A control system and method for a DCDC full-bridge converter under weak excitation conditions

Abstract

The present invention discloses a control system and method for a DCDC full-bridge converter under weak excitation conditions, belonging to the field of electronic power, and based on the soft switching process analysis of a weak excitation full-bridge converter, a switch control method and an implementation circuit are proposed according to the relationship between the weak excitation inductor current, output voltage, output current and the soft switching process under different loads. According to the full-mode weak excitation inductor current change and the load current, the present invention obtains a segmented fixed-frequency pulse width control method for soft switching conditions and a selection basis for the relevant frequency switching reference and the operating frequency, and realizes soft switching within the full load range of the full-bridge converter without adding auxiliary power devices, and provides an adaptive switch adjustment module for weak excitation inductors, which solves the problem that the fixed switch mode cannot meet the soft switching requirements of a wide load range, making the circuit operation more stable, reliable and efficient, while reducing the circuit volume, with a high upper limit of power density, and a simple and easy-to-implement control method.

Description

Technical Field

[0001] The present invention relates to the technical field of DC / DC full-bridge converter control, and in particular to a control system and method for a DCDC full-bridge converter under weak excitation conditions. Background Art

[0002] With the rapid development of electric vehicles, aerospace and other fields, higher requirements are placed on the switching frequency, power density, efficiency and stability of power modules. However, with the increase and limitation of switching frequency and power density, taking the full-bridge DC / DC converter as an example, the excitation inductance of the transformer will become smaller and smaller (referred to as weak excitation inductance in this article). This makes the current analysis gradually no longer applicable. In addition, high switching frequency means high switching loss, and soft switching becomes particularly important as an effective means to reduce switching loss.

[0003] In order to achieve soft switching within the full load range, most methods introduce auxiliary circuits. The introduction of auxiliary networks will undoubtedly increase the complexity of the circuit and reduce the power density. At the same time, most of the existing soft switching technologies require complex control strategies to accurately control the switching action, which not only increases the complexity of the system, but also may lead to insufficient control stability. At the same time, the circuit topology of some existing technologies has limitations under high frequency and high power density conditions, such as the weak excitation problem caused by the reduction of transformer excitation inductance, which limits the implementation effect of soft switching.

[0004] Therefore, it is an important topic in the current field of power electronics technology to study more efficient, stable and low-cost full-load range soft switching technology, optimize circuit topology and improve system stability. Summary of the invention

[0005] The problem to be solved by the present invention is to provide a control system and method for a DCDC full-bridge converter under weak excitation conditions, which is used to realize full-load soft switching without any auxiliary circuit and ensure reliable operation of the circuit through a corresponding switch regulation scheme.

[0006] The present invention adopts the following technical scheme: a control system for a DCDC full-bridge converter under weak excitation conditions, comprising: a first duty cycle calculation module for weak excitation natural soft switches of Q3 and Q4 tubes, a second duty cycle calculation module for weak excitation natural soft switches of Q3 and Q4 tubes, a duty cycle conversion circuit for Q3 and Q4 tubes, a weak excitation soft switch frequency switching reference generation module, an external frequency selection module, an external weak excitation full-bridge converter, and an external drive circuit.

[0007] The external weak excitation full-bridge converter comprises: a primary full-bridge inverter, a weak excitation transformer, and a full-wave rectifier circuit coupled to the weak excitation transformer;

[0008] The first duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes includes a first subtractor, a square operation circuit, a second subtractor, a square root operation circuit, a third subtractor and a proportional link circuit connected in series in sequence, and is used to generate the switch tubes Q3 and Q4 under light load i according to the weak excitation natural soft switch conduction time relationship. Lp right The duty cycle of the time; where i Lp Indicates the load current I o The converted value of the original edge is I right Indicates the right reference of the low frequency band, I left Indicates the left reference of the low frequency band;

[0009] The second duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes includes an adder and a divider connected in series in sequence, and is used to generate the switch tube Q3 and Q4 under heavy load i according to the weak excitation natural soft switch conduction time relationship. Lp >I right The duty cycle of

[0010] The duty cycle conversion circuit of the Q3 and Q4 tubes includes: a comparator, a single-pole double-throw switch S1, a voltage-controlled current source, a freewheeling switch tube S2, a resistor R1 and a resistor R2, a capacitor C, a first Schmitt trigger and a second Schmitt trigger, an AND gate and / or a NOT gate, and is used to convert the output results of the first duty cycle operation module and the second duty cycle operation module of the weak excitation natural soft switch of the Q3 and Q4 tubes into a fixed-frequency PWM signal to drive the switch tubes Q3 and Q4;

[0011] The weak excitation soft switch frequency switching reference generation module includes: a first proportional link, a fourth subtractor, a second proportional link, a third proportional link, and a fifth subtractor, which are used to generate a weak excitation soft switch frequency switching reference I right and I left , input to the external frequency selection module to provide a switching basis for switching frequency.

