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Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

Posted by: Sipxtech 2022-08-30 Comments Off on Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In these applications, switching power supplies are often required not only to control the bidirectional flow of energy, but also to achieve low-voltage, high-current output. When the switching frequency is not too high, as the output voltage decreases and the output current increases, the rectification loss becomes the main factor that affects the efficiency of the switching power supply. Therefore, in order to improve the efficiency of the switching power supply, it is necessary to try to reduce the rectification loss. The synchronous rectification technology used in this paper is an effective means to reduce the rectification loss.

Authors: Wu M, Jin Xinmin, Tong Yibin

In these applications, switching power supplies are often required not only to control the bidirectional flow of energy, but also to achieve low-voltage, high-current output. When the switching frequency is not too high, as the output voltage decreases and the output current increases, the rectification loss becomes the main factor that affects the efficiency of the switching power supply. Therefore, in order to improve the efficiency of the switching power supply, it is necessary to try to reduce the rectification loss. The synchronous rectification technology used in this paper is an effective means to reduce the rectification loss.

In the past power supply design, analog control technology has been widely used due to its advantages of fast dynamic response, no quantization error, and low price; while digital control technology is rarely used due to its cost and technical factors. In recent years, with the continuous development of semiconductor technology, the cost of digital microcontrollers has been significantly reduced and the performance has been continuously improved, which makes the full digitalization of high-frequency switching power supplies possible. Because digital control has many advantages such as simplifying system hardware, reducing the number of discrete components, and improving system reliability, it will be more and more widely used in the design of switching power supplies in the future.

1 System introduction

1.1 Basic description of the system

The overall composition of the system is shown in Figure 1. The dotted line in Figure 1 is the control part of the system. The rest is the main circuit part. The working principle of the main circuit will be analyzed in detail later.

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In the control circuit of the system, its core processor is the LPC2119 microcontroller based on the ARM7 core produced by PHILIPS (Philips). LPC2119 has many advantages such as high performance, low cost, low power consumption, etc. It is very suitable for industrial control fields that have strict requirements on cost and performance. Responsible for the A/D conversion is the 24-bit high-precision A/D converter CS5460A, which also has the characteristics of low cost and high performance, and has been widely used in various products in the past.

When the control circuit is working, after the CS5460A obtains the analog signals of the system output voltage and current, it converts them into digital quantities and transmits them to the LPC2119 through a dedicated bus. After the LPC2119 obtains these information, it will first perform software processing such as digital filtering, and then use it as a feedback quantity for the operation of the control algorithm to obtain the control quantity and its corresponding driving signal. Finally control the action of the main circuit switch tube.

1.2 Software realization of bidirectional DC/DC full digital control

As mentioned above, for the consideration of simplifying the control circuit structure and increasing system reliability, the system adopts a fully digital design with the ARM chip LPC2119 as the core of the control system. To achieve the desired control function, in addition to the basic control circuit described above, perfect and reliable control software and appropriate control strategies are also indispensable.

In terms of control software, the author develops the system control software based on the design general principles of clear hierarchy, time sequence classification, overall consideration, and writing specifications. According to the actual needs of the power Electronic software, the program is divided into three layers as a whole, namely the main control layer, the algorithm layer and the interface layer. The interface layer is the bottom layer, the main control layer is the top layer, and the algorithm layer acts as an intermediate bridge connecting the main control layer and the interface layer. Specifically, the main control layer does not involve specific operations, and is only responsible for the scheduling of each task, the scheduling of interrupts, and the processing of time and priority. The main control layer has a file, including the main function and the interrupt function. The functions of the algorithm layer are called in the main function and the interrupt function to realize the functions of the system. The algorithm layer is responsible for the execution of specific tasks, the realization of control algorithms, and the main functions of the system are all reflected in the algorithm layer. The interface layer is responsible for the interface with the hardware, and all operations related to peripherals are processed in this layer.

In terms of control strategy, this paper chooses the incremental digital PI algorithm.The main advantages of the incremental PI algorithm are

(1) The incremental algorithm does not need to accumulate, and the determination of the control quantity is only related to the recent error sampling values, that is, the error does not accumulate.

(2) The output of each time is the increment of the control amount, and the influence of misoperation is small.

In the PI algorithm, the proportional part can improve the dynamic performance of the system, and the integral part can reduce the steady-state error of the system, and theoretically, the output without static error can be realized.The digital PI algorithm expression after discretization is

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In the formula: KP is the proportional coefficient;

K1 is the integral coefficient;

e(k) is the current error;

u(k) is the output of this control quantity.

By recursion of formula (1), we can get

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

The expression of the incremental digital PI control algorithm can be obtained by subtracting the formula (2) from the formula (1) as follows:

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

△u(k) in the formula (3) is the increment of the control quantity output by the digital regulator. Therefore, the control quantity of the final output of the control algorithm is:

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In the actual work of the device, if the load is a lead-acid battery, when the energy is flowing (charging) in the forward direction, the system can apply voltage closed loop or current closed loop to control the output voltage and output current of the device as required. The voltage and current closed loop adopts the incremental PI algorithm described in this paper; when the energy flows in the reverse direction, the system only performs constant current control on the load (battery) side for practical application needs.

