lm5027mhx National Semiconductor Corporation, lm5027mhx Datasheet - Page 24

• Manufacturer Part Number: lm5027mhx
• Description: Voltage Mode Active Clamp Controller
• Manufacturer: National Semiconductor Corporation

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The hiccup mode provides the periodic cool-down time for the
power converter in the event of a sustained overload or short
Printed Circuit Board Layout
The LM5027 Current Sense and PWM comparators are very
fast and respond to short duration noise pulses. The compo-
nents at the CS, COMP, SS, UVLO, TIME1, TIME2, and
TIME3 pins should be physically close as possible to the IC,
thereby minimizing noise pickup on the PC board trace in-
ductance. Layout consideration is critical for the current sense
filter. If a current sense transformer is used, both leads of the
transformer secondary should be routed to the sense filter
components and to the IC pins. The ground side of the trans-
former should be connected via a dedicated PC board trace
to the AGND pin, rather than through the ground plane.
If the current sense circuit employs a sense resistor in the
drive transistor source, low inductance resistors should be
used. In this case, all the noise sensitive, low-current ground
trace should be connected in common near the IC, and then
a single connection made to the power ground (sense resistor
ground point). The gate drive outputs of the LM5027 should
have short, direct paths to the power MOSFETs in order to
minimize inductance in the PC board. The two ground pins
(AGND, PGND) must be connected together with a short, di-
rect connection, to avoid jitter due to relative ground bounce.
FIGURE 16. Hiccup Mode Timing
24
circuit. This off time results in lower average input current and
lower power dissipation within the power components.
Application Circuit Example
The following schematic shows an example of the LM5027
controlling a 100W active clamp forward power converter.
The input voltage range (V
voltage is 3.3V. The output current capability is 30 Amps.
Current sense transformer T3 provides information to the CS
pin for current limit protection. The error amplifier and refer-
ence U3 and U5 provide voltage feedback via opto-coupler
U2. Synchronous rectifiers Q3-Q6 minimize rectification loss-
es in the secondary. An auxiliary winding on the power trans-
former T1 provides power to the LM5027 VCC pin when the
output is in regulation. The input UVLO levels are 34V for in-
creasing V
The circuit can be shutdown by forcing the ON/OFF input J2
below 0.4V. An external synchronizing frequency can be ap-
plied to the Oscillator input. The converter output current limit
is limited at 32A.
Special care was taken in this design such that the converter
will turn on with the output pre-biased without sinking current
from the output. More information is available in AN-1976.
PWR
, and approximately 32V for decreasing V
PWR
) is 36V to 76V, and the output
30099023
PWR
.