Embedded CPU power supply ESD filter
Embedded CPU power supply ESD filter
(OP)
I'm having trouble with a PC104 on a mobile robot. It is cursed with a very dry climate and carpeted floors, so massive static discharge a constant problem - it sees +/- 40V transients on the 5V supply quite frequently due to coming in contact with metal or being handled by people.
The several on-board microcontrollers handle this just fine, however the PC104 CPU tends to reboot when these transients occur. This board is very sensitive and will sometimes reboot when running off my bench supply if there is static discharge anywhere nearby.
The robot takes power at 12-15V from batteries and I use a TI PTN78020 power module to provide the 5V supply. This has capacitor/ferrite filters on both the input and output. The 5V supply then passes through a small filter with a common-mode choke, a couple large supply caps and some smaller ceramics, and a zener TVS before hitting the PC104. This seems to take a few volts off the peaks of the transients but doesn't make much of a difference; the spikes are on the order of 10nS, so I guess the TVS doesn't clamp quickly enough?
I don't have space to add a commercial PC104 power supply to the stack, and I don't have much background in filtering. Any suggestions on what I could do to mitigate this issue? Thanks!
The several on-board microcontrollers handle this just fine, however the PC104 CPU tends to reboot when these transients occur. This board is very sensitive and will sometimes reboot when running off my bench supply if there is static discharge anywhere nearby.
The robot takes power at 12-15V from batteries and I use a TI PTN78020 power module to provide the 5V supply. This has capacitor/ferrite filters on both the input and output. The 5V supply then passes through a small filter with a common-mode choke, a couple large supply caps and some smaller ceramics, and a zener TVS before hitting the PC104. This seems to take a few volts off the peaks of the transients but doesn't make much of a difference; the spikes are on the order of 10nS, so I guess the TVS doesn't clamp quickly enough?
I don't have space to add a commercial PC104 power supply to the stack, and I don't have much background in filtering. Any suggestions on what I could do to mitigate this issue? Thanks!





RE: Embedded CPU power supply ESD filter
What value are your caps? You need some pretty small ceramic ones (0.1uF, 0.001uF, 0.001uF) to nip the 10nS rise time; the parasitics on the larger value caps will keep them from doing anything that fast. Just watch your voltage rating, but at 40V you should have no problem with ceramic capacitors.
John D
RE: Embedded CPU power supply ESD filter
There are two 1200uF electrolytics, the small ceramics are 1uF and 0.1uF. There are two of each between 5V and GND supply from the supply, and there is a 1uF and 0.1uF from each of 5V and GND to a separate connection to chassis ground.
The above setup really doesn't make much of a dent in the spikes looking directly at the PC104 inputs.
Cheers,
Mike
RE: Embedded CPU power supply ESD filter
Keith Cress
kcress - http://www.flaminsystems.com
RE: Embedded CPU power supply ESD filter
RE: Embedded CPU power supply ESD filter
Glenn
RE: Embedded CPU power supply ESD filter
These problems always have three sections - the source, the path, and the receptor. You can't do anything about the source (ESD events), you are already working on the path (filtering), so reducing the receptor susceptibility is another option.
John D
RE: Embedded CPU power supply ESD filter
I hadn't thought about the reset pin - I assumed there is an internal pull-up somewhere but I'll try adding one and see if that makes a difference.
As the CPU currently shares a common supply with the rest of the 5V electronics, an isolated supply would involve a new board revision and isolating the serial port, but may be the right solution. I've only used isolated supplies before for ADC's or audio - would one of the small modules like the TDK CC-E series likely work well for this, or should I be looking at the heavy duty industrial packages?
Thanks for all the input,
Mike
RE: Embedded CPU power supply ESD filter
Your comment that the 5V supply is shared made me wonder if the problem isn't somewhere else. You've seen the 40V spike on the 5V line; is another spike showing up on a PC104 I/O line?
John
RE: Embedded CPU power supply ESD filter
Cheers,
RE: Embedded CPU power supply ESD filter
Keith Cress
kcress - http://www.flaminsystems.com
RE: Embedded CPU power supply ESD filter
I'd left a couple unterminated wires running from the board to a PS/2 connector for a keyboard. After disconnecting this bundle, several large shocks wouldn't cause a reboot, reconnecting the wires and right back to rebooting. I'll have to do some more testing next week to confirm, but this looks pretty promising.
Cheers,
Mike
RE: Embedded CPU power supply ESD filter
Keith Cress
kcress - http://www.flaminsystems.com