[0012] Specifically, the first duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes converts the sampled output current I o The value of the positive input terminal of the first subtractor is equal to the excitation current I m , the output of the first subtractor is divided into two paths, one is sent to the square operation circuit, and the other is sent to the positive input of the third subtractor. The result of the square operation circuit is sent to the positive input of the second subtractor. The value input to the negative input of the second subtractor is:

[0013] V in 2 / L m 2 A​2 ;

[0014] Where, L m Indicates the magnitude of the excitation inductance, V in is the input voltage, C ds is the junction capacitance of the main switch, C d Represents the junction capacitance of the secondary diode;

[0015] The output of the second subtractor is connected to the square root operation circuit, and the output of the square root operation circuit is used as the input of the reverse input terminal of the third subtractor. The output of the third subtractor is connected to the duty cycle conversion circuit of the subsequent Q3 and Q4 tubes through the proportional link circuit.

[0016] Specifically, the second duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes converts the sampled output current I o It is fed into the positive output terminal of the adder and combined with the excitation current signal I m The output of the adder is fed into the divider, making 2C ds V in The output of the adder completes the division operation, and the output of the divider is connected to the duty cycle conversion circuit of Q3 and Q4 tubes in the subsequent stage.

[0017] Specifically, in the duty cycle conversion circuit of Q3 and Q4 tubes, the single-pole double-throw switch S1 is controlled by a comparator, and the positive input terminal of the comparator is the sampled load current I of the main circuit weak excitation full-bridge converter. o , the inverting input terminal is I right , when I o >I right When , the single-pole double-throw switch S1 is connected to the second terminal, otherwise it is connected to the first terminal, the output of the single-pole double-throw switch S1 is sent to the control end of the voltage-controlled current source, and the input end of the voltage-controlled current source is connected to one end of the resistor R1;

[0018] One end of the capacitor C is connected to the ground, and the other end is connected to the other end of the resistor R1. The resistor R2 and the freewheeling switch tube S2 are connected in parallel with the capacitor. The connection point of the capacitor C and the resistor R1 is connected to the first Schmitt trigger, and the outputs are used as an input of the AND gate and the NOR gate respectively.

[0019] The PWM square wave with a duty cycle of 50% is divided into three paths after passing through the second Schmitt trigger, two of which are outputted respectively to the other end of the gate and the NOR gate, and the third is outputted to the control end G of the freewheeling switch tube. When the level is high, the freewheeling switch tube S2 is turned off, and when the level is low, the freewheeling switch tube S2 is turned on. The output of the AND gate is driven as the driving signal of the switch tube Q4, and the output of the NOR gate is driven as the driving signal of the switch tube Q3.

[0020] Specifically, the weak excitation soft switching frequency switching reference generation module generates a weak excitation soft switching frequency switching reference I through the first proportional link and the fourth subtractor. right part, the method is as follows:

[0021] Sample input voltage V in , after passing through the first proportional link, it is sent to the positive input terminal of the fourth subtractor. The reference voltage of the negative input terminal of the fourth subtractor is set to: nV o / 4fL m ;

[0022] Among them, V o represents the output voltage, n represents the transformer ratio, f represents the switching frequency, L m Indicates the size of the excitation inductance. The first proportional link amplification factor is 1.3sqr(C ds / L r ), C ds Represents the switch junction capacitance, L r Indicates the leakage inductance of the transformer.

[0023] Specifically, the weak excitation soft switch frequency switching reference generation module generates the weak excitation soft switch frequency switching reference I through the second proportional link, the third proportional link, and the fifth subtractor. left part, the method is as follows:

[0024] Sample input voltage V in , after passing through the second proportional link, it is sent to the negative input terminal of the fifth subtractor. The reference voltage of the positive input terminal of the fifth subtractor is set to nL m V o , the input of the fifth subtractor passes through the third proportional link and is used as I left Benchmarks;

[0025] Among them, the second proportional link amplification factor is The amplification factor of the third proportional link is 1 / 4L m 2 f,C d Represents the junction capacitance of the secondary diode.

[0026] The technical solution of the present invention also provides: a control method for a DCDC full-bridge converter under weak excitation conditions, which is applied to the control system of a DCDC full-bridge converter under any of the above weak excitation conditions, comprising the following steps:

[0027] S1, sampling the output current I of the main circuit weak excitation full-bridge converter o As the input of the external frequency selection module, and the output of the weak excitation soft switching frequency switching reference generation module I left and I rightMake comparison and select the operating frequency of the main circuit;

[0028] S2, sampling main circuit input voltage V in , sent to the weak excitation soft switching frequency switching reference generation module, and obtained I left and I right , as the basis for the external frequency selection module to switch the frequency;

[0029] S3, sampling output current I o And use it as the input of the first duty cycle calculation module of weak excitation natural soft switch of Q3 and Q4 tubes and the second duty cycle calculation module of weak excitation natural soft switch of Q3 and Q4 tubes. According to the relationship between the output current and the soft switch of the main circuit, the current conduction time of the switch Q3 and Q4 tubes is calculated, and the corresponding PWM wave is output to drive the switch Q3 and Q4 tubes through the subsequent Q3 and Q4 tube duty cycle conversion circuit.

