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WHITE PAPER: WP003
Flyback Converters
Both of the converter designs were based as close as possible on the controller
manufacturers’ suggested circuits. As the driver requirements for eGaN FETs
are somewhat dierent to traditional MOSFET’s, it was necessary to add dis
-
crete external drivers to the eGaN converter versions as shown in gure 1.
The MOSFET and eGaN FET devices used for each of the converters are listed
in table 1.
SMALL SIZE 13 W FLYBACK CONVERTER
Figure 2 on the following page shows an implementation of a 48 V to 5 V, 13
W PoE-PD yback converter utilizing the LT1725 IC from Linear Technology
[1] which is a general purpose yback controller. The datasheet for the
LT1725 species a maximum operating frequency
of 250 kHz, however, in this implementation the fre
-
quency was adjusted to 400 kHz to show the higher
frequency advantages of the eGaN FET.
Both eGaN FET and MOSFET-based converter ecien
-
cies for operation at 300 kHz and 400 kHz are shown
in gure 3. The MOSFET (FDS2582, 150 V, 66 mΩ [2])
is compared to an EPC1012 die (200 V, 100 mΩ [3]).
It can be seen that the eGaN FET eciency results
are consistently about 2% higher than the MOSFET
converter for all but heavy load despite a 50% higher
R
DS(ON)
. In fact, the eGaN FET eciency at 400 kHz is
still higher than the equivalent MOSFET 300 kHz ver
-
sion over most of the load range.
Waveforms in gure 4 shows eGaN FET gate and
drain operation at 400 kHz, as well as the converter’s
output voltage.
Improve DC-DC Flyback Converter
Eciency Using eGaN FETs
EFFICIENT POWER CONVERSION
Alex Lidow PhD, CEO and Johan Strydom, PhD, Vice President Applications Engineering, Efficient Power Conversion Corporation
DC-DC converter designers can achieve low cost at low power
densities by using yback converters and enhancement mode
gallium nitride transistors. To evaluate the performance of eGaN
FETs in a yback converter, two dierent converter designs were
created and compared to MOSFET equivalent versions of the
same design. Both converters were targeted at Power over Eth
-
ernet (PoE) low power (13 W) Powered Device (PD) applications.
The rst of these two converters was aimed at small size; the sec
-
ond design was aimed at high eciency.
FDS2582 150 4.1 66 19 4.4 1254 290
EPC1012 200 3 100 1.9 0.9 190 90
SIR464 40 50 4.2 28.2 9 118 38
EPC1015 40 33 4 11.5 2.2 46 9
Table 1 - MOSFET and eGaN FET specications comparison
Table 1: Comparison of MOSFETs and eGaN FETs used in the yback converters
Figure 1: Discrete gate driver circuit used in eGaN yback controllers.
Figure 1 – EPC recommended eGaN FET Gate-drive Circuit
SI1029X
47 Ω
2.2 Ω
1µF
SIA513
DRV
Source
Gate
5 V
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EPC – EFFICIENT POWER CONVERSION CORPORATION | WWW.EPC-CO.COM | COPYRIGHT 2012 | | PAGE 2
Flyback Converters
Figure 2 – LT1725 based 48 V to 5 V, 13 W PoE converter schematic.
0.18 Ω
35.7 k,
1%
3.01 k,
1%
47pF
0.1цF
2.7 k
51 k
51 k
51 k
1 nF
820 k
33 k
100 pF
Vin
PGND
ISENSE
SFST
ROCMP
RCMPC
OSCAP
VC
FB 3VOUT
UVLO
SGND
MINENAB
ENDLY
TON
VCC
GATE
LT1725
FDS2582
V in
150pF
68
Ω
2.2цF,
100 V
1цF
47 K
+
15uF
22
Ω
1
2
V in
36~57 V
2.2цF,
100 V
BAS16
C1048
470 pF
18
Ω
+
150цF,
6.3V
5 V, 2 A
51
Ω,
1 W
10цF
12CWQ06
1
2
3
4
Figure 2: LT1725-based 48 V to 5 V, 13 W PoE yback converter.
Figure 3: Eciency comparison of eGaN FETs vs.
MOSFETs for a 48 V to 5 V, 13 W yback converter.
Figure 4: LT1725 eGaN FET Gate and Drain waveforms (f=400 kHz,
V
IN
=48 V, V
OUT
=5 V, I
OUT
= 2.5 A) CH1: eGaN FET Gate drive,
CH2: eGaN FET Drain, CH3: Converter output voltage.
60%
62%
64%
66%
68%
70%
72%
74%
76%
78%
80%
Eciency (%)
0 0.5 1 1.5 2 2.5
300 kHz MOSFET
300 kHz eGaN FET
400 kHz MOSFET
400 kHz eGaN FET
Output Current (A)
Output Voltage
Gate Voltage
Drain Voltage