TNY274G POWERINT [Power Integrations, Inc.], TNY274G Datasheet - Page 10

• Manufacturer Part Number: TNY274G
• Description: Energy Efficient, Off-Line Switcher with Enhanced Flexibility and Extended Power Range
• Manufacturer: POWERINT [Power Integrations, Inc.]

Table 2. Minimum Practical Power at Three Selectable Current Limit Levels.
For best performance of the OVP function, it is recommended
that a relatively high bias winding voltage is used, in the range of
15 V-30 V. This minimizes the error voltage on the bias winding
due to leakage inductance and also ensures adequate voltage
during no-load operation from which to supply the BP/M pin
for reduced no-load consumption.
Selecting the Zener diode voltage to be approximately 6 V
above the bias winding voltage (28 V for 22 V bias winding)
gives good OVP performance for most designs, but can be
adjusted to compensate for variations in leakage inductance.
Adding additional filtering can be achieved by inserting a low
value (10 Ω to 47 Ω) resistor in series with the bias winding
diode and/or the OVP Zener as shown by R7 and R3 in
Figure 14. The resistor in series with the OVP Zener also limits
the maximum current into the BP/M pin.
Reducing No-load Consumption
As TinySwitch-III is self-powered from the BP/M pin capacitor,
there is no need for an auxillary or bias winding to be provided
on the transformer for this purpose. Typical no-load consumption
when self-powered is <150 mW at 265 VAC input. The addition
of a bias winding can reduce this down to <50 mW by supplying
the TinySwitch-III from the lower bias voltage and inhibiting the
internal high voltage current source. To achieve this, select the
value of the resistor (R8 in Figure 14) to provide the data sheet
DRAIN supply current. In practice, due to the reduction of the
bias voltage at low load, start with a value equal to 40% greater
than the data sheet maximum current, and then increase the value
of the resistor to give the lowest no-load consumption.
Audible Noise
The cycle skipping mode of operation used in TinySwitch-III
can generate audio frequency components in the transformer.
To limit this audible noise generation the transformer should
be designed such that the peak core flux density is below
3000 Gauss (300 mT). Following this guideline and using the
standard transformer production technique of dip varnishing
TNY274 P or G
TNY275 P or G
TNY276 P or G
TNY277 P or G
TNY278 P or G
TNY279 P or G
TNY280 P or G
10
PRODUCT
TNY274-280
E
2/06
I
LIMIT
10.8
15.1
19.4
23.7
11.8
28
9
-1
230 VAC ±15%
10.9
15.3
19.6
28.4
32.7
I
LIMIT
12
24
OUTPUT POWER TABLE
I
LIMIT
15.1
19.4
23.7
32.2
36.6
9.1
28
+1
practically eliminates audible noise. Vacuum impregnation
of the transformer should not be used due to the high primary
capacitance and increased losses that result. Higher flux densities
are possible, however careful evaluation of the audible noise
performance should be made using production transformer
samples before approving the design.
Ceramic capacitors that use dielectrics such as Z5U, when used
in clamp circuits, may also generate audio noise. If this is the
case, try replacing them with a capacitor having a different
dielectric or construction, for example a film type.
TinySwitch-lll Layout Considerations
Layout
See Figure 15 for a recommended circuit board layout for
TinySwitch-III.
Single Point Grounding
Use a single point ground connection from the input filter capacitor
to the area of copper connected to the SOURCE pins.
Bypass Capacitor (C
The BP/M pin capacitor should be located as near as possible
to the BP/M and SOURCE pins.
Primary Loop Area
The area of the primary loop that connects the input filter
capacitor, transformer primary and TinySwitch-III together
should be kept as small as possible.
Primary Clamp Circuit
A clamp is used to limit peak voltage on the DRAIN pin at turn
off. This can be achieved by using an RCD clamp or a Zener
(~200 V) and diode clamp across the primary winding. In all
cases, to minimize EMI, care should be taken to minimize the
circuit path from the clamp components to the transformer and
TinySwitch-III.
I
LIMIT
15.1
18.5
21.8
11.8
7.1
8.4
9.2
-1
BP
)
85-265 VAC
15.3
18.6
25.4
11.9
I
8.5
9.3
LIMIT
22
I
LIMIT
11.8
15.1
18.5
21.8
25.2
28.5
7.1
+1