TI 11kW GaN three-phase ANPC inverter

4. Optimized control and driving

By optimizing the control scheme, the entire power stage (consisting of 18 power devices) can be driven with only 6 PWM signals, while the traditional scheme requires 9, greatly saving MCU resources. At the same time, ISO7741 isolators are used to achieve PWM signal isolation, and the fault signals of GaN devices can be transmitted back to the MCU in reverse, achieving real-time safe operation.

5. Bi directional operation capability

The ANPC power stage itself has the inherent ability for bidirectional energy flow. Through software configuration, it can act as an inverter (DC-AC) to feed solar or battery energy into the grid, or as a power factor corrector (PFC, AC-DC) to store grid energy in local batteries. The document states that the inverter mode has passed the test, PFC mode testing is currently underway, and the direction switching time is less than 1 millisecond.

Key system design and component selection

Design of LCL Filter

To achieve high-quality grid connected current and effectively attenuate high-frequency harmonics, LCL filters were designed and adopted. At a switching frequency of 100kHz, the inductance (L_inv) value on the inverter side can be significantly reduced. The calculated value of L_inv is approximately 360 μ H (considering the influence of bias current). The filtering capacitor (Cf) is selected to be 4.7 μ F to limit the absorbed reactive power to below 3.5% of the total power. To obtain a 5% attenuation factor, the grid side inductance (L_grid) was calculated to be approximately 10.3 μ H, and a standard value of 15 μ H was ultimately selected. The calculated resonant frequency (approximately 19.35kHz) meets the stability requirements of being much higher than the power frequency and lower than half of the switching frequency. In addition, a 0.58 Ω damping resistor is connected in series to suppress resonance peaks.

Current and voltage detection

Current detection: AMC3302 enhanced isolated current sensor is used, which integrates an isolated bias power supply, greatly simplifying the design. The sensor has a small input range of ± 50mV, coupled with a high-precision shunt resistor of 2m Ω, producing only 0.5W loss at full power (about ± 25A), achieving a perfect balance between high precision and low loss.

Voltage detection: Voltage sampling is performed at three points before and after the output relay, as well as on the positive and negative busbars. Attenuate high voltage signals through a resistive voltage divider network and add a bias voltage of 1.65V to match the input range of the ADC. This design allows the control system to lock the grid voltage and frequency before grid connection, and independently adjust the duty cycle of the positive and negative half cycles to balance the bus voltage.

Power supply and driver

Auxiliary power supply: The system is powered by a 12V main bias power supply. The TPS563200 buck converter generates 5V, and then LP5907 LDO generates 3.3V for use in analog and logic circuits. For each GaN or Si device on the power card, an SN6501 transformer driver is used in conjunction with different transformers to generate isolated 12V (GaN) or+12V/-5V (Si MOSFET) driving voltages.

Si MOSFET driver: For low-frequency Si power boards, a dual channel isolated driver UCC21541 is used, which can simultaneously drive two devices on one power board. The independent conduction and turn off paths, as well as the ferrite beads in the gate path, effectively control the driving current and suppress ringing.

Test validation and performance evaluation

The reference design has been strictly tested to verify its progressiveness in theory and reliability in practice.

Inverter mode test (resistive load)

Under open-loop and closed-loop control, the system generated a pure 50Hz sine waveform at an 800V DC bus voltage. The test results show that even near zero crossing, waveform distortion is minimal, thanks to the protection mechanism based on CLB. The stability of the current loop has been verified through the step response of the current from 1A to 6A and the reverse direction, demonstrating smooth transient transition capability.

PFC mode testing and efficiency

In PFC mode, the system operated at a grid voltage of 230Vrms and tested its efficiency at 600V and 800V DC bus voltages, respectively. The test results are remarkable:

High efficiency: Within the load range of 2kW to 11kW, the system efficiency remains above 98%, with a peak efficiency of 98.62%.

Voltage selection strategy: Data shows that using a lower 600V bus voltage is more efficient at low power, while maintaining an 800V bus voltage can bring about a 0.4% efficiency improvement at high power (high current).

Low harmonic: Under rated load, the current waveform has good sinuosity and total harmonic distortion (THD) is less than 4%.

power density

The final physical dimensions of the design are 300mm (length) x 220mm (width) x 65mm (height), with a total volume of only 4.29 liters. At a rated power of 11 kW, the achieved power density is as high as 2.57 kW/L, which fully demonstrates the huge advantage of high-frequency GaN switches in reducing the volume of passive components.