Board current consumption Vs current supplied by VME crate
The CONV-TTL-BLO board generates the +27V required for the blocking output stage, from the +12V provided on the ELMA crate. To do so, it uses the TI TSP 40210 boost controller.
In terms of power ELMA specifies 66W available on the +12V power supply (corresponding to 5.5A maximum current) compared with the 200W on the +5V power supply.
As the board can potentially operate six blocking channels, each with 3 outputs, the +27V supply may not be able to cope with current demand.
Therefore it is necessary to define a limiting value in terms of blocking outputs/crate, that can be supported by the design.
Measurement procedure:
- A conv-ttl-board is installed on an ELMA crate with an RTM board.
- All blocking outputs are terminated with a 50Ohms resistor.
- Worst case pulse frequencies are generated by the board (Board running with gateware pulsetest enabling settable output frequencies).
- The current consumption of the board will be measured for short 250ns pulses and long 1.2 us pulses.
- The SysMon on the ELMA crate allows access to current sensor data from a telnet prompt (Type command @ > current@).
- The number of channels running is gradually inctremented and current
consumption from the crate is recorded for the number of channels.
The table below shows the current consumption measured by the crate, as a function of output pulse width and frequency, as well as the number of channels running simultaneously:
Number of channels | Number of active outputs | 250ns @2MHz | 250ns @500kHz | 1.2us @100kHz | 1.2us @50kHz |
---|---|---|---|---|---|
1 | 3 | 1.7A | 1A | 2A | 1.8A |
2 | 6 | 2.7A | 1.3A | 2.6A | 2.2A |
3 | 9 | 3.75A | 1.9A | 3.4A | 2.5A |
4 | 12 | 4.6A | 2A | 3.8A | 2.7A |
5 | 15 | OVERCURRENT | 3.25A | 3.9A | 2.9A |
6 | 18 | OVERCURRENT | 3.75A | 4.2A | 3.2A |
Conclusions and recommendations:
- Maximum current consumption is fast reached at higher frequencies.
- By adopting worst case values as an indicator of extreme use of the board, a limitation may need to be imposed on the number of active blocking outputs per crate:
- As The crate will shut off the +12V power supply at 5.5A, the +12V fuse can be selected at 5Amps, which will offer extra protection to the board.
If too many blocking outputs are in operation, and if the number of
outputs exceeds the maximum recommended, then the fuse will blow causing
the blocking output to fail.
Therefore, a limit on blocking outputs/crate, must be imposed. In worst
case scenarios, each output port requires a maximum current consumption
I (blo-out) of 420mA for maximum repetition frequency (2MHz
repetition).
As a rule of thumb, the maximum number of active output ports N
(blo-out) per crate is:
N (blo-out}= I(fuse+12V)/I(blo-out) thus giving:
N(blo-out}= 5/0.4~12
-
For 250ns pulses, the limit is 12 BLO outputs per crate (Regardless of the number of CONV-TTL-BLO boards).
-
For 1.2us pulses, the limit is 18 BLO output channels per crate (Regardless of the number of CONV-TTL-BLO boards).
(red) This means that during planning and installations, it is important to not allocate any more than 12 active blocking output ports in total per crate, regardless of the number of boards in the crate (1). As the board is fuse-protected, failing to respect this limit will not damage the board, but will cause a failure in blocking repetition and will necessitate human intervention to replace the damaged fuse.
(1)Could range from 12 boards with one blocking output channel each to 1 board with only 12 channels active out of the 18 available
Denia Bouhired, 31 March 2017.