7708 datasheet(24/42 Pages) POWERINT

24 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

Brown-Out

At full load, reduce input voltage until the output just falls out of

regulation. Note the HVDC input voltage, measure the duty

cycle, and check the waveforms for complete core reset. Reduce

the input voltage further until the output just drops below

regulation (it is now in “LR mode”) while checking for complete

core reset further reduce input voltage to find the voltage at

which the converter shuts off (main UVLO)

Temperatures

Use a thermal camera and check device hotspot temperature,

and the temperatures of the snubber components, output

diodes, and magnetics.

Light Load

Examine the high-side MOSFET Source waveform at very light

load. As the load is reduced, the duty cycle will begin to

reduce, and at light enough load the high-side Source voltage

will not reach ground. Keep reducing the load and check for

misfiring in the high-side MOSFET. At 132 kHz operation, some

misfiring at very light load may occur, but it should occur at

such low duty cycles that any audio noise in the main transformer

will not be audible if the transformer is dip-varnished.

Loop Stability

As a first check, do a load step of 50% -> 100% load, and

check for oscillation or excessive ringing. Also check from

100% to peak load (be careful to avoid over-heating when

operating at peak load).

Check for cross-talk between the forward and flyback outputs.

When a load transient is applied to one output, the other output

should only show a very small perturbation, well below the

output ripple specification.

Use a gain-phase analyzer and check gain and phase margin at

full load. Also check the minimum phase at lower frequencies.

Check minimum phase at reduced load (just enough for

continuous mode operation).

7708 Datasheet(PDF) 24 Page - Power Integrations, Inc.