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APPLICATION NOTE
U-135
The UC3848 Average Current Mode Controller Squeezes
Maximum Performance from Single Switch Converters
by JOHN A. O’CONNOR
ABSTRACT
This application note describes the UC3848 average current mode PWM controller. The unique features of
this controller are discussed, which make primary side average current mode control practical for isolated
converters. The UC3848 employs a current waveform synthesizer which monitors switch current and simu-
lates the inductor current down slope, generating a complete current waveform without actual secondary
side measurement. Primarily intended for single ended converters, several additional features such as ac-
curate duty-cycle and volt-second limiting allow maximum transformer and switch utilization. A three out-
put, 200 watt off-line design example is presented which also features planar magnetics and a coupled
output inductor.
INTRODUCTION
The UC3848 represents a significant advance in
the control of single switch forward converters.
Generally considered simple and reliable, but non-
optimum in transformer and switch utilization, the
single switch forward has previously been reserved
for less demanding applications. Upon careful ex-
amination however, it is apparent that many of the
perceived limitations actually result from the con-
trol circuitry rather than the converter topology it-
self.
The advantages that an inner current loop brings to
power supply design and performance are well
known [1]. Current mode control is usually pre-
ferred over direct duty cycle control because of the
superior input supply rejection and simplified volt-
age loop closure. Average current feedback pro-
vides additional advantages over the more
common peak current feedback. Major benefits in-
clude inherent slope compensation, better noise
rejection, and the ability to operate with both con-
tinuous and discontinuous inductor current. Addi-
tionally,
average current feedback provides
significantly better closed current loop accuracy.
This further improves input supply rejection and
current limit accuracy. Average current feedback is
detailed in the references [2,3,4].
Maximum power component utilization requires
carefully defined and controlled operating mode
boundaries. While this can be said of many con-
verter topologies, it is particularly critical with the
single switch forward because of the transformer
reset mechanism. Energy in the transformer leak-
age and magnetizing inductance must be removed
after each energy transfer cycle. Above all, the
control circuit must insure that this condition is
achieved. Total losses are generally minimized by
bringing the peak power transfer as close to the
average as possible. This indicates that improve-
ments in efficiency and component utilization are
obtainable by maximizing duty-cycle. Unfortu-
nately, maximizing duty-cycle conflicts with assur-
ing transformer reset, traditionally requiring an
overly conservative design to assure reliability.
Previously, these characteristics have limited the
single switch converter to low-power, low-end ap-
plications. The UC3848 Average Current Mode
PWM Controller allows operation beyond conven-
tional limitations by employing highly accurate cir-
cuitry to provide programmable operating
boundaries, and by implementing an inner average
current feedback loop for improved control charac-
teristics and accuracy. This control circuit advance
capitalizes on unique, patented circuitry, and the
precision achievable with Unitrode’s thin-film resis-
tor process.
The UC3848 Average
Current Mode PWM Controller
The block diagram of the UC3848 shown in figure
1, illustrates a number of unique functions. Al-
though the IC can certainly be used for flyback,
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