TDA4605

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FUNCTIONAL DESCRIPTION

In free running fly-back converters, the TDA4605

assumes control of a MOS power transistor and all

necessary regulation and monitoring functions.

The serial circuit of power transistor and primary

winding of the flyback transformer is connected to

the input voltage. During the switch-on period of

the transistor, energy is stored in the transformer

and during the switch-off period it is fed to the load

via the secondary winding. By varying the switch-

on time of the power transistor, the IC controls

each portion of energy transferred to the second-

ary side such that the output voltage remains near-

ly independent of load variations.

The required control information is taken from the

input voltage during the switch-on period and from

a regulation winding during the switch-off period. A

new cycle begins as soon as the energy stored in

the transformer has been totally delivered to the

secondary side.

In different load ranges, the SMPS will behave as

follows:

– No-Load Operation: The power supply unit os-

cillates in non continuous mode at a typical fre-

quency of 20 to 40kHz. Depending upon the

transformer winding and the regulated voltage di-

vider, the output voltage can be slightly above

the nominal value.

– Normal Operation: Starting from typ. 200kHz,

the switching frequency falls with increasing load

and decreasing AC voltage. The duty cycle de-

pends primarily on the AC voltage. The output

voltage is only slightly load-dependent.

– Overload Point: Maximum output power is

available at this point of the output characteris-

tics

– Overload: The energy transferred per operation

cycle is limited at the top. Therefore, the output

voltage will fall with secondary overloading.

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