WHITE PAPER: WP003
Flyback Converters
Improve DC-DC Flyback Converter
Efficiency Using eGaN FETs
EFFICIENT POWER CONVERSION
Alex Lidow PhD, CEO and Johan Strydom, PhD, Vice President Applications Engineering, Efficient Power Conversion Corporation
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 different to traditional MOSFET’s, it was necessary to add discrete external drivers to the eGaN converter versions as shown in figure 1.
DC-DC converter designers can achieve low cost at low power
densities by using flyback converters and enhancement mode
gallium nitride transistors. To evaluate the performance of eGaN
FETs in a flyback converter, two different converter designs were
The MOSFET and eGaN FET devices used for each of the converters are listed
in table 1.
created and compared to MOSFET equivalent versions of the
same design. Both converters were targeted at Power over Ethernet (PoE) low power (13 W) Powered Device (PD) applications.
Small Size 13 W Flyback Converter
The first of these two converters was aimed at small size; the sec-
Figure 2 on the following page shows an implementation of a 48 V to 5 V, 13
W PoE-PD flyback converter utilizing the LT1725 IC from Linear Technology
[1] which is a general purpose flyback controller. The datasheet for the
LT1725 specifies a maximum operating frequency
of 250 kHz, however, in this implementation the frequency was adjusted to 400 kHz to show the higher
frequency advantages of the eGaN FET.
ond design was aimed at high efficiency.
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
Both eGaN FET and MOSFET-based converter efficiencies for operation at 300 kHz and 400 kHz are shown
in figure 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 efficiency results
are consistently about 2% higher than the MOSFET
converter for all but heavy load despite a 50% higher
RDS(ON). In fact, the eGaN FET efficiency at 400 kHz is
still higher than the equivalent MOSFET 300 kHz version over most of the load range.
Table 1: Comparison of MOSFETs and eGaN FETs used in the flyback converters
Table 1 - MOSFET and eGaN FET specifications comparison
5V
SIA513
SI1029X
1µF
DRV
Waveforms in figure 4 shows eGaN FET gate and
drain operation at 400 kHz, as well as the converter’s
output voltage.
47 Ω
2.2 Ω
Gate
Source
Figure 1: Discrete gate driver circuit used in eGaN flyback controllers.
Figure 1 – EPC recommended eGaN FET Gate-drive Circuit
EPC – EFFICIENT POWER CONVERSION CORPORATION | WWW.EPC-CO.COM | COPYRIGHT 2012 |
| PAGE 1
WHITE PAPER: WP003
Flyback Converters
36~57 V
1
V in
2
+
15uF
22 Ω
BAS16
1цF
V in
47 K
2.2цF,
100 V
2.2цF,
100 V
470 pF 18 Ω
68 Ω
35.7 k,
1%
3.01 k,
1%
PGND
GATE
ISENSE
VCC
SFST
TON
2.7 k
ROCMP ENDLY
0.1цF
RCMPC MINENAB
47pF
OSCAP
SGND
1 nF
VC
UVLO
FB
3VOUT
LT1725
Vin
820 k
12CWQ06
150pF
51 k
51 k
51 k
10цF
150цF, +
6.3V
5 V, 2 A 1
2
3
4
C1048
51 Ω,
1W
FDS2582
0.18 Ω
33 k
100 pF
Figure 2 – LT1725
48 V48toV 5toV,
PoEflyback
converter
schematic.
Figure 2: based
LT1725-based
5 V,13
13 W
W PoE
converter.
80%
78%
Output Voltage
Gate Voltage
76%
Efficiency (%)
74%
72%
70%
300 kHz MOSFET
68%
300 kHz eGaN FET
66%
64%
400 kHz MOSFET
62%
400 kHz eGaN FET
60%
0
0.5
1
1.5
2
Drain Voltage
2.5
Output Current (A)
Figure 3: Efficiency comparison of eGaN FETs vs.
MOSFETs for a 48 V to 5 V, 13 W flyback converter.
Figure 4: LT1725 eGaN FET Gate and Drain waveforms (f=400 kHz,
VIN=48 V, VOUT=5 V, IOUT= 2.5 A) CH1: eGaN FET Gate drive,
CH2: eGaN FET Drain, CH3: Converter output voltage.
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WHITE PAPER: WP003
Flyback Converters
VCC
0.1 µF
36~57 V
1
2
2.2 µF, 2.2 µF,
100 V 100 V
+
+
+4.7 µF, 20 Ω
50 V
Vin
CMPD2838
61.9 K
1SMB5936B
10 Ω
0.01 µF
0.1 µF,
100 V
15 K 3300 pF V
CC
220 pF
VIN
SS
FB RT/SYNC
COMP CS
VCC U VLO
OUT GND
LM5020
Zero
1K
2.87 K
10 Ω,
1W
470 pF
3.3 V, 4.5 A
1
2
3
4
+
100 µF
U VLO
MBRD83
CMPD2838
0.01 µF
B0695AL
100 µF
12.4 K
VCC
U VLO
1.2 K
0.01 µF
2.43 K
FDS2582
100 pF
100 Ω
0.47 Ω x2
470 µF,
6.3 V
1K
PS2801-1-L
0.01 µF
TL V431
1.47 K
Figure 5: LM5020 based 48 V to 3.3 V, 13 W flyback converter.
Figure 5 – LM5020 based 48 V to 3.3 V, 13 W PoE con Verter schematic.
90%
High Efficiency 13 W Flyback Converter
Note how little the drop in efficiency for the eGaN FET solution is when the
switching frequency is increased to 500 kHz. The drop in efficiency is about
0.5% for the eGaN FET-based solution versus about 2% for the MOSFET solution. Waveforms in figure 7 show eGaN FET gate and drain voltage at 500 kHz,
as well as the converter’s output voltage.
Summary
In this paper, two eGaN FET-based 13 W flyback converters were built and
evaluated side by side with standard MOSFET designs. One was optimized
for small size, and the other was optimized for high efficiency. In both cases,
eGaN FET flyback converters showed higher efficiencies and the potential of
reducing system costs over their MOSFET counterparts.
88%
86%
84%
Efficiency (%)
Figure 5 shows the schematic of the flyback converter using the LM5020 from
National Semiconductor [4]. This circuit is virtually identical to the application
given in the LM5020 datasheet. The converter efficiency was measured at 300
kHz and 500 kHz and the results are shown in figure 6. It can be seen that
the 300 kHz MOSFET converter and eGaN FET converter efficiency results are
almost identical despite a 50% higher RDS(ON) for the eGaN FET compared to
the MOSFET. This is a result derived from the eGaN FETs’ lower switching loss.
82%
80%
78%
300 kHz MOSFET
76%
300 kHz eGaN FET
74%
400 kHz MOSFET
72%
400 kHz eGaN FET
70%
0
0.5
1
1.5
2
2.5
3
3.5
Output Current (A)
Figure 6: Efficiency comparison of eGaN FET vs MOSFET for
48 V to 3.3 V, 13 W flyback converter.
REFERENCES:
[1] http://cds.linear.com/docs/Datasheet/1725fa.pdf
[2]thttp://www.fairchildsemi.com/ds/FD%2FFDS2582.pdf
[3] http://epc-co.com/epc/documents/datasheets/EPC1012_datasheet_final.pdf
[4] http://www.national.com/ds/LM/LM5020.pdf
Figure 7: LM5020 waveforms (f=500 kHz, VIN=48 V, VOUT=3.1V, IOUT= 4 A).
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