HV-LV DC-DC Converter Based on High-Speed IGBT
Dec 21, 2022 View: 585
This paper analyzes a soft-switching phase-shifted full-bridge DC/DC converter with a synchronous rectification based on high-speed IGBTs. The soft-switching technology of phase-shifted full-bridge topology is the mainstream key technology for high-voltage to low-voltage DC/DC converters for hybrid and electric vehicles. The industry used MOSFETs as the main power unit in the early days. As the power demand for this DC/DC converter gradually increased, the efficiency of the MOSFET-based design system dropped dramatically. It could no longer meet the application requirements.
DC/DC converter is an indispensable auxiliary electronic equipment in new energy vehicles such as electric vehicles and hybrid vehicles, which replaces the original engine of traditional vehicles through the belt-driven generator to power the vehicle voltage 12V network. It performs the separation of the vehicle propulsion system and auxiliary power supply system. It provides convenience to improve the efficiency of the whole vehicle system.
Its input is a high-voltage storage power battery system, and its output is a low-voltage 12-volt power network, so it is called a high-voltage - low-voltage DC/DC converter (HV-LV DC/DC Converter). This DC/DC converter usually has a power of 1~3kW.
A phase-shifted full bridge with zero voltage switching is a common topology for this application. The advantage is that the parasitic system parameters (transformer leakage Leak and switching device output capacitance Coss) are utilized through phase shift modulation. This soft-switching topology operates at a fixed switching frequency, which is favorable for selecting parasitic device parameters.
Typical phase-shifted full-bridge converters for electric and hybrid vehicles require a high-voltage input from a high-voltage battery pack with a voltage of about 200V to 400V; an output section connected to a low-voltage battery and a weak load with a voltage of about 14V. Table 1 gives the typical specifications of this DC/DC converter. Based on the 100kHz switching frequency and input voltage range specifications, most of the switching devices for this application are currently super-junction field-effect tubes (Super-junction MOSFETs).
IGBTs were originally used in switching frequency applications from 1 kHz to 20 kHz. With the improvement of the structure and the reduction of switching losses, high-speed IGBTs are gradually used in higher switching frequencies.
IGBT technology for high-frequency switching operation
Based on the charge compensation principle, MOSFETs in super junction technology entered the market as early as 1998, creating a revolution in the 600 V withstand voltage class. Its most important advantage is its three-dimensional structure with thin vertical PN strips in the active layer of the parasitic diode, which maintains the same blocking voltage but allows a proportional reduction of the on-resistance due to the reduced width of the vertical PN strips. This method can reduce the on-resistance per unit area by 5 to 10. Before super junction technology, IGBTs with excellent conduction losses were inevitably used in 600 V to withstand voltage-level applications. The switching frequency was always below 20 kHz due to the characteristic trailing current and resulting switching losses of IGBTs. Both mainstream IGBTs of the time (PT and NPT) suffered from this trailing current.
The landmark technological advancement that changed this phenomenon was achieved by the trench-gate terminated structure IGBT (made by Infineon) and the soft feed-through junction IGBT (made by ABB). The trench-gate termination IGBT was born in 2000 and improved the turn-off trailing current waveform of IGBTs. Subsequently, the trench-gate termination IGBTs were further optimized for different applications. The optimized IGBTs operate at switching frequencies of 20 kHz to 40 kHz in welding machines, solar inverters, and UPS. Infineon released a 600V trench-gate terminated IGBT optimized for high-frequency hard switching in 2010 and a series of trench-gate terminated IGBTs for different applications in 2012.
DCDC converter circuit design
The DC/DC converter uses Infineon's 650V 50A high-speed IGBTs and the fast diode module Easy module 1B.
Test verification results
A series of tests were performed at 100kHz switching frequency to evaluate the suitability and potential benefits of the high-speed IGBTs for this application. Instead of using an external resonant inductor for cost and space reasons, the design uses the transformer's leakage inductance for resonance. From the basic performance of the same voltage and current IGBT, the chip area is only one-sixth of MOSFET. MOSFET has the advantage in low current and low-temperature conditions. But as the working junction temperature increases, the current increases, the IGBT's current capability increases rapidly, and the conduction loss is significantly lower than MOSFETs.
Conclusion
Contemporary high-speed IGBTs (such as Infineon HS3 series), compared to conventional trench-gate field termination IGBTs, significantly improve the trench gate without increasing the collector-to-emitter saturation voltage drop, with significant improvements in trailing current and turn-off losses.
Through circuit design and practical testing, the possibility of replacing super junction MOSFETs in this soft-switching phase-shifted full-bridge DC/DC converter application is realized while surpassing the performance of superjunction MOSFETs at higher power operating conditions. At the same time, the chip area is significantly reduced compared to MOSFETs, and therefore the chip cost is reduced.
The design uses a 13:1 turns ratio with phase-shifted full bridge and full wave synchronous rectifier topology and resonance-free inductor characteristics to achieve an optimal efficiency of 93% in the power range of 220V to 400V. A very gentle efficiency degradation plateau, providing a new direction of power device design options for high voltage - low voltage DC/DC converter design.