2 Analysis of the working process of the circuit

The main circuit topology proposed in this paper is shown in Figure 2, which mainly includes: the filter capacitor C1 on the input side of the power supply; the main switch tube S1 and the SNUBBER circuit of S1 composed of R1, C2, and D2; the transformer T and its primary side for magnetic reset The third winding and the clamp circuit composed of R2, C3, D3 for magnetic reset for its secondary side; rectifier tube S2, freewheeling tube S3 and output filter links L and C4 and other parts.

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

2.1 Analysis of the working process when the energy is flowing forward

For the convenience of analysis, it is assumed that the load is a battery at this time. When the circuit controls the forward flow of energy, the work of each cycle of the main circuit can generally be divided into two stages, namely the forward flow stage and the freewheeling stage. However, in order to prevent the rectifier tube S2 and the freewheeling tube S3 from being turned on at the same time and causing the secondary side of the transformer to be short-circuited, the complementary pulses of the two tubes need to be added to the dead zone, so the final working process of the circuit can be divided into 4 parts. The driving signals of the main tube S1, the rectifier tube S2, and the freewheel tube S3 are shown in FIG. 3, and 1 to 4 in FIG. 3 correspond to the four stages of the circuit operation respectively.

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

When the energy flows in the forward direction, if the output current flows through the parasitic body diode of the MOS tube with a large tube voltage drop, it will bring about a large rectification loss and freewheeling loss. To this end, we have applied synchronous rectification technology, so that the current flows through the MOS tube with only 6mΩ on-resistance, which greatly reduces the loss and improves the efficiency. The following is a detailed analysis of the 4 working stages when the energy is flowing forward.

Phase 1 (Energy Forward Flow) At the beginning of this phase, the main pipe S1 and the rectifier S2 are triggered to conduct. The input current flows into the homonymous end of the primary winding of the transformer, and the output current flows out of the homonymous end of the secondary winding of the transformer. At this time, the way of energy transmission from the input side to the load side is basically the same as that of the traditional single-ended forward converter, and its current flow is shown in Fig. 4(a). i1 in Figure 4(a) represents the primary current of the transformer, and i2 represents the secondary current of the transformer (the same below). This process does not end until the mains switch off.

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

Stage 2 (Dead Time 1) At the beginning of this stage, the main tube S2 and the rectifier tube S2 are turned off, and the freewheel tube S3 is still not turned on but its body diode has been turned on. Due to the limitation of the leakage inductance of the transformer, the current on the secondary side of the transformer gradually decreases from the output current, while the current flowing through the body diode of the freewheeling tube increases gradually from zero. In this stage, the output current is converted from the rectifier loop to the freewheeling loop. The current direction for this process is shown in Fig. 4(b). i2a and i2b in Figure (b) represent the two parts of the load current flowing through the rectifier tube and the freewheeling tube, respectively.

Stage 3 (freewheeling stage) At the beginning of this stage, the freewheeling tube S3 is triggered to conduct, so the output current is mainly freewheeling through S3, so the loss is greatly reduced. This stage will continue until the freewheeling tube S3 is turned off, and its current flow is shown in Figure 4(c).

Stage 4 (Dead Time 2) At the beginning of this stage, the freewheeling tube S3 is turned off, but its body diode is still conducting freewheeling. The output current is completely freewheeling through the body diode of the freewheeling tube. This stage will not be terminated until the supervisor is turned on. The current direction of this process is shown in Fig. 4(d). So far, one cycle of work of the main circuit has ended. When the circuit operates next time, the main tube S1 and the rectifier tube S2 will be turned on again, and the circuit will re-enter the working state of stage 1.

2.2 Analysis of the working process when the energy flows in reverse

When the energy flows in the reverse direction, the working process of the circuit is basically the same as that of the BOOST circuit, which can be roughly divided into two stages.

Stage 1 (freewheeling) In this stage, the freewheeling tube is turned on, the rectifier tube is turned off, the battery discharge current i1 flows through the Inductor coil L, the current increases linearly, and the electrical energy is stored in the inductor coil L in the form of magnetic energy. The current direction of this process is shown in Fig. 5(a).

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

Stage 2 (reverse discharge) During this stage, the freewheeling tube is turned off and the rectifier tube is turned on. The inductor L converts the stored magnetic energy into electrical energy and discharges it to the input side together with the battery. Its current flow is shown in Fig. 5(b).

