TNY274-280
For lower no-load input power consumption, the bias winding
may also be used to supply the
TinySwitch-III
device. Resistor
R8 feeds current into the BP/M pin, inhibiting the internal high
voltage current source that normally maintains the BP/M pin
capacitor voltage (C7) during the internal MOSFET off time.
This reduces the no-load consumption of this design from
140 mW to 40 mW at 265 VAC.
Under-voltage lockout is configured by R5 connected between
the DC bus and EN/UV pin of U1. When present, switching
is inhibited until the current in the EN/UV pin exceeds 25
µA.
This allows the startup voltage to be programmed within the
normal operating input voltage range, preventing glitching of
the output under abnormal low voltage conditions and also on
removal of the AC input.
In addition to the simple input pi filter (C1, L1, C2) for
differential mode EMI, this design makes use of
E-Shield™
shielding techniques in the transformer to reduce common
mode EMI displacement currents, and R2 and C4 as a damping
network to reduce high frequency transformer ringing. These
techniques, combined with the frequency jitter of TNY278,
give excellent conducted and radiated EMI performance with
this design achieving >12 dBµV of margin to EN55022 Class
B conducted EMI limits.
For design flexibility the value of C7 can be selected to pick one
of the 3 current limits options in U1. This allows the designer
to select the current limit appropriate for the application.
•
•
Standard current limit (I
LIMIT
) is selected with a 0.1
µF
BP/M
pin capacitor and is the normal choice for typical enclosed
adapter applications.
When a 1
µF
BP/M pin capacitor is used, the current
limit is reduced (I
LIMITred
or I
LIMIT
-1) offering reduced RMS
device currents and therefore improved efficiency, but at
the expense of maximum power capability. This is ideal
for thermally challenging designs where dissipation must
be minimized.
When a 10
µF
BP/M pin capacitor is used, the current
limit is increased (I
LIMITinc
or I
LIMIT
+1), extending the power
capability for applications requiring higher peak power or
continuous power where the thermal conditions allow.
Key Application Considerations
TinySwitch-lll
Design Considerations
Output Power Table
The data sheet output power table (Table 1) represents the
minimum practical continuous output power level that can be
obtained under the following assumed conditions:
1. The minimum DC input voltage is 100 V or higher for
85 VAC input, or 220 V or higher for 230 VAC input or
115 VAC with a voltage doubler. The value of the input
capacitance should be sized to meet these criteria for AC
input designs.
2. Efficiency of 75%.
3. Minimum data sheet value of I
2
f.
4. Transformer primary inductance tolerance of ±10%.
5. Reflected output voltage (V
OR
) of 135 V.
6. Voltage only output of 12 V with a fast PN rectifier diode.
7. Continuous conduction mode operation with transient K
P
*
value of 0.25.
8. Increased current limit is selected for peak and open frame
power columns and standard current limit for adapter
columns.
9. The part is board mounted with SOURCE pins soldered to
a sufficient area of copper and/or a heatsink is used to keep
the SOURCE pin temperature at or below 110 °C.
10. Ambient temperature of 50 °C for open frame designs and
40 °C for sealed adapters.
*Below a value of 1, K
P
is the ratio of ripple to peak primary
current. To prevent reduced power capability due to premature
termination of switching cycles a transient K
P
limit of ≥0.25
is recommended. This prevents the initial current limit (I
INIT
)
from being exceeded at MOSFET turn on.
For reference, Table 2 provides the minimum practical power
delivered from each family member at the three selectable current
limit values. This assumes open frame operation (not thermally
limited) and otherwise the same conditions as listed above.
These numbers are useful to identify the correct current limit
to select for a given device and output power requirement.
Overvoltage Protection
The output overvoltage protection provided by
TinySwitch-III
uses an internal latch that is triggered by a threshold current
of approximately 5.5 mA into the BP/M pin. In addition to an
internal filter, the BP/M pin capacitor forms an external filter
providing noise immunity from inadvertent triggering. For the
bypass capacitor to be effective as a high frequency filter, the
capacitor should be located as close as possible to the SOURCE
and BP/M pins of the device.
•
Further flexibility comes from the current limits between adjacent
TinySwitch-III
family members being compatible. The reduced
current limit of a given device is equal to the standard current
limit of the next smaller device and the increased current limit is
equal to the standard current limit of the next larger device.
E
2/06
9
POWERINT [ POWER INTEGRATIONS, INC. ]
