Abstract: This paper designs a full-bridge zero-voltage zero-current converter (ZCC). By connecting a saturated inductor in series with the primary winding of the transformer, the ZCS of the lagging arm is achieved, enabling soft switching of all switching transistors and improving system efficiency. Furthermore, the working principle and operating modes of the converter are analyzed, and an experimental circuit is designed to verify the correctness of the theoretical analysis and design concept.
Keywords: Full-bridge converter; Soft switching;
REACH ON HIGH FREQUENCY FULL BRIDGE electric operating power supply
Abstract: This paper design a full bridge DC/DC converter as an electric operating power supply. By using saturable inductor and block capacitor, the bridge devices could achieve soft switching, so the converter can get a high efficiency. Analysis of the converter's working principle and mode of work. Final design of the experimental circuit is completed, and to verify the theoretical analysis and the correctness of design ideas.
Key words: full bridge converter, soft switching;
1. Introduction
Power operating power supplies (220VDC, 110VDC) play a crucial role in modern power systems, primarily used in power plants and substations as vital power sources for DC mechanisms (opening and closing), relay protection, signals, automatic control, emergency lighting, instrumentation, and emergency loads. Their performance and quality directly impact the safe operation of the power grid and the safety of equipment. With the rapid development of power semiconductor technology, computer control technology, and VLSI manufacturing processes, high-frequency switching power supplies entered practical application in the late 1980s and gradually matured in the 1990s. However, most power supplies used in my country's power system still employ phase-controlled power supplies. Phase-controlled power supplies suffer from high ripple and high-order harmonic interference, low efficiency, large size, inadequate monitoring, and difficulty in achieving intelligent battery charging and discharging management. To improve the overall efficiency and reliability of my country's power grid, the development of power operating power supplies in my country is currently based on high-frequency switching conversion technology, with trends characterized by high frequency, high efficiency, high power, pollution-free operation, and modularity. In terms of management, combined with the development of computer control technology and bus technology, a multi-level computer network centralized monitoring and management system is being formed. This paper mainly focuses on the research and discussion of high-frequency power supplies with full-bridge structures.
2. System Principle Design and Analysis
FB-ZVZCS converters have various topologies, but their basic operating mechanism is the same: ZVS (Zero-Voltage-Side Flow) is achieved by utilizing the energy storage of parallel capacitors in the power devices and the output filter inductor. Because the output filter inductor is generally large, the leading arm can achieve ZVS over a wide input voltage range and output load regulation range. During the primary-side freewheeling phase, a reverse blocking voltage source is introduced to quickly reset the primary-side current, thus achieving ZCS in the lagging arm.
Full-bridge ZVZCS converters can achieve primary-side current reset using different topologies. One approach utilizes the reverse avalanche breakdown of the lead-arm switch to completely dissipate the energy stored in the transformer leakage inductance in the lead-arm IGBT, providing zero-current switching conditions for the lagging arm. This topology is simple and requires no auxiliary circuitry. However, the energy stored in the leakage inductance is entirely dissipated in the IGBT, and the low (15~30V) and fixed reverse avalanche breakdown voltage of the IGBT limits the converter's maximum controllable duty cycle.
A capacitor is connected in parallel at the output of the rectifier on the secondary side of the transformer. During the zero-crossing of the primary voltage, the voltage on the secondary capacitor is reflected back to the primary side as a reverse blocking voltage source, which quickly resets the primary current and creates zero-current switching conditions for the lagging arm switch. The advantage of this converter is that it can achieve a large duty cycle and high efficiency. However, this converter introduces an active clamping switch, which makes the system control more complex. In addition, the active clamping switch uses a hard switch, which affects the reliability of the system.
Using a blocking capacitor and a saturation inductor on the primary side of the transformer, the voltage across the blocking capacitor is used as a reverse blocking voltage source during the primary voltage zero-crossing period, resetting the primary current and providing zero-current switching conditions for the lagging arm. This converter achieves zero-current switching (ZCS) of the lagging arm by matching the blocking capacitor and the saturation inductor. The blocking capacitor in this converter has a dual function in the circuit: one is to achieve ZCS of the lagging arm as mentioned above, and the other is to suppress the transformer's bias magnetization. In a full-bridge converter, there is a problem of magnetic flux imbalance between the two half-cycles. If a non-polarized capacitor is connected in series on the primary side of the transformer, it can prevent the core from unidirectionally saturating and the current from rising sharply, thus preventing damage to the switching transistor and improving the reliability of the system. Compared with the ZVZCS converter that uses IGBT avalanche breakdown, the duty cycle control range is larger; compared with the ZVZCS converter that uses active lossless clamping on the secondary side, the control circuit is simpler and more reliable.
Combining the advantages and disadvantages of the above structures, this paper uses blocking capacitors and saturated inductors to realize ZVZCS, and the topology is shown in Figure 1.
Figure 1 ZVZCS topology
The converter can be considered to operate in 6 modes within one switching cycle:
Mode 1: Energy Transfer Stage. In this stage, switches Q1 and Q4 are turned on, and the input power supply transfers energy to the load through the transformer. The primary current ip = nIo remains constant, the saturation inductor is in a forward saturation state, charging the blocking capacitor Cb. The voltage across the blocking capacitor rises linearly, with a peak voltage VCbp .
(1)
Mode 2: When the forearm switch Q1 is turned off, the forearm parallel capacitor C1 starts to charge, and C2 starts to discharge.
(2)
When the voltage on Q2 drops to zero, the anti-parallel diode on it naturally turns on, creating conditions for Q2 to turn on at zero voltage.
