SC4901ITSTRT datasheet(16/20 Pages) SEMTECH

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 2005 Semtech Corp.

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SC4901

POWER MANAGEMENT

Combi-Sync Topology

Combi-Sync is a unique secondary side topology that

overcomes most of the problems associated with

synchronous rectification of isolated outputs. It also

incorporates synchronous post regulation, making it the

ideal solution for low voltage, high current outputs.

Independently regulated multiple outputs can be derived

from a common transformer winding. The output stage

replaces the conventional rectifiers and regulators with

three MOSFETs, two of which switch at zero voltage. The

topology inherently eliminates turn on shoot through

without complicated timing or look ahead circuits to

maximise efficiency. All secondary switching circuits are

naturally synchronised to primary which simplifies noise

suppression. There are no separate synchronising, current

sensing or gate driving signals crossing the isolation

boundary. In most cases, there will be no need for a

separate bias supply on the secondary side further

simplifying the system design.

The primary side in a Combi-Sync circuit is a typical single

ended forward convertor which may be regulated or free

running. An additional benefit of the Combi-Sync topology

is the zero current turn on and turn off for the primary

MOSFET as well. The free running mode is preferred when

there are multiple outputs without minimum load and cross

regulation constraints. Input voltage feedforward is

recommended to achieve volt-second clamp and minimise

core losses in the free running mode.

Background on Synchronous Rectification and Post

Regulation

The synchronous rectifier technology is widely used in non

isolated DC-DC convertors but its use has been limited in

isolated convertors because of various difficulties. An

example of synchronous rectification on the secondary side

of a forward convertor is shown in Fig 9. DF and DR are

the parasitic body diodes of their respective FETs. The

forward MOSFET QF is turned ON when the transformer

secondary voltage goes positive and the rectifying MOSFET

QR is turned ON when the transformer secondary is

negative. Two approaches have been used to drive the

MOSFETs. One is the self driven scheme where the

transformer secondary itself provides the gate voltage for

the appropriate FET. While the scheme is simple and has

a very low cost, it has several limitations.

Fig 9) Isolated Synchronous Rectification

One is that QR can conduct synchronously only while the

transformer is being reset. Thereafter there is no gate

voltage to drive it and the circuit must employ diode.

Secondly since the gate voltages, and the peak of

transformer secondary, must be with 4.5V to 20V under

all conditions, the scheme may fail at lower voltage and

wide input ranges.

An alternative is to use a control driven approach where a

synchronous controller provides the gate drive. This

provides the low loss FET conduction over the entire cycle

and is not limited by output or input ranges. However it is

not without its own problems. At the instant transformer

voltage turns positive the body diode of QF gets forward

biased. At the same time, QR would also be fully conducting

and the result is a shorted winding just when the primary

switch is trying to turn ON. To prevent catastrophe it is

necessary to turn QR OFF prior to transformer voltage going

positive. This requires an advanced signal from the primary

side crossing the isolation boundary. Attempts have been

made to avoid this by complex timing or look ahead circuits

on the secondary side itself and several patents have been

issued for them.

It should be understood that all these techniques for

isolated synchronous rectification have been restricted to

a single unregulated secondary output..... The existing

circuits only rectify the output but do not synchronously

regulate it any further. Nor is it possible to generate multiple

outputs from the same winding. Post regulation of the

isolated outputs has been implemented so far using either

the saturable magnetic inductor or a power MOSFET in

series with the forward diode. The saturable magnetic

element is bulky and inefficient at high frequencies. The

circuit with series MOSFET is widely known and well

documented. Fig 10) shows a standard implementation.

Application Information (Cont.)

••

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