Siemens TDA4605 Datasheet
Control IC for Switched-Mode Power Supplies
using MOS-Transistors
Features
● Fold-back characteristic provides overload protection for
external components
● Burst operation under short-circuit conditions
● Loop error protection
● Switch-off if line voltage is too low (undervoltage switch-off)
● Line voltage compensation of overload point
● Soft-start for quiet start-up
● Chip-over temperature protection (thermal shutdown)
● On-chip parasitic transformer oscillation suppression
circuitry
Type Ordering Code Package
P-DIP-8-1
TDA 4605
Bipolar IC
TDA 4605 Q67000-A8078 P-DIP-8-1
The IC TDA 4605-1 controls the MOS-power transistor and performs all necessary regulation and
monitoring functions in free running flyback converters. Since good load regulation over a wide load
range is attained, this IC is applicable tor consumer and industrial power supplies.
The serial circuit of power transistor and primary winding of the flyback transformer is connected to
the input voltage. During the switch - on period of the transistor, energy is stored in the transformer
and during the switch - off period it is fed to the load via the secondary winding. By varying switchon 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 ot 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.
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TDA 4605
In the different load ranges the switched-mode power supply (SMPS) behaves as follow:
No load operation:
The power supply unit oscillates at its resonant frequency typ. 100 kHz to 200 kHz. Depending upon
the transformator windings the output voltage can be slightly above nominal value.
Nominal operation:
The switching frequency declines with increasing load and decreasing AC-voltage. The duty factor
primarly depends on the AC-voltage. The output voltage is load-dependent only.
Overload point:
Maximal output power is available at this point ot the output characteristic.
Overload:
The energy transferred per operation cycle is limited at the top. Therefore the output voltage
declines by secondary overloading.
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TDA 4605
Pin Definitions and Functions
Pin No. Function
1 Regulating Voltage: Information input concerning secondary voltage.
By comparing the regulating voltage - obtained from the regulating winding ot the
transformer - with the internal reference voltage, the output impulse width on pin 5
is adapted to the load ot the secondary side (normal, overload, short-circuit, no
load).
2 Primary Current Simulation: 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 ot external RC-element. When a value is reached that is 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
between the thresholds V3Hand V
V
< V3L: SMPS is switched OFF (line voltage too low).
3
V
> V3H: Compensation of the overload point regulation (controlled by pin 2)
3
3L(V3H
> V3 > V3L).
V
is moving
3
starts at V3H: V3L = 1.7.
4 Ground
5 Output: Push-pull-output provides ± 1 A for rapid charge and discharge of the
gate capacitance ot the power MOS-transistor.
6 Supply Voltage Input: A stable internal reference voltage
the supply voltage also the switching thresholds V6A, V6E, V
the supply voltage detector. If V6 > V6E then V
is switched on and swiched off
REF
when V6< V6A. In addition the logic is only enable for V
6 min
V
is derived from
REF
and V
6 max
< V6< V
6 min
6 max
for
.
7 Soft-Start: Input for soft-start. Start-up will begin with short pulses by connecting
a capacitor from pin 7 to ground.
8 Zero Detector: Input tor the oscillation feedback. After starting oscillation, every
zero transit of the feedback voltage (falling edge) triggers an output impulse at
pin 5. The trigger threshold is at + 50 mV typical.
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TDA 4605
Block Diagram
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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 resistorR1; 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 R2 - C4 - D2 circuitry limits overshoot peaks, and R3 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 VC1 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 R4, R5 must be dimensioned correctly
in order to prevent driving the transformer core into saturation.
The R10/R11 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
, R6 and R7. The R6- C7 network suppresses parasitic overshoots (transformer oscillation).
5
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 R9 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 R12, R14 and R19 to R21 are used as bleeder resistors.
Fusable resistors R15 to R18 protect the rectifiers against short circuits in the output circuits, which
are designed to supply only small loads.
