I am fairly sure the answer is no.
Is there an easy way to check a capacitor's dielectric?
I have some pre production buck converters that aren't stable where as the prototypes were OK. The input and output caps are specified as 4u7
50V X7R. The caps on the boards look suspiciously thin for the value. 1210s but only about 1.0mm thick. I bet someone has used Y5V to save money.
Measure its voltage:capacitance curve. That's what will matter here.
ESR might matter too. A fairly simple setup could measure both.
What voltage are they running at?
John
24V input, 8.4V output.
A Y5V might lose 40% of its capacitance at 17% (8.4/50) of its rated voltage. Add a little temperature effect and you could be down over half. Freeze spray the caps (-60% delta-C or so for Y5V) and see how that affects stability.
The input side cap could be down over 80%, but that wouldn't be as important to loop stability.
Measure them!
John
As a quick check we replaced the 4u7s and more than halved the output ripple. I'll measure them tomorrow.
The problem isn't cured so the next step is to replace all capacitors that affect stability and check them.
For a given brand, the firs thing is to get/derive volume VS capacitance for the various formulations (NPO, X7R, Y5U, etc) and see which type fits what you have; should stand out like a sore thumb. Next way is measure capacitance VS temperature for curve matching.
They sent me the data on the capacitor and it is Murata and should be
2.5mm thick. They are claiming Miurata also do a 1mm thick version and I have told them I do not believe them.
High value compact ceramics capacitance is very voltage dependant which is probably your problem (assuming it is not temperature related).
Measure the capacitance under changing bias and compare with what you think is a good one. If you don't have a meter capable then you could lash up something with a signal generator and scope/meter to ring it with an inductor under bias.
If you want to do a voltage-bias sensitivity test, just put two capacitors in series and put a few Mohm resistor to a bias voltage on the center tap. You can snap together any number of 9V batteries to get to the target bias.
Some dielectrics have 'memory', so it's worth reversing the bias battery for a second test. I'm not familiar with X7R and Y5V characteristics, but this is an easier test than a real temperature dependence test would be (test tube of oil, dunk in boiling or ice water).
May be a mistake on your part..did the specs include dielectric type, ie Y5U?
The 4u7 caps were Y5V but that wasn't the cause of the instability although the supplier has been castigated. They claim Murata sent the wrong part on the reel, not that I believe them.
I had replaced an axial 4.7uH inductor with a surface mount part and improved the power tracking to reduce pcb losses. The better inductor and reduced damping were enough to cause a problem.
Fortunately linking out the inductor fixes the problem. The spectrum analyser shows no crap getting out of the board even without the inductor. It looks like the multilayer ground planes and 7 caps from
47n to 4u7 have killed any conducted RF.ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.