[0030] Specifically, in step S1, the external frequency selection module switches the load to the reference I at the weak excitation soft switching frequency according to the weak excitation natural soft switching condition and the frequency and circuit parameter constraints. left and I right is the boundary point, divided into three intervals: 0-I left , I left -I right , I right -I omax ; Among them, I omax is the value of the maximum output current converted to the primary side of the transformer,

[0031] Sampling main circuit output current I o , when the load current is converted to the value of the primary side of the transformer I Lp Greater than I right and less than I left When I Lp In I right and I left When the converter is operated in the low frequency mode f low , the original mode of the main circuit I Lp Approximately equal to I m The hard switching state during the period is transformed into I Lp m Soft switching state during the period;

[0032] Among them, the switching frequency in the low frequency range is f low , the expression is:

[0033]

[0034] Where V o ​represents the output voltage, f is the switching frequency, L r is the transformer leakage inductance, n is the transformer ratio, L m Indicates the magnitude of the excitation inductance, V in is the input voltage, C ds is the junction capacitance of the main switch, C d Represents the junction capacitance of the secondary diode.

[0035] Specifically, in step S2, in the weak excitation soft switching frequency switching reference generation module, the realization of the soft switch of the weak excitation full-bridge converter is jointly determined by the weak excitation inductance and leakage inductance circuit parameters, and the threshold of the low-frequency working range depends on the soft switching condition obtained by the full-mode weak excitation inductance current change and the load current. o and input voltage V in , and the corresponding I is obtained and output through the following formula left and I right , providing a basis for the external frequency selection module to switch frequency:

[0036]

[0037] when i Lp Less than I right Or greater than I left If the system still works at the original high switching frequency, the main circuit enters the hard switching state. Lp Down to I right Or rise to I left When the system switching frequency is switched, the hard switching state is avoided.

[0038] Specifically, in step S3, the first duty cycle calculation module and the second duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes sample the output current I according to the relationship between the output voltage, output current and the soft switching process under different loads under the premise that all modes consider the change of the excitation current. o , based on the relationship between soft switching and circuit parameters under the influence of weak excitation inductor current in the full load range, calculate the corresponding conduction time of switch tubes Q3 and Q4;

[0039] Q3 and Q4 tubes are weakly excited and naturally soft switched. The first duty cycle calculation module is used. When the switch tubes Q3 and Q4 are under light load, Lp right When the switch Q3 and Q4 are configured, the corresponding conduction time t1 is expressed as follows:

[0040]

[0041] Q3 and Q4 tubes are weakly excited and naturally soft-switched. The second duty cycle calculation module is used. When the switch tubes Q3 and Q4 are under heavy load,​Lp >I right When the switch Q3 and Q4 are configured, the corresponding conduction time t2 is expressed as follows:

[0042]

[0043] In the formula, Function f x Indicates the switching frequency at that time;

[0044] The calculation results are converted into corresponding PWM signals through the duty cycle conversion circuit of Q3 and Q4 in the subsequent stage to drive the switch tubes Q3 and Q4, so that the switch tubes are turned on immediately after the junction capacitance is charged and discharged, thereby realizing zero-voltage turn-on of the switch tubes in the main circuit.

[0045] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0046] 1. The control system and method of the DCDC full-bridge converter under weak excitation conditions of the present invention improve the current situation of insufficient research on weak excitation conditions, and realize soft switching of the full-bridge converter within the full load range without adding any auxiliary circuits and devices. The circuit volume is reduced, the high switching loss caused by high frequency is avoided, and the upper limit of power density is high, which is in line with the current background of high frequency and high power density.

[0047] 2. The present invention is used for the control system of the DCDC full-bridge converter under weak excitation conditions, and provides an adaptive switch regulation module for weak excitation inductance, which solves the problem that the fixed switch mode cannot meet the soft switching requirements of a wide load range, making the circuit operation more stable, reliable and efficient.

[0048] 3. The control method of the DCDC full-bridge converter circuit under weak excitation conditions of the present invention is essentially a simple fixed-frequency duty cycle control, which is simple to implement and low in cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A topological diagram of a weak excitation full-bridge DC / DC converter, which is an application object of the present invention;

[0050] Figure 2 The control system principle block diagram of the present invention for a DCDC full-bridge converter under weak excitation conditions;

[0051] Figure 3 This is a schematic diagram of a voltage stabilizing module according to an embodiment of the present invention;

[0052] Figure 4 This is a block diagram of the frequency selection module implementation of the present invention;

[0053] Figure 5The weak excitation soft switching frequency switching reference I of the embodiment of the present invention Left and I right Generate circuit diagrams;

[0054] Figure 6 This is a principle block diagram of the duty cycle conversion part of the first and second duty cycle calculation modules of the weak excitation natural soft switch of Q3 and Q4 tubes in an embodiment of the present invention;

[0055] Figure 7 This is an application circuit diagram of the duty ratio conversion part of the first and second duty ratio calculation modules of the weak excitation natural soft switch of Q3 and Q4 tubes in the embodiment of the present invention;

[0056] Figure 8 A schematic diagram of a frequency conversion circuit according to an embodiment of the present invention;

[0057] Fig. 9 This is a working waveform diagram of the weak excitation full-bridge converter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the application is further elaborated in detail below in conjunction with the accompanying drawings. The described embodiments are only a part of the embodiments involved in the present invention. All non-innovative embodiments of other researchers in the field on this embodiment belong to the protection scope of the present invention. At the same time, for the step numbering in the embodiment of the present invention, it is only set for the convenience of explanation, and the order between the steps is not limited in any way. The execution order of each step in the embodiment can be adaptively adjusted according to the understanding of those skilled in the art.

