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DN94 डेटा पत्रक PDF( Datasheet डाउनलोड )


डेटा पत्रक - High performance rectifiers significantly improve server power supply efficiency - Diodes

भाग संख्या DN94
समारोह High performance rectifiers significantly improve server power supply efficiency
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DN94 pdf
DN94
In the ‘standby’ or ‘off’ mode, it delivers power to the external circuits that perform soft power
control, Wake On LAN (WOL), Wake On Modem (WOM) or suspend state activities. If an external
USB device stimulates the server to transit from Standby to Active mode, the power supply could
be requ ired t o pro vide 5 A curr ent fo r a f ew second s, t herefore t he po wer sup ply must be
designed to that peak power.
A high level of integration is desirable for the standby circuit which is made feasible by using a
PWM regulator that incorporates a 650V MOSFET. The PWM regulator also provides over current
protection to en sure the +5VSB supply will not be damaged under output fault conditions. The
converter is nor mally designed fo r Critical Conduction Mode to red uce MOSFE T t urn o n
switching loss . Furthermore, the flybac k transf ormer size can be redu ced owing to the lowe r
average energy storage whilst its smaller ma gnetizing inductance also yields a b etter transient
line/load response. At light loads the IC will operate in skip cycle mode, reducing its switching
losses and ensuring high efficiency throughout the load range.
The seconda ry w indings of the tr ansformer a re re ctified and filtered t o pr oduce t wo o utputs,
where the +5Vstb main output channel is closed loop regulated through an optocoupler U2. The
low power +12VCC_S is used to supply the supervisory IC monitoring the 12V, +5V and+3.3V rails.
The superv isory IC w ill norma lly shut dow n the ac tive PFC st age and the m ain downst ream
forward converter if those outputs are not sensed at their nominal value. In the design example,
a third bias output +14VCC_P is capable of supplying 100mA to the P FC controller and forward
converter PWM regulator ICs during normal operation.
Critically, much of the flyback converter inefficiency is caused by the Schottky diodes normally
used fo r second ary side re ctification on t he +5VSB out put. Re placing the old ex isting diode
technologies with a more efficient rectifier is recognized as a clear means of drastically improving
power supply ef ficiency. Two diff erent secondary side rectif ier approaches in Figure 1 were
considered including synchronous MOSFET rectifier and the Super Barrier Rectifier.
High performance rectifier #1. Synchronous rectification with ZXGD3101
The ZXGD31 01 ca n emulat e t he performance of an idea l rectif ier by driving a sy nchronous
MOSFET effectively. Figure 2 shows a typical circuit configuration for low side rectification. The
controller can dr aw its power directly from the regulated +12 VCC_S output via emitter-follower
transistor Q1. In ot her cases wh ere a regulated v oltage above 8 V is una vailable, the
recommendation is t o provide a ded icated supply through auxiliary transformer wind ing. RREF
and RBIAS are chosen to be 3kand 1.8kwhich sets the controller turn-off threshold value to -
20mV.
+ 12 VC C_ S, 50 m A
A1
C8
1nF
R9
100 Ω
Q3
Q1
FM M T491
U5
DRA IN
Vcc
GATEL
GATEH GND
Z XG D 3101
R EF
BIAS
RR EF
R BIAS
B1
O ption # 1: Synchr onous r ec tifier
R8
10 kΩ
ZD1
C9
1 μF
Figure 2 - Synchronous rectifier control circuit
www.zetex.com
www.diodes.com
2 Issue 1 - November 2008
© Diodes Incorporated 2008

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