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High efficiency DC-DC PoL conversion using the
DMS3015SSS
Dean Wang, Applications Engineer, Diodes Inc.
Introduction
This application note describes the benefits of using the DIOFET™ DMS3015SSS in the low-side
MOSFET position of synchronous buck point-of-load (PoL) converters. The DIOFET™ products
monolithically integrates a power MOSFET and anti-parallel Schottky diode into a single die. This
technology reduces both RDS(ON) and anti-parallel diode VSD induced losses, ultimately improving the
efficiency of PoL converters. The electrical and thermal performance benefits of the DMS3015SSS are
illustrated through comparison with competing solutions in a popular synchronous buck converter.
Low-side MOSFET considerations for synchronous Buck converter
Microprocessor based computing, telecom and industrial systems have become increasingly
sophisticated and ever more powerful. This places stringent demands on the power density and
dissipation of the PoL converters. The synchronous buck converter is the most popular topology for
PoL converters due to its low conduction loss and high switching frequency enabling miniaturization of
magnetic component.
The primary building blocks of a synchronous buck converter are, as illustrated in Figure 1: the highside control MOSFETs (Q1 and Q3); low-side synchronous MOSFETs (Q2 and Q4); output inductors
(L1 and L2) and PWM controller. The PWM controller is selected basedon its ability to supply sufficient
current to drive the MOSFETs at high frequency and to provide simple, single feedback loop, voltage
mode control with fast transient response. In this example, the PWM IC is a dual-output step down
controller that operates from a 3V to 28V input voltage.
Figure 1. Dual-channel POL converter using DMS3015SSS and DMG4466SSS
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The majority of the power losses in a PoL converter are due to losses in the external MOSFETs.
These are:
•
Conduction losses in low-side synchronous MOSFETs — Q2 and Q4
•
Switching losses in high-side MOSFETs — Q1 and Q3
•
Body diode conduction in Q2 and Q4
•
Reverse-recovery charge losses due to Q2 and Q4 body diode
The efficiency of a PoL converter can be improved if these losses can be mitigated. As the duty cycle
of the PWM IC is low then the conduction cycle of the synchronous MOSFET can be as high as 73%.
Therefore the biggest improvement in PoL performance will be achieved by selecting a synchronous
MOSFET with low RDS(ON), to minimize these conduction losses.
Furthermore, conduction losses can be further reduced by ensuring that the forward voltage drop of
the synchronous MOSFET’s anti-parallel diode is as low as possible. The anti-parallel diodes, which
are normally the body diodes of Q2 and Q4, conduct during the PWM controller’s dead time. It is for
this reason that external Schottky diodes are often used in parallel with the low-side MOSFET since
the VSD of a Schottky is much lower than the intrinsic body diode of the MOSFET.
Schottky diodes also provides softer reverse recovery characteristic, lowering the turn ON losses of
Q1 and Q3 at high frequency. However, the disadvantage of such implementation is that the external
diode adds capacitance to the circuit, increasing the MOSFET turn OFF switching loss. . Extra care is
also needed to minimize layout’s parasitic inductance; otherwise the effectiveness of the Schottky will
be reduced—if not negated.
DIOFET™ improves POL’s efficiency and reliability
The DMS3015SSS has a low leakage Schottky structure interdigitated within the MOSFET cell to
create an ideal solution for low voltage, fast switching conversion. It simplifies the design and
provides:
•
Low VSD is achieved without compromising the RDS(ON) value
•
Integrated Schottky diode has lower QRR and softer reverse recovery characteristic with
respect to intrinsic body diode
•
20V gate breakdown voltage to ensure robustness against voltage spike
Table 1. comparison of DMS3015SSS electrical parameters against two competing SchottkyMOSFET solutions
Parameters
SYMBOL
Drain-Source
Voltage
Gate Threshold
Voltage
On- Resistance
Test
Condition
VDSS
VGS(th)
RDS(ON)
Gate-Source
Charge
Gate-Drain
Charge
Diode Forward
Voltage
Qgs
Qgd
VSD
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VGS=VDS,
ID=250uA
1
DMS3015SSS
Competitor A
Competitor B
UNIT
30
30
30
V
1.5
2.5
VGS=10V
8.5
VGS=4.5V
9.5
1
1.6
3
11.9
10
14.9
12
1.3
1.65
2.5
12
11.5
14
15
17
21
V
mΩ
VGS=10V,
VDS=15V
VGS=0V, IS=1A
3.40
2.10
2.00
4.30
3.00
3.90
nC
0.45
1.00
0.60
0.70
0.43
0.50
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V
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Performance evaluation
The performance of DMS3015SSS was evaluated against that of competitor ‘A’ and ‘B’ in a two output
buck converter as shown in figure 1. The low gate charge, fast switching DMG4496SSS was selected
as the high side switch (Q1 and Q3) and the DMS3015SSS, competitor A and competitor B were in
turn evaluated in the synchronous position (Q2 and Q4). The efficiency of the 3.3V and 5V output
voltages were measured whilst the converters output load was then varied from 0.8A to 8A in 1A steps.
