Lowest ESR ceramic caps for LED drivers?

[VanIsleDSM]

Trying to maximize efficiency for an LED driver I'm building, and I'm looking for the lowest possible ESR (equivalent series resistance) ceramic capacitors.

In the past I have used the panasonic ECJ series capacitors, which are "low ESR" caps, but they do not actually give a value. I have been looking through as many datasheets as I can, but I'm on a very poor connection right now in the middle of the frozen north at work, so it's hard to view a ton of PDF files.

Often the datasheets don't even mention ESR, which I find to be quite useless.

Does anybody know of any documentation comparing capacitors for use in DC/DC converters that makes mention of their ESR ratings? I really want to find the best ones possible, without going insane on price. Seems to be tough information to dig up.

Thanks in advance!

 

[jtr1962]

It's not going to make a measureable difference in efficiency trying to find the lowest possible ESR caps. Usually ceramic caps are used to bypass the driver IC in order to prevent instability in the system. You need a certain minimum ESR and capacitance for that, but generally, decreasing the ESR further gives you very little in terms of either enhanced stability or efficiency. If you want to maximize efficiency start with finding the lowest resistance inductor which physically fits into the space available. That can get you a few percent gains. After that look for the best MOSFET. Again, a few more percent. Even better is to use a driver IC which uses synchronous rectification. That one thing alone can increase efficiency by 5% or more, especially if the output voltage is a lot less than the input voltage. And use an IC which requires a very low sense voltage, such as 100 mV, in order to minimize your losses in the sense resistor.

 

[VanIsleDSM]

Thanks for the reply jtr,

I've already found the lowest possible resistance inductor I can fit, while paying attention to magnetic losses in the core as well, along with the other points you have mentioned.

ESR, while not playing a huge roll in efficiency, will still add to it if lower, just trying to get all I can out of it, but you're probably right, I'm stretching a bit.

 

[jtr1962]

ESR, while not playing a huge roll in efficiency, will still add to it if lower, just trying to get all I can out of it, but you're probably right, I'm stretching a bit.

You might be able to squeeze out another 0.1% or 0.2% ( if you're really lucky ), but at some point the extra cost just isn't worth it. Also, I've run into similar problems finding data on the ESR of ceramic caps. Apparently it's low enough that the manufacturers don't consider it worth putting in the data sheets as they often do with electrolytics. I do know for a fact that ESR of ceramic caps is lower than an equivalent-sized tantalum cap, and usually lower than one much higher. I had a circuit once which was marginally unstable with a 100 uF tantalum bypass cap. I put in a 4.7 uF ceramic, and no problems. I'm reasonably sure the ESR of most ceramic bypass caps falls in the area of a few tens of milliohms or less at the frequencies switching drivers operate at. You can always put a few caps in parallel just to test how much the efficiency of your driver improves. My guess is probably not enough to even be reliably measured.

I had forgot to mention core losses, but yes, that's important also. If the inductor gets close to saturation, efficiency really tanks.

 

[BillyNoMates]

Try using Murata - plenty of information on their website.

Here is an example for a 22uF in 1210 SMD size showing the ESR bottoming out at 2mOhm.

 

[J_C]

Don't expect efficiency increase from capacitor ESR, I agree 0.2% is a high/lucky idealism. If it is higher, all you end up with is roughly same average current but ripple resulting in periodic higher/lower momentary light. When higher, LED is less efficient, when lower, more.

My advice: LEDs have a strong point of long life if driven and heatsunk reasonably, choose low-ESR, including ceramic or tantalum when possible, to make the driver hopefully last as long as the LED otherwise could.

In other words, don't use electrolytic caps if you can avoid it. The real question is, what is the point? No matter how exotic the circuit to save a trivial amount of power, in a few months a better LED will come along to swamp all efforts towards the goal. I don't mean *one* killer LED, I meant the progress the industry keeps making.

Even if it were going on the Space Shuttle, this is not a reasonable concern.