tda46053, elektronika, KATALOGI
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Control IC for Switched-Mode Power Supplies
using MOS-Transistor
TDA 4605-3
Bipolar IC
Features
l
Fold-back characteristics provides overload protection for
external components
l
Burst operation under secondary short-circuit condition
implemented
l
Protection against open or a short of the control loop
l
Switch-off if line voltage is too low (undervoltage switch-off)
l
Line voltage depending compensation of fold-back point
l
Soft-start for quiet start-up without noise generated by the
transformer
P-DIP-8-1
l
Chip-over temperature protection implemented (thermal
shutdown)
l
On-chip ringing suppression circuit against parasitic
oscillations of the transformer
l
AGC-voltage reduction at low load
Type
Ordering Code
Package
TDA 4605-3
Q67000-A5066
P-DIP-8-1
The IC TDA 4605-3 controls the MOS-power transistor and performs all necessary control and
protection 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.
Semiconductor Group
74
06.94
TDA 4605-3
In the different load ranges the switched-mode power supply (SMPS) behaves as follows:
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.
Nominal operation
The switching frequency is reduced with increasing load and decreasing AC-voltage.
The output voltage is only dependent on the load.
Overload point
Maximal output power is available at this point of the output characteristic.
Overload
The energy transferred per operation cycle is limited at the top. Therefore the output voltages
declines by secondary overloading.
Semiconductor Group
75
TDA 4605-3
Pin Definitions and Functions
Pin No.
Function
1
Information Input Concerning Secondary Voltage.
By comparing the
regulating voltage - obtained trom 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, overload, short-circuit, no load).
2
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-element. When a voltage level is reached thats derived from the regulating
voltage at pin 1, the output impulse at pin 5 is terminated. The RC-element serves
to set the maximum power at the overload point set.
3
Input for Primary Voltage Monitoring:
In the normal operation
V
3
is moving
between the thresholds
V
3H
and
V
3L
(V
3H
>
V
3
>
V
3L
).
V
3
<
V
3L
: SMPS is switched OFF (line voltage too low).
V
3
>
V
3H
: Compensation of the overload point regulation (controlled by pin 2)
starts at
V
3H
:
V
3L
= 1.7.
4
Ground
5
Output:
Push-pull output provides
±
1 A for rapid charge and discharge of the
gate capacitance of the power MOS-transistor.
6
Supply Voltage Input:
A stable internal reference voltage
V
REF
is derived from
the supply voltage also the switching thresholds
V
6A
,
V
6E
,
V
6 max
and
V
6 min
for
the supply voltage detector. If
V
6
>
V
6E
then
V
REF
is switched on and swiched off
when
V
6
<
V
6A
. In addition the logic is only enable for
V
6 min
<
V
6
<
V
6 max
.
7
Input for Soft-Start.
Start-up will begin with short pulses by connecting a
capacitor from pin 7 to ground.
8
Input for the Oscillation Feedback.
After starting oscillation, every zero
transition of the feedback voltage (falling edge) through zero (falling edge)
triggers an output pulse at pin 5. The trigger threshold is at + 50 mV typical.
Semiconductor Group
76
TDA 4605-3
Block Diagram
Semiconductor Group
77
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
C
1
. 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
C
2
, under steady state conditions the IC receives its
supply voltage from transformer winding
n
1
via diode D1. The switching transistor T1 is a BUZ 90.
The parallel connected capacitor
C
3
and the inductance of primary winding
n
2
determine the
system resonance frequency. The
R
2
-
C
4
-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
-
C
5
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
-
C
5
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
C
5
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
n
1
during
the off period of T1 is rectified by D3, smoothed by
C
6
and stepped down at an adjustable ratio by
R
5
,
R
6
and
R
7
. The
R
8
-
C
7
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
C
8
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
n
3
to
n
7
rectified by D4 to
D8 and smoothed by
C
9
to
C
13
. 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
78
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