2.3 Selection of Transformer, Inductor and Capacitor Parameters

Considering many factors such as power supply volume, system efficiency, control accuracy, device withstand voltage, etc., the operating frequency selected in this paper is f=55 kHz, T=1/f, and the maximum duty cycle Dmax is 0.4, then the maximum conduction of S1 is in charge of The time toNmax is

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

3.1 Calculation of Transformer

The transformer secondary voltage Vs is calculated according to formula (6).

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In the formula: Vo represents the output voltage;

Vf represents the tube voltage drop on the secondary side of the transformer and the voltage drop across the output filter inductor.

Then the minimum voltage on the secondary side of the transformer should be

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

If the minimum value of the input voltage Vp is VPmin, then the transformation ratio n can be obtained as

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In the formula: Bm is the maximum working magnetic flux density of the iron core;

S is the effective cross-sectional area of ​​the transformer core.

Therefore, the number of turns N1 of the primary winding of the transformer can be obtained as:

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

When calculating the third winding, the reset voltage Vr should first be calculated according to the principle of volt-second product balance as

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

In the formula: tDFFmin is the shortest turn-off time of the supervisor S1;

VPmax is the maximum input voltage.

Then the number of turns N3 of the third winding responsible for the reset of the magnetic flux on the primary side of the transformer can be obtained as

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

2.3.2 Calculation of output filter inductance L

To calculate the inductance of the output filter inductor, first determine the size of the current ΔIL flowing through the inductor. Considering the dimensions, cost, and transition response of the inductor coil, it is more appropriate for ΔIL to take 10% to 30% of the output current Io. In this paper, in order to better limit the ripple content in the output current, ΔIL is taken as 10% of the output current Io. To sum up, the size of the inductance L can be obtained from the formula (13).

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

2.3.3 Calculation of output capacitor C4

The size of the output capacitor is determined by the limit of the output ripple voltage suppression, that is, determined by ΔIL and the equivalent series resistance ESR of the output capacitor. Usually the output ripple voltage is taken as 0.3%~O of the output voltage. 5%, in this paper, the ripple voltage is taken as 0.3%.Therefore, it can be obtained

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

After the ESR is obtained, the appropriate filter capacitor can be selected according to the product manual provided by the manufacturer.

3 Experimental results

IRL3803, the diode used in the magnetic reset circuit of the primary side of the transformer is BYV26G of PHILIPS (Philips). It must be pointed out here that when selecting the rectifier tube and the freewheeling tube, in addition to considering the withstand voltage and current capacity of the power device, special attention should also be paid to the size of the on-resistance value. The IRL3803 selected in this article is a MOS tube specially used for synchronous rectification introduced by IR Company. The on-resistance is only 6mΩ, which can minimize the conduction loss and thus reduce heat generation. Calculated by formula (5) to formula (14), the transformation ratio of the primary side, the secondary side and the third winding of the transformer is 170:3:255; the output filter inductance is 14.72μH; the capacitance is 9900μF. The load is a single lead-acid battery.

The main technical conditions of the experiment are as follows: the switching frequency is 55kHz. When working in the forward direction, the input voltage Vi is 400(1±5%)V, the rated output voltage Vo is 2V, and the output current is 20A; when working in the reverse direction, the input voltage is 2(1±10%)V.

After testing, the accuracy of voltage stabilization and current stabilization can be less than O when the system is working. 5% of the design requirements; the highest efficiency of the device is 86.7%. The main experimental waveforms are shown in Figures 6 to 8; when the energy flows forward/reversely, the efficiency curve of the system is shown in Figure 9.

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

Design of Bidirectional DC/DC Converter Based on ARM7 Core LPC2119 Chip

Figure 6 shows the measured waveforms of the driving signals of the rectifier tube and the freewheeling tube when charging the battery. At this time, the primary side main waveform is completely synchronized with the rectifier. In Fig. 6, channel 1 is the driving waveform of the rectifying power device, and channel 2 is the driving waveform of the freewheeling power device. Figure 7 shows the measured waveforms of the driving signals of the rectifier power device and the freewheeling power device when the energy flows in the reverse direction. At this time, the primary power device does not operate. In Figure 7, channel 1 is the driving waveform of the rectifier power device, and channel 2 is the driving waveform of the freewheeling power device. Figure 8 shows the experimental waveform of the DC/DC converter outputting a 2V voltage when the energy is flowing forward. As can be seen from Figure 8, the output voltage regulation accuracy is high and the voltage ripple is small.

4 Conclusion

This paper presents an all-digital bidirectional DC/DC converter based on the microcontroller LPC2119. Its main features are:

(1) Adopt full digital control, simple hardware design and high reliability;
(2) The application of synchronous rectification can effectively reduce the on-state loss, and the system efficiency is high;
(3) The output voltage stabilization and current stabilization accuracy are high, and the system control performance is good;
(4) The system cost is low.

Experiments have proved that the converter is correct in principle, safe and reliable in operation, and has good control performance. It can be applied to various occasions such as single battery charging/discharging where both output low voltage and large current and control energy flow in both directions. It has broad market prospects. .

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