eGaN FETs enable a leap in DC transformer capability

By John Glaser,
Director of Applications Engineering,
Efficient Power Conversion,
www.epc-co.com

 

 

One
of the key technologies that drove the adoption of electricity as the dominant
and most versatile form of energy harnessed by the human race was the
transformer. This simple device enabled the efficient conversion of voltages,
currents, and impedances, which in turn made practical the electrification of
the world and all electrical and electronic devices to follow. Since the early
transformers were AC devices, they drove the world towards AC power generation,
distribution, and use, despite Thomas Edison and his best efforts to promote DC
over AC [1].

 

However,
today we live in an electronic world, and a digital electronic one at that.
This digital electronic world is a DC world. This world is enabled by
switch-mode power electronics, which provides the same benefits as the original
AC transformer to the DC world, as well as new ones like regulation and
control. However, there is a particular incarnation of a DC-DC converter that
is very true to its AC transformer ancestor, namely the DC transformer, often referred
to by DCX [2]. The term DCX typically refers to an unregulated DC-DC converter
that converts voltage and current with a fixed conversion ratio, just like an
AC transformer. The DCX is especially useful in distributed DC power systems
found in data centers and compute farms.

 

In
this article, we see how the revolution in wide bandgap semiconductors, in the
form of eGaN FETs, enables a leap in DCX capability with an example of a 700 W
hard-switched 4:1 DCX with a 48 V nominal input and 12-V output in an 1/8th
brick format.

 

What
is a DCX?

 

A
DCX generally consists of an inverter, an AC transformer, and a rectifier, as
shown in Fig. 1. A modern DCX uses semiconductor switches to form the
inverter and rectifier, and runs at high frequency to minimize the size of the
transformer. If the rectifier is implemented with active switches (synchronous
rectifiers or SRs), the DCX can be capable of bi-directional power flow, and
the roles of the inverter and rectifier can be exchanged [3]. The switches are
controlled in such a manner that the DCX obeys the standard transformer
equations as closely as possible, that is;

 

VIN
= NVOUT

 

NIIN
= IOUT

 

Note
that a DCX does not provide regulation as part of normal operation. This allows
the circuit to be highly optimized for maximum efficiency and power density. A
modern DCX is often expected to provide some additional features, which may
include overcurrent detection, current limiting, enable/disable power control,
and measurement and diagnostic capability.

 

fapo_Power_EPC01_700WdcTransformer_oct2016

 

Fig.
1: Block diagram of modern DC transformer (DCX).

 

 

There
has been a great deal of effort in developing various implementations of the
DCX. Some use a variation of the series resonant converter running at or very
close to resonance [4], [5]. This has the benefits of soft-switching and
inherent transformer-like action while running the inverter and rectifier at
fixed duty cycle, but RMS currents are higher than a PWM converter, and
implementation of current limiting is a challenge. Another promising approach
is the dual-active bridge (DAB) [6], [7]. This approach can make use of
soft-switching and still have low RMS currents. However, the DAB does not
inherently follow the transformer equations, and thus active control is
required at all times.

 

The
simplest DCX is implemented with a standard hard-switched topology, shown in Fig. 2. The converter operates as a
buck converter running at or very close to the maximum possible effective duty
cycle D = 1, equivalent to all switches in Fig. 2 running at 50%. This
maximizes transformer utilization, and allows the use of a very small inductor
value due to the low volt-seconds applied to the inductor. In fact, the
inductor is not strictly necessary for ideal DCX operation, but a small
inductor allows current limiting when necessary and serves to filter switching
spikes and associated ringing.

 

fapo_Power_EPC02_700WdcTransformer_oct2016

 

Fig.
2: Isolated hard-switch DCX based on buck converter.

 

 

Where
is it used?

 

The
typical application of DCX converters is as part of a distributed power system,
as shown earlier in Fig. 3. Such a
system only needs close voltage regulation at the load, but the other benefits
of a distributed power system, such as high efficiency, increased safety and
reduced power bus cost are still desired. The benefits of such systems are well
known due to the history of utility power and the widespread use of AC
transformers [8].

 

fapo_Power_EPC03_700WdcTransformer_oct2016

 

Fig.
3: Typical use of DCX in a distributed DC power system.

 

 

eGaN
DCX

 

The
superior FOMs for eGaN FETs are well documented [9], and the efficiency gains
provided by eGaN FETs have already shown that power density of hard-switched
PWM brick-type converters can be increased by nearly 70% [10], [11]. It makes
sense to see what kind of performance can be achieved in a DCX converter using
eGaN FETs.

 

Approach

 

In
order to evaluate the performance benefits of eGaN FETs in a DCX converter, it
was decided to take the simplest approach possible, namely the use of a
standard hard-switched converter (Fig. 2). Since EPC has already designed an
eight-brick demonstration board, the EPC9115 500 W 50 V to 12 V nominal, fully
regulated bus converter (Fig. 4), it
made sense to start with this as the baseline design [12]. In order to run as a
DCX, only a few simple changes were made. First, the inductor was changed from
a 470 nH, 0.9 mΩ molded powder core inductor to a 180 nH, 0.3 mΩ gapped ferrite
inductor. Note that running as a DCX would actually allow a smaller-value
inductor, but there were no commercially smaller-value inductors with lower DCR
that were big enough to connect to the pads on the PCB. Second, the maximum
duty cycle clamp was changed in the software from 0.98 to 0.985, to allow a
slightly higher output for a given input. Finally, deadtime was decreased from
25 ns to 15 ns.

 

fapo_Power_EPC04_700WdcTransformer_oct2016

 

 

Fig.
4: EPC9115 Eighth-brick demonstration board used as basis for DCX.

