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11-Feb-13
RDR-91 12 V, 1 A, Universal Input Supply
4.7 Peak Primary Current Limit Selection
The value of the capacitor installed on the BP/M pin allows the current limit of U1 to be
selected. The power supply designer can change the current limit of the power MOSFET
by simply changing the capacitance value connected to the BP/M pin (see the
TinySwitch-III data sheet for more details).
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Installing a 0.1 ? F capacitor on the BP/M pin selects the standard current
limit of the IC, and is the normal choice for enclosed adapter applications.
Installing a 1 ? F capacitor on the BP/M pin reduces the power MOSFET
current limit, which lowers conduction losses and improves efficiency (at
the expense of reducing the maximum power capability of the IC).
A 10 ? F capacitor on the BP/M pin will raise the power MOSFET current
limit and extend the power capability of the IC (for higher power
applications that do not have the thermal constraints of an enclosed
adapter, or to supply short-duration, peak load demands).
The EP91 demonstration board comes with a 0.1 ? F capacitor installed as C7, which
causes U1 to select the standard current limit specified in the TinySwitch-III data sheet. If
C7 were replaced by a 1 ? F capacitor (C8 in the BOM, section 6), the current limit of U1
will be the same as the standard current limit for a TNY277 device. If a 10 ? F capacitor is
installed, the current limit of U1 will be the same as the standard current limit for a
TNY279 device. The flexibility of this option enables the designer to do three things.
First, it allows the designer to measure the effect of switching to an adjacent device
without actually removing and replacing the IC. Second, it allows a larger device to be
used with a lower current limit, for higher efficiency. Third, it allows a smaller device to be
used with a higher current limit in a design when higher power is not required on a
continual basis, which effectively lowers the cost of the supply.
4.8 Undervoltage Lockout
The RD-91 circuit board has a location where an optional undervoltage (UV) lockout
detection resistor (R5) can be installed. When installed, MOSFET switching is disabled at
start-up until current into the EN/UV pin exceeds 25 ? A. This allows the designer to set
the input voltage at which MOSFET switching will be enabled by choosing the value of
R5. For example, a value of 3.6 M ? requires an input voltage of 65 VAC (92 VDC across
C2) before the current into the EN/UV pin exceeds 25 ? A. The UV detect function also
prevents the output of the power supply from glitching (trying to restart) after output
regulation is lost (during shutdown), by disabling MOSFET switching until the input
voltage rises above the undervoltage lockout threshold.
Power Integrations
Page 9 of 35
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