CROSS REFERENCE

This application is based upon and claims priority to Chinese Patent Application No. 201911363275.0, filed on Dec. 26, 2019, the entire contents thereof are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to the field of DC/DC high-frequency isolation conversion technologies, and more particularly, to a hybrid five-level bidirectional DC/DC converter and a voltage match modulation method thereof.

BACKGROUND

A dual active bridge-isolated bidirectional DC/DC converter (DAB) based on phase shift control technologies has the advantages such as high power density, fast dynamic response, easy implementation of soft switching, and bidirectional power flow, and thus is widely popular in the fields of uninterruptible power supplies, electric vehicles, and solid state transformers, etc. The phase shift control is a common DAB converter control mode. A two-level or multi-level voltage square wave with a relative phase shift is generated at a primary-side port and a secondary-side port of a high-frequency transformer. In the meanwhile, by regulating the relative phase shift between the primary-side square wave and the secondary-side square wave and a duty ratio of each level, the power flowing through the converter is regulated and the waveform of the current flowing through the high-frequency transformer is controlled.

In a conventional two-level DAB converter, commonly used modulation modes include: single phase shift modulation (SPSM), dual phase shift modulation (DPSM), extended phase shift modulation (EPSM), and triple phase shift modulation (TPSM), etc. Having three independent control variables, the TPSM serves as the most flexible modulation mode for the two-level DAB converter. For the two-level DAB converter, when the voltage transmission ratio is approximate to 1, that is, when an input/output voltage is approximately matched, an effective value of the current and a current stress are smaller, a soft switching range is wider, and a transmission efficiency is higher. The voltage transmission ratio is calculated by multiplying the output voltage by the transformation ratio and dividing by the input voltage. When a port voltage of the converter changes within a large range, for example, when the voltage of a vehicle-mounted power battery sometimes may vary within a wide range of 200V˜450V, in such an application scenario, even if an optimized TPSM modulation technology is employed, the two-level DAB converter still has larger current stress and larger RMS(root mean square) value of the current in some working regions, then the energy transmission efficiency is low, and the converter still needs to be further improved in comprehensive performance.

To ensure the converter to adapt to an ultra-wide port voltage range, the hybrid five-level bidirectional converter may be additionally equipped with a five-level unit so as to obtain extra intermediate levels, for which duty ratios may be independently regulated, one quarter of the input voltage, one half of the input voltage, and three-quarters of the input voltage respectively. When the voltage transmission ratio is in the range of one quarter to one, the duty ratios of the intermediate levels may be optimized such that the converter can work in the state of approximate voltage match, which can significantly reduce the current stress and the RMS value of the current during operation, improve the conversion efficiency of input/output power within the entire voltage range and power range, and implement global optimization operation.

SUMMARY OF THE INVENTION

In view of the abovementioned technical problems, the present disclosure provides a hybrid five-level bidirectional DC/DC converter and a voltage match modulation method thereof. The hybrid five-level bidirectional DC/DC converter has a topological structure, which can adapt to workplaces where port voltage has an ultra-wide range variation, can efficiently reduce the RMS value of current and a current stress for an isolation transformer in the converter, and can improve an operation performance of the DAB.

To solve the aforementioned technical problems, the present disclosure adopts following technical solutions.

An aspect of the present disclosure provides a hybrid five-level bidirectional DC/DC converter, which includes a first input filter capacitor Cinpand a second input filter capacitor Cinn, an output filter capacitor Co, a DC voltage source, a primary-side hybrid five-level unit, a primary-side two-level half bridge, a secondary-side single-phase full bridge H2, a high-frequency isolation transformer M1, a high-frequency inductor Ls, and a controller, meanwhile,

