Control IC for Switched-Mode Power Supplies
q Fold-back characteristics provides overload protection for
q Burst operation under secondary short-circuit condition
q Protection against open or a short of the control loop
q Switch-off if line voltage is too low (undervoltage switch-off)
q Line voltage depending compensation of fold-back point
q Soft-start for quiet start-up without noise generated by the
q Chip-over temperature protection implemented (thermal
q On-chip ringing suppression circuit against parasitic
oscillations of the transformer
The IC TDA 4605-2 controls the MOS-power transistor and performs all necessary regulation and
monitoring 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 follow:
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.
The switching frequency is reduced with increasing load and decreasing AC-voltage. The duty
factor primarily depends on the AC-voltage.
The output voltage is only dependent on the load.
Maximal output power is available at this point of the output characteristic.
The energy transferred per operation cycle is limited at the top. Therefore the output voltages
declines by secondary overloading.
Pin Definitions and Functions
Information Input Concerning Secondary Voltage. By comparing the
regulating voltage - obtained from the regulating winding of the transformer - with
the internal reference voltage, the output impulse width on pin 5 is adjusted to the
load of the secondary side (normal load, overload, short-circuit, no load).
Information Input Regarding the Primary Current. The primary current rise in
the primary winding is simulated at pin 2 as a voltage rise by means of external
RC-circuit. If a voltage level is reached which is derived from the control voltage
at pin 1, the output impulse at pin 5 is terminated. The RC-circuit is used to set
the maximum power of the foldback point.
Input for Primary Voltage Monitoring: In the normal operation V3 is moving
between the thresholds V3H and V3L (V3H > V3 > V3L).
V3 < V3L: SMPS is switched OFF (line voltage too low).
V3 > V3H : Compensation of the overload point regulation (controlled by pin 2)
starts at V3H : V3L = 1.7.
Output: Push-pull output for charging or discharging the gate capacity of the
Supply Voltage Input. From the voltage at pin 6 a stable internal reference
voltage VREF and the switching thresholds V6A , V6E , V6 max and V6 min for the
supply voltage detector are derived. If V6 > V6E then VREF is switched on. The
reference voltage will be switched off if V6 < V6A . In addition the logic is only
enable, for V6 min < V6 < V6 max .
Input for Soft-Start and Integrator Circuit. The capacitor connected to ground
causes a slow increase of the duration of the output pulse during start-up and an
integrating response of the control amplifier.
Input for the Feedback of the Oscillator. After the oscillations of the SMPS
started, every transition of the feedback voltage through zero (falling edge)
triggers an output pulse at pin 5. The trigger threshold is at + 50 mV typical.
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