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AFL28XXD 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 idle 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

output.

The sync input port permits synchronization of an AFL

connverter 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 +0.8V and a minimumigh

level 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 indicted, 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 synch output has adequate

drive reserve to synchronize at least five additional

converters. A typical synchronization connection option is

illustrated in Figure III.

Power

Input

(Other Converters)

Share Bus

1

6

AFL

7

12

- Output

Enable 2

+ Output

Return

Trim

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

1

6

AFL

7

12

- Output

Enable 2

+ Output

Return

Trim

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

1

6

AFL

7

12

- Output

Enable 2

+ Output

Return

Trim

Share

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

Optional

Synchronization

Connection

to Positive Load

to Negative Load

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 current

sharing among the members of a set whose load current

exceeds the capacity of an individual AFL. An important

feature of 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.