7-Feb-2006
EP-91 12 V, 1 A, Universal Input Supply
15 Extended and Reduced Current Limit (ILIMIT) Operation
Additional capacitors (C8 and C9 on the BOM in Section 6) have been included in the
DAK-91 kit for the convenience of trying out the ILIMIT+1 and ILIMIT–1 operation of TNY278
in the EP-91 reference board. When C7 (0.1 µF) is replaced with a 10 µF capacitor (C9),
the TNY278 will operate in the ILIMIT+1 mode, which increases the maximum primary
current limit from the standard maximum limit of 0.55 A to 0.65 A (equal to that of a
TNY279). This allows a TNY278 to deliver from 15% to 25% more output power
(depending on the output voltage and current).
CAUTION: Because EP-91 was designed for standard ILIMIT operation, It should not be
loaded with more than 1.25 A at an elevated temperature for very long (a few minutes)
when verifying the performance of TNY278 in the ILIMIT+1 mode, since the other power
components (transformer, input bulk capacitors, output diode, output capacitors and
primary clamp network) are not sized for sustained operation at more than 12 W.
When C7 is replaced with a 1 µF capacitor (C8), the TNY278 will operate in the ILIMIT–1
mode, which reduces the maximum current limit from the standard maximum limit of
0.55 A to 0.45 A (equal to that of a TNY277). Although this reduces the maximum output
power that the supply can deliver, it typically will increase the efficiency, especially at
lower output power levels. To take the fullest advantage of the increase in efficiency that
can be obtained from ILIMIT–1 operation, the power transformer would need to be
redesigned slightly.
16 TNY277 and TNY279 Operation in EP-91
A TNY277 device used in the ILIMIT+1 mode (a 10 µF installed in place of C7) will work in
the EP-91 reference board, and deliver output power equal to that of a TNY278 device.
This flexibility allows a design engineer the option of using a lower cost part in
applications with less demanding thermal requirements.
A TNY279 device used in the ILIMIT–1 mode (a 1 µF installed in place of C7) will deliver
the same output power as a TNY278 in the standard ILIMIT configuration. This can
improve efficiency and lower the temperature rise of the device, which can give greater
thermal margin to a design that must operate in high ambient temperature environments.
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