[0059] In one embodiment of the present invention, the topology of the application object weak excitation full-bridge DC / DC converter is as follows: Figure 1 As shown, Q1, Q2 and Q3, Q4 are complementary switch tubes respectively. Q1 and Q2 constitute the leading arm controlled by the voltage stabilizing part, mainly maintaining the stability of the output voltage. Q3 and Q4 constitute the lagging arm, which is controlled by the first and second duty cycle operations of the weak excitation natural soft switch of Q3 and Q4 tubes and the duty cycle conversion part of Q3 and Q4, and is used to meet the requirements of the soft switch for the switching state under different circuit conditions.

[0060] In the figure, V in is the input voltage, C ds1 -C ds4 is the junction capacitance of the main switch, L r , L m are the transformer leakage inductance and magnetizing inductance respectively. The secondary side adopts a full-wave rectifier circuit, where D R1 , D R2 is the secondary diode, C DR1 , C DR2 is the junction capacitance of the secondary diode.o and C o are the inductance and current of the filter respectively, and R represents the load.

[0061] In this embodiment, the control principle of the control system for the DCDC full-bridge converter under weak excitation conditions is as follows: Figure 2 As shown, its main circuit is a weak excitation full-bridge converter, which is equipped with Q3 and Q4 tube weak excitation natural soft switching duty cycle operation 1, Q3 and Q4 tube weak excitation natural soft switching duty cycle operation 2, Q3 and Q4 tube duty cycle conversion circuit, weak excitation soft switching frequency switching reference generation and external frequency selection module, voltage stabilization module, external weak excitation full-bridge converter, and external drive circuit.

[0062] The external frequency selection module obtains the output load current I according to the system sampling o Determine the working range of the system, when the primary component of the output load current i Lp In I right and I left When the system works in the low frequency band, the switching frequency is f low , otherwise the system operates at the normal switching frequency f.

[0063] The weak excitation soft switching frequency switching reference generation module is based on the sampled input voltage V in Provides the basis for selecting the frequency switching point for the frequency selection module right and I left The voltage regulator module continuously compares the output voltage obtained by sampling with the set voltage V ref By comparison, the duty cycle of the output control switch tubes Q1 and Q2 is obtained to maintain the stability of the output voltage.

[0064] Q3 and Q4 tubes are weakly excited and the natural soft switching duty cycle operations 1 and 2 convert the output current I sampled by the system o As input, the calculation result is then converted into the driving signal of Q3 and Q4 of the switching tubes through the duty cycle conversion circuit of the subsequent Q3 and Q4 tubes to ensure that the system can achieve natural soft switching in the full load range.

[0065] The following is a detailed description of each control module in the system.

[0066] First, in order to ensure the stability of the output voltage, this embodiment adopts Figure 3 The circuit shown controls the switches Q1 and Q2, corresponding to Figure 2 Voltage regulator module.

[0067] Among them, V o is the output voltage, which is sampled by voltage division and compared with the set reference value V refThe voltage loop regulator is compared and sent to the voltage loop regulator, which is implemented by PID control. The modulation signal of the voltage loop is compared with the carrier signal generated previously to obtain the required PWM duty cycle, which is supplied to the main circuit switch tubes Q1 and Q2, thereby achieving the purpose of controlling the output voltage. In order to prevent the output duty cycle from being too large, a limiting circuit is added after the PID output.

[0068] Then, the specific implementation steps of the external frequency selection module are:

[0069] Under weak excitation conditions, the energy for charging and discharging the switch tube of the full-bridge converter is provided by the excitation inductance and leakage inductance. The realization of soft switching mainly depends on the excitation current and the load current. However, in a constant frequency weak excitation full-bridge converter, when the load current is converted to the primary value i Lp At the excitation current I m When it is near, the energy that the excitation inductance can provide is very limited, and the energy of the leakage inductance is often not enough to meet the requirements of soft switching.

[0070] In order to avoid the above conditions when i Lp In I m Therefore, this embodiment provides a segmented fixed-frequency pulse width control method suitable for the natural resonance soft switching mode of weak excitation inductance, such as Figure 8 shown.

[0071] With output load current I o Converted to the original value i Lp Based on the size, the load is divided into three intervals: 0-I left , I left -I right , I right -I omax Among them, I omax It is the value of the maximum output current converted to the primary side of the transformer.

[0072] When the load current converted to the primary side of the transformer is greater than I right and less than I left When the converter operates in high frequency mode f, Lp In I right and I left When the converter is operated in the low frequency mode f low .

