eGaN® TECHNOLOGYHow2AppNote 022

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MotivationTo accommodate the increasing power requirement in the server applications, there is increasing demand on extracting more power from standard 48 V bus converters. This application note presents the design of a 1 kW, 4:1 conversion ratio, eGaN FET-based LLC resonant converter in the 1/8th power brick size for the 48 V server applications. The EPC9149 [5] converter module achieves 97.5% peak efficiency and 96.7% full-load efficiency.

Design Overview The schematic of the LLC resonant topology adopted in this work is illustrated in Figure 1. It consists of a full-bridge primary side, a center-tapped secondary side with parallel-connected synchronous rectifiers to reduce the conduction loss. A series-connected 2 × 2:1:1 matrix transformer integrated in a single core is designed. The high output current is distributed among multiple secondary stages, ensuring low interconnect inductance between the transformer and the synchronous rectifiers, and reduces winding loss.

eGaN FET Selection for the LLC Resonant Converter eGaN FETs are well suited for the soft-switching LLC resonant converters [1]. Compared to Si MOSFETs of similar rating, their lower gate charge (QG) and the 5 V gate operation result in much lower gate power consumption. In addition, GaN FETs have much lower output capacitance and therefore need much less charge to achieve zero voltage switching (ZVS). This would either reduce the dead-time and increase the effective power delivering time, or reduce the required magnetizing current, circulating energy and conduction losses.

The 100 V, 3.2 mΩ EPC2218 [2] and 40 V, 1.5 mΩ EPC2024 [3] as shown in Figure 2 are selected for the primary and secondary-side power devices respectively. Both eGaN FETs can operate at up to 150°C junction temperature. The small form factor of GaN FETs makes it possible to fit in 8 FETs in the limited 1/8th power brick size for the synchronous rectifiers.

System Overview The block diagram of the circuit design is shown in Figure 3. The design also includes on-board housekeeping power supply, digital controller, and input and output voltage sensing. The PWM signals are generated by the dsPIC controller dsPIC33CK32MP102-I/2N [4] from Microchip. On-board housekeeping power supply generates the 5 V for the gate drivers, and the 3.3 V for the controller.

How to Design a High-Efficiency 48 V, 1 kW LLC Resonant Converter in a 1/8

th Brick Size Using eGaN FETs EFFICIENT POWER CONVERSION

Figure 1. Power architecture schematic of the 48 V, 1 kW LLC resonant converter.

Figure 2. Photo of the bump side of EPC2218 (left) and EPC2024 (right).

2 mm2.3 mm

3.5 mm6.05 mm

Housekeepingpower supply

3.3 V 5 V

PWM’s

Measure Measure

Gate drive signals

VIN

VOUT

Digitalcontroller

Gate drivers

Power stage

Figure 3. Block diagram of the LLC resonant converter.

CIN COUT

Q1

CrTR1

SR2

SR6

SR1

SR5

SR4

SR8

SR3

SR7

Lr 2:1:1

TR22:1:1

Lm

Lm

Q3

Q2

Q4

VINVOUT

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Figure 5. EPC9149 and motherboard assembly showing the input and output connections.

Figure 4. The 1 kW, 4:1 ratio, LLC resonant converter using EPC2218 and EPC2024.

Figure 6. Power efficiency as function of output power at 48 V input voltage and 12 V output voltage.

Experimental ValidationThe EPC9149 1 kW, 4:1 ratio GaN FET-based LLC resonant converter shown in Figure 4 is built to verify the design.

The EPC9149 converter module is set up on a motherboard for evaluation. Figure 5 shows the motherboard and EPC9149 installed on it. The main input and output connections, measurement ports, bulk input and output capacitors, USB and communication port are located on the motherboard.

HVhousekeeping PSU

Controller

Programming& commsport

Resonant capacitor bank:ultra low ESR, voltage stable

Transformer

Transformer- Embedded in PCB- ML91S core

Recti�erEPC2024’s

Recti�er

Outputcapacitor

Input Capacitors

EPC2218’s (2x)

Top Side

Bottom Side

Primary HB 2

Resonant capacitor bank

3.3 V housekeeping PSU

Primary HB 1

HVhousekeeping PSU

USB

Communications connection

LLC module48 Vconnection

12 V connection-2

12 V connection-1

12 V Kelvin Sense48 V Kelvin Sense

0

98

97

96

95

94

93

92

91

40

35

30

25

20

15

10

550 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 972

POUT (W)

E�cie

ncy (

%)

Loss

(W)

97.5% peak e�ciency at 400 W

96.7% e�ciency at 1 kW

48 V input to 12 V output

70 V

60 V

50 V

40 V

30 V

20 V

10 V

0

-10 V-238 ns -33 ns 164 ns 384 ns 584 ns 784 ns 984 ns 1.184 μs 1.384 μs 1.584 μs 1.784 μs

Synchronous recti�er FETs drain

Primary side switch-node

200 ns/div20 V/div

200 ns/div20 V/div

200 ns/div4 A/div

Figure 7. (a) Switching waveforms at 48 V input voltage and 1 kW load condition, (b) Resonant tank current waveform.

