Excelsys ultimod, Designer'S Manual, Page: 7 / 15

7
UltiMod Designer Manual
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sig
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an
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l
Using equation (5) I
trim
= FI
lim
= CF - 0.53
I
trim
= 1.45
Using equation (7) R
Itrim
1320 I
trim
2.47- I
trim
R
Itrim
= 1,873 ohms
To calculate the ratio of R
1
to R
2
use the formula above
Use equation (11) to get the value of VD
VD = 0.53
Use equation (9) to get the value of R1
R1 = R
Itrim
V
out
FI
fb1
(1 + R
Itrim
) - 2R
Itrim
+1 - VD
1320 1320
R1 = 9110 ohm
Use equation (10) to get the value of R2
R2 = (R1)(R
Itrim
)
R1 - R
Itrim
Over Voltage Protection (OVP)
Over-voltage protection is implemented on each powerMod
output. OVP level is fixed relative to Vmax (110-125%). OVP is
Latching and it may be reset by removing and reinstating AC
power from the powerPac input.
Power Limit
Each powerMod has a number of levels of protection in order to
ensure that UltiMod is not damaged if used in overload
conditions. See graph.
When Vset is less than or equal to Vnom, current limit is
employed at the current limit set point. However if Vset is
greater than Vnom, the power limit is employed to ensure that
the powerMods does not exceed its power rating.
e.g. XgK is adjustable between 12V and 30V. Imax is 9.2A.
Power rating is 220W.
At 24V the powerMod can deliver 9.2A continuously, i.e 220W.
At 30V, the powerMod can still deliver 220W, however this
equates to 7.33A continuous.
Remote Sense
To compensate for voltage drops in the output leads, use
remote sensing. Remote sensing is available on all single
output and on the first output (V1) of the dual output module.
Output powerMod Power Limit
Remote sensing may be implemented by connecting the
Positive Sense pin (J3 pin1) to the positive side of the remote
load and the Negative Sense pin (J3 pin2) to the negative side
of the remote load. The maximum line drop, which can be
compensated for by remote sensing, is 0.5V, subject to not
exceeding the maximum module voltage at the output terminals.
Observe the following precautions when remote sensing:
1. Use separate twisted pairs for power and sense wiring.
2. Route the sensing leads to prevent pick up, which may
appear as ripple on the output.
3. Never disconnect the output power rail with the sensing still
connected to the load.
In certain applications where there is a high dynamic impedance
along the power leads to the sensing point, remote sensing may
cause system instability. This system problem can be overcome
by using resistors in the sense leads (Positive sense lead: R
1
=
100ohm , Negative sense lead: R
2
=10ohm ), together with local
AC sensing, by using 22uF capacitors between the remote
sense pins and the output terminals.
The resistance of the power cables must be so that the voltage
drop across the cables is less than 0.5V (to ensure remote
sensing operates correctly).
R
cable
< 0.5
Iout
e.g. for an XgH, 5V/36A. The R
cable
must be less than
12.5mohms.
Measurement of Ripple & Noise
As with all switched mode power supplies, it is important to
ensure that the correct method is used to verify ripple & noise.
Care should be taken to ensure that a loop antenna is not
formed by the tip and ground lead of the oscilloscope probe as
this would lead to erroneous readings consisting mainly of
pickup from remnant radiation in the vicinity of the output
connectors. Excelsys recommends the use of a x1 probe with
the ground sheath of the probe tip used for ground connection.
In some applications, further erroneous readings may result from
CM currents. These can be reduced by looping a few turns of the
scope lead through a suitable high permeability ferrite ring.
As most loads powered by a power supply will have at least
small values of differential capacitors located near the load,
Excelsys also recommends the use of small value of
capacitance (approx 1uF) positioned at the point of
measurement.
Remote Sense of Output Voltage