TDA4605-2 Datasheet, PDF (7/23 Pages) Siemens Semiconductor Group – Control IC for Switched-Mode Power Supplies using MOS-Transistors

TDA 4605-2
Circuit Description
Application Circuit
The application circuit shows a flyback converter for video recorders with an output power rating of
70 W. The circuit is designed as a wide-range power supply for AC-line voltages of 180 to 264 V.
The AC-input voltage is rectified by the bridge rectifier GR1 and smoothed by C₁ . The NTC limits
the rush-in current.
The IC includes an internal circuit to avoid the turn-on of the power transistor T1 because of static
charges applied to the transistors gate, during the turn-off state of the IC. The resistor R ₁₃ helps to
limit the spectrum of the radiated noise.
During the conductive phase of the power transistor T1 the current rise in the primary winding
depends on the winding inductance and the mains voltage.
The network consisting of R ₄-C₅ is used to create a model of the sawtooth shaped rise of the
collector current. The resulting control voltage is fed into pin 2 of the IC. The RC-time constant given
by R ₄-C₅ must be designed that way that driving the transistor core into saturation is avoided.
The ratio of the voltage divider R ₁₀/R ₁₁ is fixing a voltage level threshold. Below this threshold the
switching power supply shall stop operation because of the low mains voltage. The control voltage
present at pin 3 also determines the correction current for the foldback point.
This current added to the current flowing through R ₄ and represents an additional charge to C₅ in
order to reduce the turn-on phase of T1. This is done to stabilize the fold-back point even under
higher mains voltages.
The control of the switched-mode power supply is done by means of a control voltage applied to
pin 1. The control voltage of winding n₁ during the off-period of T1 is rectified by D3 smoothed by C₆
and stepped down at an adjustable ratio by R ₅ , R ₆ and R ₇ . The primary peak current, is adjusted
by the IC so that the voltage applied across the control winding, and hence the output voltages, are
at the desired level.
When the energy stored in the transformer is transferred into the load the control voltage passes
through zero. The IC detects the zero crossing via the series R ₉ connected to pin 8. But zero
crossings of the control voltage can also be produced by ringing of the transformer after the turn-off
of the power transistor for T1 or when a short-circuit is applied to the output of the SMPS.
The capacitor C₈ is connected to pin 7. During the start-up phase this capacitor assures pulses with
a shorter duty cycle in order to keep the operating frequency outside the audible frequency range.
On the secondary side of the transformer 3 output voltages are produced using the windings n ₂ to
n₅ , rectified by D4 to D6 and smoothed by C₉ to C₁₁ . The resistor R ₁₂ is used as a bleeder resistor,
the resistors with implemented fuse R ₁₅ and R ₁₆ protect the rectifies against short circuits in the
output circuits, which are designed to supply only small loads.
Version 2.0
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1 Jul 2002