(a)

(b)

The overall power loss and efficiency at 48 V input and 12 V output including the housekeeping power consumption is given in Figure 6 with 97.5% peak efficiency and 96.7% full-load efficiency.

The measured switching waveforms, at 48 V input, 12 V and 83 A output are shown in Figure 7(a). ZVS was achieved on the primary side devices as evident by the absence of overshoot and ringing. The resonant current flowing in the tank circuit is also shown in Figure 7(b). The circuit is tuned for operation above resonance to reduce conduction loss on the secondary side FETs.

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For More Information

Please contact info@epc-co.com or your local sales representative

Visit our website: epc-co.com

Sign-up to receive EPC updates at

bit.ly/EPCupdates or text “EPC” to 22828 EFFICIENT POWER CONVERSION

eGaN is a registered trademark of Efficient Power Conversion Corporation

M2 x 10 mm screws (2x)Top heatsink

Heatsink TIM

Secondary FET TIM (8x)Secondary FETs

M1.4 x 16 mm screws (2x)M1.4 nuts (2x)

Copper heat spreader(topside)

Primary FET TIM (2x)Primary FETs

Bottom Heat Spreader

Bottom Heat Sink

Heatsink TIM

Top heat spreader

A combination of custom shape heat spreaders and a finned heatsink for the top and bottom side of the EPC9149 board are designed. The thermal solution assembly is shown in Figure 8. Copper heat spreaders are placed on top of both primary and secondary side FETs to spread their heat to the outer structure. The copper heat-spreaders include contour features that allow parts of the heat-spreader to rest on the printed circuit board to provide addition cooling, mechanical stability and define the correct spacing between the heat-spreader and the FET top surface. It only requires a gap filler TIM to be added underneath of the heat spreader pieces to provide insulation and high thermal conductivity between the components and the metal surface of heat spreaders. Mechanical screws are inserted on the board to hold the entire mechanical structure together.

Figure 9 shows the converter module assembled with the heatsink and the direction of air flow.

PCB

eGaN(side wall)

Copper heat-spreader

Heatsink

Test �xtureboard

Air-�ow direction

Figure 10. Thermal image of the EPC9149 operating at 48 VIN , 12 V, 83.3 A output and 400 LFM forced air cooling, thermal steady

state reached after 10 minutes, highest board temperature.

The EPC9149 is intended for bench evaluation at normal ambient temperature. The addition of a heat-spreader or heatsink and forced air cooling can significantly increase the current rating of the power devices, but care must be taken to not exceed the absolute maximum die junction temperature of 150°C. Figure 10 shows the thermal image of the module at steady state condition.

ConclusionThe EPC9149 is a 48 V input, 1 kW output, 4:1 conversion ratio, LLC resonant converter in the 1/8th power brick size built using eGaN FETs. Measured peak efficiency is 97.5% and full power efficiency is 96.7% including housekeeping power consumption. The low gate capacitance, output charge and on-resistance and the small form factor of the eGaN FETs are the key to achieving this at a power density exceeding 1227 W/in3.

References[[1] A. Lidow, M. de Rooij, J. Strydom, D. Reusch, and J. Glaser, GaN Transistors for Efficient Power Conversion, 3rd ed. Wiley, 2019.

[2] EPC. (2021). “EPC2218 datasheet,” [online]. Available: https://epc-co.com/epc/Portals/0/epc/documents/datasheets/epc2218_datasheet.pdf

[3] EPC. (2021). “EPC2024 datasheet,” [online]. Available: https://epc-co.com/epc/Portals/0/epc/documents/datasheets/epc2024_datasheet.pdf

[4] Microchip. (2018-2020). [online]. Available: https://www.microchip.com/wwwproducts/en/dsPIC33CK32MP102

[5] EPC. (2021). “EPC9149 QSG,” [online]. Available: https://epc-co.com/epc/Portals/0/epc/documents/guides/epc9149_qsg.pdf

Figure 9. EPC9149 thermal measurement setup.

Figure 8. Thermal solution assembly process for the EPC9149 module.