PRM48JH480T250A00 Datasheet, PDF (40/44 Pages) Vicor Corporation – High Efficiency Converter

Input Filter Stability
The PRM can provide very high dynamic transients. It is therefore
very important to verify that the voltage supply source as well as the
interconnecting lines are stable and do not oscillate. For this purpose,
the converter dynamic input impedance magnitude rEQ_ IN is
provided in Figures 21, 22, 23. It is recommended to provide
adequate design margin with respect to the stability conditions
illustrated in the previous sections.
Inductive source and local, external input decoupling capacitance
with negligible ESR (i.e.: ceramic type)
The voltage source impedance can be modeled as a series R_LINE L_LINE
circuit. The high performance ceramic decoupling capacitors will not
significantly damp the network because of their low ESR; therefore
in order to guarantee stability the following conditions must be
verified:
R_line

(C_{IN _ INT}
L_line
C_{IN _ EXT} ) u
r_{EQ _ IN}
(13)
R_line  r_{EQ _ IN}
(14)
It is critical that the line source impedance be at least an octave
lower than the converter’s dynamic input resistance, 14. However,
R_LINE cannot be made arbitrarily low otherwise equation 13 is
violated and the system will show instability, due to under-damped
RLC input network.
Inductive source and local, external input decoupling capacitance
with significant R_{CIN_EXT} ESR (i.e.: electrolytic type)
In order to simplify the analysis in this case, the voltage source
impedance can be modeled as a simple inductor L_line. Notice that the
high performance ceramic capacitors C_{IN_INT} within the PRM, should
be included in the external electrolytic capacitance value for this
purpose. The stability criteria will be:
r  R _{EQ_ IN}
C_{IN _ EXT}
(15)
L_line
C u R ^{ r} _{IN _ EXT}
C_{IN _ EXT}
EQ _ IN
(16)
Equation 16 shows that if the aggregate ESR is too small – for
example by using very high quality input capacitors (C_{IN_EXT}) – the
system will be under-damped and may even become destabilized.
Again, an octave of design margin in satisfying 15 should be
considered the minimum.
Layout Considerations
Application Note AN:005 details board layout recommendations
using VI Chip® components, with details on good power connections,
reducing EMI, and shielding of control signals and techniques to
reference them to SGND.
Avoid routing control signals (ENABLE, TRIM, AL etc.) directly
underneath the PRM. It is critical that all control signals (aside from
VC and VT) are referenced to SGND, both for routing and for pull-
down and bypassing purposes. VC and VT provide control and
feedback from a VTM, and must be referenced to –OUT of the PRM
(-IN of the VTM).
SGND is connected to –IN internally to the PRM. SGND should not be
tied to any other ground in the system.
PRM^TM Regulator
Page 40 of 44
Rev 1.2
11/2015
PRM48JH480x250A00
Input Fuse Recommendations
A fuse should be incorporated at the input to each PRM, in series
with the +IN pin. A 10 A or smaller input fuse (Littelfuse® NANO2®
451/453 Series) is required to safety agency conditions of
acceptability. Always ascertain and observe the safety, regulatory, or
other agency specifications that apply to your specific application.
Thermal Considerations
VIChip products are multi-chip modules whose temperature
distribution varies greatly for each part number as well as with the
input / output conditions, thermal management and environmental
conditions. Maintaining the top of the PRM48JH480x250A00 case to
less than 100ºC will keep all junctions within the VI Chip module
below 125ºC for most applications. The percent of total heat
dissipated through the top surface versus through the J-lead is
entirely dependent on the particular mechanical and thermal
environment. The heat dissipated through the top surface is typically
60%. The heat dissipated through the J-lead onto the PCB board
surface is typically 40%. Use 100% top surface dissipation when
designing for a conservative cooling solution.
It is not recommended to use a VI Chip module for an extended
period of time at full load without proper heat sinking.
vicorpower.com
800 927.9474