UWE-Q12 Series
Eighth-Brick DOSA-Compatible
Wide Input Isolated DC-DC Converters
Positive ("P" suffix) logic models are enabled when the On/Off pin is left open or
is pulled high (see specifications) with respect to the –Input. Positive-logic
devices are disabled when the on/off pin is pulled low with respect to the –Input.
Negative (“N” suffix) logic devices are off when the On/Off pin is left open or is
pulled high (see specifications), and on when the pin is pulled low with respect to
the –Input. See specifications.
Floating Outputs
Since these are isolated DC-DC converters, their outputs are “floating” with
respect to their input. Designers will normally use the –Output as the ground/
return of the load circuit. You can however, use the +Output as ground/return
to effectively reverse the output polarity.
Dynamic control of the remote on/off function is best accomplished with a
mechanical relay or an open-collector/open-drain drive circuit (optically isolated if
appropriate). The drive circuit should be able to sink appropriate current (see
Performance Specifications) when activated and withstand appropriate voltage
when deactivated.
Thermal Shutdown
The UWE-Q12 series converters are equipped with thermal-shutdown
circuitry. If environmental conditions cause the temperature of the DC-DC
converter to rise above the designed operating temperature, a precision
temperature sensor will power down the unit. When the internal temperature
decreases below the threshold of the temperature sensor, the unit will self-
start.
The thermal shutdown is set to a point where the semiconductors should
never exceed their “maximum ratings”. The thermal shutdown is set to avoid
“nuisance” shutdown under fault conditions. i.e. if the air conditioning goes
down in the data center, the module can run at a higher temperature for
some time. We do not recommend that you run the module continuously
above the thermal derating curve recommendations.
It is recommended that you fully understand the “recommended operating
temperature” and verify that under normal operating conditions the module
temperature is not exceeded in your application.
Figure 2. ON/OFF Control Circuit
Remote Sense
See Performance/Functional Specifications.
Note: The Sense and Vout lines are internally connected through low-value
resistors. Nevertheless, if the sense function is not used for remote regulation,
the user should connect the +Sense to +Vout and –Sense to –Vout directly at the
DC-DC converter pins. UWE series converters employ a sense feature to provide
point of use regulation, thereby overcoming moderate IR drops in PCB conductors
or cabling. The remote sense lines carry very little current and therefore require
minimal cross-sectional-area conductors. The sense lines, which are coupled to
their respective output lines, are used by the feedback control-loop to regulate
the output. As such, they are not low impedance points and must be treated with
care in layouts and cabling. Sense lines on a PCB should be run adjacent to dc
signals, preferably ground.
Output Over-Voltage Protection
Vout is controlled via a closed loop system and monitored for fault conditions
(over voltage, over current) such as an over-voltage condition. If Vout for any
reason rises above the specified OVP set point the converter will shut down
causing Vout to decrease rapidly (depending on load conditions). Following a
time-out period the module will restart causing Vout to ramp to its specified
set-point. If the fault condition persists and Vout again exceeds the OVP set
point the converter will again enter the shutdown cycle. This on/off cycling is
referred to as “hiccup” mode. When the fault condition has been corrected
the module will return to normal operations.
[Vout(+) – Vout(-)] – [Sense(+) – Sense(-)] ≤ 10%×Vout
In cables and discrete wiring applications, twisted pair or other techniques should
be used. Output over-voltage protection is monitored at the output voltage pin,
not the Sense pin. Therefore, excessive voltage differences between Vout and
Sense in conjunction with trim adjustment of the output voltage can cause the
over-voltage protection circuitry to activate (see Performance Specifications for
over-voltage limits).
Power derating is based on maximum output current and voltage at the
converter’s output pins. Use of trim and sense functions can cause output
voltages to increase, thereby increasing output power beyond the converter’s
specified rating, or cause output voltages to climb into the output over-voltage
region. Therefore, the designer must ensure:
Current Limiting
As soon as the output current increases to approximately 130% of its rated
value, the DC-DC converter will go into a current-limiting mode. In this
condition, the output voltage will decrease proportionately with increases in
output current, thereby maintaining somewhat constant power dissipation.
This is commonly referred to as power limiting. Current limit inception is
defined as the point at which the full-power output voltage falls below the
specified tolerance. See Performance/Functional Specifications. If the load
current, being drawn from the converter, is significant enough, the unit will
go into a short circuit condition as described below.
(Vout at pins) × (Iout) ≤ rated output power
Short Circuit Condition
When a converter is in current-limit mode, the output voltage will drop as the
output current demand increases. If the output voltage drops too low, the
magnetically coupled voltage used to develop primary side voltages will also
drop, thereby shutting down the PWM controller. Following a time-out period,
the PWM will restart causing the output voltage to begin ramping to their
appropriate value. If the short-circuit condition persists, another shutdown
cycle will be initiated. This on/off cycling is referred to as “hiccup” mode. The
hiccup cycling reduces the average output current, thereby preventing
internal temperatures from rising to excessive levels. The UWE-Q12 Series is
capable of enduring an indefinite short circuit output condition.
Figure 3. Remote Sense Circuit
On/Off Control
The input-side, remote On/Off Control function can be ordered to operate with
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