[0073] The original mode i Lp Approximately equal to I m , the hard switching state during this period changes to i Lp m ​The soft switching state during the period is realized, thereby realizing soft switching in the full load range without the help of any auxiliary devices. The corresponding implementation circuit example is Figure 4 As shown, corresponding to Figure 2 The frequency selection module in .

[0074] Among them, two hysteresis comparators and diodes constitute a frequency selection circuit, and the right side is a carrier generation circuit. The equivalent capacitance formed by capacitors C1 and C2 controls the output carrier frequency.

[0075] When the output current is higher than the set I right or below I left When , the two hysteresis comparators output high level, and after passing through the inverter, they get low level, which turns off the switch tube S. The equivalent capacitance of the carrier generation part that determines the frequency decreases, which means that the output at this time is a high-frequency carrier.

[0076] When the output current is I left and I right When , the two hysteresis comparators output low level, and after passing through the inverter, the high level drives the switch tube S to turn on, so that the equivalent capacitance of the carrier generation part increases and the output carrier frequency decreases, thereby reducing the switching frequency.

[0077] In the buffer zone of the hysteresis comparator of the frequency selection circuit, the circuit can realize soft switching in both high-frequency and low-frequency working modes, thereby preventing the main circuit from frequently changing frequency due to interference and other factors.

[0078] When in the low frequency range, the excitation current becomes larger, making the circuit Lp Approximately equal to I m The state changes to i Lp m In order to make the excitation current and leakage inductance current after frequency conversion sufficient to realize the soft switching of the system, the low switching frequency expression that meets the soft switching requirements is obtained as follows, which can be used as the basis for selecting capacitors C1 and C2.

[0079]

[0080] Next, the specific implementation steps of the weak excitation soft switching frequency switching reference generation module are as follows:

[0081] When the system leakage inductance and other device parameters are set, the output voltage V in , the output corresponding to I right and I left , providing a basis for the frequency selection module to switch frequencies.

[0082] when i Lp Less than I right Or greater than I​left If the system still works at the original high switching frequency, the circuit will enter the hard switching state. Lp Down to I right Or rise to I left When the system switching frequency is switched immediately, the hard switching state can be effectively avoided.

[0083] In this embodiment, the weak excitation soft switching frequency switching reference I right and I left Generate modules such as Figure 5 As shown, it includes a first proportional link, a fourth subtractor, a second proportional link, a third proportional link, and a fifth subtractor.

[0084] First, the sampled output voltage is sent to the positive input of the subtractor after passing through the first proportional link. The reference voltage of the negative input of the fourth subtractor is set to nV. o / 4fL m .

[0085] The output of the fourth subtractor is used as the weak excitation switching frequency switching reference I right The I right The first proportional link amplification factor is 1.3sqr(C ds / L r ), V o represents the output voltage, n represents the transformer ratio, L m represents the excitation inductance, f represents the normal switching frequency, C ds Represents the switch junction capacitance, L r Indicates the leakage inductance of the transformer.

[0086] Sample input voltage V in The signal is sent to the negative input terminal of the fifth subtractor after passing through the second proportional link. The reference voltage of the positive input terminal of the fifth subtractor is set to nL m V o The input of the fifth subtractor is then used as the weak excitation switch frequency switching reference I after passing through a third proportional link. left The second proportional link amplification factor is The amplification factor of the third proportional link is 1 / 4L m 2 f.

[0087] Weak excitation soft switching frequency switching reference output I right and I left The relationship with the input voltage expression is as follows:

[0088]

[0089] That is, whenLp Less than I right Or greater than I left If the system still operates at the original high switching frequency, the circuit will enter the hard switching state.

[0090] Finally, the working principles and specific implementation steps of Q3 and Q4 tube weak excitation natural soft switching duty cycle calculation 1, 2 and Q3, Q4 tube duty cycle conversion are as follows:

[0091] Through the analysis of the principle of the weak excitation inductor full-bridge converter, the changes in the excitation current and the load current will have a great impact on the zero voltage opening of the circuit. Since the junction capacitance charging and discharging time of the switch tube is related to the load current, the time when the voltage of the switch tubes Q3 and Q4 drops to 0 is also different as the load current changes. In order to enable the weak excitation converter to drive the switch tube to turn on when its switch tube voltage just drops to 0, thereby further reducing the loss, the Q3 and Q4 tube weak excitation natural soft switching duty cycle calculation 1, 2 and the subsequent Q3 and Q4 tube duty cycle conversion circuit are designed, such as Figure 6 shown.

[0092] Among them, the first path is i Lp right First, the output current is sampled and used as the negative input of the first subtractor. The positive input value of the first subtractor is I m , thus achieving I m -i Lp The square operation circuit performs a square operation on the output of the first subtractor to obtain (I m -i Lp ) 2 .

[0093] The square operation circuit sends the calculation result to the positive input terminal of the second subtractor to complete (I m -i Lp ) 2 -V in 2 / L m 2 A 2 Calculation of V in 2 / L m 2 A 2 is a constant.

