Research on Multi-channel Bidirectional DC/DC Converter

1 Introduction

In recent years, environmental and energy issues have become hot issues of concern to countries around the world. With the worsening of environmental pollution and energy crisis, hybrid power systems that can significantly reduce fuel consumption and pollution have become an important research direction in the field of new power vehicles. In hybrid power systems, bidirectional DC/DC converters are an important link in energy flow. Traditional bidirectional DC/DC converters used in this system are single-channel, and often require multiple bidirectional DC/DC converters to connect multiple energy storage devices such as batteries and supercapacitors to the DC bus, which inevitably increases the number of inductors and switches. When energy is transferred between multiple energy storage devices and the DC bus, each bidirectional DC/DC converter needs to work in a certain logical order, which requires multiple control signals, so the structure and control method of this system are very complex.

Here, a multi-channel bidirectional DC/DC converter is proposed. This converter not only simplifies the system structure, but also uses solar energy to charge the energy storage devices in the system when the load is not working. By replacing the diode with a power MOSFET, the energy can be transferred between multiple energy storage devices and the DC bus through only one bidirectional DC/DC converter.

2 Topology and Working Principle

Figure 1 shows a multi-channel DC/DC converter. Its topology is essentially a bidirectional Buck-Boost circuit. VQ1, VQ3 and VQ5 control whether energy can pass through the supercapacitor (SC) channel, DC Bus channel and battery (VRLA) channel respectively. VQ6 and VQ7 form a bidirectional switch. Among them, only VQ2 or VQ4 works in PWM mode, and the other switches mostly work in normally open or normally closed state, cooperating with each other to control the flow of energy between channels.

When the energy storage device transfers energy to the DC bus terminal, the circuit works in the Boost state in the forward direction. VQ4 is the main switch and works in PWM mode. Apply opening signals to VQ1 and VQ5 respectively, so that VQ1 and VQ5 work in the reverse conduction state, VQ2 works in the synchronous rectification state, and VQ3 remains on. At this time, if VQ7 is turned on, the current will flow into the PV channel, the DC bus will not be able to obtain the maximum power, and the photovoltaic cell will be damaged; if VQ6 is turned on, current will flow out of the PV channel, interfering with normal power transmission, so the bidirectional switch composed of VQ6 and VQ7 should be in the off state to avoid the occurrence of the above two unallowed states. The equivalent circuit at this time is shown in Figure 2.

When the circuit works in reverse in the Buck state, the energy storage device is in the charging state. At this time, VQ2 is the main switch and works in the PWM mode. If the load is in the working state, the energy is fed back from the DC bus terminal, and a trigger signal with a duty cycle of 1 is applied to VQ3, making it work in the reverse conduction state. VQ4 works in the synchronous rectification state.

The control of VQ1 is determined by judging whether the SC channel is full. If it is not full, VQ1 is turned on and VQ5 is turned off, and energy can be fed back to the SC channel from the DC bus direction; if it is full, VQ1 is turned off and VQ5 is turned on, and energy is fed back to the VRLA channel from the DC bus direction. If the energy storage devices are all full, VQ1 is kept off and VQ5 is turned on, so that VRLA is in a floating charge state. VQ6 and VQ7 remain off to avoid the previously mentioned impermissible state. The equivalent circuit at this time is shown in Figure 3.

If the load is not in working state, and the DC bus side has stopped feeding back energy to the energy storage device, VQ3 is turned off, VQ6 is forward-conducted, and VQ7 is reverse-conducted, and energy flows from the PV channel to the energy storage device. The working conditions of other switch tubes are the same as when the system is in working state. At this time, the energy storage device is charged by the photovoltaic cell, and while using clean energy, it is ensured that the energy storage device will not be powered off. When the light is insufficient, the load needs to continue to work to charge the energy storage device. The equivalent circuit at this time is shown in Figure 4.

