Gallium Nitride Transistors Switch In The Sub-Nanosecond Timeframe

High frequency GaN FET applications can take advantage of switching transition speeds in the sub nano-second range using Efficient Power Conversion Corporation new third generation devices. Among the applications that require this speed are envelope tracking in RF power amplifiers and highly resonant wireless power transfer systems for wireless charging of mobile devices.

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Before describing these two applications here are details of the new GaN FETs. They have features that enable higher speed designs, including:

· Reduction in Q_GD, which reduces voltage transient switching losses

· Improved Miller ratio that provides high dv/dt immunity

· Low inductance pads provide improved connection to both gate and drain circuits

· Orthogonal current flow between the gate and drain circuits permit enhanced common source inductance (CSI) reduction

· Separate gate return connection also enables enhanced CSI reduction.

Table 1 lists the characteristics of this EPC8000 family.

Envelope tracking is an RF design approach in which the power amplifier constantly adjusts its applied power supply voltage to ensure it is operating at peak efficiency for its instantaneous output power requirements. Without envelope tracking, amplifiers operating with a constant supply voltage become less efficient as the signals crest factor increases, because the amplifier spends more time operating below peak power and, therefore, spends more time operating below its maximum efficiency.

10 MHz Buck Converter For Envelope Tracking

The EPC8005 (third generation part) can be used in a 42 V to 20 V, 20 W buck converter operating at 10 MHz, as shown in Fig. 1.

Fig. 1. Envelope tracker buck converter schematic.

Where:

L_Buck= 2.2 μH

C_OUT= 2 x 4.7 μF

The main supply (V_DD) bus caps are 100 nF. To ensure the highest efficiency, the board was designed using an optimal layout technique. The choice of low inductance supply bus capacitors is also critical even when using the optimal layout.

Fig. 2. Evaluation board showing the EPC8005 devices and LM5113 gate driver.

Fig. 2 is an evaluation board with EPC8005 devices and the LM5113 gate driver IC. The right image shows the details of the power circuit with the gate driver IC. To maintain low inductance in the gate circuit, two parallel connected size 0201 resistors were used side by side for the gate resistors, as well as keeping the gate driver IC very close to the devices. The area occupied by the converter is smaller than the footprint of a SO-8 package.

Fig. 3. Efficiency plots for 5 MHz and 10 MHz operation of the envelope tracking converter.

The converter was tested at both 10 MHz and 5 MHz, and the efficiency is in Fig. 3. The plots show a respectable 89% peak efficiency while operating at 10 MHz and 92% while operating at 5 MHz. The inductor used in the 5 MHz operation is the same as in the 10 MHz operation. Selecting a more optimal inductance can lead to further improvement.

Class D Wireless Power Transfer

A 6.78 MHz voltage mode class D wireless power transfer converter was previously demonstrated using the EPC2014 (second generation part). It was noted at the time that the EPC2014 was the smallest device available for this demonstration, and that the performance could improve given the correct size selection of the FETs. The original power converter stage (using the EPC2014) was replaced with an EPC9024 (development board) populated with EPC8004 devices, using the same coil and load set of the original 2014 version.

Fig. 4. Voltage mode class D wireless power transfer.

Fig. 5. Wireless transfer system experimental setup.

Fig. 4 is the basic schematic of the power stage and Fig. 5 shows the wireless power transfer system. The setup comprises the source board, source coil, device coil, and device load. The EPC8004 FETs (third generation part) were mounted to the EPC9024 board (shown in Fig. 2), and the gating signals were generated from the voltage feedback control signal using a phase follower controller.

Fig. 6. Wireless converter efficiency plots when using the EPC8004 and EPC2014 FETs.

The EPC8004 based wireless system was tested by varying the input supply voltage from 8 V through 24 V, and the efficiency is shown in Fig. 6. Overall, there is at least a 2% increase in efficiency over the EPC2014 based version despite the higher R_DS(ON)of the EPC8004 (125 mΩ) in comparison to the EPC2014 (16 mΩ). This is mainly due to a reduction in losses associated with C_GD(turn off).

Fig. 7. Loss breakdown in the wireless energy transfer system comparing the EPC8004 with the EPC2014.

Fig. 6 shows that the efficiency peaked at the high end and that the efficiency of the EPC2014 based system could potentially increase further. Fig. 7 gives the loss breakdown of the wireless system when operating at 15 W load with a 22 V input. The EPC2014 has high switching losses while the EPC8004 has high conduction losses, but overall the total losses are lower than the EPC2014 for the same operating condition.

Development Board

The EPC9027 development board, featuring the EPC8007 devices and the LM5113 gate driver IC in a half bridge configuration, is available. Additional development boards will be available to support designers in evaluating and incorporating other EPC8000 family products into their power conversion systems.