IX9907N datasheet(7/10 Pages) IXYS

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INTEGRATED CIRCUITS DIVISION

IX9907

R02

www.ixysic.com

2.2 Normal Operation

Because the IX9907 employs quasi-resonant

operation, its PWM switch-on is set by the zero

crossing of the auxiliary winding voltage, and the

switch-off is set by the current sense voltage.

2.2.1 Zero Crossing & Switch-On Determination

The application example in Figure 1 shows that the

voltage from the auxiliary winding is connected to the

zero crossing pin, ZCV, through an RC network. This

network provides a delay so that switch-on can occur

at a voltage valley, thus enhancing efficiency. The

required time delay,

t, should be approximately

one-fourth of the oscillation period (determined by the

inductance of the transformer’s primary and the

drain-source capacitance of the built-in power switch

MOSFET) minus the propagation delay from

zero-cross detect to power switch-on, tdelay.

This time delay,

t, should be matched by adjusting

the RC network.

After the power switch is turned off, its VDS will show

some oscillation. This will also show on the ZCV input.

To avoid a mis-triggered switch turn-on, a ringing

suppression circuit is implemented. The suppression

time has two values that depend on the voltage at

ZCV. If VZCV is greater than 0.7V, then the time is

2.5

s nominal. If V

ZCV is less than 0.7V, then the time

is 42

s nominal. Turn-on of the power switch cannot

occur during the suppression time, but does occur

after a zero-crossing is detected. In the case of a

missed zero-crossing, a maximum off-time is

implemented. After the power switch has been off for

s nominal (t

offMax), it is turned back on.

2.2.2 Switch-Off Determination

In the application circuit the primary current is sensed

by RCS. The voltage across this resistor, VCS, is

applied to the CS input of the device. It is processed

internally (VCSINT = VCS(3) + 0.7V), and compared to

the voltage at the VR pin, which is a scaled version of

the rectified line voltage. When VCSINT > VR, the

power switch is turned off.

Leading-edge blanking is used to prevent a false

trigger caused by the voltage spike across RCS at the

moment of power switch turn-on. This blanking time,

tBLKCS, is nominally 450nS. To prevent transformer

saturation, a maximum on-time circuit is implemented.

Max on-time for the power switch is 30

s nominal.

2.2.3 Foldback Point Correction

When the AC line voltage increases, the power switch

on-time decreases, which increases the operating

frequency. As a result, with a constant primary current

limit, the output power increases. To provide output

power regulation with respect to line voltage, the

internal foldback point correction circuit varies the VCS

limit. The VCS limit is decreased in response to an

increase in AC line voltage. The relationship between

VCSMax and VIN is shown in Figure 3.

Figure 3 VCSMax vs. VIN

t

OSC

-----------

delay

–

ZCV

ZCV1

ZCV2

---------------



IN (V)

100

150

200

250

300

350

400

450

0.6

0.7

0.8

0.9

1.0

1.1

Variation of V

CS Limit Voltage

With Respect to V

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