STR2A100 SERIES APPLICATION NOTE

Rev. 0.3

Copy Right: SANKEN ELECTRIC CO., LTD.

Page.13

Figure 7-13

Output current at OCP

without input compensation

Variation resulting

from detection delay time

Low AC input

High AC input

Output current, IOUT

During the transition to the burst-oscillation, if VCC terminal voltage decreases to VCC(BIAS)= 9.5V(TYP), Bias

Assist Function is activated and stabilizes the standby mode operation, because ISTARTUP is provided to VCC

terminal so that VCC terminal voltage does not decrease to VCC(OFF).

However, if Bias Assist Function is always activated during standby mode, the power loss increases. Therefore

VCC terminal voltage should be more than VCC(BIAS), for example, by adjusting the turns ratio between auxiliary

winding and secondary winding and/or R2 in figure 7-5.

7.5

Random Switching Function

The IC modulates its switching frequency randomly within △f= 5kHz of Average Switching Frequency,

fOSC(AVE）= 67kHz(TYP), by superposing the modulating frequency on fOSC(AVE) in normal operation.

The conduction noise with this function is smaller than that without this function, and this function simplifies

noise filtering of the input lines of power supply.

7.6

Latch Function

The latch function stops switching operation and a protection state is latched when OVP and/or TSD protection

functions activate.

After stopping switching operation with latch mode, VCC terminal voltage drops. When VCC terminal voltage

decreases to VCC(BIAS)= 9.5V(TYP), Bias Assist Function is activated to prevent VCC terminal voltage from

reaching VCC(OFF), and then the latch function is maintained.

Releasing the latch function can be done when VCC terminal voltage reaches VCC(OFF) or less by simply turning

off the AC input.

7.7

Overcurrent Protection Function (OCP)

Overcurrent Protection Function (OCP) detects each drain peak current level of a power MOSFET on

pulse-by-pulse basis, and limits the output power. This function incorporates AC Input Compensation Function

to reduce OCP point variation for AC input voltage, without any additional external components.

This OCP function detects the drain current by a current detection resistor, ROCP, which is connected between

S/OCP terminal and GND terminal. When the voltage drop on both sides of ROCP increases to internal OCP

threshold voltage, the power MOSFET is turned off.

PWM control ICs usually have some detection delay time on

OCP detection. The steeper the slope of actual drain current in

high AC input voltage is, the longer the actual detection point is,

compared with internal OCP threshold voltage. And thus the

actual OCP point limited the output current usually has some

variation depending on AC input voltage as shown in figure 7-13.

The IC incorporates a built-in Input Compensation Function that

superposes a signal with a defined slope to the detection signal on

S/OCP terminal as shown in figure 7-14.

When AC input voltage is lower and the ON-duty cycle is wider,

the OCP compensation level increases. And thus the OCP point

in low AC input voltage increases to minimize the difference of

OCP points between low AC input voltage and high AC input

voltage.