As people pursue a healthy and close-to-nature lifestyle, the demand for outdoor portable power supply increases. According to the industrial development report of China Industrial Association of Power Sources, only 52,000 portable energy storage devices were shipped worldwide in 2016. It is estimated that the shipment will reach 4.83 million units in 2021, with the compound annual growth rate of 148%. At the same time, with the change of application occasions in portable power supply, the proportion of large-capacity batteries will increase year by year, and with it, the demand for matched power supply design will also rise from hundreds of watts to kilowatts.
This paper introduces our design considerations of a 3KW bidirectional converter that meet the demand for high-power portable energy storage products. At the beginning, we compare the performance of several power devices in the design and figure out that SiC devices can improve the performance of bidirectional converters because of their excellent switching characteristics and minimal reverse recovery performance of their body diodes. Next, we analyze the design challenges of the whole system, and figure out that meeting the gain requirements of the charging and inverter bidirectional DC/DC is a major difficulty. And next, we design a solution, and build a bidirectional LLC resonant network by adding a resonant inductor to meet the gain requirements of the bidirectional LLC. In the end, the test results and data are given.
Table 1 shows the design specifications of this design, and Fig. 1 shows the power circuit of this design. When working in the forward direction, it charges the battery. At the same time of charging, the output AC can be connected to the inverter output through the relay, the inverter port takes power from the input AC; In the case of inversion, the system first boosts the battery voltage to about 400V, and then inverts 400V DC into 220Vac sine wave AC for the load.
Table 1: Design Specifications of 3kW Bidirectional ConverterA
When the system works in the charging mode, the input AC voltage needs to be converted to DC400V by the front stage PFC circuit, and then the rear stage DC/DC converts the 400V DC voltage to the constant current source to charge the output battery.
The theoretical analysis of totem PFC is very mature. Let’s briefly review it here.
Fig. 2~ Fig. 5 shows the working mode of totem pole PFC. In the positive half cycle of AC, Q2 is the main switch and Q1 is the freewheeling switch, Q4 is always on and Q3 is always off. In the AC auxiliary half-cycle, Q1 is used as the main switch and Q2 as the freewheeling switch. At the same time, Q3 is always on and Q4 is always off.
It can be seen that:
The working frequencies of two bridge arms of totem PFC are different; Q1&Q2 constitute a high-frequency bridge arm, and its working frequency is the switching frequency set by us. The bridge arm composed of Q3&Q4 works in a low-frequency state, and its working frequency is the input AC frequency.
Within the positive and negative half of the totem PFC, the main switch and freewheeling switch need to switch.
Totem PFC works as a synchronous rectifier boost circuit, and it is usually designed in the state of CCM inductor current in high-power applications, which puts forward higher requirements for the performance of the device body diode at this position.
Mosfet has very low on resistance, good switching characteristics and low switching loss. However, in the design of totem pole PFC, since totem pole PFC can be equivalent to synchronous rectification boost circuit, when the switch works as a freewheeling switch, in order to prevent the upper and lower MOS from going short through, the inductor current will inevitably flow through the body diode, so the performance of the body diode will be an important consideration in this application design.
Fig. 6 selects the body diode parameters of the best Mosfet on the market at present. Its Qrr reaches 1.2uC. If the bus voltage of 400V and the switching frequency of 45KHz are considered, the loss caused by Qrr reaches 21.6W through calculation, which brings great efficiency reduction. More importantly, the problem of device temperature rise will become more serious. Therefore, the high-frequency switching MOS should not only have extremely low conduction and switching losses, but also have very good reverse recovery characteristics of its body diode.
IGBT devices are widely used in high-power design because of their low saturation conduction voltage drop and competitive cost advantages. However, IGBT also has its disadvantages. First, IGBT itself can’t work in the 3rd quadrant like Mosfet, so when it is used as a freewheeling switch, there is no freewheeling channel, and the inductor current can only freewheel through the anti-parallel diode; Secondly, the current tailing effect of IGBT increases the switching loss of IGBT and limits the switching frequency of IGBT. In order to overcome the above shortcomings, the hybrid IGBT of LUXIN Semiconductor is selected in this design. LUXIN Semiconductor provides optimized IGBT performance and integrates anti-parallel SiC diodes inside. The switching frequency of totem pole PFC in this design can be increased from 20KHz to 45KHz.