Mode 3: Primary current reset stage. Under the influence of the voltage V<sub> Cb</sub> across the blocking capacitor C<sub> b </sub>, the primary current of the transformer decreases, and the secondary current decreases accordingly. When the secondary current is less than the output current, all the secondary output rectifier diodes are turned on, entering the freewheeling stage, and the secondary side of the transformer is short-circuited. The entire voltage across the blocking capacitor is applied to the transformer leakage inductance L<sub> lk </sub>, and the primary current rapidly drops to zero.
Mode 4: Saturated inductance prevents primary current from flowing in reverse. During resonance, under the action of the blocking capacitor voltage V<sub> Cb</sub> , the primary current may increase in reverse. However, at this time, the saturated inductor L<sub> s </sub> has desaturated and exhibits a large inductance, preventing the current from flowing in reverse.
Mode 5: Switch Q4 is turned off. This stage is the dead time of the lagging arm switch. With Q4 off, Q3 is not turned on, and the primary current of the transformer is zero. Therefore, Q4 achieves zero-current turn-off, and the saturated inductor is still in the desaturation state.
Mode 6: When switch Q3 is turned on, the primary current of the transformer begins to increase in the reverse direction. Because the saturated inductor is not yet saturated, the primary current lags behind before it begins to rise rapidly. After the saturated inductor saturates, the DC bus voltage and the blocking capacitor voltage are applied to the leakage inductance, causing the primary current to increase rapidly in the reverse direction.
(3)
3. Design of the control section
This system uses the MSP430F155 microcontroller. The overall system structure diagram is as follows:
Figure 2 System Overall Structure Diagram
The software design of this system uses the C programming language. It adopts a modular structure design, with each functional module designed as an independent programming and debugging block. This not only facilitates future functional expansion but also makes debugging and linking easier, and further improves program portability and modification. Its structure is shown in the figure below.
Figure 3 Software Structure Diagram
The system program consists of several components, including a data acquisition module, a parameter calculation module, an interrupt alarm module, an internal storage module, a communication interrupt module, and a control module. The main program flow is shown in Figure 4.
Figure 4 Main Program Flowchart
4. Experimental Results and Analysis
Figure 5 shows the experimental waveforms of the leading arm under the conditions of 530V input, 250V output voltage, and 3A output current. The lower waveform is the drive waveform uce of the switching transistor, and the upper waveform is the collector-emitter voltage uce of the switching transistor. As can be seen from the experimental waveforms in Figure 5, under the influence of the junction capacitance of the switching transistor and the leakage inductance of the transformer, after the collector-emitter voltage uce of the leading arm switching transistor drops to zero, the drive signal uge turns on the switching transistor, thus achieving zero-voltage turn-on. This demonstrates that the system can achieve zero-voltage turn-on of the leading arm switching transistor under relatively light load conditions.
Figure 5 Experimental waveforms of the lead-arm switching transistor under light load.
Figure 6 Experimental waveforms of the light-load time-delay arm switching transistor.
Figure 7 Experimental waveform of the lead-arm switching transistor under full load.
Figure 8 Experimental waveform of the switching transistor under full load time lag.
Figure 6 shows the experimental waveforms of the lagging arm under the conditions of an output voltage of 250V and an output current of 3A. The top waveform is the transformer primary current waveform ip, and the bottom waveform is the driving voltage uge of the lagging arm switch. As can be seen from the waveforms in Figure 6, the transformer primary current waveform ip is reset to zero under the action of the blocking capacitor, after which the lagging arm switch is turned off, thus achieving zero-current turn-off of the lagging arm switch. This demonstrates that the system can achieve zero-current turn-off of the lagging arm under relatively light load conditions.
Figure 7 shows the experimental waveforms of the upper arm under the conditions of 530V input, 250V output voltage, and 40A output current. In Figure 7, the lower part shows the drive waveform uge of the switching transistor, and the upper part shows the collector-emitter voltage uce of the switching transistor. As can be seen from the experimental waveforms in Figure 7, under the influence of the junction capacitance of the switching transistor and the leakage inductance of the transformer, after the collector-emitter voltage uce of the upper arm switching transistor drops to zero, the drive signal uge turns on the switching transistor, thus achieving zero-voltage turn-on of the upper arm under full load.
Figure 8 shows the experimental waveforms of the lagging arm under the conditions of an output voltage of 250V and an output current of 40A. The top waveform is the transformer primary current waveform ip, and the bottom waveform is the driving voltage uge of the lagging arm switch. As can be seen from the waveforms, the transformer primary current waveform ip is reset to zero under the action of the blocking capacitor, after which the lagging arm switch turns off, thus achieving zero-current turn-off of the lagging arm under full load. The experimental results are consistent with the theoretical analysis and simulation results.
5. Conclusion
Power supply systems play a crucial role in modern power systems, and their performance and quality directly affect the safe operation of the power grid and the safety of equipment. This paper designs a full-bridge zero-voltage zero-current converter, which enables soft switching and exhibits strong stability and reliability. Therefore, this converter has broad development prospects.
References
[1] Wang Cong. Soft-switching power converters and their applications. Science Press, 2000: 139~146
[2] Ruan Xinbo, Yan Yangguang. Analysis of Zero-Voltage-Zero-Current Switching PWM DC/DC Full-Bridge Converter. Journal of Electrical Engineering. 2000, 15(2):73~77
[3] Lin Weixun. Modern Power Electronic Circuits. Zhejiang University Press, 2002: 74