a positive pole of a DC bus of the primary-side hybrid five-level unit is coupled to a positive pole of the corresponding DC voltage source and to a positive pole of the input filter capacitor Cinprespectively. A negative pole of the DC bus of the primary-side hybrid five-level unit is coupled to a negative pole of the corresponding DC voltage source and to a negative pole of the input filter capacitor Cinnrespectively. A terminal of the primary-side hybrid five-level unit is coupled to a midpoint between the first input filter capacitor Cinpand the second input filter capacitor Cinnconnected in series. The primary-side hybrid five-level unit is coupled to a primary side of the high-frequency isolation transformer M1through the high-frequency inductor Ls, and a midpoint of the primary-side two-level half bridge is coupled to another terminal of the primary side of the high-frequency transformer;

a positive pole of a DC bus of the secondary-side single-phase full bridge H2is coupled to a positive pole of a corresponding DC load and to a positive pole of the output filter capacitor Corespectively. A negative pole of the DC bus of the secondary-side single-phase full bridge H2is coupled to a negative pole of the corresponding DC load and to a negative pole of the output filter capacitor Corespectively, and an AC side of the secondary-side single-phase full bridge H2is coupled to a secondary side of the high-frequency isolation transformer.

an input terminal of a control signal of the primary-side hybrid five-level unit, an input terminal of a control signal of the primary-side two-level half bridge, and an input terminal of a control signal of the secondary-side single-phase full bridge are respectively coupled to an output terminal of a switch signal corresponding to the controller.

Preferably, a transformation ratio of the high-frequency isolation transformer M1is N:1.

Preferably, nine levels are produced by an AC port voltage vpof the primary-side five-level unit:

±vin,±3⁢vin4,±vin2,±vin4
and 0, wherein vinrepresents an input DC voltage of the hybrid five-level bidirectional DC/DC converter. An AC port voltage vsof the secondary-side single-phase full bridge H2produces three levels as follows: ±voutand 0, wherein voutrepresents an output voltage of the hybrid five-level bidirectional DC/DC converter.

Preferably, the controller includes a sampling unit, a PI controller, and a modulation unit, wherein,

the sampling unit includes two signal input terminals, and the two signal input terminals respectively measure the input voltage vinand the output voltage vout.

the PI controller uses an error obtained by comparing the output voltage voutand a reference voltage vrefthereof as an input, and uses an output x obtained after being processed by the PI controller as a modulation signal.

the modulation unit uses the input voltage vim, the output voltage vout, the reference voltage vrefand the output x as input signals, and obtains a voltage transmission ratio F and a drive signal of each switch based on the input voltage vin, the output voltage vout, and the transformation ratio N of the high-frequency isolation transformer.

Another aspect of the present disclosure provides a voltage match modulation method for the aforementioned hybrid five-level bidirectional DC/DC converter, including:

setting a PI controller parameter of a controller, specifically, setting a proportionality coefficient kpand an integration coefficient kifor the controller, and calculating an output x of the PI controller to serve as a modulation signal;

outputting the voltage transmission ratio F and the output x of the PI controller respectively to a modulation unit of the controller to calculate a control variable; and

driving, based on the control variable, a fully-controlled switch device of the primary-side hybrid five-level unit, a fully-controlled switch device of the primary-side two-level half bridge, and a fully-controlled switch device of the secondary-side single-phase full bridge to control actions of the fully-controlled switch devices to implement optimized operation.

Preferably, the voltage transmission ratio F is:

F=Nvoutvin

wherein the transformation ratio N of the high-frequency isolation transformer serves as an initial value presetting.

Preferably, the output x of the PI controller is:
x=kp(vref−vout)+ki∫(vref−vout)dt

wherein 0.1≤kp≤10, 0.001≤ki≤1, vrefrepresents a reference voltage; and the output x of the PI controller ranges from −0.5 to 1.

Preferably, the number of the control variable is six: Dp0, Dp2, Dp3, Dp4, Ds0, and Dss; wherein,

Dp0controls a zero level of the primary-side hybrid five-level unit;

Dp2controls a ½ level of the primary-side hybrid five-level unit;

Dp3controls a ¾ level of the primary-side hybrid five-level unit;

Dp4controls a ±vinlevel of the primary-side hybrid five-level unit;

Ds0controls a zero level of the secondary-side single-phase full bridge H2; and

Dsscontrols a relative phase shift between the primary-side hybrid five-level unit and the secondary-side single-phase full bridge H2.

