AUTOMOTIVE
Figure 2: Digital average current mode control diagram
sensing. However, shunt resistors that are rated for above 70A usually have a large footprint and therefore also high parasitic inductance that can result in high noise, which can saturate the current-sense amplifier and thus void the measurement. A simple solution to overcome this problem was to add an RC filter network with a matched time constant that cancels the shunt inductance. This design uses a current-sense amplifier with a maximum bandwidth of 500kHz and 50V/V gain, which results in 10mV/A total current-sensing gain when used with a 200µΩ shunt resistor. It is also critical to ensure a symmetrical layout between the two
phases, so that phase currents are balanced and any effects due to mismatching of gate drive delay, switching transition speed, overshoot, or other parameters, are minimised. The internal vertical loop approach when designing with GaN power devices is to place decoupling capacitors close to the FETs and position a solid ground plane beneath. The microcontroller chosen for this application has a high-
resolution PWM module that allows accurate control of the duty cycle and dead-time of 0.25ns, permitting these to be optimised to take full advantage of the GaN FET’s performance. Digital average current mode control is implemented for both buck
and boost modes, as shown in Figure. 2. Using the same current reference, IREF, for the two independent current loops regulates the current in both inductors to the same value. The bandwidth of the two inner current loops is set to 6kHz, and the outer voltage loop bandwidth set to 800Hz. The GaN FETs require a heatsink to operate at the full output power
of 1.5kW. A standard commercially available 1/8th-brick heatsink is used. Four metal spacers are installed on the PCB to provide the appropriate clearance for the heatsink mounting. An electrically
Figure 3: The EPC9137 converter with the EPC2206 GaN FETs
insulating thermal interface material with thermal conductivity of 17.8W/mK was applied between the FETs and heatsink.
Performance analysis Figure 3 shows an image of the EPC9137 converter. With the heatsink installed and 1700 LFM airflow, the converter was operated at 48V input, 13.8V output and tested at both 250kHz and 500kHz. Figure 4 shows the efficiency results. At 250kHz, using a 2.2µH inductor, the converter achieved a peak efficiency of 97%. When operated at 500kHz, using a 1.0µH inductor, the peak efficiency was 95.8%. The EPC9137 converter was also tested at 13.8V input and 48V output
for boost-mode operation, as shown in Figure 5. At full load, EPC eGaN FETs can operate with 96% efficiency at 250kHz
switching frequency, enabling 750W/phase compared to silicon- based solutions, which are limited to 600W/phase due to the limitation on the inductor current at the 100kHz maximum switching frequency. Car makers face demands to increase the pace of vehicle
electrification, both to compete in the marketplace and meet toughening environmental legislation. This design example for a bi- directional DC-DC converter shows how EPC’s automotive-qualified eGaN FETs, such as the EPC2206, can help integrate a 48 V bus, needed to electrify power-hungry loads and meet rising power demands throughout the vehicle. When transferring power between 48V and 12V domains, the EPC9137 converter achieves maximum efficiency greater than 96% with 250kHz switching frequency, and above 95% at 500 kHz.
EPC
www.epc-co.com
Figure 4: Measured converter efficiency of the EPC9137 at 250kHz and 500kHz, 48V input and 13.8V output
Figure 5: Measured EPC9137 converter efficiency at 250kHz, 13.8V input and 48V output
DECEMBER/JANUARY 2022 | ELECTRONICS TODAY 9
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42
