Single-ended forward bidirectional DC/DC converter based on single-chip microcomputer control

1 Introduction
    In the application of aviation power supply system, electric vehicle and other vehicle power supply, shipboard power supply, battery energy storage and other applications, both sides are DC voltage or DC active load, where the input end is connected to the DC bus and the output end is connected to the energy storage device (battery). In this case, in order to realize charging and discharging, energy must be able to flow in both directions, so a bidirectional DC/DC converter is required. With the development of science and technology, the application of bidirectional DC/DC converters is gradually expanding, especially for occasions where batteries need to be charged/discharged. As a new form of DC/DC converter, the position of bidirectional DC/DC converters in industrial applications is becoming more and more prominent.
    The current trend of switching power supply development is low voltage and high current, which makes the use of synchronous rectification technology in the secondary rectification circuit a high-efficiency and low-loss method. The design of bidirectional DC/DC converter mainly considers the selection of main circuit topology and control mode. This paper introduces the design process of a bidirectional DC/DC converter with a single-ended forward conversion circuit as the main circuit and a C8051F020 microcontroller as the controller. The converter uses synchronous rectification technology and full digital control, making the entire design have the characteristics of simple circuit, high conversion efficiency, simple control, reliable operation, and bidirectional energy flow. The feasibility of the scheme has been verified through PSPICE simulation and prototype testing. This converter can be used for charging and discharging of various types of batteries and the core part of DC power supply.

2 Main circuit topology
    At present, the bidirectional DC/DC converter topology structure that is widely used has the disadvantages of complex circuit, more links in the energy transmission process, low converter efficiency, and difficulty in suppressing the switch tube voltage. The circuit of the single-ended forward converter is relatively simple and is one of the more commonly used methods in medium and small power supplies. Figure 1 shows the main circuit topology of the proposed bidirectional DC/DC converter.

    The system consists of transformer T and its magnetic reset circuit, main switch tube V1, rectifier tube V2 and freewheeling tube V3, output filter inductor L, capacitor C and other parts. Compared with common bidirectional DC/DC converters of the same power level, this topology has the characteristics of simple structure, low system cost, high working efficiency and simple control method, and has certain advantages in industrial applications.
2.1 Analysis of forward working process
    Figure 2 shows the main waveform of the converter when the forward working current is continuous. Its working process is divided into 4 stages.

    Phase 1 [0, t1] V1 and V2 are turned on. At t=0, V1 is turned on, and the power supply voltage Ui is applied to the primary winding N1, that is, uN1=Ui, so the core is magnetized and the core magnetic flux φ increases, that is; N1dφ／dt =Ui. In this switching mode, the increase of φ is:
    △φ(+)=UiDyTs／N1 (1)
    The excitation current iM of the transformer increases linearly from zero, and iM=Uit／Lm, Lm is the excitation inductance of the primary winding. Then the voltage on the secondary winding N2 is:
    uN2=N2Ui／N1=Ui／K12 (2)
    Where: K12 is the turns ratio of the primary and secondary windings, K12=N1／N2.
    At this time, V2 is turned on, V3 is turned off, and the filter inductor current iL increases linearly, which is the same as when the switch tube in the Buck converter is turned on, except that the voltage is Ui/K12, and: diL/dt=(Ui/K12-Ui)/L. [page]

    Phase 2 [t1, t2] V1 is in the off state. At t1, V1 is turned off, and no current flows through the primary and secondary windings. At this time, the transformer performs magnetic reset through the reset winding, and iM is fed back from the reset winding N3 to the input power supply through VD4. Then the voltage of the reset winding uN3 = -Ui. In this way, the voltages on the primary and secondary windings are: uN1 = -K13Ui, uN2 = -K23Ui. K13 is the turns ratio of the primary winding to N3, K13 = N1/N3; K23 is the turns ratio of the secondary winding to N3, K23 = N2/N3. At this time, V2 and V3 are turned off, and iL
    continues to flow through VD3. Phase 3 [t2, t3] V1 is still in the off state, V3 is turned on, which greatly reduces the conduction loss, and iL continues to flow through V3. This stage will last until V3 is triggered to turn off.
    Phase 4 [t3, t4] V3 is turned off, but its body diode is still turned on. The body diode continues to flow, and there is no current in all windings, and their voltages are all zero. This phase ends when V1 is triggered to turn on. At this point, a working cycle of the main circuit ends.
2.2 Analysis of the reverse working process
    The working process of the circuit in reverse working is basically the same as that of the Boost circuit, and can be divided into two stages. Its main working waveform is shown in Figure 3. At this time, V1 does not work.