These measurements were taken under two input voltage conditions: at 19V’s, to simulate the input
voltage of a notebook PC from the AC Adaptor, and then at 9V to simulate the output voltage from a
notebook PC battery pack. The results of these efficiency measurements are shown in Figures 2 and
3.
Figure 2a and 3a demonstrate that under 19V input conditions the DMS3015SSS increases the output
of the PoL converter by upto 1% when compared with competitor B and by upto 0.5% when compared
against competitor A. Furthermore, under 9V battery conditions the DMS3015SSS increases
efficiency by upto 1% when compared against competitor B and is marginally better than competitor A.
Efficiency Vi=9V DMG4466SSS + DMS3015SSS
Efficiency Vi=9V DMG4466SSS + Competitor A
Efficiency Vi=9V DMG4466SSS + Competitor B
98%
98%
97%
97%
96%
96%
95%
95%
Efficiency (%)
Efficiency (%)
Efficiency Vi=19V DMG4466SSS + DMS3015SSS
Efficiency Vi=19V DMG4466SSS + Competitor B
Efficiency Vi=19V DMG4466SSS + Competitor A
94%
93%
94%
93%
92%
92%
91%
91%
90%
90%
0.08
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.08
1.00
2.00
3.00
Load (A)
4.00
5.00
6.00
7.00
8.00
Load (A)
(a)
(b)
Figure 2. 5V output POL converter efficiency at (a) Vin = 19V and (b) Vin = 9V
Efficiency Vi=9V DMG4466SSS + DMS3015SSS
Efficiency Vi=9V DMG4466SSS + Competitor A
Efficiency Vi=9V DMG4466SSS + Competitor B
98%
100%
97%
99%
96%
98%
95%
97%
Efficiency (%)
Efficiency (%)
Efficiency Vi=19V DMG4466SSS + DMS3015SSS
Efficiency Vi=19V DMG4466SS + Competitor A
Efficiency Vi=19V DMG4466SSS + Competitor B
94%
93%
96%
95%
92%
94%
91%
93%
90%
92%
0.08
1.00
2.00
3.00
4.00
Load (A)
(a)
5.00
6.00
7.00
8.00
0.08
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
Load (A)
(b)
Figure 3. 3.3V output POL converter efficiency at (a) Vin = 19V and (b) Vin = 9V
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Another important circuit consideration is the limitation of shoot-through or cross conduction current in
the circuit. At high switching frequencies there is a risk that a temporary shoot-through or cross
conduction could happen. This occurs during the switch-on interval of the high-side MOSFET as a
very high dV/dt on the phase node (see Figure 1) that induces a gate voltage on the synchronous
MOSFETs (Q2 and Q4). If the synchronous MOSFET sees an induced voltage greater than the gate
threshold VGS(th), then it could be turned ON whilst the high-side switch is ON. This causes excessive
power dissipation in both MOSFETs, and could ultimately lead to devices failure.
Shoot through can be minimized by selecting a synchronous MOSFET that has a low gate
capacitance ratio (Qgd/ Qgs). The DMS3015SSS has a gate capacitance ratio of 1.1 which is much
lower than that of either of the competing solutions summarized in table 1. Figure 4 illustrates that no
shoot-through was observed when the DMS3015SSS was used as the synchronous MOSFET even
when the phase node is subjected to rate of change of 600V/ns.
Figure 4 Operating waveforms at the turn-on transition of the high-side switch (Pink: Low-side
MOSFET’s VGS; Blue: High-side MOSFET’s VGS; Cyan: Phase voltage)
Furthermore, a thermal camera was used to measure the temperature of the high side and
synchronous MOSFETs during these efficiency measurements. The PoL evaluation board was
convection cooled in 25ºC ambient during the recording of the data. As can be seen in Figure 5 the
DMS3015SSS operates at a temperature that is 5% lower than that of either of the competing
Schottky-MOSFET solutions.
This lower operating temperature reduces conduction loss in the surrounding components and
increases reliability, every 10ºC reduction in the junction temperature of the MOSFET will double the
lifetime reliability of the PoL converter.
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Figure 5. 5V output, 19V input POL converter thermal measurements for (a) DMS3015SSS, (b)
Competitor ‘A’ and (c) Competitor ‘B’
(a)
(b)
(c)
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Conclusion
It has been demonstrated that the efficiency of the PoL converter can be up to 1 % higher when the
DMS3015SSS is used than when competing solutions are used. Furthermore, the DMS3015SS
provides this increase in performance whilst operating at a lower temperature, reducing conduction
losses in the surrounding components and doubling the reliability of the PoL converter. Furthermore,
the DMS3015SSS operates at a higher efficiency enabling the PoL converter to have a higher current
handling capability or operate with a lower device junction temperature.
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IMPORTANT NOTICE
Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or
other changes without further notice to any product herein. Diodes Incorporated does not assume any liability arising out of the
application or use of any product described herein; neither does it convey any license under its patent rights, nor the rights of
others. The user of products in such applications shall assume all risks of such use and will agree to hold Diodes Incorporated
and all the companies whose products are represented on our website, harmless against all damages. Diodes Incorporated
does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels.
LIFE SUPPORT
Diodes Incorporated products are specifically not authorized for use as critical components in life support devices or systems
without the express written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body, or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use
provided in the labeling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or to affect its safety or effectiveness.
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