Research and modeling of a new bidirectional DC/DC topology based on lithium battery formation

With the development of society, the contradiction between energy, environmental protection and development has become increasingly prominent. The development of lithium batteries can effectively improve this problem. Lithium batteries are widely used in electric vehicle energy systems, aerospace power systems, solar photovoltaic power systems, mobile communication systems and mobile terminal devices due to their high working voltage, small size, light weight, no memory effect, no pollution, low self-discharge and long cycle life. Bidirectional DC/DC converters are an important part of managing the charging and discharging of lithium batteries, and their working performance directly affects the efficiency and life of lithium batteries.

At present, there are two main types of topological structures of bidirectional DC/DC converters: non-isolated converters and isolated converters. Non-isolated Buckboost converters have high efficiency and simple structure, but have no isolation capability and cannot be used in situations where the input and output voltage difference is large. Isolated converters have bidirectional push-pull structures, bidirectional half-bridge structures and bidirectional full-bridge structures.

Among them, the push-pull structure is more efficient than the half-bridge bidirectional DC/DC structure. When the input voltage on the high-voltage side is large, the switch tube is subjected to large voltage stress, and the transformer winding is complex; the half-bridge structure transformer has no center tap and is simple in design. When the voltage on the low-voltage side is low, the boost capability is insufficient during the boost conversion process due to capacitor voltage division; the full-bridge structure has the highest efficiency and can achieve soft switching control, but the control circuit is complex and the cost is high. This paper proposes a bidirectional DC/DC converter based on digital control, which adopts a two-stage conversion structure, one stage adopts fixed pulse drive; the other stage adopts dual closed-loop control, which can effectively convert between 3V lithium battery voltage and 400V power supply voltage.

1 Bidirectional DC/DC main circuit structure and working principle

This paper adopts a two-stage bidirectional DC/DC converter structure, as shown in Figure 1. The first stage adopts an isolated half-bridge conversion structure, using a transformer to isolate the high-voltage side from the low-voltage side. The switch tubes V1, V2, V3, and V4 use fixed pulse control to achieve conversion from 400 V bus voltage to 20 V intermediate voltage. The second stage adopts a non-isolated Buckboost converter. The switch tubes V5 and V6 use closed-loop control to achieve secondary conversion between 20 V intermediate voltage and 3 V lithium battery voltage.

1.1 Buck working mode

The bus side input voltage is 400 V, and after C1 and C2 voltage division, the upper and lower bridge arm input voltage is 200 V. The controller sends a fixed pulse to TG1 and TG2 to make the switch tubes V1 and V2 work in the switching state. The diodes in V3 and V4 and D3 and D4 form a full-wave rectifier circuit, which is filtered by C0 to reduce the voltage from 400 V to 20 V; the closed-loop controller outputs a PWM signal to the switch tube V5, so that V5, D6, L1, and C11 form a Buck converter to reduce the voltage from 20 V to 3 V. The output voltage can be adjusted by adjusting the duty cycle of the drive waveform of the input switch tube V5. The relationship between the input voltage and the output voltage during the step-down conversion is:

Where: N 1 is the number of turns on the high-voltage side of the transformer; N2 is the number of turns on the low-voltage side of the transformer, V400 is the input voltage on the high-voltage side; D1 is the input pulse duty cycle of the switch tube V5.

Figure 1 Two-stage bidirectional DC/DC main circuit Figure

1.2 Boost working mode

The battery side inputs 3 V voltage, which is filtered by C11 and sent to the boost converter composed of V6, D5, L1, and C0. The boost converter increases the voltage from 3 V to 20 V. By adjusting the duty cycle of the pulse sent to V6, the output voltage can be adjusted; the voltage boosted to 20 V by the first-stage converter is divided by C3 and C4 and sent to the half-bridge converter, giving a fixed pulse to TG3 and TG4, so that the switch tubes V3 and V4 work in the switching state, and the voltage is boosted to 200 V by the transformer. The internal diodes of V1 and V2, D1, D2, and C1 and C2 form a full-wave voltage doubler rectifier circuit to stabilize the output voltage at 400 V.

The relationship between the input voltage and the output voltage during the boost conversion is:

Where: N1 is the number of turns on the high-voltage side of the transformer; N2 is the number of turns on the low-voltage side of the transformer; Vbat tery is the battery voltage; The input pulse duty cycle of the D2 switch tube V6.