    Phase 1 [0, t1] V3 is turned on, V2 is turned off, the battery is discharged, the current flows through L, iL increases linearly until t1, iL reaches the maximum value, and the electrical energy is stored in L in the form of magnetic energy. During the period when V3 is turned on, the increment of iL is:
   
    Phase 2 [t1, t2] V3 is turned off, V2 is turned on. L converts the magnetic energy into electrical energy and discharges it to the input side together with the battery. iL decays linearly until t2, when iL reaches the minimum value. During the period when V3 is turned off, the reduction of iL is:
   

3 Control system design
3.1 Control system structure and main hardware design
    The bidirectional DC/DC converter includes a power main circuit composed of power components, a control circuit and a drive circuit, etc., as shown in Figure 4.

    Considering the problem that the external input signal may cause a short circuit to the driving circuit, an integrated circuit drive form is adopted, and the IR2110 chip is selected. Since the output current cannot be directly obtained by the single-chip microcomputer, it is necessary to design a current detection circuit to accurately and timely measure the current value. The UGN-3501M Hall sensor is used here, which has the characteristics of high sensitivity and wide operating temperature range (-20～85℃). The detection circuit uses the integrated AD522 chip as the amplification stage. AD522 is a dual-end input and single-end output measurement amplifier with high input impedance, good linearity, and high accuracy.

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3.2 System Software Design
    The system operation is divided into two processes: step-down conversion and step-up conversion. In step-down conversion, the sampled voltage signal is converted by A/D, and the duty cycle is adjusted by the incremental digital PI algorithm to generate a PWM waveform to control the output voltage. In step-up conversion, the sampled current signal is converted by A/D, and the duty cycle is adjusted by the incremental digital PI algorithm to generate a PWM waveform to control the output current. The main program flow is shown in Figure 5.

4 System Simulation Analysis
    PSPICE is used here to simulate the main circuit of the system. The simulation parameters are: input voltage 400 V, output voltage 2 V, inductance 14.2 μH, capacitance 9 900 μF, switching frequency 55 kHz, transformer ratio 170:3, maximum duty cycle 0.4, load resistance 1 kΩ. Figure 6 shows the simulation waveform.

    From top to bottom in Figure 6a, the driving voltages of V1 to V3 when the energy flows forward and the driving voltages of V2 and V3 when the energy flows reversely are shown. It can be seen that when the energy flows forward, ugV1 and ugV2 are generated synchronously, ugV2 and ugV3 are complementary, and dead time is added; when the energy flows reversely, V2 and V3 are turned on alternately to ensure the normal transmission of energy, and the two also have overlapping conduction time to ensure that the current completes the necessary commutation.
    Figure 6b shows the output voltage Uo of the DC/DC converter when the energy flows forward and the output current Io waveform when the energy flows reversely. It can be seen that the system voltage has a good dynamic response and realizes energy conversion from 400 to 2 V. When the converter works in reverse, the output current of the battery remains constant, the ripple is small, and the inductance design is more accurate.

5 Experimental Analysis
    The main components and parameters of the experimental prototype are as follows: V1 uses a power MOSFET of model IXFN100N50P according to the working conditions such as the input voltage of 400 V; V2 and V3 use MOSFET tube IRL3803 specially used for synchronous rectification; the energy storage inductor L=14.2 μH; the output filter capacitor is 9900 μF; the load is a battery. The experimental results are shown in Figure 7. Figure 7a shows the PWM drive waveforms of V2 and V3 when charging the battery. Since V1 is synchronized with V2 at this time, it can be clearly seen that the two drive signals are complementary and have a dead zone, which is completely consistent with the theoretical analysis. Figure 7b shows the PWM drive waveforms of V2 and V3 when the energy flows in the reverse direction, and V1 does not work at this time. It can be seen from the experimental waveform that the switching frequency is approximately 55 kHz, and the duty cycle of PWM is approximately 0.4, realizing the bidirectional flow of energy.

6 Conclusion
    A bidirectional buck-boost DC/DC converter design based on single-chip microcomputer control is introduced in detail. Through simulation and experimental analysis, the feasibility of the converter scheme is verified. It works safely and reliably and has good power supply characteristics. The entire system has low cost and adopts full digital control, simple hardware design and high reliability, so it is convenient for applications that require bidirectional energy flow control.