TDA4605 Datasheet, PDF (5/21 Pages) STMicroelectronics – CONTROL CIRCUIT FOR SWITCH MODE POWER SUPPLIES USING MOS TRANSISTORS

TDA 4605
Circuit Description
Application Circuit
Application circuit shows a flyback converter for video recorders with a power rating of 50 W. The
circuit is designed as a wide-range power supply tor AC-line voltages ot 90 to 270 V. The AC-input
voltage is rectified by 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 supplied 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 ot primary winding 112 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.
While T1 conducts, the current rise in the primary winding depends on the winding’s inductance
and the V_C1 voltage. A voltage reproduction ot the current rise is tabbed using the R₄ - C₅ network
and forwarded into pin 2 ot the IC. The RC-time constant ot R ₄ , R ₅ must be dimensioned correctly
in order to prevent driving the transformer core into saturation.
The R ₁₀/R ₁₁ divider ratio provides the line voltage threshold controlling the undervoltage control
circuit in the IC. The voltage present at pin 3 also determines the overload. Detection of overload
together with the current characteristic at pin 2 controls the on period ot T1. This keeps the cut-off
point stable even with higher AC-line voltages.
Regulation of the switched-mode power supply is 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 ₆ - 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 specitied 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 ftrequency outside the audible range during start-up.
On the secondary side, tive 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
37