2 Digital control system design

With the improvement of battery performance, higher requirements are put forward for the formation power supply. The formation power supply is required to have not only high precision and high reliability, but also small size, high safety, strong networking ability, fast charge and discharge response speed, and no impact in the process to extend the service life of the battery. The traditional analog formation power supply can no longer meet these new requirements. In addition, due to the limitations of lithium battery production process, small-capacity lithium batteries are usually used in parallel, which requires multiple bidirectional DC/DC converters to be used in parallel in large-scale formation equipment to realize the formation of large-scale lithium batteries. In order to complete the management and monitoring of multi-point lithium batteries, the bidirectional DC/DC converter designed in this paper uses dsPIC30F2010 as the core control device. dsPIC30F2010 is a high-performance 16-bit microprocessor with only 28 pins. It adopts Harvard architecture and has 1 16-bit CPU and 1 DSP core.

The peripheral resources of dsPIC30F2010 include 6 PWM output channels; 3 16-bit timer/counters, which can be paired to form a 32-bit timer module; 4 16-bit capture input functions, 2 16-bit comparison/PWM outputs; 1 addressable UART module with FIFO buffer; 6 10-bit analog-to-digital converters (A/D), 500 KS/s (for 10-bit A/D) conversion rate. In this design, the chip completes the control of each switch tube, lithium battery current, voltage, temperature measurement, equipment working condition display, host computer communication and other functions, and the structure is shown in Figure 2.

Figure 2 Hardware structure diagram

In order to ensure the stability of the output voltage and current of the two-stage converter, this design adopts average current control. The principle of average current control is shown in Figure 3. This control method adopts voltage outer loop control and current inner loop control. Ur is a given reference signal, Uback is the output voltage of the non-isolated Buckboost converter, Ur and Uback are output to the proportional integrator after the error device to obtain the reference signal Ir of the current loop; the inductor current Iback of the non-isolated Buckboost converter is obtained through the current sampling resistor, and Iof is obtained through the proportional device. After passing through the operator and the proportional integrator, the error voltage Ue is obtained. The error voltage Ue is compared with the triangle wave Tr1 to obtain the PWM wave, which controls the conduction or cutoff of the switch tubes V5 and V6.

Figure 3 Average current control diagram

During software design, set the PMOD position of the PWMCON1 register to 1 to set the PWM of the dsPIC30F2010 to independent working mode; set the PT MR register to obtain the reference time base, configure the value of the period register PTPER, obtain the required frequency of the triangle wave, send the AD sampling result to the PDC, and set the duty cycle. 3 Simulation experiment and result analysis

The SIMULINK module in Mat lab is used to simulate and verify this design. The first-stage half-bridge structure adopts open-loop control, and the second-stage non-isolated Buckboost adopts outer voltage loop and inner current loop control. As shown in Figure 4.

Figure 4 Overall simulation circuit diagram

In the half-bridge converter structure, in order to prevent the upper and lower bridge arms from being turned on at the same time, a certain dead time needs to be set to allow the upper and lower bridge arms to be turned on alternately. The switching frequency f = 50 kHz, the duty cycle of the upper and lower bridge arms is 0.3 respectively, the input voltage is 400 V, under the conditions of ripple current of 10% and ripple voltage of 1%, the calculated output filter inductance is L1 = 25 H, the output filter capacitor is C11 = 612.5 F, the load resistance R5 = 0.15, and the transformer ratio is Ns/Np = 20.

Figure 5 shows the transformer primary and secondary voltage waveforms and the battery charging voltage and current waveforms during step-down. As shown in Figure 5 (a), when the input voltage is 400 V, due to the primary capacitor voltage division, the voltage amplitude on the primary winding is 200 V, and the secondary winding voltage is 10 V. Figure 5 (b) shows the output voltage and current waveforms. After a rising period after the start-up, the charging voltage stabilizes at 3 V, charging current is stable with 20 A.

Figure 5 Step-down simulation result waveform Figure

6 shows the battery side discharge current and voltage waveforms, bus side current and voltage waveforms during boost, as shown in Figure 6 (a) battery discharge voltage and current waveforms, the battery discharge voltage is 3 V, and the output current is stable at 20 A after one cycle; Figure 6 (b) bus side voltage and current waveforms during discharge, the output voltage is 400 V after one cycle, and the current is constant at 0.05 A.

Figure 6 Boost simulation result waveform

4 Conclusion

This paper proposes a two-stage bidirectional DC/DC topology. The circuit structure of this topology is simple. It is composed of a bidirectional half-bridge converter and a bidirectional Buckboost converter. It improves the shortcoming of poor conversion performance of a single-stage half-bridge bidirectional converter when the voltage input is low. By performing boost simulation on the converter when the low voltage side is 3 V and buck simulation on the high voltage side at 400 V, the analysis results show that the bidirectional DC/DC topology can achieve conversion of large input and output voltage differences.