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AFL28XXS Series

Figure III. Preferred Connection for Parallel Operation

Synchronization of Multiple Converters

Parallel Operation-Current and Stress Sharing

Internally, these ports differ slightly in their function. In use,

a low on Enable 1 completely shuts down all circuits in the

converter, while a low on Enable 2 shuts down the secondary

side while altering the controller duty cycle to near zero.

Externally, the use of either port is transparent to the user

save for minor differences in stndby current. (See

specification table).

When operating multiple converters, system requirements

often dictate operation of the converters at a common

frequency. To accommodate this requirement, the AFL

series converters provide both a synchronization input and

a synchronization output.

The sync input port permits synchronization of an AFL

converter to any compatible external frequency source

operating between 500KHz and 700KHz. This input signal

should be referenced to the input return and have a 10% to

90% duty cycle. Compatibility requires transition times less

than 100ns, maximum low level of +0.8Vand a minimum

highvel of +2.0V. The sync output of another converter

which has been designated as the master oscillator provides

a convenient frequency source for this mode of operation.

When external synchronization is not required, the sync in

pin should be left open (unconnected )thereby permitting

the converter to operate at its’ own internally set frequency.

The sync output signal is a continuous pulse train set at

550 ± 50KHz, with a duty cycle of 15 ± 5%. This signal is

referenced to the input return and has been tailored to be

compatible with the AFL sync input port. Transition times

are less than 100ns and the low level output impedance is

less than 50

Ω. This signal is active when the DC input

voltage is within the specified operating range and the

converter is not inhibited. This output has adequate drive

reserve to synchronize at least five additional converters.

A typical connection is illustrated in Figure III.

Figure III. illustrates the preferred connection scheme for

operation of a set of AFL converters with outputs operating

in parallel. Use of this connection permits equal sharing

among the members of a set whose load current exceeds

the capacity of an individual AFL. An important feature of

Optional

Synchronization

Connection

Power

Input

(Other Converters)

Share Bus

1

6

AFL

7

12

- Sense

Enable 2

+ Vout

Return

+ Sense

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

1

6

AFL

7

12

- Sense

Enable 2

+ Vout

Return

+ Sense

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

1

6

AFL

7

12

- Sense

Enable 2

+ Vout

Return

+ Sense

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

to Load

the AFL series operating in the parallel mode is that in addition

to sharing the current, the stress induced by temperature

will also be shared. Thus if one member of a paralleled set

is operating at a higher case temperature, the current it

provides to the load will be reduced as compensation for

the temperature induced stress on that device.