3 Modulation strategy of multi-channel bidirectional DC/DC converter

Synchronous rectification technology is a new technology to reduce the conduction loss of DC/DC converters. It uses power MOSFET with extremely low on-resistance to replace the rectifier diode, which can greatly reduce the switch loss under high current conditions. When the power tube is used as a synchronous rectifier, it is completely different from when it is used as a switch. The synchronous rectifier is to reverse the drain and source of the power tube. Because of its low reverse conduction impedance, when it is turned on, it is equivalent to short-circuiting its body diode, so the conduction loss of the converter is reduced. Usually, power MOSFET is used as a switch, so the reverse conduction characteristic is rarely used.

At the entrance of each channel in the converter, there is a switch tube to control whether energy can pass through this channel. If energy needs to flow into the channel, the switch tube is in the forward conduction state; if energy needs to flow out, the switch tube is in the reverse conduction state. The working principle at this time is similar to synchronous rectification. It uses the low conduction impedance of the synchronous rectifier to replace the voltage drop on the PN junction, so that part of the power MOSFET works in the reverse normal on state or forward off state, and the different working states of multiple power MOSFETs cooperate with each other to control the flow direction of energy.

In the forward boost circuit, if the converter is in the continuous inductor current condition and the converter loss is assumed to be zero, the output voltage average value expression is:

Uo=Uin／(1-D) (1)

From the definition, we know that the duty cycle D≤1, D=Ton／(Ton+Toff). Therefore, the Boost circuit is different from the Buck circuit, and D cannot be equal to 1. In the reverse Buck circuit, the output voltage expression when the inductor current is in the continuous working mode is:

Uo=HU (2)

There is no special restriction on the duty cycle in formula (2). In the Buck circuit, the inductor stores energy when VQ2 is turned on, and forms a release loop through the synchronous rectifier when the switch tube is turned off. During the operation of the Boost circuit, the output needs to be connected to the load, otherwise the energy stored in the inductor cannot be consumed, which will cause the output voltage to increase. This is also different from the Buck circuit.

4 Experimental Results

According to the above analysis, in order to verify the feasibility of the principle, a 50 W experimental prototype was made in the laboratory. The switch tube is IRF540N, and its on-resistance is 44 mΩ. The experimental parameters are: the output DC voltage range of the voltage regulator is 0-30 V, the load voltage range is 0-50 V, the maximum power is 50 W, and the switching frequency is 20 kHz.

The duty cycle of the driving signal is set to 0.4. In order to retain the dead time of the synchronous rectification signal and the driving signal, the duty cycle of the synchronous rectification signal is also set to 0.4. The driving waveform is shown in Figure 5a.

When the system is working in the Boost state, if the input voltage is 30 V and the load is 50 Ω. At this time, the input current is 1.7 A, the load voltage value is 49 V, and the circuit efficiency is calculated to be about 94.2%. The output voltage waveform on the load is shown in Figure 5b. The inductor current is in a continuous state, and its waveform is shown in Figure 5c. When the system is working in the Buck state, if the input voltage is 30 V and the load is 50 Ω. At this time, the input current is 0.1 A, the load voltage is 12 V, and the efficiency is calculated to be about 96%. The load output voltage waveform is shown in Figure 5d (top). The inductor current is in a continuous state, and its waveform is shown in Figure 5d (bottom).

5 Conclusion

By improving the structure of bidirectional DC/DC converter in traditional hybrid power system, a multi-channel bidirectional DC/DC converter is introduced here. The operation principle and control strategy are analyzed, and its feasibility is verified by experimental prototype. The converter can connect multiple energy storage devices and DC bus at the same time, which simplifies the structure of connecting energy storage devices and DC bus with multiple bidirectional DC/DC converters in traditional system. The synchronous rectification principle is applied and generalized, and the different states of each switch tube are used to cooperate with each other to control the direction of energy flow in the converter. The system efficiency is guaranteed while simplifying the structure.