TDA4605-3 Datasheet, PDF (5/20 Pages) Siemens Semiconductor Group – Control IC for Switched-Mode Power Supplies using MOS-Transistor

TDA 4605-3
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.
In the period before the switch-on threshold is reached the IC is suppled via resistor R ₁ ; during the
start-up phase it uses the energy stored in C₂ , under steady state conditions the IC receives its
supply voltage from transformer winding n₁ via diode D1. The switching transistor T1 is a BUZ 90.
The parallel connected capacitor C₃ and the inductance of primary winding n ₂ determine the
system resonance frequency. The R ₂-C₄-D2 circuitry limits overshoot peaks, and R ₃ protects the
gate of T1 against static charges.
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 fold-back 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.
Regulation of the switched-mode power supplies via 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 R ₈-C₇ network suppresses parasitic overshoots (transformer oscillation). The
peak voltage at pin 2, and thus 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 transformer has supplied its energy to the load, the control voltage passes through zero.
The IC detects the zero crossing via series resistors R ₉ connected to pin 8. But zero crossings are
also produced by transformer oscillation after T1 has turned off if output is short-circuited. Therefore
the IC ignores zero crossings occurring within a specified period of time after T1 turn-off.
The capacitor C₈ connected to pin 7 causes the power supply to be started with shorter pulses to
keep the operating frequency outside the audible range during start-up.
On the secondary side, five output voltages are produced across winding n₃ to n₇ rectified by D4 to
D8 and smoothed by C₉ to C₁₃ . Resistors R ₁₂ , R ₁₄ and R ₁₉ to R ₂₁ are used as bleeder resistors.
Fusable resistors R ₁₅ to R ₁₈ protect the rectifiers against short circuits in the output circuits, which
are designed to supply only small loads.
Semiconductor Group
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