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TDA4605-15

Features

Fold-back characteristics provides overload protection for
external components
Burst operation under secondary short-circuit condition implemented
Protection against open or a short of the control loop
Switch-off if line voltage is too low (undervoltage switch-off)
Line voltage depending compensation of fold-back point
Soft-start for quiet start-up without noise generated by the transformer
Chip-over temperature protection implemented (thermal shutdown)
On-chip ringing suppression circuit against parasitic oscillations of the transformer

AGC-voltage reduction at low load

The IC TDA 4605-3 controls the MOS-power transistor and performs all necessary control and
protection functions in free running flyback converters. Because of the fact that a wide load range
is achieved, this IC is applicable for consumer as well as industrial power supplies.

The serial circuit and primary winding of the flyback transformer are connected in series to the input
voltage. During the switch-on period of the transistor, energy is stored in the transformer. During the
switch-off period the energy is fed to the load via the secondary winding. By varying switch-on time
of the power transistor, the IC controls each portion of energy transferred to the secondary side
such that the output voltage remains nearly 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 will start if the transformer has transferred the stored
energy completely into the load.

In the different load ranges the switched-mode power supply (SMPS) behaves as follows:

No load operation

The power supply is operating in the burst mode at typical 20 to 40 kHz. The output voltage can be
a little bit higher or lower than the nominal value depending of the design of the transformer and the
resistors of the control voltage divider.

Nominal operation

The switching frequency is reduced with increasing load and decreasing AC-voltage.
The output voltage is only dependent on the load.

Overload point

Maximal output power is available at this point of the output characteristic.

Overload

The energy transferred per operation cycle is limited at the top. Therefore the output voltages
declines by secondary overloading.