page
date
5 of 9
08/2009
PART NUMBER:
VHK75W
4. INPUT CURRENT
DESCRIPTION:
chassis mount dc-dc converter
The input voltage source must be able to provide enough current to the converter, otherwise it may not start up or operate properly. A
typical symptom is not starting or unusually low output voltage. In general, it is recommended to be able to provide at least:
η
Ipeak = 150%*Pout/(η *Vmin)
where Pout is the maximum output power, Vmin is the minimum input voltage and
η
is the converter's
efficiency. As an example, for VHK75W-Q24-S5 to operate with 9~36V input, 75W output and an efficiency of 82%, the minimum
source current is recommended to be:
Ipeak = 150% * 75 / (82% * 9) = 15.24A.
5. INPUT FUSE
To limit the input current and to facilitate input reversal protection and input OVP protection, a fast-acting input fuse is recommended
for the input line. The fuse rating will depend on the input range and should allow for the maximum current at the lowest input
η
voltage, as shown in this equation:
Ipeak = 150%*Pout/(η *Vmin).
In the previous example of VHK75W-Q24-S5, the peak input current at 9V was calculated to be 15.24A. A 10A or a 15A fuse may be
suitable for this application. Make sure the fuse voltage rating is higher than the maximum input voltage.
6. INPUT OVP
It is important to ensure the input voltage does not exceed the maximum rated input voltage for that model. To suppress voltage
transients of short durations, the converter includes a transient voltage suppression device (TVS) at the input. The built-in TVS has a
rated breakdown voltage of 39V for the VHK75W-Q24-XX models and 82V for the VHK75W-Q48 models. With small transients of
short-durations, the TVS will limit the input voltage without interrupting the converters operations. For large transients, the TVS may
conduct a large amount of current that may trip the input fuse. Without the input fuse, the converter may suffer permanent damages.
When the fuse is open, replace it with one of same type and ratings.
7. INPUT REVERSAL PROTECTION
If the input voltage is reversed for any reason, the built-in protection circuits in the converter will limit the reverse voltage to one diode
drop which is no more than 1V. With an external input fuse connected, the fuse will open and thus remove power from the converter.
Check the wiring and make corrections as needed. The input fuse will need to be replaced. Make sure the new fuse is of the same
type and rating. Without the input fuse, the converter may suffer permanent damages in a reversal situation.
8. REMOTE SENSE
The converter provides regulated outputs at the output terminals. When there is a large current and/or the output cable is of some
length, the voltage at the end of the output cable may be noticeably lower than at the terminals. The converter can compensate up to
0.5V of voltage drop through remote sense terminals. To ensure accurate regulation, run two separate wires (twisted) from the
desired regulation points to the remote sense terminals, as shown below. Even if the load current is low, still connect +Vo to +S and
-Vo to -S.
+Vo
+S
Trim
(-)
-S
-Vo
FIGURE 2. REMOTE SENSE
(+)
Load
20050
SW
112
th
Ave. Tualatin, Oregon
97062
phone
503.612.2300
fax
503.612.2383
www.v-infinity.com
CUI [ CUI INC ]
