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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

PTQA430025, PTQA430033, PTQA420050

SLTS261A – MAY 2006 – REVISED DECEMBER 2006

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 and the requirements for operational isolation. 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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