Rev. D 08/16
10
TNY284-290
www.power.com
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 BYPASS/MULTI-FUNCTIONAL 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 ltering can
be achieved by inserting a low value (10 W to 47 W) resistor in series
with the bias winding diode and/or the OVP Zener as shown by R7 and
R3 in Figure 16. The resistor in series with the OVP Zener also limits
the maximum current into the BYPASS/MULTI-FUNCTIONAL pin.
Reducing No-load Consumption
As TinySwitch-4 is self-powered from the BYPASS/MULTI-
FUNCTIONAL pin capacitor, there is no need for an auxiliary 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-4 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 16) 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-4 can
generate audio frequency components in the transformer. To limit
this audible noise generation the transformer should be designed
such that the peak core ux density is below 3000 Gauss (300 mT).
Following this guideline and using the standard transformer production
technique of dip varnishing 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
ux 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 lm type.
TinySwitch-4 Layout Considerations
Layout
See Figure 17 for a recommended circuit board layout for TinySwitch-4.
Single Point Grounding
Use a single point ground connection from the input lter capacitor to
the area of copper connected to the SOURCE pins.
Bypass Capacitor (CBP)
The BYPASS/MULTI-FUNCTIONAL pin capacitor must be located
directly adjacent to the BYPASS/MULTI-FUNCTIONAL and SOURCE
pins.
If a 0.1 μF bypass capacitor has been selected it should be a high
frequency ceramic type (e.g. with X7R dielectric). It must be placed
directly between the ENABLE and SOURCE pins to lter external noise
entering the BYPASS pin. If a 1 μF or 10 μF bypass capacitor was
selected then an additional 0.1 μF capacitor should be added across
BYPASS and SOURCE pins to provide noise ltering (see Figure 17).
ENABLE/UNDERVOLTAGE Pin
Keep traces connected to the ENABLE/UNDERVOLTAGE pin short and,
as far as is practical, away from all other traces and nodes above
source potential including, but not limited to, the bypass, drain and
bias supply diode anode nodes.
Primary Loop Area
The area of the primary loop that connects the input lter capacitor,
transformer primary and TinySwitch-4 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. To reduce EMI, minimize
the loop from the clamp components to the transformer and
TinySwitch-4.
Thermal Considerations
The SOURCE pins are internally connected to the IC lead frame and
provide the main path to remove heat from the device. Therefore all
the SOURCE pins should be connected to a copper area underneath
the TinySwitch-4 to act not only as a single point ground, but also as
a heat sink. As this area is connected to the quiet source node, this
area should be maximized for good heat sinking. Similarly for axial
output diodes, maximize the PCB area connected to the cathode.
Y Capacitor
The placement of the Y capacitor should be directly from the primary
input lter capacitor positive terminal to the common/ return terminal
of the transformer secondary. Such a placement will route high
magnitude common mode surge currents away from the TinySwitch-4
device. Note – if an input π (C, L, C) EMI lter is used then the
inductor in the lter should be placed between the negative terminals
of the input lter capacitors.
Optocoupler
Place the optocoupler physically close to the TinySwitch-4 to
minimizing the primary-side trace lengths. Keep the high current,
high-voltage drain and clamp traces away from the optocoupler to
prevent noise pick up.
Output Diode
For best performance, the area of the loop connecting the secondary
winding, the output diode and the output lter capacitor, should be
minimized. In addition, sufcient copper area should be provided at
the anode and cathode terminals of the diode for heat sinking. A
larger area is preferred at the quiet cathode terminal. A large anode
area can increase high frequency radiated EMI.
PC Board Leakage Currents
TinySwitch-4 is designed to optimize energy efciency across the
power range and particularly in standby/no-load conditions. Current
consumption has therefore been minimized to achieve this performance.
The ENABLE/UNDERVOLTAGE pin under-voltage feature for example
has a low threshold (~1 μA) to detect whether an undervoltage
resistor is present.
Parasitic leakage currents into the ENABLE/UNDERVOLTAGE pin are
normally well below this 1 μA threshold when PC board assembly is in
a well controlled production facility. However, high humidity conditions
together with board and/or package contamination, either from
no-clean ux or other contaminants, can reduce the surface resistivity
enough to allow parasitic currents >1 μA to ow into the ENABLE/
UNDERVOLTAGE pin. These currents can ow from higher voltage
exposed solder pads close to the ENABLE/UNDERVOLTAGE pin such
as the BYPASS/MULTI-FUNCTIONAL pin solder pad preventing