switch Q1 is connected to the positive terminal of the input power supply. The source of the main switch Q1 is connected to one end of the inductor L and the drain of the freewheeling transistor Q2. The other end of the inductor L is connected to one end of the capacitor Co. The source of the freewheeling transistor Q2 is connected to the ground GND of the input power supply. The other end of the capacitor Co is connected to the ground GND. The freewheeling transistor Q2 can be a diode or a MOSFET.

[0034] One end (drain) of the switching transistor Q3 is connected to the positive terminal of the input power supply of the synchronous rectifier BUCK converter, and the other end (source) is connected to one end of the inductor La and the cathode of the diode D2. The other end of the inductor La is connected to one end of the capacitor Ca and the anode of the diode D1. The cathode of the diode D1 is connected to the drain Vs of the freewheeling transistor Q2. The anode of the diode D2 and the other end of the capacitor Ca are connected to the ground GND of the input power supply of the synchronous rectifier BUCK converter.

[0035] Figure 4 This document presents the control timing and waveforms of the synchronous rectification BUCK converter schematic diagram of this invention. It is now combined with... Figure 4 The five stages of each cycle (from time t0 to time t5, denoted as T) are explained as follows:

[0036] t0-t1 stage: At the initial time t0, the current IL of inductor L reaches its negative maximum value. At this time, freewheeling transistor Q2 is turned off, and the driving voltage Vgs2 of freewheeling transistor Q2 becomes low level. The negative current charges the junction capacitance of freewheeling transistor Q2, the junction capacitance of main switch transistor Q1 discharges, and the voltage of drain Vs of freewheeling transistor Q2 begins to rise. Until time t1, the voltage of drain Vs of freewheeling transistor Q2 rises to near the voltage Vin of the input power supply. During this stage, switch transistor Q3 and diodes D1 and D2 are not conducting.

[0037] t1-t2 stage: At the initial time t1, the main switch Q1 is turned on with zero voltage, and the drive voltage Vgs1 of the main switch Q1 becomes high. The input power supply supplies power to the output capacitor Co and the load through the main switch Q1 and the inductor L. During this stage, the switch Q3 and diodes D1 and D2 are not conducting.

[0038] t2-t3 stage: At the initial time t2, the switch Q3 is turned on, and the driving voltage Vgs3 of the switch Q3 becomes high. The input power supply charges the capacitor Ca through the switch Q3 and the inductor La until time t3, when the voltage Vca on the capacitor Ca is charged to be consistent with the voltage Vin of the input power supply.

[0039] t3-t4 stage: At the initial time t3, the main switch Q1 is turned off, the drive voltage Vgs1 becomes low level, the switch Q3 is turned off, the drive voltage Vgs3 becomes low level. Since the voltage on capacitor Ca is equal to the input power supply voltage Vin, the voltage across the main switch Q1 is clamped at 0V, achieving zero voltage turn-off. After the main switch Q1 is turned off, capacitor Ca discharges to the output capacitor and load through diode D1 and inductor L until time t4, when the voltage Vca on capacitor Ca is zero.

[0040] t4-t5 stage: At the initial time t4, the freewheeling diode Q2 is turned on with zero voltage, the driving voltage Vgs2 becomes high level, the inductor L is demagnetized, and at time t5, the negative inductor current IL reaches its maximum value.

[0041] Thus, one cycle of the first embodiment of the present invention has ended.

[0042] Note that during the t2~t3 phase, the required conduction time of the switching transistor Q3 is fixed and depends only on the selection of inductor La and capacitor Ca, and does not change with the input power supply voltage Vin. The specific theoretical calculation process is as follows:

[0043] Where Vin is the voltage of the input power supply (i.e., the input voltage of the converter), u la U is the voltage across inductor La. ca Let be the voltage across capacitor Ca, La be the inductance of inductor La, and Ca be the capacitance of capacitor.

[0044]

[0045] Solving the above equations simultaneously, we can obtain u ca =Vin, It can be seen that the time T (T=t3-t2) required to charge the voltage across capacitor Ca to the input power supply voltage Vin is only related to the product of inductance La and capacitance Ca.

[0046] This invention also has the same improvement effect on conventional BUCK converters with high current output (conventional BUCK converters operate in CCM mode with a current peak of less than 2Io). The specific working process is similar to the synchronous rectification BUCK converter implementation process described above, and will not be repeated here.

[0047] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A converter, comprising a switching transistor Q1, a switching transistor Q2, an inductor L, and a capacitor Co; the drain of the switching transistor Q1 is connected to an input power supply, the source of the switching transistor Q1 is connected to one end of the inductor L and the drain of the switching transistor Q2, the other end of the inductor L is connected to one end of the capacitor Co, the source of the switching transistor Q2 is connected to ground, and the other end of the capacitor Co is connected to ground, characterized in that, The converter also includes an auxiliary circuit, which has a switching transistor Q3, an inductor La, a capacitor Ca, a diode D1, and a diode D2. One end of the switching transistor Q3 is connected to the input power supply, and the other end of the switching transistor Q3 is connected to one end of the inductor La and the cathode of the diode D2. The other end of the inductor La is connected to one end of the capacitor Ca and the anode of the diode D1. The cathode of the diode D1 is connected to the drain of the switching transistor Q2, and the anode of the diode D2 and the other end of the capacitor Ca are connected to ground.

2. The converter according to claim 1, characterized in that: The capacitor Ca is a collection of multiple capacitors connected in series or in parallel, and the switch Q3 is a collection of multiple switches connected in series or in parallel.

3. A control method for the converter according to claim 1, characterized in that: include: The switch Q1 is turned on. When the switch Q1 is turned on, the input power supply supplies power to the capacitor Co and the load of the converter through the switch Q1 and the inductor L. During the conduction of the switch Q1, the switch Q3 is controlled to conduct for a fixed time. During the conduction of the switch Q3, the input power supply charges the capacitor Ca through the switch Q3 and the inductor La. The switching transistors Q1 and Q3 are turned off. After the switching transistor Q1 is turned off, the capacitor Ca supplies power to the capacitor Co and the load through the diode D1 and the inductor L until the capacitor Ca is completely discharged, and then the switching transistor Q2 is turned on.

4. The control method according to claim 3, characterized in that: During one switching cycle, the conduction time T of the switching transistor Q3 is related to the inductance value of the inductor La and the capacitance value of the capacitor Ca by the following formula: Where La is the inductance of the inductor La, and Ca is the capacitance of the capacitor.

5. The control method according to claim 3, characterized in that: The conduction time of the switch Q3 is less than the conduction time of the switch Q1, and the start time of the conduction of the switch Q3 is not earlier than the start time of the conduction of the switch Q1, and the turn-off time of the switch Q3 is not later than the turn-off time of the switch Q1.

6. The control method according to claim 5, characterized in that: When the switch Q3 is turned off, the voltage across the capacitor Ca is equal to the voltage of the input power supply, and the turn-off time of the switch Q3 is synchronized with the turn-off time of the switch Q1.

7. The control method according to claim 3, characterized in that: When the output load current of the converter is less than a certain value, the switch Q3 will not operate.

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