SC4901ITSTRT datasheet(17/20 Pages) SEMTECH

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 2005 Semtech Corp.

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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

••

OUTP UT

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