While looking at TI's power supply design page I am seeing capacitors listed with a resistance value e.g. (100nF / 29.6mOhms). How critical is this esr rating in switch mode supplies? There are several 100nF capacitors in their design example, all with different Ohm values. The Ohm values range from 1mOhm to 29.6mOhms. As it happens all these capacitor part numbers supplied by TI are in the extended parts offerings on JLCPCB's site. It would be nice if I could limit their extended part setup fee by using only 1 part number 100nF cap.

 

It is very critical, and a forum post is hardly the place for you to learn this. I suggest a discrete conversation with an experienced and knowledgeable colleague to avoid an embarrassing faux pas. If such a colleague is not available, then a textbook or online resources may be your only alternative.

In short:

1. The ESR contributes to losses which affect the efficiency, which can affect the service life.
2. It can sometimes affect the converter's control characteristics. This will be a nasty problem to find and fix late in the design cycle.
3. Startup behavior is affected by ESR

There may be additional items that are peculiar to specific converter topologies, but these are the main ones I am familiar with.

 

There is a chart with the ESR meter that lets you know what ESR a typical application might need.
In this video the ecap in circuit on the scope has a higher ESR then he switches to a lower ESR.
In some applications the ESR is critical as demonstrated. An acceptable level of ESR depends on the application.
If it works don't fix it because ESR is relative to a specified tolerance of a circuit.
If the circuit waveform has issues then the capacitor of higher ESR can be replaced with a lower value ESR.
The video fast forwards to t = 7:40 first high ESR then Lower ESR

 

Both of the responses are correct! A switch-mode power supply design is probably the most complex common system around, because every portion, including the interconnections, has an effect on the operation.
So while the basic theory of operation is fairly simple, the translation into a practical device is not at all simple.

 

Since the cost of using extended parts in a JLCPCB design is much greater than the part cost could I replace all the higher esr caps with ones of lesser esr? In other words, is there a reason that TI selected a range of 100nF caps in their design other than cost savings?

 

Since the cost of using extended parts in a JLCPCB design is much greater than the part cost could I replace all the higher esr caps with ones of lesser esr? In other words, is there a reason that TI selected a range of 100nF caps in their design other than cost savings?

I don't exactly understand this question. If the cost of using the extended parts is much greater where is the cost savings? Did it ever occur to you that the cost of fabricating and grading capacitors for their ESR characteristics might have costs associated with them? They used to do this with ICs. They all came off the same line, but some were better, and some were worse. Instead of binning the parts that did not meet their original specifications they tested and graded the parts according to their capabilities to make them sellable thus increasing their yields. In the early days of semiconductor manufacturing the yields were horrible while the process engineers were tweaking their processes. I would bet modest sums, up to my betting limit, that TI, not being a stupid enterprise, is doing it for economic reasons.

 

Thanks. The cost increase for using JLCPCB extended parts only applies to the 1st part. If you have 10 of the same parts on a board you only get charged the extended part setup fee once.

 

It may also be worth the effort of locating other sources of the capacitors. That may involve looking at data sheets and determining which properties match the designs specifications.
AND, what do you mean by "Extended parts"??? That is a term I have not heard before. It is rather ambiguous, really. WHAT are you talking about there???

 

It may also be worth the effort of locating other sources of the capacitors. That may involve looking at data sheets and determining which properties match the designs specifications.
AND, what do you mean by "Extended parts"??? That is a term I have not heard before. It is rather ambiguous, really. WHAT are you talking about there???

PCB houses that provide assembly services typically have an inventory of parts that they maintain. If you use any parts not on that list, it is referred to as an extended part (they don't all use the same terminology) and they charge you a setup fee to have them populate your board with those parts. In some cases, if you provide the parts to them in a suitable form (such as tape and reel), they may waive the setup fee or it might be pretty minimum. But if you expect them to order the parts, the setup fee can be much higher as it is custom work that they are doing just for your project. Sometimes, the part you are specifying is one that they have just never been asked to use before, but is one that they expect other customers will likely use if it were in their inventory list, so they might waive or discount the setup fee. If it isn't a part they are interested in adding to their list or keeping on hand (which is the more common situation), then they will usually bill you for the entire cost of the part, and will usually include the remainder of the parts with your boards.

 

I was considering the option of the TS doing a bit of the assembly themselves. since the 1930's most hobby class experimenters and constructors build their own projects, until quite recently.