[0094] The output of the second subtractor is converted into sqrt((I m -i Lp ) 2 -V in 2 / L m ​2 A 2 ) is then fed to the positive input of the third subtractor to complete the operation. Then the output of the third subtractor is passed through the proportional operation circuit to obtain (2L m / V in )(sqrt((I m -i Lp ) 2 -V in 2 / L m 2 A 2 )-(I m -i Lp )).

[0095] The second path is similar to i Lp >I right The operation circuit at this time will sample the I o Send to the adder to complete the operation I m +i Lp The output of the adder is then combined with the 2C ds V in Divide and we get 2C ds V in / (I m +i Lp ).

[0096] The output of the two channels is selected by the single-pole double-throw switch S1 to connect to the duty cycle conversion circuit of the subsequent Q3 and Q4 tubes. The single-pole double-throw switch S1 is controlled by the comparator. Lp right When , the output of the first channel is allowed to be sent to the next stage, otherwise the output of the second channel is allowed to be sent to the next stage.

[0097] The outputs of the first two paths serve as control signals for the subsequent voltage-controlled current source to continuously change its output current. The output current of the voltage-controlled current source charges capacitor C through R1. R2 is connected in parallel across C so that the maximum voltage on C does not exceed I max *R2 / (R1+R2), where I max Represents the maximum output current of the voltage-controlled current source.

[0098] ​The freewheeling switch S2 is controlled by a square wave with a 50% duty cycle. In order to prevent interference, Schmitt trigger 2 is used for filtering. When PWM is at a high level, S2 is turned off, and the current source current charges the capacitor C, causing its voltage to rise slowly. After the predetermined time, the voltage of C rises to a certain level, and Schmitt trigger 1 outputs a high-level signal. When PWM is at a low level, the freewheeling switch S2 is turned on, causing the voltage on capacitor C to drop rapidly to 0, and Schmitt trigger 1 outputs a low level. The output of the Schmitt trigger and PWM pass through the NOR gate and the AND gate to obtain the drive signal of the switch Q3 and Q4.

[0099] The conduction time of the Q3 and Q4 drive signals satisfies the following expression:

[0100]

[0101] In the formula, L m Indicates the magnitude of the excitation inductance, f x Indicates the switching frequency at that time, C ds is the junction capacitance of the main switch, V in is the input voltage, V o Indicates the input voltage, i Lp Indicates the primary value converted from the load current.

[0102] Figure 7 Gives a Figure 6 The specific circuit implementation method is that the weak excitation natural soft switching duty cycle operations 1 and 2 of Q3 and Q4 tubes are realized by multi-stage operational amplifiers, as follows:

[0103] The first, second and third subtractors are implemented by a subtraction circuit, wherein the positive input terminal is the amount to be subtracted and the negative input terminal is the amount to be decremented; the inputs of the multiplication circuit are directly connected to implement a square operation; the open operation consists of a square operation and a proportional operation, and the square operation is used as a negative feedback loop of the proportional operation, so that the output is an inverse function of the square.

[0104] In this embodiment, the amplification ratio of the proportional circuit is 1, the input is connected to the reverse input terminal, and the adder is composed of a common-phase proportional addition operation circuit to realize the sum of two input quantities; the divider is similar to the square root operation circuit, and the multiplier is used as a negative feedback loop of the geometric operation to realize the inverse operation of multiplication, that is, the division operation.

[0105] The inverting input terminal of the geometric operation circuit is the dividend, and the idle terminal of the multiplier is the divisor. The voltage-controlled current source of the duty cycle conversion circuit of the subsequent Q3 and Q4 tubes is realized by a voltage-controlled current source, and the freewheeling switch tube S2 is a PMOS tube.

[0106] In this embodiment, the working waveform of the weak excitation full-bridge converter is as follows: Fig. 9As shown, the left side is the value i of the load current converted to the primary side of the transformer Lp Less than I m The waveform on the right is i Lp Greater than I m The waveform when .

[0107] From the above description, it can be seen that the present invention is a control method for a full-bridge DC / DC converter based on weak excitation conditions. It can realize soft switching within the full load range without adding any power devices. It improves the upper limit of power density and is suitable for today's high-frequency and high-power density environment. It has adaptive Q3 and Q4 switch control and regulation modules, which solves the problem that the fixed switch mode cannot meet the soft switching requirements of a wide load range. The control method is simple and easy to implement.

