How to decouple the input of a DC-DC converter? - Page 1
Forum section: electronics / projects

author: Hobgoblin612
author_rank: Contributor
timestamp: March 05, 2013, 10:28:41 am
content: I am building an analog version of Dave's power supply and realised quite late that the panel meters I was using need an isolated supply. So I thought the best way to solve this was to add a small pre-made isolated DC-DC converter (http://www.farnell.com/datasheets/1662783.pdf) and run it from the 5V rail I already had. This works but it is causing about 20mV of ripple at about 160kHz (I assume this is the switching frequency of the converter, also note that it was measured with an uncalibrated old Tek scope) on the main 5V supply which is also used as an auxiliary output. What value and type of capacitor should I use to help this problem? Is there a better solution?The regulator (7805) already has a 100n cap on its output and a 1uF across the output terminals.Any sugestions would be appreciated, Thanks, Timothy.

author: Rerouter
author_rank: Super Contributor
timestamp: March 05, 2013, 10:38:05 am
content: best thing to do is give the switcher a decent bit of input capacitance with very low ESR, e.g. a big ceramic, (1-2.2uF) as close as humanly possible to its input terminals, even soldered half way up the legs with minimal lead length on the cap is good,as for the 7805 what do you mean a 100n on its output and a 1uF across the output terminals? do you mean one is the input as a typo or a 100n and a 1uF are parrelleled on its output,equally for the 7805, to really bring down the amount of noise, stick a decent very low esr cap right on its input terminals, the output is generally a bit happier with a little esr, (low tens of milli-ohms)

author: Hobgoblin612
author_rank: Contributor
timestamp: March 05, 2013, 11:11:15 am
content: Sorry, I wasn't clear; there is a 100nF film cap on the input and output of the 7805 right next to it on the PCB. There is also a 1uF electro across the binding posts (after the switch) on the front panel.What type of capacitor will have the lowest ESR? I plan on looking through datasheets but I need a place to start.I also have thought of another option which is to use a converter with a higher input voltage and run it from the main unregulated supply (i.e. after the big filter caps), will this work or will it put too much noise on the output after the LT3080?Thanks for your help Rerouter.

author: Rerouter
author_rank: Super Contributor
timestamp: March 05, 2013, 11:30:43 am
content: just so your aware the 3080 isnt some magical chip, if you only need 5V as opposed to variable a 7805 is fine given some capacitcance,a film cap should be good for the 7805 just how close to the input terminals is it? regulators like when its nice and cozy with its input buffer cap, (but when electro's not so cozy it cooks it... for later readers)and for the switcher a standard ceramic will have stupidly low ESR thus why i mentioned one,

author: Hobgoblin612
author_rank: Contributor
timestamp: March 05, 2013, 12:25:25 pm
content: The 100nF caps are about 4mm from the 7805. The 3080 is the main regulator as this is a lab supply. Sorry I missed that you said ceramic in your first reply.

author: SeanB
author_rank: Super Contributor
timestamp: March 05, 2013, 05:42:46 pm
content: Put another 7805 to feed the converters only. Input and output a 100n ceramic and a 1000uf ( voltage depending on input, probably 35V) low ESR 105C capacitor on input, and 470uF 25V 105C electrolytic on output. No more ripple on the switched 5V rail then. You should have enough room to put the regulator in the case on a heatsink.

author: Hobgoblin612
author_rank: Contributor
timestamp: March 06, 2013, 05:55:40 am
content: It only needs to power 2 panel meters (for voltage & current) at a couple of mA each so can I get away with a separate 78L05?Also will the inherent ripple on the output affect the panel meters? (CX101B type) At this stage it doesn't seem to.Also I might not have mentioned that I am going to re-spin the board anyway so I can easily add this now.Thanks for the help.

author: SeanB
author_rank: Super Contributor
timestamp: March 06, 2013, 07:10:34 pm
content: Don't use the L version, use the 1A version, as then you do not need a heatsink if the power is low. The L may be able to do 100mA, but cannot dissipate more than a few hundred milliwatts, the bigger package is more capable, even if you are at a low current it runs a lot cooler.