PI3740-00 Datasheet, PDF (23/48 Pages) Vicor Corporation – Parallel capable with single wire current sharing

Filter Considerations
The PI3740-00 requires low impedance ceramic input capacitors
(X7R/X5R or equivalent) to ensure proper start up and high
frequency decoupling for the power stage. The PI3740-00
will draw nearly all of the high frequency current from the
low impedance ceramic capacitors when the main high side
MOSFET(s) are conducting. During the time the MOSFET(s) are
off, the input capacitors are replenished from the source. Table 1
shows the recommended input and output capacitors to be used
for the PI3740-00. Divide the total RMS current by the number
of ceramic capacitors used to calculate the individual capacitor’s
RMS current. Table 2 includes the recommended input and
output ceramic capacitor. It is very important to verify that the
voltage supply source as well as the interconnecting line are
stable and do not oscillate.
Input Filter case 1; 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:
( ) ^{Rline >}
L_line
C + C _{IN_INT}
IN_EXT
• r_{EQ_IN}
(3)
R_line << r_{EQ_IN}
(4)
Where r_{EQ_IN} can be calculated by dividing the lowest line
voltage by the full load input current. It is critical that the
line source impedance be at least an octave lower than the
converter’s dynamic input resistance, Equation (4). However,
Rline cannot be made arbitrarily low otherwise Equation (3)
is violated and the system will show instability, due to under-
damped RLC input network.
PI3740-00
Input Filter case 2; 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 PI3740-00 should be included in the external electrolytic
capacitance value for this purpose. The stability criteria will be:
r > R _{EQ_IN}
CIN_EXT
(5)
L_line
C • R _{IN_INT}
CIN_EXT
< r_{EQ_IN}
(6)
Equation (6) 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
Equation (5) should be considered the minimum.
Note: When applying an electrolytic capacitor for input
filter damping the ESR value must be chosen to avoid loss of
converter efficiency and excessive power dissipation in the
electrolytic capacitor.
C_INPUT
(see Table 2)
C_OUTPUT
(see Table 2)
5 X 2.2µF
8 X 10µF or 2.2µF
Table 1 — Minimum recommended input and
output capacitance
Part Number
GRM32ER72A225KA35
GRM32ER71H106KA12
Description
2.2µF Capacitor, X7R 20% 100V, 1210
10µF Capacitor X7R 20% 50V, 1210
Table 2 — Capacitor manufacturer part numbers
MFG Description
Murata
Murata
Cool-Power^® ZVS Switching Regulators
Page 23 of 48
Rev 1.4
04/2017
vicorpower.com
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