This time I will write a short article about the DC/DC Converter which we will use to drive our Shape Memory Alloy based Hold Down Release Mechanism.
As the resistance of the SMA spring is quite low, we need a DC/DC Converter which provides a low voltage at high current. We also want our DC/DC Converter to have a constant current limit in overload conditions (blue curve in the picture below). Most commercial DC/DC Converter implement a current foldback (more infos on Wikipedia http://en.wikipedia.org/wiki/Foldback_%28power_supply_design%29), a hiccup mode (the converter turns on and off all the time while in overload condition) or the converter simply shuts down.
Finally the converter shall be powered directly by the REXUS Service Module 28V power bus. For this reason we decided to design our own DC/DC converter. After a little bit of search I found the LT3840 step down converter IC, which fulfills all these requirements:
- Wide input range up to 60 V
- Low output voltage down to 1.25 V
- Integrated 7.5 V boost-buck converter to drive the MOSFETs (less heat than LDO)
- Constant current operation with current monitoring (integrated shunt amplifier)
- Soft start feature
To test this DC/DC converter, we built a little demo board, which is already capable of driving our test mechanism.