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Product(s)

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Product(s)

AN1645 - APPLICATION NOTE

5/22

Figure 5: CCM waveform and Ck circuit

In a typical Forward converter for telecom application, the DC input voltage has a 1:2 variability range

(typically 36V-72V). Consequently the secondary winding voltage has also 1:2 variability. The resistor

divider can be calculated in order to have about 2.8V at the Ck pin at 36V input; at 72V input, the Ck pin

reaches 5.6V. Even if this value is higher than the maximum voltage on the CK pin, it can be accepted

limiting the current flowing into the pin to 10mA.

Figure 6: Synchronization with a resistor divider

In case the Forward converter has a higher variability range 1:3 or 1:4, at minimum input voltage, 2.8V

must be guaranteed at the Ck pin. At maximum input voltage, the voltage at the Ck pin will be 7.5V or

10V. This voltage exceeds the absolute maximum ratings of the device. If R2 limits the current flowing

into the Ck pin to a value below the maximum Ck current value indicated in the datasheet (10mA), the

On Time

Off Time

Turn-Off

Spike

On Time

Off Time

Turn-Off

Spike

+5V

+5V

C1

R1

R2

D1

STSR2P

4

5

2

6

3

Ck

INHIBIT

Vcc

SGLGND

SETANT

FREE WHEEL

FORWARD