# Using capacitor with voltage rating close to input voltage

I am using a buck converter in my design. The input voltage is 12V and output voltage is 5V.

The capacitor C331 and C332 are 10 uF 25 V in 1210 package. The inventory have already 16 V 0805 capacitors. All these are ceramic capacitors (C331, C332 and caps present in the inventory).

Can I use 16 V capacitor for an input voltage of 12 V?

• Your 12V input voltage - where does it come from? A very well regulated 12V plugpack is a different story from an automobile that intentionally hits 14.5V to charge the battery. Nov 16, 2023 at 4:45
• @Hari Automotive applications need more margin. There are few environments noisier than an automobile's electrical system. I wouldn't use 16 V capacitors here, though some might say I'm being overly cautious. Nov 16, 2023 at 5:01
• Given the reputation automotive has, I don't know if I'd even want 25V caps. Nov 16, 2023 at 6:17
• Do you have 80 V load dump requirement? Nov 16, 2023 at 8:01
• When I worked on my car's electronics with a scope, I saw noise spikes up to some 20V, and that was under normal operation. During more interesting conditions (starter motor engage, disengage, alternator start/stop, connecting/disconnecting heavy load), I'd expect worse. There's a reason automotive tests usually go up to 80 or 90V. Nov 16, 2023 at 8:18

If you take the ISO-16750 standard (electrical standard for road vehicles) as a reference, or if it's a must for your project to meet, then the input voltage capacitors' voltage rating should be greater than or equal to 18 VDC.

That's because the standard requires the equipment to be able to work with input voltage range of 9-18 VDC (or 18-36 VDC for two-battery vehicles such as trucks/lorries and coaches) at room temperature, although the nominal input voltage is 14 VDC (or 28 VDC) where the operational parameters are rated at.

Some further requirements are load dump (the input voltages can be 40V or even higher) and endurance (running with 14V or 16V at 50°C). These would be more stressful for your capacitors. Although some good quality capacitors can withstand the voltages equal to twice their voltage ratings for some limited time, it still doesn't allow us to use them without risk.

It's also worth to mention that Class-II MLCCs (judging from the value you should be using X5R or X7R) have terrible capacitance change behaviour with DC bias compared to Class-I (C0G/NP0) i.e. as the DC voltage across a Class-II MLCC reaches its rating the effective capacitance reduces further. The graph below is nicked from a Kemet application note:

As can be seen from above, we can fairly assume that the effective capacitance of a 100n/16V capacitor when used under 12V or 14V will exhibit less than or equal to half its capacitance. So you may want to consider this as well for EMI/EMC and other purposes, though I can't see any filter network in your schematic. You should have one as the automotive standards also cover and require EMC.

The inventory have already 16 V 0805 capacitors.

If you don't want to buy/get 25V-rated capacitors and if you are not size-limited (i.e. if there's enough space) then use two of those 16V-rated capacitors in series. Two 0805-case capacitors placed side-by-side occupy almost the same space as a 1210-case one does (see the image below), but you'll end up with higher number of capacitors to reach the same capacitance.