[0108] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A control system for a DCDC full-bridge converter under weak excitation conditions, used to achieve full-load soft switching without auxiliary circuits, characterized in that: include: Q3 and Q4 tube weak excitation natural soft switch first duty cycle calculation module, Q3 and Q4 tube weak excitation natural soft switch second duty cycle calculation module, Q3 and Q4 tube duty cycle conversion circuit, weak excitation soft switch frequency switching reference generation module, external frequency selection module, external weak excitation full-bridge converter, external drive circuit; The external weak excitation full-bridge converter comprises: a primary full-bridge inverter, a weak excitation transformer, and a full-wave rectifier circuit coupled to the weak excitation transformer; The first duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes includes a first subtractor, a square operation circuit, a second subtractor, a square root operation circuit, a third subtractor and a proportional link circuit connected in series in sequence, and is used to generate the switch tube Q3 and Q4 under light load i according to the relationship between the weak excitation natural soft switch conduction time t1. Lp right The duty cycle of​ The on-time t1 is calculated as follows: Among them, i Lp Indicates the load current I o The converted value of the original edge, I right Indicates the right reference of the low frequency band, I left Indicates the left reference of the low frequency band, L m Indicates the magnitude of the excitation inductance, V in is the input voltage, I m is the excitation current, function f x represents the switching frequency at that time, C ds is the junction capacitance of the main switch, C d Represents the junction capacitance of the secondary diode; The second duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes includes an adder and a divider connected in series in sequence, and is used to generate the switch tube Q3 and Q4 under heavy load i according to the relationship between the weak excitation natural soft switch conduction time t2. Lp >I right The duty cycle of The on-time t2 is calculated as follows: The duty cycle conversion circuit of the Q3 and Q4 tubes includes: a comparator, a single-pole double-throw switch S1, a voltage-controlled current source, a freewheeling switch tube S2, a resistor R1 and a resistor R2, a capacitor C, a first Schmitt trigger and a second Schmitt trigger, an AND gate and / or a NOT gate, and is used to convert the output results of the first duty cycle operation module and the second duty cycle operation module of the weak excitation natural soft switch of the Q3 and Q4 tubes into a fixed-frequency PWM signal to drive the switch tubes Q3 and Q4; The weak excitation soft switch frequency switching reference generation module includes: a first proportional link, a fourth subtractor, a second proportional link, a third proportional link, and a fifth subtractor, which are used to generate a weak excitation soft switch frequency switching reference I right and I left , input to the external frequency selection module to provide a switching basis for switching frequency; I left and I right The calculation is as follows: Among them, V o represents the output voltage, n represents the transformer ratio, L r is the transformer leakage inductance, and f is the switching frequency.

2. The control system for a DCDC full-bridge converter under weak excitation conditions according to claim 1, characterized in that: The first duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes converts the sampled output current I o The value of the positive input terminal of the first subtractor is equal to the excitation current I m , the output of the first subtractor is divided into two paths, one is sent to the square operation circuit, and the other is sent to the positive input of the third subtractor. The result of the square operation circuit is sent to the positive input of the second subtractor. The value input to the negative input of the second subtractor is: V in 2 / L m 2 A 2 ; The output of the second subtractor is connected to the square root operation circuit, and the output of the square root operation circuit is used as the input of the reverse input terminal of the third subtractor. The output of the third subtractor is connected to the duty cycle conversion circuit of the subsequent Q3 and Q4 tubes through the proportional link circuit.

3. The control system for a DCDC full-bridge converter under weak excitation conditions according to claim 2, characterized in that: The second duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes converts the sampled output current I o It is fed into the positive output terminal of the adder and combined with the excitation current signal I m The output of the adder is fed into the divider, making 2C ds V in The output of the adder completes the division operation, and the output of the divider is connected to the duty cycle conversion circuit of Q3 and Q4 tubes in the subsequent stage.

4. The control system for a DCDC full-bridge converter under weak excitation conditions according to claim 3, characterized in that: In the Q3 and Q4 duty cycle conversion circuit, the single-pole double-throw switch S1 is controlled by a comparator, and the positive input terminal of the comparator is the sampled load current I of the weak excitation full-bridge converter of the main circuit. o , the inverting input terminal is I right , when I o >I right When , the single-pole double-throw switch S1 is connected to the second terminal, otherwise it is connected to the first terminal, the output of the single-pole double-throw switch S1 is sent to the control end of the voltage-controlled current source, and the input end of the voltage-controlled current source is connected to one end of the resistor R1; One end of the capacitor C is connected to the ground, and the other end is connected to the other end of the resistor R1. The resistor R2 and the freewheeling switch tube S2 are connected in parallel with the capacitor. The connection point of the capacitor C and the resistor R1 is connected to the first Schmitt trigger, and the outputs are used as an input of the AND gate and the NOR gate respectively. The PWM square wave with a duty cycle of 50% is divided into three paths after passing through the second Schmitt trigger, two of which are outputted respectively to the other end of the gate and the NOR gate, and the third is outputted to the control end G of the freewheeling switch tube. When the level is high, the freewheeling switch tube S2 is turned off, and when the level is low, the freewheeling switch tube S2 is turned on. The output of the AND gate is driven as the driving signal of the switch tube Q4, and the output of the NOR gate is driven as the driving signal of the switch tube Q3.

5. The control system for a DCDC full-bridge converter under weak excitation conditions according to claim 4, characterized in that: The weak excitation soft switch frequency switching reference generation module generates a weak excitation soft switch frequency switching reference I through a first proportional link and a fourth subtractor. right part, the method is as follows: Sample input voltage V in , after passing through the first proportional link, it is sent to the positive input terminal of the fourth subtractor. The reference voltage of the negative input terminal of the fourth subtractor is set to: nV o / 4fL m ; Among them, V o represents the output voltage, n represents the transformer ratio, f represents the switching frequency, L m Indicates the size of the excitation inductance. The first proportional link amplification factor is 1.3sqr(C ds / L r ), C ds Represents the switch junction capacitance, L r Indicates the leakage inductance of the transformer.