Preferably, the step of calculating the control variable includes:

classifying, based on a value of the voltage transmission ratio F, the converter into five operation conditions to calculate six control variables:

in the first operation condition, 0<F<¼, specifically, 4×Nvout<vin, calculation of each control variable being as follows:
Dss=x
Ds0=0
Dp4=0
Dp3=0
Dp2=0
Dp0=1−4F

in the second operation condition, ¼<F<½, specifically, vin<4×Nvout<2vin, calculation of each control variable being as follows:
Dss=x
Ds0=0
Dp4=0
Dp3=0
Dp2=4F−1
Dp0=0

in the third operation condition, ½<F<¾ specifically, 2vin<4×Nvout<3vin, calculation of each control variable being as follows:
Dss=x
Ds0=0
Dp4=0
Dp3=4F−2
Dp2=3−4F
Dp0=0

in the fourth operation condition, ¾<F<1, specifically, 3vin<4×Nvout<4vin, calculation of each control variable being as follows:
Dss=x
Ds0=0
Dp4=4F−3
Dp3=4(1−F)
Dp2=0
Dp0=0

in the fifth operation condition, 1<F, specifically, vin<Nvout, calculation of each control variable being as follows: and

Dss=x⁢⁢Ds⁢0=1-1FDp⁢⁢4⁢=0Dp⁢3=0Dp⁢2=0Dp⁢0=0

based on the aforementioned modulation method, following equation is satisfied such that an input/output voltage is approximately matched;
∫0t5vp(t)dt=∫0t5Nvs(t)dt

wherein vprepresents an AC port voltage of the primary-side five-level unit, and vsrepresents an AC port voltage of the secondary-side single-phase full bridge H2.

Preferably, the modulation unit is set based on:

a duty ratio Dp0of the zero level of the primary-side five-level unit;

a duty ratio Dp2of the ½ level of the primary-side five-level unit;

a duty ratio Dp3of the ¾ level of the primary-side five-level unit;

a duty ratio Dp4of the ±vinlevel of the primary-side five-level unit;

a duty ratio Ds0of the zero level of the secondary-side single-phase full bridge H2; and

a relative phase shift Dssbetween the primary-side five-level unit and the secondary-side single-phase full bridge H2.

A drive signal pulse is inputted in chronological order, and the fully-controlled switch device of the hybrid five-level unit and the fully-controlled switch device of the primary-side two-level half bridge H2are controlled to complete a modulation process.

Adoption of the present disclosure has the following beneficial effects.

(1) According to the hybrid five-level bidirectional DC/DC converter and the voltage match modulation method thereof provided by the present disclosure, compared with a full five-level bidirectional DC/DC scheme, the hybrid five-level scheme may reduce the number of semiconductor devices, reduce costs, and enhance system reliability.

(2) According to the hybrid five-level bidirectional DC/DC converter and the voltage match modulation method thereof provided by the present disclosure, a control objective is to reduce the RMS value of the current flowing through the isolation transformer of the DAB converter, to reduce system loss and improve the power conversion efficiency.

(3) According to the hybrid five-level bidirectional DC/DC converter and the voltage match modulation method thereof provided by the present disclosure, six-degree-of-freedom global optimization control is implemented based on six control variables as follows: a duty ratio of the zero level of the primary side of the hybrid five-level bidirectional DC/DC converter; a duty ratio of a high level of the primary side, a duty ratio of the ½ level of the primary side, a duty ratio of the ¾ level of the primary side, a duty ratio of the zero level of the secondary side, and a relative phase shift between the primary side and the secondary side. By coordinating the six control variables of the converter, the RMS value of the current is reduced, and the operation efficiency of the converter is improved.

(4) According to the hybrid five-level bidirectional DC/DC converter and the voltage match modulation method thereof provided by the present disclosure, a voltage range of an input/output side may be increased, a conduction loss may be reduced, and an energy conversion efficiency within an ultra-wide voltage range may be improved.

