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APPLICATION INFORMATION
Operating Features and System Considerations for the PTQA Series of DC/DC Converters
Overcurrent Protection
Output Overvoltage Protection
Overtemperature Protection
Undervoltage Lockout
Primary-Secondary Isolation
Input Current Limiting
Thermal Considerations
SLTS261B – MAY 2006 – REVISED NOVEMBER 2007
To protect against load faults, these converters incorporate output overcurrent protection. Applying a load to the
output that exceeds the converter's overcurrent threshold (see applicable specification) will cause the output
voltage to momentarily fold back, and then shut down. Following shutdown the module will periodically attempt to
automatically recover by initiating a soft-start power-up. This is often described as a hiccup mode of operation,
whereby the module continues in the cycle of successive shutdown and power up until the load fault is removed.
Once the fault is removed, the converter automatically recovers and returns to normal operation.
Each converter incorporates protection circuitry that continually senses for an output overvoltage (OV) condition.
The OV threshold is set approximately 20% higher than the nominal output voltage. If the converter output
voltage exceeds this threshold, the converter is immediately shut down and remains in a latched-off state. To
resume normal operation the converter must be actively reset. This can only be done by momentarily removing
the input power to the converter. For fail-safe operation and redundancy, the OV protection uses circuitry that is
independent of the converter’s internal feedback loop.
Overtemperature protection is provided by an internal temperature sensor, which closely monitors the
temperature of the converter’s printed circuit board (PCB). If the sensor exceeds a temperature of approximately
105
°C, the converter will shut down. The converter will then automatically restart when the sensed temperature
drops back to approximately 95
°C. When operated outside its recommended thermal derating envelope (see
data sheet SOA curves), the converter will typcially cycle on and off at intervals from a few seconds to one or two
minutes. This is to ensure that the internal components are not permanently damaged from excessive thermal
stress.
The Undervoltage lockout (UVLO) is designed to prevent the operation of the converter until the input voltage is
at the minimum input voltage. This prevents high start-up current during normal power-up of the converter, and
minimizes the current drain from the input source during low input voltage conditions. The UVLO circuitry also
overrides the operation of the Remote On/Off control.
These converters incorporate electrical isolation between the input terminals (primary) and the output terminals
(secondary). All converters are production tested to a withstand voltage of 1500 VDC. This specification complies
with UL60950 and EN60950 requirements. This allows the converter to be configured for either a positive or
negative input voltage source. The data sheet Pin Descriptions section provides guidance as to the correct
reference that must be used for the external control signals.
The converter is not internally fused. For safety and overall system protection, the maximum input current to the
converter must be limited. Active or passive current limiting can be used. Passive current limiting can be a fast
acting fuse. A 125-V fuse, rated no more than 10 A, is recommended. Active current limiting can be implemented
with a current limited Hot-Swap controller.
Airflow may be necessary to ensure that the module can supply the desired load current in environments with
elevated ambient temperatures. The required airflow rate may be determined from the Safe Operating Area
(SOA) thermal derating chart (see typical characteristics).
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