The integrated circuit TDA 4601/D is designed for driving, controlling and protecting the switching
transistor in self-oscillating flyback converter power supplies as well as for protecting the overall
power supply unit. In case of disturbance, the rise of the secondary voltage is prevented. In addition
to the ICs application range including TV-receivers video tape recorders, hifi devices and active
loudspeakers, it can also be used in power supply units for professional applications due to its wide
control range and high voltage stability during increased load changes.

Type

Ordering Code

Package

TDA 4601

Q67000-A2379

P-SIP-9-1

Control ICs for Switched-Mode Power Supplies

Bipolar IC

TDA 4601

P-SIP-9-1

Features

Direct control of the switching transistor

Low start-up current

Reversing linear overload characteristic

Base current drive proportional to collector current

Protective circuit in case of disturbance

Semiconductor Group

06.94

Semiconductor Group

TDA 4601

Circuit Description

The TDA 4601 is designed for driving, controlling and protecting the switching transistor in flyback
converter power supplies during start-up, normal and overload operations as well as during
disturbed operation. In case of disturbance the drive of the switching transistor is inhibited and a
secondary voltage rise is prevented.

Start-Up

The start-up procedures (on-mode) include three consecutive operating phases as follows:

1. Build-Up of Internal Reference Voltage

The internal reference voltage supplies the voltage regulator and effects charging of the coupling
electrolytic capacitor connected to the switching transistor. Current consumption will remain at

< 3.2 mA with a supply voltage up to

approx. 12 V.

2. Enabling of Internal Voltage - Reference Voltage

= 4 V

Simultaneously with

reaching approx. 12 V, an internal voltage becomes available, providing

all component elements, with the exception of the control logic, with a thermally stable and
overload-resistant current supply.

3. Enabling of Control Logic

In conjunction with the generation of the reference voltage, the current supply for the control logic
is activated by means of an additional stabilization circuit. The integrated circuit is then ready for
operation.

The start-up phase above described are necessary for ensuring the charging of the coupling
electrolytic capacitor, which in turn supplies the switching transistor. Only then is it possible to
ensure that the transistor switches accurately.

Normal Operating Mode / Control Operating Mode

At the input of pin 2 the zero passages of the frequency provided by the feedback coil are registered
and forwarded to the control logic. Pin 3 (control input, overload and standby identification) receives
the rectified amplitude fluctuations of the feedback coil. The control amplifier operates with an input
voltage of approx. 2 V and a current of approx. 1.4 mA. Depending on the internal voltage reference,
the overload identification limits inconjunction with collector current simulator pin 4 the operating
range of the control amplifier. The collector current is simulated by an external RC-combination
present at pin 4 and internally set threshold voltages. The largest possible collector current
applicable to the switching transistor (point of return) increases in proportion to the increased
capacitance (10 nF). Thus the required operating range of the control amplifier is established. The
range of control lies between a DC-voltage clamped at 2 V and a sawtooth - shaped rising AC-
voltage, which can vary up to a max. amplitude of 4 V (reference voltage). During secondary load
reduction to approx. 20 W, the switching frequency is increased (approx. 50 kHz) at an almost
constant pulse duty factor (1:3). During additional secondary load decreases to approx. 1 W, the
switching frequency increases to approx. 70 kHz and pulse duty factor to approx. 1:11. At the same
time collector peak current is reduced to < 1 A.

Semiconductor Group

TDA 4601

The output levels of the control amplifier as well as those of the overload identification and collector
current simulator are compared in the trigger and forwarded to the control logic. Via pin 5 it is
possible to externally inhibit the operations of the IC. The output at pin

pin 8 will be inhibited when voltages of

� 0.1 are present at pin 5.

Flipflops for controlling the base current amplifier and the base current shut-down are set in the
control logic depending on the start-up circuit, the zero passage identification as well as on the
enabling by the trigger. The base current amplifier forwards the sawtoothspahed

voltage to the

output of pin 8. A current feedback with an external resistor (

= 0.68

) is present between pin 8

and pin 7. The applied value of the resistor determines the max. amplitude of the base driving
current for the switching transistor.

Protective Operating Mode

The base current shut-down activated by the control logic clamps the output of pin 7 to 1.6 V. As a
result, the drive of the switching transistor is inhibited. This protective measure is enabled if the
supply voltage at pin 9 reaches a value

6.7 V or if voltages of

� 0.1 are present at pin 5.

In case of short-circuits occurring in the secondary windings of the switched-mode power supply,
the integrated circuit continuously monitors the fault conditions. During secondary, completely load-
free operation only a small pulse duty factor is set. As a result the total power consumption of the
power supply is held at

= 6 ... 10 W during both operating modes. After the output has been

inhibited for a voltage supply of

6.7 V, the reference voltage (4 V) is switched off if the voltage

supply is further reduced by

= 0.6 V.

Protective Operating Mode at Pin 5 in Case of Disturbance

The protection against disturbances such as primary undervoltages and/or secondary over-
voltages (e.g. by changes in the component parameters for the switched-mode power supply) is
realized as follows:

Protective Operating Mode with Continuous Fault Condition Monitoring

In case of disturbance the output pulses at pin 8 are inhibited by falling below the protective
threshold

, with a typical value of

/2. As a result current consumption is reduced (

14 mA

= 10 V).

With a corresponding high-impedance start-up resistor *), supply voltage

will fall below the

minimum shut-down threshold (5.7 V) for reference voltage

will be switched off and current

consumption is further reduced to

3.2 mA at

10 V.

Because of these reductions in current consumption, the supply voltage can rise again to reach the
switch-on threshold of

12.3 V. The protective threshold at pin 5 is released and the power

supply is again ready for operation.

REF

------------

REF

------------

Semiconductor Group

TDA 4601

In case of continuing problems of disturbance (

/2 � 0.1 V) the switch-on mode is interrupted

by the periodic protective operating mode described above, i.e. pin 8 is inhibited and

is falling,

etc.

Switch-On in the Wide Range Power Supply (90 Vac to 270 Vac)
(application circuit 2)

Self-oscillating flyback-converters designed as wide range power supplies require a power source
independent of the rectified line voltage for TDA 4601. Therefore the winding polarity of winding
11/13 corresponds to the secondary side of the flyback converter transformer. Start-up is not as
smooth as with an immediately available supply voltage, because TDA 4601 has to be supplied by
the start-up circuit until the entire secondary load has been charged. This leads to long switch-on
times, especially if low line voltages are applied.

However, the switch-on time can be shortened by applying the special start-up circuit (dotted line).
The uncontrolled phase of feedback control winding 15/9 is used for activating purposes.
Subsequent to activation, the transistor T1 begins to block when winding 11/13 generates the
current supply for TDA 4601. Therefore, the control circuit cannot be influenced during operation.

Электронный компонент: TDA4601