Board features

AN1094

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1.1

VIPer100-E pin description

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charging current. It is also connected to the inverted input of the error amplifier.

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Osc: to be connected to an external R-C network to fix the switching frequency.

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Comp: the control pin of the device. Its voltage sets the peak current of the power

switch, and it is the output of the error amplifier. As this amplifier is transconductance,

the compensation network on that pin is simply grounded.

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Drain & source: the two power pins of the device. The source is also the ground

reference of the device for compensation and oscillation networks.

2

Board features

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DC output voltage / load: Vout = + 12 V, 4 A

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Operating frequency: 100 kHz

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Par. 3 gives the complete schematics of this off-line flyback converter.

2.1

Feedback loop

This evaluation board allows the user to choose between primary and secondary regulation.

Primary regulation is done through the primary auxiliary winding which delivers the low level

supply voltage on the Vdd pin of the device. This voltage represents also the secondary

output voltage, if correctly filtered in order to avoid the spikes at the beginning of each off

phase. This mode of running is achieved by putting one strap on JP1 in the "P" position.

Secondary regulation is made with an optocoupler directly from secondary output voltage. It

acts on the "COMP" pin, and the auxiliary winding is delivering the low level supply voltage

at a lower value than when in primary regulation configuration, thanks to R6. The internal

error amplifier is consequently saturated in high state, sourcing a constant current of about

0.5 mA on the "COMP" pin. To address this configuration, just put two straps on JP1 in the

"S" position. In any case, a strap in "G" position must be always present.

2.2

Special operations

No limitation has been set on the VIPer device. This limitation can be implemented on the

"COMP" pin with a zener or a Vbe multiplier, or whatever else able to clamp the voltage on

this pin at about 3 V. When the voltage is clamped, the evaluation board is able to deliver up

to 130 W of power, in continuous running. This operation must not last for more than 5

seconds. The evaluation board is fully protected against short circuits. This type of condition

leads to a burst operation, with peak current of up to 25 A on the output. As the burst duty

cycle is rather low, no risk exists and this situation can continue indefinitely. The board

withstands the no-load condition. Notice that in primary regulation, the output voltage rises

at about 20 V. Secondary regulation provides better behavior by maintaining the output

voltage at about 12 V, and has also the lowest consumption on the main lines: less than 1

watt on a 325 VDC input voltage.