TNY253/254/255
PI—2253-062398
V
EN
CLOCK
V
IN
0V
DC
MAX
I DRAIN
VDRAIN
0V
0
.2
.4
.6
.8
1
Time (ms)
V DRAIN
Figure 7. TinySwitch Power-Up Timing Diagram.
PI—2251-062398
PI-2261-061198
V
IN
Figure 6. TinySwitch Operation at Light Load.
130 kHz Switching Frequency (TNY255)
The switching frequency (with no cycle skipping) is set at
130kHz. This allows the TNY255 to deliver 10W while still
using the same size, low cost transformer (EE16) as used by the
TNY253/254 for lower power applications.
VDRAIN
12 V
0V
BYPASS Pin Capacitor
The BYPASS pin uses a small 0.1
µF
ceramic capacitor for
decoupling the internal power supply of the
TinySwitch.
12 V
0V
Application Examples
Television Standby
TinySwitch
is an ideal solution for low cost, high efficiency
standby power supplies used in consumer electronic products
such as TVs. Figure9 shows a 7.5 V, 1.3 W flyback circuit that
uses TNY253 for implementing a TV standby supply. The
circuit operates from the DC high voltage already available
from the main power supply. This input voltage can range from
120 to 375VDC depending on the input AC voltage range that
the TV is rated for. Capacitor C1 filters the high voltage DC
supply, and is necessary only if there is a long trace length from
the source of the DC supply to the inputs of the TV standby
circuit. The high voltage DC bus is applied to the series
combination of the primary winding of T1 and the integrated
high voltage MOSFET inside the TNY253. The low operating
frequency of the TNY253 (44kHz), allows a low cost snubber
circuit C2 and R1 to be used in place of a primary clamp circuit.
In addition to limiting the DRAIN turn off voltage spike to a
safe value, the RC snubber also reduces radiated video noise by
0
100
200
300
400
500
Time (ms)
Figure 8. TinySwitch Power Down Timing Diagram.
lowering the dv/dt of the DRAIN waveform, which is critical for
video applications such as TV and VCR. On fixed frequency
PWM and RCC circuits, use of a snubber will result in an
undesirable fixed AC switching loss that is independent of load.
The ON/OFF control on the
TinySwitch
eliminates this problem
by scaling the effective switching frequency and therefore,
switching loss linearly with load. Thus the efficiency of the
supply stays relatively constant down to a fraction of a watt of
output loading.
The secondary winding is rectified and filtered by D1 and C4 to
create the 7.5V output. L1 and C5 provide additional filtering.
The output voltage is determined by the sum of the optocoupler
U2 LED forward drop (~ 1 V) and Zener diode VR1 voltage.
The resistor R2, maintains a bias current through the Zener to
improve its voltage tolerance.
C
7/01
5
POWERINT [ POWER INTEGRATIONS, INC. ]