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Block Diagram
Pin 1
TDA 4605
The regulating voltage forwarded to this pin is compared with a stable internal reference voltage V
in the regulating and overload amplifier. The output of this stage is ted to the stop comparator.
Pin 2
A voltage proportional to the drain current ot the switching transistor is generated there by the
external RC-combination in conjunction with the primary current transducer. The output of this
transducer is controlled by the logic and referenced to the internal stable voltageV2B. If the voltage
V
exceeds the output voltage of the regulating amplifier, the logic is reset by the stop comparator
2
and consequently the output ot pin 5 is switched to low potential. Further inputs tor the logic stage
are the output for the start impulse generator with the stable reference potential VST and the
supply voltage monitor.
Pin 3
The down-divide primary voltage applied there stabilizes the overload point. In addition the logic is
disabled in the event of low voltage by comparison with the internal stable voltageVV in the primary
voltage monitor block.
Pin 4
Ground
R
Pin 5
In the output stage the output signals produced by the logic are shifted to a leved suitable for MOSpower transistors.
Pin 6
From the supply voltage V6 are derived a stable internal referenceV
V
, V6E, V
6A
derived from V
the logic is released only for V
6 max
REF
and V
for the supply voltage monitor. All reference values (VR, V2B, VST) are
6 min
. If V6 > VVE the V
6 min
is switched on and switched off whenV6 < V6A. In addition,
REF
< V6 < V
6 max
.
and the switching threshold
REF
Pin 7
The output of the overload amplifier is connected to pin 7. A load on this output causes a reduction
in maximal impulse duration. This function can be used to implement a soft start, when pin 7 is
connected to ground by a capacitor.
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TDA 4605
Pin 8
The zero detector controlling the logic block recognizes the transformer being discharged by
positive to negative zero crossing of pin 8 voltage and enables the logic for a new pulse. Parasitic
oscillations occurring at the end of a pulse cannot lead to a new pulse (double-pulsing), because an
internal circuit inhibits the zero detector for a finite time tUL after the end of each pulse.
Start-Up Behaviour
The start-up behaviour of the application circuit per sheet 48 is represented on sheet 50 for a line
voltage barely above the lower acceptable limit voltage value (without soft-start). After applying the
line voltage at the time t0 to the tollowing voltages built up:
– V6 corresponding to the half-wave charge current over R
– V2 to V
(typically 6.6 V)
2 max
1
– V3 to the value determined by the divider R10/R11.
The current drawn by the IC in this case is less than 1.6 mA. If V6 reaches the threshold V6E (time
point t1), the IC switches on the internal reference voltage. The currentdraw max. rises to 12 mA.
The primary current- voltage reproducer regulates V2 down to V2E and the starting impulse
generator generates the starting impulses from time point t5 to t6. The feedback to pin 8 starts the
next impulse and so on. All impulses including the starting impulse are controlled in width by
regulating voltage of pin 1. When switching on this corresponds to a short-circuit event, i.e. V1 = 0.
Hence the IC starts up with "short-circuit impulses" to assume a width depending on the regulating
voltage feedback (the IC operates in the overload range). The maximum pulse width is reached at
time point t2 (V2 = V
). The IC operates at the overload point. Thereafter the peak values ot V
2 max
decrease rapidly, as the IC is operating within the regulation range. The regulating loop has built
up. If voltage V6 falls below the switch-off threshold V
before the reversal point is reached, the
6 min
starting attempt is aborted (pin 5 is switched to low). As the IC remains switched on, V6 further
decreases to V6. The IC switches off; V6 can rise again (time point 14) and a new start-up attempt
begins at time point t1. If the rectified alternating line voltage (primary voltage) collapses during
load, V3 can fall below V3A, as is happening at time point t3 (switch-on attempt when voltage is too
low). The primary voltage monitor then clamps V3 to V3S until the IC switches off (V6 < V6A). Then
a new start-up attempt begins at time point t1.
2
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