 

Results

 

The
modified EPC9115 converter was tested at three input voltages (48 V, 50 V, 52
V) over a range of load current up to a maximum load current of 62 A. Figure 5 shows efficiency results for
the eGaN DCX at 25C (not thermal steady-state). A very flat efficiency curve
reaches 97% over a wide range of current for all input voltages, and the output
power at 62 A is 710 W for the 48 V input and 771 W for the 52 V input. Worst
case efficiency at 62 A load is still 96.6%.

 

fapo_Power_EPC05_700WdcTransformer_oct2016

 

Fig.
5: eGaN DCX efficiency and output voltage for 3 values of VIN (48 V,
50 V, 52 V). Note maximum output current of 62 A, corresponding to 710 W at VIN
= 48 V and 771 W at VIN = 52 V.

 

The
results of Fig. 5 are useful for a baseline comparison with other converters,
but do not represent a realistic operating condition. Figure 6 shows a thermal image of the converter at thermal steady
state with 400 LFM airflow at 24°C, with VIN = 48 V and IOUT
= 58.4 A for an output power of 667 W. In this condition, the maximum
temperature of 108°C is on the transformer core. The secondary side eGaN FETs
are running at 106°C, with the primary side eGaN FETs running at a relatively
cool 84°C.

 

fapo_Power_EPC06_700WdcTransformer_oct2016

 

Fig.
6: Thermal image of eGaN DCX with VIN = 48 V. IOUT = 58.4
A and POUT = 667 W, running at thermal steady state with 400 LFM (2
m/s) airflow at 24°C.

 

 

 

The proven efficiency and power
density of eGaN FETs in a hard-switching converter far exceeds what is possible
with silicon MOSFETs in a similar converter. While it may be possible to
achieve similar performance by using silicon MOSFETs in a soft-switched
converter, such a design is challenging and has numerous limitations as
previously discussed. In addition, it is likely that eGaN FETs will benefit
such an approach due to the superior figures of merit.

 

Note than all testing was done without
heat sinks. Most high-power
silicon-based brick converters employ an integral heat sink. It’s been shown
that the top-side heat removal capability of eGaN FETs can allow up to 30%
higher current, hence large improvements could be made [13].

 

Finally, the DCX performance was
evaluated based on modification of the fully regulated EPC9115 eight-brick
converter. However, designing a converter as a DCX up front allows many further
optimizations, such as control simplification, improvement of the transformer,
and using a smaller inductor to shorten the high-current output path. At
60 A, a 1 m resistor dissipates 3.6 W. For A 700W DCX, this is
a 10-15% increase in losses for each milliohm. At these high output currents, a
key challenge is how to get the current out of such a small converter.

 

 

The superior performance of eGaN FETs
enable engineering to push the limits of traditional hard-switched DCX
performance far beyond what is possible with silicon MOSFETs.

 

 

 

[1]     T. S. Reynolds and T. Bernstein, “Edison
and ’the chair’,” IEEE Technology and
Society Magazine
, vol. 8, no. 1, pp. 19–28, Mar. 1989.

 

[2]     R. P. Severns and G. E. Bloom, Modern DC-to-DC Switchmode Power Converter
Circuits
. Van Nostrand Reinhold, 1985.

 

[3]     R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics,
Second. Springer, 2001, p. 912.

 

[4]     Y. Ren, M. Xu, J. Sun, and F. C. Lee, “A
family of high power density unregulated bus converters,” Power Electronics, IEEE Transactions on, vol. 20, no. 5, pp.
1045–1054, 2005.

 

[5]     D. Reusch and J. Strydom, “Evaluation of
gallium nitride transistors in high frequency resonant and soft-switching DC-DC
converters,” in Applied Power Electronics
Conference and Exposition (APEC), 2014 Twenty-Ninth Annual IEEE
, 2014, pp.
464–470.

 

[6]     D. Costinett, H. Nguyen, R. Zane, and D.
Maksimovic, “GaN-FET based dual active bridge DC-DC converter,” in Applied Power Electronics Conference and
Exposition (APEC), 2011 Twenty-Sixth Annual IEEE
, 2011, pp. 1425–1432.

 

[7]     M. H. Kheraluwala, “High-power
High-frequency Dc-to-dc Converters,” University of Wisconsin, Madison, 1991.

 

[8]     J. W. Coltman, “The transformer [historical
overview],” IEEE Industry Applications
Magazine
, vol. 8, no. 1, pp. 8–15, Jan. 2002.

 

[9]     A. Lidow, J. Strydom, M. de Rooij, and D.
Reusch, GaN Transistors for Efficient
Power Conversion
, 2nd ed. Wiley, 2015.

 

[10]   J. Glaser, J. Strydom, and D. Reusch, “High
Power Fully Regulated Eighth-brick DC-DC Converter with GaN FETs,” in PCIM Europe 2015; International Exhibition
and Conference for Power Electronics, Intelligent Motion, Renewable Energy and
Energy Management; Proceedings of
, 2015, pp. 406–413.

 

[11]   D. Reusch and J. Glaser, DC-DC Converter Handbook – A Supplement to GaN Transistors for
Efficient Power Conversion
, 1st ed. Power Conversion Publications, 2015.

 

[12]   Efficient Power Conversion Corporation, EPC9115 Demonstration Board, (http://epc-co.com/epc/Products/DemoBoards/EPC9115.aspx)
2015.

 

[13]   D. Reusch, J. Strydom, and A. Lidow, “Thermal
Evaluation of Chip–Scale Packaged Gallium Nitride Transistors,” in Applied Power Electronics Conference and
Exposition (APEC), 2016 IEEE
, 2016.

 

 

Originally published on Power Electronics News