As the third-generation semiconductor, SiC Mosfet provides excellent performance with extremely low on-resistance and switching loss. At the same time, it can work in the 1st or 3rd quadrant. When freewheeling, the inductor current flows through the conductive channel, realizing real synchronous rectification. The body diode only works in the dead time of the upper and lower transistors, which greatly reduces the device loss. At the same time, the loss of reverse recovery of the body diode of SiC mosfet mentioned above is also very small. Therefore, SiC devices are very suitable for the high-frequency bridge arm of totem pole PFC.
Fig. 7 compares the forward voltage drop of a 50A IGBT and a 40 mω SiC Mosfet. It can be seen from the figure that the forward voltage drop of SiC devices is linear, which can be calculated by Rdson*Ids. At the same time, with the increase of temperature, the turn-on voltage drop will also increase because of the increase of Rdson. The voltage drop curve of IGBT is a nonlinear curve. It can be seen from the figure that within Vce voltage drop of 1.2V, SiC’s on-voltage drop is superior to IGBT’s. At the same time, IGBT has the lowest threshold voltage of 0.8V. It can be expected that the efficiency advantage of SiC devices is more obvious under light load.
Fig. 8 and Fig. 9 show the current loops of SiC and IGBT in freewheeling state. When IGBT works in freewheeling mode, the inductor current can only flow through its anti-parallel diode, because IGBT does not have the ability to reverse current. However, when the SiC device is used in the freewheeling state, the inductor current can flow through the conductive channel of SiC. Because the Rdson of SiC devices is low enough, the voltage drop caused by current flowing through Rdson will be much lower than that of IGBT’s anti-parallel diode, which will further improve the efficiency of SiC circuits.
As mentioned above, when working in the forward direction, totem pole PFC converts AC voltage into 400V DC bus, and the second stage DC/DC converts 400V bus voltage into constant current source to charge the battery. When working in reverse, the battery voltage is boosted by LLC, and the totem pole PFC circuit will work in an inverter state, converting the DC voltage into 220V AC voltage.
Different from the design of totem pole PFC, LLC’s device selection will be much simpler, because LLC can realize ZVS of devices and ZCS of rectifier diodes in a full range. The design difficulty of LLC lies in how to meet the system requirements of voltage gain in two directions. Table 2 shows the system gain requirements in two modes.
Table 2: Gain Design Requirements of LLC
According to the gain requirements in Table 2, the forward and reverse gain curves of LLC are drawn as follows.
Fig. 10 shows the gain curve of LLC in charging mode. It can be seen that the gain curve of LLC can meet the system design requirements under the condition of forward operation;
Fig. 11 shows the gain curve of LLC in Inverter mode. It can be seen from the figure that the gain of LLC is always less than 1 in inverter state, which can’t meet the gain requirements of the system.
In order to meet the bidirectional system gain requirements, there are several different solutions. The first method is to use roof of the world DC/DC converter, and add a synchronous rectification bidirectional buck boost circuit between PFC and LLC. Through the buck boost converter, the required bus voltage can be flexibly adjusted when working in both directions to support the design requirements in both directions, and LLC can also work at a better frequency point to realize the optimal LLC design. However, the increase of the third buck boost circuit will make the system and control more complicated and the cost higher.
In this design, a new method is adopted. Firstly, the bus voltage will be adjusted with the battery voltage. In order to further widen the adjustment range, 500V electrolytic capacitor is adopted. In the forward direction, the PFC voltage will be adjusted with the battery voltage in the range of 380V to 460V; When working in the reverse direction, the bus voltage will be adjusted in the range of 360V to 460V V. Secondly, in order to achieve the design goal of LLC reverse gain greater than 1, we add an inductor between the midpoints of the high-voltage bridge arms. This inductor, resonant inductor and resonant capacitor form a new resonant network. The specific circuit is shown in Fig. 12, when working in the forward direction, Lr,Cr and the excitation inductance Lm of the transformer form a forward LLC resonant network; In reverse, the increased inductances L2,Lr and Cr constitute a reverse LLC resonant network. By increasing the external inductance, the design goal of reverse boost of LLC converter is realized.
