Any problems with using X5R/X7R caps as buck converter output capacitors?

[Joerg]

Hi Joerg,

almost all of my SMPS are Peak Current-Mode Controlled - that being
said, the ones I am doing now are voltage-mode controlled, but with peak
current limits.

we are talking about the same thing, but I shall elucidate.

at FET turn-on, the gate current flows the current sense resistor Rs,
causing a spike. for low-power converters that switch quickly, this
spike can be large - possibly even troublesome. I often re-route the
gatedrive current away from Rs to avoid this.

and also when the FET turns on, it has to charge the xfmr/snubber/etc
capacitance. this Cx*dV/dt current also flows thru Rs, again causing
problems. not many cute tricks to get rid of this - but in DCM can turn
on nice and slow (ware start-up though, it wont be DCM). of course the
FET Cds doesnt cause trouble, other than heating up the FET.

making a very efficient low power PCMC supply that runs from high
voltage can thus be a real PITA - low power = large Rs; high efficiency
generally means fast FET switching hence high CdV/dt current into Rs

I found that a nice ceramic cap across Rsense takes care if this quite
well. Large enough to muffle the spike but small enough not to skew the
ramp too much. After all, it's only used to detect when a desired
cut-off has been reached.

Many of my designs have to go from full bore all the way down to zero
output current, and sometimes also zero volts meaning it has to be a
forward converter or SEPIC. Never been a problem really. Below a certain
load they go into "pulse rate stretch" or in modern SMPS lingo pulse
skipping, although technically stretching has nothing to do with skipping.

 

[Joel Koltner]

So the equivalent ESR decrease from Esr (C2/(C1+C2))^2 at low frequency,
with a 12dB/oct rate above a corner frequency given by Esr and C1//C2

Let me try to rephrase this a little: Choosing the ESR of the electrolytic
capacitor is still important in the case where its capacitance (C2) is
significantly larger than the total of all those 100nF (or whatever) ceramics
sprinkled around the board (C1) (assuming assuming the switcher's loop gain
starts falling before you hit the zero of the electrolytic at ESR*C2, which I
would think can be taken as a given)... right?

Thanks,
---Joel

 

[Joerg]

Let me try to rephrase this a little: Choosing the ESR of the electrolytic
capacitor is still important in the case where its capacitance (C2) is
significantly larger than the total of all those 100nF (or whatever) ceramics
sprinkled around the board (C1) (assuming assuming the switcher's loop gain
starts falling before you hit the zero of the electrolytic at ESR*C2, which I
would think can be taken as a given)... right?

But: Nobody is going to notify you when the manufacturer improves the
caps. Suddenly their ESR becomes a lot lower, the regulator chip gets
all choked up ... phssst .. BANG. IMHO relying on a cap ESR is the same
as relying on prop delays. In a production environment it's like playing
with fire. Hence no LDOs in this here lab

 

[Terry Given]

But: Nobody is going to notify you when the manufacturer improves the
caps. Suddenly their ESR becomes a lot lower, the regulator chip gets
all choked up ... phssst .. BANG. IMHO relying on a cap ESR is the same
as relying on prop delays. In a production environment it's like playing
with fire. Hence no LDOs in this here lab

OR, use a very low ESR electrolytic, in series with a resistor.

the other reason using electrolytic ESR for damping/loop compensation
isnt so crash hot is its behaviour at low temperatures - expect a
10-fold increase at -20C

Cheers
Terry

 

[Joel Koltner]

It's usually a loop stability or layout issue. Can you post a schematic?

It's over in ABSE now. It is an "all in one" switcher (LTC3404) .

I looked at it again today before dumping the schematic/PCB into a PDF and
it's actually not as bad as I had previously recalled, but right around 3V
input there's jitter around 15% of the switching period

---Joel

 

[Joerg]

It's over in ABSE now. It is an "all in one" switcher (LTC3404) .

I looked at it again today before dumping the schematic/PCB into a PDF and
it's actually not as bad as I had previously recalled, but right around 3V
input there's jitter around 15% of the switching period

Hasn't shown up yet but I'll keep looking. Might be the weekend though.
Got a job to finish tomorrow. Plus my missus wants a new commode in the
master bath. Rear-outlet plumbing and all those nasties. Oh what fun!

 

[Joel Koltner]

Hasn't shown up yet but I'll keep looking. Might be the weekend though.

Sure, understood -- I'm happy you're willing to take a look in the first
place.

Got a job to finish tomorrow. Plus my missus wants a new commode in the
master bath. Rear-outlet plumbing and all those nasties. Oh what fun!

Would one of those Home Depot "How to do Anything to your Home" books for Xmas
be received well if you got her a copy? ;-)

---Joel

 

[Nico Coesel]

Before I start dropping some decent fraction of a dollar per capacitor (sorry,
Joerg, the PCB layout guy claims he doesn't have room for electrolytics), I
was wondering if anyone here has had any problems (or successes) using regular
ceramic chip caps with X5R or X7R dielectrics as their main output capacitors
in a buck converter? I need ESR under 250milliohms, which -- while ceramic
caps don't normally spec this -- seems much higher than any guesstimate at ESR
I can make by looking at impedance graphs from the likes of AVX (my
guesstimates range from about 10-100milliohms depending on the particular cap
I look at). I'm looking for some tens of uF here... probably a 22uF cap would
work well.

It works well. But you have to look after 2 things:
- the regulator loop may require extra compensation
- the ceramic capacitor's capacitance varies by the voltage applied to
it. Read the specs of the capacitor you want to use carefully.

 

[Joerg]

Sure, understood -- I'm happy you're willing to take a look in the first
place.

I did, and commented over on a.b.s.e. since that's where the images are.

Would one of those Home Depot "How to do Anything to your Home" books for Xmas
be received well if you got her a copy? ;-)

She's quite versed when it comes to repairs. But plumbing jobs with
heavy duty lifting or anything high on a ladder fall into my job
description. Got the old wall-hung commode out, whew. Behind it I found
a totally weird mount. Of course. And you can't get a hand behind the
bracket because they closed off the wall after mounting it. Of course.
Stuff like this obviously isn't meant to be easy.

 

[krw]

I did, and commented over on a.b.s.e. since that's where the images are.



She's quite versed when it comes to repairs. But plumbing jobs with
heavy duty lifting or anything high on a ladder fall into my job
description. Got the old wall-hung commode out, whew. Behind it I found
a totally weird mount. Of course. And you can't get a hand behind the
bracket because they closed off the wall after mounting it. Of course.
Stuff like this obviously isn't meant to be easy.

Of course. Plumbers earn their money. IMO the only way to do
plumbing is by starting with a chain saw and finish up the delicate
work with a sawsall. Levers help too.

 

[Joerg]

Of course. Plumbers earn their money. IMO the only way to do
plumbing is by starting with a chain saw and finish up the delicate
work with a sawsall. Levers help too.

Just finished the demo. It's scary how easy it is to crunch away
drywall. Now I have to do the face lift, mending drywall back in. Don't
like it much, I am used to the good stuff. Laying brick, mortar and such.

 

[krw]

Just finished the demo. It's scary how easy it is to crunch away
drywall. Now I have to do the face lift, mending drywall back in. Don't
like it much, I am used to the good stuff. Laying brick, mortar and such.

But that's the nice thing about sheetrock; it's so easy to work with
(against). The trick is to pull it down, rather than breaking it up
too much. I cut close to the seams and use a pry bar to pull it off
the wall. Mych less messy than using a hammer. THEN you take you
the chain saw (Sawsall) to the plumbing.