6. The control system for a DCDC full-bridge converter under weak excitation conditions according to claim 5, characterized in that: The weak excitation soft switch frequency switching reference generation module generates a weak excitation soft switch frequency switching reference I through a second proportional link, a third proportional link, and a fifth subtractor. left part, the method is as follows: Sample input voltage V in , after passing through the second proportional link, it is sent to the negative input terminal of the fifth subtractor. The reference voltage of the positive input terminal of the fifth subtractor is set to nL m V o , the input of the fifth subtractor passes through the third proportional link and is used as I left Benchmarks; Among them, the second proportional link amplification factor is The amplification factor of the third proportional link is 1 / 4L m 2 f,C d Represents the junction capacitance of the secondary diode.

7. A control method for a DCDC full-bridge converter under weak excitation conditions, applied to the control system of the DCDC full-bridge converter under weak excitation conditions according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1, sampling the output current I of the weak excitation full-bridge converter of the main circuit o As the input of the external frequency selection module, and the output of the weak excitation soft switching frequency switching reference generation module I left and I right Make comparison and select the operating frequency of the main circuit; S2, sampling main circuit input voltage V in , sent to the weak excitation soft switching frequency switching reference generation module, and obtained I left and I right , as the basis for the external frequency selection module to switch the frequency; S3, sampling output current I o And use it as the input of the first duty cycle calculation module of weak excitation natural soft switch of Q3 and Q4 tubes and the second duty cycle calculation module of weak excitation natural soft switch of Q3 and Q4 tubes. According to the relationship between the output current and the soft switch of the main circuit, the current conduction time of the switch Q3 and Q4 tubes is calculated, and the corresponding PWM wave is output to drive the switch Q3 and Q4 tubes through the subsequent Q3 and Q4 tube duty cycle conversion circuit.

8. The control method for a DCDC full-bridge converter under weak excitation conditions according to claim 7, characterized in that: In step S1, the external frequency selection module switches the load to the reference I at the weak excitation soft switching frequency according to the weak excitation natural soft switching condition and the frequency and circuit parameter constraints. left and I right is the boundary point, divided into three intervals: 0-I left , I left -I right , I right -I omax ; Among them, I omax is the value of the maximum output current converted to the primary side of the transformer, Sampling main circuit output current I o , when the load current is converted to the value of the primary side of the transformer I Lp Greater than I right and less than I left When I Lp In I right and I left When the converter is operated in the low frequency mode f low , the original mode of the main circuit I Lp Approximately equal to I m The hard switching state during the period is transformed into I Lp m Soft switching state during the period;​ Among them, the switching frequency in the low frequency range is f low , the expression is: Where V o represents the output voltage, f is the switching frequency, L r is the transformer leakage inductance, n is the transformer ratio, L m Indicates the magnitude of the excitation inductance, V in is the input voltage, C ds is the junction capacitance of the main switch, C d Represents the junction capacitance of the secondary diode.

9. The control method for a DCDC full-bridge converter under weak excitation conditions according to claim 7, characterized in that: In step S2, in the weak excitation soft switching frequency switching reference generation module, the realization of the soft switch of the weak excitation full-bridge converter is determined by the weak excitation inductance and leakage inductance circuit parameters. The threshold of the low-frequency working range depends on the soft switching condition obtained by the full-mode weak excitation inductance current change and the load current. The output voltage V o and input voltage V in , get and output the corresponding I left and I right , providing a basis for the external frequency selection module to switch frequency; when i Lp Less than I right Or greater than I left If the system still works at the original high switching frequency, the main circuit enters the hard switching state. Lp Down to I right Or rise to I left When the system switching frequency is switched, the hard switching state is avoided.

10. The control method for a DCDC full-bridge converter under weak excitation conditions according to claim 7, characterized in that: In step S3, the first duty cycle calculation module and the second duty cycle calculation module of the weak excitation natural soft switch of Q3 and Q4 tubes sample the output current I according to the relationship between the output voltage, output current and the soft switching process under different loads under the premise that the excitation current changes are considered in all modes. o , based on the relationship between soft switching and circuit parameters under the influence of weak excitation inductor current in the full load range, calculate the corresponding conduction time of switch tubes Q3 and Q4; Q3 and Q4 tubes are weakly excited and naturally soft switched. The first duty cycle calculation module is used. When the switch tubes Q3 and Q4 are under light load, Lp right When , configure the switch Q3 and Q4 tubes to have a corresponding conduction time t1;​ Q3 and Q4 tubes are weakly excited and naturally soft-switched. The second duty cycle calculation module is used. When the switch tubes Q3 and Q4 are under heavy load, Lp >I right When , configure the switch Q3 and Q4 tubes to have a corresponding conduction time t2; The calculation results are converted into corresponding PWM signals through the duty cycle conversion circuit of Q3 and Q4 in the subsequent stage to drive the switch tubes Q3 and Q4, so that the switch tubes are turned on immediately after the junction capacitance is charged and discharged, thereby realizing zero-voltage turn-on of the switch tubes in the main circuit.

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