參數(shù)資料
型號(hào)： SC4901
廠商： Semtech Corporation
英文描述： COMBI-SYNC Secondary Side Synchronous Rectifier and Regulator
中文描述： 客貨兩用，同步二次側(cè)同步整流和穩(wěn)壓器
文件頁(yè)數(shù)： 17/20頁(yè)
文件大小： 598K
代理商： SC4901
17
2005 Semtech Corp.
www.semtech.com
SC4901
POWER MANAGEMENT
Application Information (Cont.)
The series FET QS is turned ON and OFF in a controlled
manner synchronously with the secondary waveform. The
width of the ON time pulse is varied to regulate the output.
Fig 10) Synchronous Post Regulation
Again it should be understood that all previous attempts
have used a
non synchronously rectified secondary.
That
is, the transformer secondary was rectified using power
diodes and the series MOSFET was added to synchronously
regulate it further.
The Combi Sync Technique
The Combi Sync, as the name indicates is a unique
secondary configuration that combines synchronous
rectification and post regulation on transformer isolated
multiple secondary voltages. The proposed implementation
as well as typical waveforms are shown in Fig 11) on the
next page. The highlights of its operation are as follows
a) Prior to the transformer voltage going positive, the
rectifying MOSFET QR is conducting. When the voltage goes
positive, QR is turned OFF.
b) After a delay, both QF and QS are turned ON. This delay
prevents both high and low side FETs conducting at the
same time and shorting the secondary winding.
c) The forward pair of QS and QF acts as a bidirectional
switch and can be turned off in a controlled manner
irrespective of the polarity of the transformer voltage. This
is the key to the topology. With SC4901 the forward pair is
turned off on any of the following conditions i) end of the
active PWM duration ii) peak current crosses the overload
limit or, iii) transformer secondary voltage begins to fall.
d) After the forward FETs QS and QF are turned OFF, the
rectifying FET QR is turned after a short delay. QR continues
to conduct until the beginning of the next cycle when
transformer secondary voltage goes positive again. By
modulating the ON time of the high side power MOSFETs
output regulation is achieved.
This arrangement of MOSFETs and their control provides
a highly efficient combination of synchronous rectification
and simultaneous control of isolated secondary voltages.
Trailing Edge Modulation
The Combi Sync topology offers the option of both leading
and trailing edge modulations to achieve synchronous post
regulation. With leading edge modulation, QS and QF are
turned on during the forward mode, with a PWM dictated
delay, but the turn off is synchronised to the falling edge
of the transformer voltage. This results in zero current turn
on for the primary switch but a hard turn off with full load.
On the other hand, in conventional secondary side post
regulation, the trailing edge modulation results in hard turn
on and zero current turn off for the primary switch. However,
the Combi Sync topology already has an inherent turn on
delay for the forward FETs on the secondary side. This
ensures zero current turn on for the primary switch,
irrespective of the modulation scheme used. It is therefore
advantageous to use trailing edge modulation which now
results in both zero current turn on and turn off for the
primary switch. If some form of ZVS is used in primary
control, all of the switching losses may be eliminated on
the primary side. Trailing edge modulation is the method
employed in SC4901.
Switching Waveforms
Theoretical switching waveforms of the Combi Sync
topology are shown on the next page. The first is the
transformer secondary voltage which acts as the reference.
Second and third are the gate drives for forward and
rectifying FETs respectively. Rectified and pulse width
modulated output that appears before the LC filter stage
is shown next. Notice that this rectified output rises after a
delay which is crucial to isolated synchronous rectification.
In the simple isolated synchronous rectifiers, there is no
QS, and QF begins to conduct (initially through its body
diode) at the same instant as the transformer voltage going
positive. Transformer secondary and inductor currents are
also shown. The delay T1-T0 is set by the capacitor CT and
the T3-T2 delay is fixed internally in SC4901
DS
QS
T
C
OUTPUT
相關(guān)PDF資料
PDF描述
SC4901ITSTRT COMBI-SYNC Secondary Side Synchronous Rectifier and Regulator
SC4905A_06 High Performance Voltage Mode PWM Controller
SC4905AIMSTR High Performance Voltage Mode PWM Controller
SC4905AIMSTRT High Performance Voltage Mode PWM Controller
SC4905B High Performance Voltage Mode PWM Controller
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
SC4901ITSTRT 制造商:Semtech Corporation 功能描述:Secondary Side PWM Cntrlr SYNC POST REGUL 1MHz 16-Pin TSSOP T/R 制造商:Semtech Corporation 功能描述:Controller IC
SC4905A 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:HIGH PERFORMANCE VOLTAGE MODE PWM CONTROLLER
SC4905A_06 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Voltage Mode PWM Controller
SC4905AIMSTR 制造商:Semtech Corporation 功能描述:IC, VOLTAGE MODE PWM CTRL, 12V, MSOP-10; Input Voltage:48V; Output Voltage:500mV; Output Current:1mA; Supply Voltage Min:0V; Supply Voltage Max:12V; Digital IC Case Style:MSOP; No. of Pins:10; Frequency:1MHz; Control Mode:Voltage ;RoHS Compliant: No
SC4905AIMSTRT 制造商:Semtech Corporation 功能描述:Voltage Mode PWM Controller 10-Pin MSOP T/R
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