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PDF DN94 Data sheet ( Hoja de datos )
| Número de pieza | DN94 | |
| Descripción | High performance rectifiers significantly improve server power supply efficiency | |
| Fabricantes | Diodes | |
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A Product Line of
Diodes Incorporated
DN94
High performance rectifiers significantly improve server
power supply efficiency
Dr Yong Ang, Diodes Incorporated
Introduction
Alternative appr oaches ca n improve the existing me thods of using Schottky diod es in h igh
reliability, h igh ava ilability and low downt ime se rver sta ndby power su pplies. T wo dif ferent
secondary circuit approaches, including syn chronous rectification a nd Su per Barrier Rectifie r
(SBR™) were used to find the highest practica l efficiency in a +5VSB power supply capable of
delivering 27W p eak p ower. Ef ficiency improvements were mea sured i n a n experimental
converter, at 3.5% for synchronous rectification and 0.5% for SBR over the Schottky solution. This
design note discusses the details.
Server standby power supplies
Desktop derived servers are de signed to ope rate in high re liability a nd availability a pplication
environments whe re it must b e working continuously wit h ext remely low unsch eduled
downtime. T ypically t he architecture of t he power supply fo llows a t wo st age conve rsion
approach as show n in F igure 1. The front en d stage is a Continu ous Co nduction Mode active
power factor correcting Boost converter and delivering a constant 400V DC rail to a downstream
forward DC-DC converter processing the tightly regulated +/-12V, +5V and +3.3V rails required by
the system.
AC input EM I f ilt e r
and
re ctif ie r
Ac tive
pow er
facto r
co rrecti o n
+400V
Cb ulk
Iso lated
DC- D C
co n ver ter
+12V, +5V,
+3.3V, -12V
4-ch an n el
seco n d ar y
moni to rin g IC
Auxiliary power supply
+12VC C_P ,
100m A
R1
100kΩ
C1
1nF
1kV
D1
S1M
R2
D2
C2
22μ F
O ption #2:
T1 A2 Super B arri er B 2
R ectifie r
L1
2.4μ H
C4
470μ F
O ption #1:
A1 Sync hr onous B1
R ectifie r
RTN
Aux
D3
U S1J
SM A
C6
47μF
U4
Z SR1200
Voltage
r egul ator
SO T 223
+5VSB, 5A
C5
470 μ F
C7
1μF
R3
Vcc HV
GND FB
U1
D
R4
200Ω
R5
+ 5V S B
C3
R6
5. 1kΩ
RTN
10kΩ
U2 0.1μF
C Y1
GND
U3
Z T L431
R7
4.7 kΩ
1nF
Figure 1 - Server power supply architecture diagram
A se cond f lyback DC- DC con verter is re quired to ge nerate a isolate d 5V ou tput wit h ±5 %
tolerance. Thi s voltage sou rce is active whe never the input AC vol tage source i s appl ied an d
remains operational even if the main output rails from the isolated DC-DC converter are disabled.
Issue 1 - November 2008
© Diodes Incorporated 2008
1
www.zetex.com
www.diodes.com
http://www.Datasheet4U.com
1 page  
DN94
Another side effect in synchronous rectification is the MOSFET output capacitance COSS on the
added sy nchronous MOSFE T intr oduces a stra y cap acitance, which reson ates with the
transformer leakage inductance and leads to a larger voltage spike at turn off. To overcome this,
use RC damping components in Figure 2 or redesign the transformer to reduce the voltage spike.
Variation of operating f requency at f ull load could also be observ ed wit h th e sy nchronous
rectifier. The synchronous MOSFET capacitance is also reflected across the transformer and adds
to the total output capacitance of the primary switch, therefore reducing the operating frequency
from 67 kHz t o 6 0kHz. Nev ertheless, this doe s not de grade pe rformance o f t he sy nchronous
rectifier compared with diode rectification.
High performance rectifier #2 - Super Barrier Rectifier™
The synchronous rect ifier prov ides a sign ificant ef ficiency impro vment over a Sch ottky diode.
However if the extra complexity of the synchronous rectifier is not desirable, the Super Barrier
Rectifier™ (SBR™) can be used to give power supply designers an a dditional lever to improve
the ov erall e fficiency o f t heir po wer sup ply de sign simp ly by r eplacing t he ou tput re ctifying
diodes. The key to the SBR™ technology lies in the patented structure with a MOS channel region
formed under the thin gate oxide layer, Figure 7, where the “super” barrier for majority (electron)
carriers is created without the unreliable Schottky contact.
The “supe r” ba rrier maint ains a simila r or be tter f orward bias per formance ove r Sch ottky but
with higher reliability . Moreover, the SBR™ impr oves the re verse leakage performance. The
potential barrier lowering due to the image charge, which is essential for the Schottky diode, is
absent in SBR. The SBR reverse current is typical of a P-N diode, where reverse current consists
of the constant injection and growing ionization currents. This improves thermal stability of the
device at elevated temperature.
Figure 7 -
The Low VF SBR™ therefore has lower forward voltage than the competitive Schottky devices in
the ma rket, red ucing the fo rward conduction lo sses of th e output rect ifiers and impro ving the
auxiliary power supply efficiency.
Evaluation results
The te st re sults for t he se rver’s st andby po wer sup ply ar e shown in F igure 7. This part icular
design operates from the 400V PFC rail and provides three outputs at a continuous output power
of 27W . The power supply efficiency with MBRB30H60CT as the output r ectifier on the h igh
current +5VSB output is 81.2% at 20% loading and it increases to 82.8% at full load.
Issue 1 - November 2008
© Diodes Incorporated 2008
5
www.zetex.com
www.diodes.com
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet DN94.PDF ] | |
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