M51995AP/AFP

REJ03D0835-0300 Rev.3.00 Jun 06, 2008

Page 33 of 40

2. Over current limiting curve

(1) In case of feed forward system

(a) Feed forward system

(b) Primary and secondary current

CLM

Figure 31 Primary and Secondary Current Waveforms Under the Current Limiting Operation

Condition on Feed Forward System

Figure 31 shows the primary and secondary current wave-forms under the current limiting operation.

At the typical application of pulse-by-pulse primary current detecting circuit, the secondary current depends on

the primary current. As the peak value of secondary current is limited to specified value, the characteristics

curve of output voltage versus output current become to the one as shown in figure 32.

The demerit of the pulse by pulse current limiting system is that the output pulse width can not reduce to less

than some value because of the delay time of low pass filter connected to the CLM terminal and propagation

As the frequency becomes higher, the delay time must be shorter. And as the secondary output voltage becomes

higher, the dynamic range of on-duty must be wider; it means that it is required to make the on-duration much

more narrower.

So this system has the demerit at the higher oscillating frequency and higher output voltage applications.

Output Current

Figure 32 Over Current Limiting Curve on Feed Forward System

To improve these points, the oscillating frequency is set low using the characteristics of VF terminal.

When the current limiting circuit operates under the over current condition, the oscillating frequency decreases

in accordance with the decrease of VF terminal voltage, if the VF is lower than 3.5V.And also the dead time

becomes longer.

Under the condition of current limiting operation, the output current I2 continues as shown in figure 31. So the

low pass filter as shown in figure 33.

It is recommended to use 15 k

Ω for R