Design and Analysis of Bidirectional DC-DC Converters
[Copy link]
This article mainly introduces the design and analysis of a new bidirectional DC-DC converter. This new topology and its control strategy completely solve the voltage spike problem existing in traditional bidirectional DC-DC converters (limited power capacity and efficiency). The converter can not only be used as a battery pack and DC bus interface, but also can work efficiently in both directions (battery charging direction and bus supporting direction). In addition, this article also analyzes the working principle of each block in the circuit and system implementation. Experimental results show that high efficiency can be achieved in both directions. The 300W input (charging the battery) 1500W output (supporting the bus) prototype has an efficiency of up to 92.9% (300W) for charging the battery and 93.6% (1500W) for supporting the bus. Higher power levels can be easily achieved by reconfiguration or paralleling.
Introduction
As part of the battery manufacturing process, battery cells or battery packs must be tested to ensure they properly maintain battery capacity and function properly. The standard way to implement such a test system consists of a power circuit that charges the battery in the correct manner and a load that can be used to discharge the battery during the test. In this configuration, the system efficiency is 0%, meaning that all the energy used to test the battery is dissipated.
Using a bidirectional DC-DC converter, the dissipated energy can be returned to the system, thus realizing the recycling of battery test charging energy. The returned energy can then be used to test subsequent battery cells, and the power consumption generated only comes from the loss of charge and discharge power conversion efficiency, and no power loss is generated due to the discharged load.
Another application of high-efficiency DC-DC converters is as an interface for battery backup systems (BBU). In the event of a power failure, information systems such as data centers usually need to continue to operate for a period of several minutes after the power is cut off, and then restore power via a backup power source (such as a generator). During this period, battery packs are generally used to maintain the functionality of the equipment. When the battery pack is discharged, a voltage drop will appear on the battery pack, so a power conversion interface is required to maintain the appropriate bus voltage. In addition, the battery pack also requires a power supply to replenish and maintain the power lost after the event. If the battery charging and bus interface functions can be realized in a single bidirectional DC-DC converter, great cost and size advantages can be obtained.
Figure 1: Existing isolated bidirectional DC-DC converter topology
Figure 1 shows a widely used existing isolated bidirectional DC-DC converter topology. The input DC voltage can be first inverted into an AC voltage, and then transformed and rectified into an output DC voltage through a transformer. This topology is not suitable for high-power applications because leakage inductance energy storage and discharge will cause high-voltage spikes on the switching MOSFET. To solve this problem, this topology has been derived into a large number of versions [a – j]. However, most of these topologies focus on reducing the application of this voltage spike through a damping circuit or a clamping circuit, which has a certain improvement effect but cannot fundamentally solve the problem.
This article mainly introduces the design and analysis of a new bidirectional DC-DC converter. It is bidirectional, so no other DC-DC converter or AC-DC converter is needed to charge the battery. This article uses a battery backup system application to illustrate the working principle of the converter.
New High Efficiency Isolated Bidirectional DC-DC Converter
Figure 2 shows the topology of this new isolated bidirectional DC-DC converter. It consists of three functional blocks: Block 1, Block 2, and Block 3. Block 2 not only isolates the input and output voltages, but also provides a fixed ratio of voltage rise and fall between them. It is bidirectional, and current can flow in both directions. Block 1 and Block 3 provide accurate voltage regulation, and they are functionally identical blocks except that the input and output voltages are in opposite directions. For Block 1, the battery is at the output. For Block 3, the busbar is at the output.
Block 2
The function of Block 2 is to provide isolation as well as fixed ratio voltage step-up and step-down. By adding a small capacitor to the transformer, the natural resonant frequency of this small capacitor and the leakage inductance of the transformer provide zero current switching [k – l]. Using the natural resonant frequency of the primary current, the MOSFET can be switched at the zero crossing of its resonant portion. When the resonant current reaches zero, S5, S6, S7 and S8 are always turned on and off. When S5 and S7 are turned on (during t1 to t2), the primary resonant current IP flows in the form of a sine wave until it reaches zero. Then, S6 and S8 are turned on, and the primary resonant current IP still maintains the shape of a sine wave, flowing in the opposite direction, as shown during t2 to t3. As shown in Figure 3, the same switching sequence can be operated in both directions, so the circuit is naturally bidirectional.
