Design Guide & Applications Manual
For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies
vicorpower.com
800-735-6200 Applications
Engineering
1-800-927-9474
Rev. 2.1
Page 28 of 88
9. EMC Considerations
RAM / MI-RAM OPERATION
The RAM / MI-RAM attenuates output noise in two ways.
First, an LC filter in the RAM / MI-RAM attenuates high
frequency components associated with the switching
frequency. Secondly, the RAM / MI-RAM contains an active
filter that attenuates low frequency components associated
with the input to the converter. These frequencies are on
the order of 60 – 120 Hz and harmonics would require
very large output LC if a passive approach were to be
used. Essentially, the active circuit looks at the output
ripple from the converter, multiplies it by –1 (inverts) and
adds it to the output. This effectively cancels out the low
frequency components.
The RAM does not contain any common-mode filtering,
so whatever common-mode noise is present is passed
through. It only provides differential filtering of noise that
is present on one output pin relative to the other.
The use of the RAM / MI-RAM is very straightforward, but
a couple of precautions should be noted. The LC filter is in
the positive output lead, so if that lead is shorted then the
high frequency attenuation is compromised. The active
circuit is in the negative output lead, so if that lead is
shorted the low frequency attenuation is compromised.
The RAM must be used with a common-mode choke at
the input of the converter.
The RAM is intended to be used with the Vicor VI-200 /
VI-J00, and the MI-RAM is intended to be used with Vicor
MI-200 / MI-J00 Family of DC-DC converter modules. It is
also available in a chassis mounted version as VI-LRAM-xx
(MegaMod package) or VI-RAM-xx-B1 (BusMod package)
.
NOTE: Do not use if load is inductive as instability
may result. The addition of the RAM will increase the
converter’s current limit setpoint by ~ 14%.
3 Amp Load
15 Amp Load
30 Amp Load
(Overload Condition)
C1
=
100
µF
C2a – C2b = 4,700 pF (Vicor Part # 01000)
C3a – C3b = 0.01 µF (Vicor Part # 33643)
C4 = 220 µF (Electrolytic)
Conditions
Light Load = 3 A
Full Load = 15 A
Overload Condition = 30 A
+IN
–IN
+OUT
–OUT
C3a
C3b
GATE
IN
GATE
OUT
+S
TRIM
–S
C2a
C2b
C1
+IN
–IN
–OUT
–S IN
+S IN
+OUT
RAM
+S
–S
+
–
L1
C4
+
Figure 9–13
— Output noise, with Ripple Attenuator Module (RAM)
Output Ripple vs. Load
RAM Output Filter
Typical Vicor Module (VI-230-CV) 48 V Input, 5 V Output with VI-RAM-C2