Fig. 13 shows the gain curve of LLC in reverse inverter mode after adding external inductance. It can be seen from the gain curve that the improved gain curve can meet the gain requirements of the system.
Fig. 14 shows the block diagram of the whole system. The control core of the whole system is a MCU, SPC1168. SPC1168 is a floating-point MCU with Arm4 core, which has abundant peripheral resources to realize most digital power control. The MCU is placed on the battery side of the system to facilitate communication with BMS management. Crisscross side sampling, including AC input voltage, inverter output voltage and PFC bus voltage, all realize isolated voltage sampling through the isolated voltage sampling chip NSI1311 of Novosense; AC input current and inverter output current are sampled by MCA1101 series current sensor of Aceinna. In addition, LLC primary resonance current is isolated and sampled by CT transformer, and all isolation sampling devices meet the requirements of reinforced insulation of primary and secondary sides. DC battery side sampling includes sampling of output voltage and output current. Because the output voltage is shared with MCU, the output voltage is sampled by a simple voltage divider resistor. The output current passes through the current sampling resistor, and the differential operation amplifier circuit is adopted to realize the accurate sampling of the output current.
When working in the forward direction, the MCU samples the input AC voltage, the input inductor current and the PFC bus voltage, and obtains the frequency of the input AC voltage by phase locked loop, thus completing the PFC control; At the same time, the output voltage and current information are sampled, and according to the output voltage information, the setting of PFC bus voltage is adjusted in real time, so that LLC works as close as possible to the resonant working point, and the output constant voltage and constant current control is completed. When working in the reverse direction, the MCU controls the inverter output, and the inverter output adopts double closed-loop control of output voltage and current, the current loop adopts PI regulation, and the voltage loop adopts PR regulator, which realizes high gain at characteristic frequency and no static error regulation of the inverter output voltage.
The successful use of MCU in this design has replaced the traditional dual DSP scheme, reduced the system cost, and at the same time reduced the user’s design dependence on DSP, which not only has obvious cost advantage, but also has more stable supply, and eased the customer’s worries about DSP supply.
Based on the above analysis results, a 3KW prototype was made in our laboratory. The efficiency of SiC device and hybrid IGBT in totem pole PFC and the whole system is compared. Part of LLC adopts IXFH34N65X2 of Littelfuse, and the secondary rectifier samples the 100V device SRT10N047H of Sanrise. A MCU SPC1168 with Arm4 core of Spintrol completes the charging and inverter control of the whole system. At the same time, in order to meet the safety requirements of the system, the isolated sampling chip NSI1311 and isolated driving chip NSI6602B from Novosense, the 65A current sensor MCA1101 of Aceinna, etc. meet the requirements of strengthening the insulation of the system.
The test data and experimental waveforms are as follows. Figs. 15-16 show the efficiency curves of PFC level and system level when SiC devices and IGBT devices are used respectively. It can be seen from the figure that the totem pole PFC with SiC is 3.5% higher than the totem pole PFC with IGBT at light load and 0.6% higher at full load. Under light load (250W), the efficiency of the whole machine is improved by 3%, and under full load, it is improved by 0.5%.
From the test results, the efficiency improvement is obvious when SiC device is used under light load, which is significant for outdoor energy storage system. The 3% efficiency improvement means that the 20W charger can last 270 minutes longer, and the 65W charger can last 83 minutes longer.
Figs. 17~18 show the test data of PF value of forward PFC and voltage THD of reverse inverter. At full load, it reaches the PF value of 0.995. At reverse inverter output and resistive load, the voltage THD is within 2% in the whole load range. Under RC and RL loads, the AC output voltage THD can also be kept within 3%.