The switching losses in this converter are close to zero, which allows the converter to operate at very high switching frequencies, up to several MHz, thus achieving very high power density. In addition, very high efficiency is achieved by achieving complete zero current switching (ZCS) on the secondary side and partial ZCS on the primary side (the error is caused by the magnetizing current, and zero voltage switching (ZVS) on the primary side is used to make the switching losses negligible).
Block 2 uses resonance to achieve zero current switching, thus effectively solving the problem of high voltage spikes on the switching MOSFET. The other topologies in [a–j] can only provide improvements in reducing the magnitude of the voltage spikes. The resonant frequency of Block 2 can be as high as several MHz. Therefore, Block 2 can achieve very high power density at very high efficiency.
Block 1 / Block 3
Block 1 / Block 3 can provide precise voltage regulation. They have the same topology and provide bidirectional operation at the system level, so the directions are opposite. Taking Block 1 as an example, as shown in Figure 4, in the first stage, S1 and S4 are turned on, and the current flowing through the inductor IL will increase at a rate proportional to VIN. Then S3 is turned on and S4 is turned off, entering the second stage; IL may be flat, or it may decrease or increase, depending on the voltage difference between the input and output. Then, S2 is turned on and S1 is closed, turning to the third stage; IL will decrease at a rate proportional to VOUT. Finally, S4 is turned on and S3 is closed, entering the fourth stage; a small negative current flows through the inductor. In this conversion process, the zero-voltage switching buck-boost controller can be used to achieve zero voltage conversion [m – n].
High efficiency and power density can also be achieved in Block 1/Block 3 due to the use of ZVS switching.
In this application, a simple control approach for the converter is to set the regulated VOUT of Block 3 to a relatively low bus voltage—lower than the nominal bus voltage most of the time, but still able to support the bus load. In this configuration, the bus voltage is higher than the regulated VOUT of Block 3 most of the time, so Block 3 only consumes no-load power. At the same time, most of the time, the bus charges the battery through Block 1 and Block 2. When the bus voltage suddenly disappears, Block 3 immediately loads and current flows through Block 2 and Block 3 to support the bus.
The advantage of this configuration is that it can achieve high efficiency and high power density when operating in both directions, especially in this busbar battery interface application.
It needs to provide different power levels for battery charging and discharging modes. When in battery charging mode, the required power level should be much lower than in supporting bus mode. In fact, it is better to limit the charging power below a certain level to ensure safety. In this configuration, n of Block 3 can be connected in parallel to achieve this bus power level, and 1 or m (m may be significantly smaller than n) of Block 1 should be sufficient to provide charging power. Therefore, although an independent Block 1 or Block 3 is not bidirectional, they work together to cover both directions and the overall size/power consumption is close to that of Block 1's n. The advantage of this configuration is significant due to the high power ratio of supporting bus and charging battery.
Figure 2: New isolated bidirectional DC-DC converter topology (change the modules in the figure to blocks )
Figure 3: Block 2: Bidirectional flow of primary and secondary resonant currents: (a) to charge the battery; (b) to support the busbar
Figure 4: Block 1: Current flows through the inductor during the ZVS interval
Experimental results
48V is used as the bus voltage and 12V is used as the battery voltage. Therefore, the conversion ratio of block 2 needs to be designed to be 4:1.
When VIN=48V, the power is 300W, and the module conversion ratio of block 2 is 4:1, the efficiency of the test exceeds 96% after the load exceeds 50%, and the peak efficiency is 96.2%. When the load is lower than 50%, the efficiency decreases, but 85.5% efficiency can still be achieved at a load of 10%. All these tests were performed at room temperature. Figure 5 (a) shows the efficiency matrix test under different input voltage and load conditions. The input voltage can be designed to be 26-55V, so that the battery voltage of 6.5-13.75V can reversely support the bus. This wide range enables more battery configurations and, more importantly, helps to extend the time that the battery supports the bus.
