The TIDA-010039 reference design from Texas Instruments provides an overview on how to implement a 10kW/10kVA three-level, three-phase, SiC-based ac-dc converter with digital control and bidirectional functionality. A high switching frequency of 50kHz reduces the size of magnetics for the filter design and as a result a higher power density. SiC MOSFETs with low switching loss enable higher dc bus voltages of up to 800V and lower switching losses with a peak efficiency of >97%.
This design is configurable to work as a two-level or three-level rectifier. Targeted applications include dc charging stations for electric and plug-in vehicles, energy storage system power conversion systems, and three-phase uninterruptible power systems.
This reference design is comprised of four separate boards that intercommunicate. The following boards work in tandem to form this three-phase converter reference design:
- A power board, comprising all of the switching device, LCL filter, sensing electronics, and power structure
- A TMS320F28377D Control Card to support the DSP
- Three gate driver cards, each with two ISO5852S and two UCC5320 gate drivers
- A high-voltage, isolated dc link voltage sensing card using the AMC1311 device
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Summary of Reference Design Features
- Rated nominal input of 380- to 400-Vac peak, with output of 800Vdc
- Maximum 10-kW, 10-kVA output power at 400-Vac 50- or 60-Hz grid connection
- High-voltage (1200V) SiC MOSFET-based full-bridge ac-dc converter for peak efficiency of >97%
- Compact filter by switching rectifier at 50 kHz
- Isolated driver ISO5852S with reinforced isolation for driving high-voltage SiC MOSFET and UCC5320S for driving middle Si IGBT
- Isolated current sensing using the AMC1301 for load current monitoring
- TMS320F28379D control card for digital control
SiC MOSFET and IGBT Selection
The main switching device needs to support the full switching voltage. To support the 1000-V dc link voltage of this design, use 1200-V FETs; however, at this voltage, the migration to SiC is necessitated by several factors:
- The switching speed of a 1200-V SiC MOSFET is significantly faster than a traditional IGBT, leading to a reduction in switching losses.
- The reverse recovery charge is significantly smaller in the SiC MOSFET, resulting in reduced voltage and current overshoot.
- A lower temperature dependence at due to reduced conduction loss increase at full load.
The middle switches are only exposed to half of the dc link voltage, or 500V in this design. As such, a 650-V device is suitable. A full SiC solution provides the best performance due to these same features; however, the cost would be higher. To reduce overall system cost, traditional Si switching devices can be used. A few factors dictate the choice of device:
- Si MOSFETs have a resistive feature that helps to reduce conduction loss at light load conditions compared with IGBT, but the high reverse recovery of the body diode increases voltage and current overshoot. Because SiC MOSFETs switch much faster than Si devices, the reverse recovery is much more severe.
- Si IGBTs have higher conduction loss at light load, but the reverse recovery can be lower if a fast recovery diode is used as the antiparallel diode. Moreover, because an IGBT is a unidirectional device, the current always conducts through one anti-parallel diode in T-type topology. The light load efficiency will be reduced.
For this design, the reverse recovery loss and voltage overshoot limits the device selection. As such, a 1200-V SiC MOSFET + 650-V IGBT solution is used.
Conduction loss is mainly determined by the RDS(on) of the 1200-V SiC MOSFET and the on voltage drop of the 650-V IGBT. The 80-mΩ SiC devices have a good high-temperature performance, and the RDS(on) only increases 30% at 150°C junction temperature. With the high temperature I-V curve in the data sheet, calculate the conduction loss on the devices.
Switching loss is a function of the switching frequency and switching energy of each switching transient, the switching energy is related with device current and voltage at the switching transient. Using the switching energy curve in the data sheet, one can estimate the total switching loss. Note that the switching energy curve in the data sheet is measured with SiC diode freewheeling, but in a T-type converter, the freewheeling device is the Si diode in IGBT. The switching loss is expected to be higher than calculated result.
Similarly, the conduction loss and switching loss can be estimated for all the devices and efficiency can be estimated. With the thermal impedance information of the thermal system design, the proper device rating can be selected. The 1200-V/80-mΩ SiC MOSFET and 650-V/30-A IGBT is a good tradeoff among thermal, efficiency and cost.