Figs 19~20 show the working waveforms of each part of the system. Fig. 18 shows the waveforms of input voltage and input current when working in the forward direction. The input current tracks the output voltage, and good PFC effect is achieved. Fig. 19 shows the working waveform of LLC. LLC works in boost mode at this time, and the driving signal of synchronous rectifier is turned off in advance to prevent the current from flowing in the reverse direction.
Figs 21~22 show the test waveforms during inversion. Fig. 20 is the startup waveform in the inverter mode. It can be seen from the waveform that the output 220V AC voltage is gradually established. Fig. 21 shows the output voltage and current waveforms at resistive full load. It can be seen that when the resistance is fully loaded, the output voltage has no distortion, and a good control effect is achieved.
In order to meet the needs of the whole portable energy storage power supply, besides the 3KW bidirectional converter, we also designed a 1200W solar MPPT power board. Fig. 22 shows the photo of the prototype, with a bidirectional conversion size of 200mm*320mm*55mm, which is designed according to the size requirements of the battery pack. MPPT board supports the maximum charging power of 1200W, which is convenient for users to use solar panels to quickly replenish batteries in real time when outdoors. Table 3 shows the design specifications of MPPT power board. MPPT board supports a wide voltage range of 10~150V, and users can flexibly configure the series and parallel connection of solar panels. In order to realize a wide range of input and output voltage conditions, MPPT design adopts 4-switches buck-boost circuit, which converts the voltage of solar panel into a constant current source of 20A to charge the battery. The MPPT control algorithm is also realized by MCU SPC1168 of Spintrol, and the MPPT efficiency reaches 99%.
The MPPT board also integrates a 300W buck converter. The voltage of the battery is converted to 24V output by the step-down converter, which supplies power to the PD part of the system. PD board provides 4 outputs, 2 of port A and 2 of port C. The maximum output power of port A is 20W, and that of port C is 100W. Table 4 shows the design specifications of PD board.
Therefore, a portable energy storage system needs to include battery BMS, bidirectional converter, solar charging MPPT circuit and PD port. Our system design is showed as Fig. 23. We have completed the power design required in the system, and BMS system is provided by battery manufacturers in most cases.
Table 4: Design Specifications of PD Board
This paper introduces some design considerations of 3KW bidirectional converter in portable energy storage applications. The advantages and disadvantages of traditional Mosfet, IGBT and SiC Mosfet in totem pole PFC application are compared. It is pointed out that SiC device is the most suitable power device for totem pole PFC application because of its excellent performance and diode characteristics.
It analyzes the gain demand of LLC part under charging and inverter conditions, and several methods to meet the gain demand. It is concluded that adding an external inductor is the simplest way to meet the bidirectional gain of LLC. By adding an external inductor between the bridge arms, the gain requirements of LLC under both charging and inverter conditions are met.
The system sampling and control of bidirectional converter are introduced. The application of MCU in bidirectional converter reduces the system cost, as well as the user’s design dependence on DSP and supply worries. It brings great reference value to users.
In the end, according to these analyses, a 3KW bidirectional converter prototype is made in the laboratory, and the test results are given. The effects of SiC devices and IGBT devices on the efficiency of the whole system are compared. SiC device can greatly improve the light-load efficiency of the system, with an increase of 3%. Through the efficiency increase of 3%, 3KW portable energy storage can provide an additional charging time of nearly 4 hours for the 20W mobile phone charger. Provide an additional 1 hour and 20 minutes for the 65W computer adapter.
In addition to 3KW bidirectional converter, we have also made 1200W MPPT board and 300W PD board. Customers can easily use these mature reference designs to complete the whole portable energy storage system design. For more schemes and detailed design information, please visit www.apl-power.com or contact sales@szapl.com.
I did. You definitely need two runs of SDI per device as they are unidirectional. My original issue beyond that was my camera’s firmware. Once I had everything (converters and BMPCC cameras) at the newest firmware, and two strands of SDI, everything worked perfectly. Have had no further problems with them.
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