Figure 5(b) shows the experimental efficiency test results of the Block 2 module in the bus-supporting direction, which is defined as the reverse direction in this paper. In this experiment, a deep-cycle marine lead-acid 12V battery (part number 24DC-1, 140 minutes of battery capacity, cold and marine starting current of more than 500 amps) was used to support the bus through the Block 2 module. Because the battery terminal voltage drops as the supply current increases, VIN drops from 11.7V (IOUT = 0.6A 4) to 10.9V (I OUT = 6.3A 4). The peak efficiency is 96.9%. Note that the efficiency in the bus-supporting direction is even higher than the efficiency in the battery charging direction, which is very beneficial for this application because the power level required for the battery to support the bus in the reverse condition is much higher than that in the battery charging direction. Higher efficiency in the bus-supporting direction will simplify thermal management design for high-power applications.
For the 500W Block 1/Block 3 modules, the experimental efficiency test results are shown in Figure 6. The peak efficiency is 97.3%.
These modules can enable the function through the control circuit, so that the disabled power consumption is significantly lower than the no-load power consumption. At a temperature of 25°C and a rated voltage of 48V, the typical disabled power consumption of the 4:1 conversion ratio Block 2 module used with a 500W Block 1 module or Block 3 module is 0.04W and the no-load power consumption is 5.3W.
Figure 5: Efficiency test results of Block 2 modules (300W, 4:1 ratio) in the following directions: (a) battery charging, (b) supporting bus
Figure 6: Efficiency test results of Block 1/Block 3 modules (500W, room temperature)
System Implementation
A 7×9-inch PCB prototype of this bidirectional DC-DC converter was built for this application, as shown in Figure 7, with three Block 3 modules (500W each) connected in parallel and five Block 2 modules (300W each) connected in parallel.
Figure 7: System Implementation As shown in the topology in Figure 2, simply paralleling the modules and putting them together will make the converter work. The regulated VOUT of the Block 3 modules is set to the voltage of the relatively low bus, which is lower than the nominal bus voltage most of the time, but still high enough to support the bus load. In this way, no additional system control circuitry is required. Once in support bus mode, all five Block 2 modules can immediately handle the power. The disadvantage of this configuration is that all modules are always active and some of them are in light load/no load power consumption state for most of their operating time.
To save this light load/no-load power consumption, the modules can be disabled when they are not needed to remain operational. Some modules need to be restored from disabled mode to enabled mode once the bus voltage disappears. During this time, the bus voltage is supported by the energy storage capacitor. Make sure to add enough capacitance to the bus to support the module during the time interval when it restarts quickly. The system-level control circuit in this board can be used to disable/enable the module to eliminate unnecessary power consumption.
In the battery charging direction, four modules in block 2 can be disabled and three modules in block 3 can be disabled, which can provide 300W of battery charging power.
In the direction of supporting the bus, the modules of block 1 can be disabled, which can provide 1500W of supporting bus power. In this configuration, the system can charge the battery at 300W/25A and support the 48V bus at 1500W/31A. With 140 minutes of battery capacity, it takes 2.3 hours from fully discharged to fully charged, and then it can provide power to the bus (1500W load) for 28 minutes. Higher power levels can be easily achieved by reconfiguring or paralleling.
In both forward and reverse modes, the efficiency of block 1/block 3 modules is maintained at 97.3%, and the efficiency of block 2 modules can reach 96.2%. 0.78W is the disabled power consumption of block 1/block 3 modules, and 0.04W is the disabled power consumption of block 2 modules. Therefore, in this battery charging mode, the peak efficiency is:
In the bus-support mode, the peak efficiency is:
Conclusion
This paper mainly introduces the design and analysis of a new bidirectional DC-DC converter. It can be used to connect the battery pack and the DC bus in both directions (battery charging direction and supporting the bus). In addition, this paper analyzes the working principle of each block in the circuit and system implementation. The experimental results show that the method achieves high efficiency in both power flow directions. We built a 300W input (battery charging) 1500W output (supporting the bus) bidirectional DC-DC converter prototype for this application. With the 140-minute lead-acid battery capacity, it takes 2.3 hours to charge from fully discharged to fully charged, and then it can provide power to the bus (1500W load) for 28 minutes. With the system control circuit on the board, the prototype can charge the battery with 92.9% efficiency (300W) and support the bus with 93.6% efficiency (1500W). Reconfiguration or paralleling can easily achieve higher power levels.
| 
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
