TNY274PG Ver la hoja de datos (PDF) - Power Integrations, Inc

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TNY274PG

Enhanced/ Energy Efficient/ Low Power Off-line Switcher

Power Integrations, Inc 

load is proportional to the primary inductance of the transformer

and peak primary current squared. Hence, designing the supply

involves calculating the primary inductance of the transformer

for the maximum output power required. If the TinySwitch-III

is appropriately chosen for the power level, the current in the

calculated inductance will ramp up to current limit before the

DCMAX limit is reached.

Enable Function

TinySwitch-III senses the EN/UV pin to determine whether or

VEN

CLOCK

DCMAX

IDRAIN

VDRAIN

Figure 6. Operation at Near Maximum Loading.

PI-2749-082305

TNY274-280

not to proceed with the next switching cycle. The sequence of

cycles is used to determine the current limit. Once a cycle is

started, it always completes the cycle (even when the EN/UV

pin changes state half way through the cycle). This operation

results in a power supply in which the output voltage ripple

is determined by the output capacitor, amount of energy per

switch cycle and the delay of the feedback.

The EN/UV pin signal is generated on the secondary by

comparing the power supply output voltage with a reference

voltage. The EN/UV pin signal is high when the power supply

output voltage is less than the reference voltage.

In a typical implementation, the EN/UV pin is driven by an

optocoupler. The collector of the optocoupler transistor is

connected to the EN/UV pin and the emitter is connected to

the SOURCE pin. The optocoupler LED is connected in series

with a Zener diode across the DC output voltage to be regulated.

When the output voltage exceeds the target regulation voltage

level (optocoupler LED voltage drop plus Zener voltage), the

optocoupler LED will start to conduct, pulling the EN/UV pin

low. The Zener diode can be replaced by a TL431 reference

circuit for improved accuracy.

ON/OFF Operation with Current Limit State Machine

The internal clock of the TinySwitch-III runs all the time. At

the beginning of each clock cycle, it samples the EN/UV pin to

decide whether or not to implement a switch cycle, and based

on the sequence of samples over multiple cycles, it determines

the appropriate current limit. At high loads, the state machine

sets the current limit to its highest value. At lighter loads, the

state machine sets the current limit to reduced values.

VEN

CLOCK

DCMAX

VEN

CLOCK

DCMAX

IDRAIN

IDRAIN

VDRAIN

VDRAIN

PI-2667-082305

Figure 7. Operation at Moderately Heavy Loading.

Figure 8. Operation at Medium Loading.

PI-2377-082305

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