EPR-48 Datasheet, PDF (6/20 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP48 - 1.4 W Non-Isolated Buck Converter Using LNK304P

EP48 – 12 V, 1.44 W, Non-Isolated Buck Converter
02-May-2005
Here, the device is configured in a buck converter. The supply is designed to operate in
mostly discontinuous conduction mode (MDCM), with the peak L1 inductor current set by
the LNK304P internal current limit. The control scheme used is similar to the ON/OFF
control used in TinySwitch^®. The on-time for each switching cycle is set by the
inductance value of L1, LinkSwitch-TN current limit and the high voltage DC input bus
across C5.
Output regulation is accomplished by skipping switching cycles in response to an
ON/OFF feedback signal applied to the FEEDBACK (FB) pin. This differs significantly
from traditional PWM schemes that control the duty factor (duty cycle) of each switching
cycle.
Unlike TinySwitch, the logic of the FB pin has been inverted in LinkSwitch-TN. This
allows a very simple feedback scheme to be used when the device is used in the buck
converter configuration. Current into the FB pin greater than 49 µA will inhibit the
switching of the internal MOSFET, while current below this allows switching cycles to
occur.
In the event of a fault condition such as output overload, output short circuit, or an open
loop condition, LinkSwitch-TN enters into auto-restart operation. If no feedback is
received for >50 ms, the internal MOSFET is disabled for 800 ms and auto-restart
alternately enables and disables the switching of the power MOSFET until the fault
condition is removed and feedback is received.
4.3 Output Rectification
During the ON time of U1, current ramps in L1 and is simultaneously delivered to the
load. During the OFF time the inductor current ramps down via free-wheeling diode D1
into C2 and is delivered to the load. Diode D1 should be selected as an ultra-fast diode
(t_rr ≤50 ns) with a voltage rating greater than the maximum DC voltage across C5, 600 V
in this case. In designs that operate in continuous conduction mode, t_rr of ≤35 ns is
recommended. Capacitor C2 should be selected to have an adequate ripple current
rating (low ESR type).
4.4 Output Feedback
The voltage across L1 is rectified and smoothed by D1 and C2 during the off-time of U1.
To a first order, the forward voltage drops of D1 and D2 are identical and therefore, the
voltage across C3 tracks the output voltage. To provide a feedback signal, the voltage
developed across C3 is divided by R1 and R3 and connected to U1’s FB pin. The values
of R1 and R3 are selected such that at the nominal output voltage, the voltage on the
FB pin is 1.65 V. This voltage is specified for U1 at an FB current of 49 µA with a
tolerance of +/-7% over a temperature range of –40 to 125 ^oC. This allows this simple
feedback to meet the required overall output tolerance of +/-10% at rated output current.
Operation down to 0 mA output current can be accomplished while still meeting +/-10%
by increasing the size of the preload from 3.5 mA to 5 mA.
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