General circuit description

AN1344

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General circuit description

The power supply has been designed for the upper voltage range. The lower voltage range

utilizes a voltage doubler to raise the bulk voltage to 2 times the peak of the input line

voltage. In the doubling mode, the current charges one capacitor for each phase of the line,

therefore doubling the voltage. When SW1 is open, both capacitors are charged in series

resulting in a bulk voltage equal to the peak of the line input.

A wire jumper can be installed at production for units destined for countries using the lower

range. The switching frequency operates at 100 kHz. The output can deliver 18 V from no

load to 6 A continuous. The mode of operation ranges from discontinuous at high line

minimum load to continuous at low line max load. This mode of operation was chosen to

minimize the high peak currents of the discontinuous mode of operation.

The VIPer100A-E can be regulated in secondary mode with an optocoupler giving excellent

output of the auxiliary winding. Depending on the coupling of the transformer, a 15%

regulation can be achieved. In this application, by taking advantage of the dual regulation, a

current limit scheme is obtained. This VIPer100A-E advantage, along with the transformer

design, constitutes the overcurrent circuit. The transformer is designed for a turn ratio of

operation for a universal input and an inductance to run in continuous conduction mode at

one-half the output load. The coupling between the secondary to auxiliary winding along

with the VIPer100A-E dual regulation plays an important part in the current limit.

Under typical operation, the output is tightly regulated through U2 and U3, the optocoupler

and TL431 respectively. As the output current increases, it causes the voltage at the

output current exceeds the maximum limit. At this point, primary regulation takes over and

the output starts to fold-back.

The output uses an STMicroelectronics 100 V Schottky diode for better efficiency. C9 and

C10 are low ESR capacitors which manage the ripple current. U3 provides the reference

and the feedback to tightly regulate the output. C7, C8, and R6 form the feed back loop

compensation to optimize stability during transients.

Table 1.

Electrical specification

Parameter

Results

Input voltage

Output voltage J2

Load regulation (0.6 to 6 A) from set point

+/- 0.6%

Line regulation (at max load)

+/- 0.05%

Efficiency

Output ripple voltage

15 mV max

Input power at no load

1.5 W typical

Transient response, 50% load step

+/- 350 mV, +/- 1.9%, 200 µs settling time

EMI

EN55022 and FCC class B