Power distribution units (PDUs) for military applications have several useful features for monitoring and controlling power paths to devices in the system. One useful feature is the adjustable current limit, which can also be used during channel startup to provide a monotonic voltage ramp to a capacitance inside the downstream Line Replaceable Unit (LRU). This is done by setting a current limit value that is lower than that required for the continuous LRU load. By charging this capacitance with a constant-current source, the voltage ramp to the LRU can be set to a slower voltage ramp, averting high inrush currents that are common with switching into a capacitance.
Well-designed PDUs will have two current limits – one for setting the I2T curve (OVERLOAD) and the other for a catastrophic level (CURR_LIMIT) designed to protect the electronic switch. These can be used to successfully enable power to the channel over capacitance.
Simple current-limit on startup work well in cases where the LRU capacitance is relatively low, but can be problematic with large capacitances (i.e. 2mF or greater). In such cases, the high current can appear to be a short to the PDU channel and will trip out. This situation is fairly common in MIL-STD-704 systems where high capacitance can be installed to provide energy storage for power interruptions from switches and relays. For these scenarios, a pre-charge procedure is recommended using these steps:
1) SSPDU channel should be off initially.
2) Set CURR_LIMIT to anything below the OVERLOAD limits. Usually, it is best to set CURR_LIMIT high enough to support a typical load current for this LRU. (This may require some experimentation to achieve the correct value.)
3) Set BATTLE mode on. This disables the OVERLOAD current limit.
4) Enable the output channel.
5) Allow the output to charge up to VIN.
6) Set CURR_LIMIT back to 125A or any limit higher than OVERLOAD.
7) Set BATTLE mode off. Now the OVERLOAD current limit is re-enabled.
Ideally, it is best to have the downstream LRU disabled so the only concern is charging the large capacitance, but this is usually not possible since the LRU is not powered at this point. Properly designed LRUs could also prevent their internal startup until the internal capacitance is charged, but this is also often not the case. In general, some experimentation is required to successfully power up an LRU with a large unknown capacitance and varying startup load current. A number of cycles may be required to charge this LRU capacitance, but once charged, the OVERLOAD current limit may be set to its desired setting for the typical load current and good protection will be achieved.
