Maker Pro
Maker Pro

LM3478 design gets insanely hot

H

Henrik [7182]

Jan 1, 1970
0
Hello Experts,

This is my first time designing a SEPIC type switchmode power supply with
the LM3478MM from national as my controller.

My design is pretty much the standard application as shown in the datasheet.

The design is made to supply 24V out from a 15-32V input. Ouput current is
1A.

My problem is that my two inductors and the MOSFET gets insanely hot. Much
more hot that I had anticipated. So I wonder if there is somewhere I have
missed something vital.

I am using CDRH127 series 22uH inductors from Sumida, which are rated at
3.3Amp and has a DCR of 43mR

I use a 39K resistor to obtain approx. 400kHz switch frequency.

I use 7mR as Rsense and MBRS360 as my power diode.

My Compensation-filter consists of 100nF and 1K.

My Mos-Fet is NP55N055 from NEC which has 12.5mR on-resistance.

Since I am a novice in the field of SMPS, this has me baffled quite a bit,
and I hope that someone could lead me on right path.


Thanking you all in advance.

Best regards
Henrik
 
J

Joerg

Jan 1, 1970
0
Henrik said:
Hello Experts,

This is my first time designing a SEPIC type switchmode power supply with
the LM3478MM from national as my controller.

My design is pretty much the standard application as shown in the datasheet.

The design is made to supply 24V out from a 15-32V input. Ouput current is
1A.

My problem is that my two inductors and the MOSFET gets insanely hot. Much
more hot that I had anticipated. So I wonder if there is somewhere I have
missed something vital.

I am using CDRH127 series 22uH inductors from Sumida, which are rated at
3.3Amp and has a DCR of 43mR

IIRC those are 3.6A but could still be a bit marginal for this design.

I use a 39K resistor to obtain approx. 400kHz switch frequency.

I use 7mR as Rsense and MBRS360 as my power diode.

My Compensation-filter consists of 100nF and 1K.

My Mos-Fet is NP55N055 from NEC which has 12.5mR on-resistance.

Since I am a novice in the field of SMPS, this has me baffled quite a bit,
and I hope that someone could lead me on right path.

Time to take scope shots and post them. But better not on a.b.s.e.
because some attorney general has shot that tool for most of us.

Probe the gate directly at the FET, and source and drain. Those are the
most important vitals for a switcher. I did a 6-9V to 0-100V adjustable
SEPIC with the LM3478 a few years ago. Don't have scope plots but I kind
of remember how it looked. Worked like a champ.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
It cannot be insanely hot if it is stil working.
 
H

Henrik [7182]

Jan 1, 1970
0
Joerg said:
Time to take scope shots and post them. But better not on a.b.s.e. because
some attorney general has shot that tool for most of us.

Probe the gate directly at the FET, and source and drain. Those are the
most important vitals for a switcher. I did a 6-9V to 0-100V adjustable
SEPIC with the LM3478 a few years ago. Don't have scope plots but I kind
of remember how it looked. Worked like a champ.

Dear Joerg,

Thank you for your valued reply. I have taken screenshots at the three
mos-fet terminals. They are located here: www.impc.dk/files/lm3478_files.rar

I hope that you will take a look and perhaps your past experience with the
device could shed some light on my problems.

All shots have been taken with 1uS for T/Div and the Gate has 2V/Div, the
Drain is 10V/div and the Source is 500mV/Div

Thanking you in advance.

Best regards
Henrik
 
J

Joerg

Jan 1, 1970
0
Henrik said:
Dear Joerg,

Thank you for your valued reply. I have taken screenshots at the three
mos-fet terminals. They are located here: www.impc.dk/files/lm3478_files.rar

I hope that you will take a look and perhaps your past experience with the
device could shed some light on my problems.

All shots have been taken with 1uS for T/Div and the Gate has 2V/Div, the
Drain is 10V/div and the Source is 500mV/Div

Thanking you in advance.

Sorry but I cannot read this kind of format. Maybe a Linux format which
I don't have. Can you post them in png, tiff, jpeg or some other more
common format?
 
L

legg

Jan 1, 1970
0
Hello Experts,

This is my first time designing a SEPIC type switchmode power supply with
the LM3478MM from national as my controller.

My design is pretty much the standard application as shown in the datasheet.

The design is made to supply 24V out from a 15-32V input. Ouput current is
1A.

My problem is that my two inductors and the MOSFET gets insanely hot. Much
more hot that I had anticipated. So I wonder if there is somewhere I have
missed something vital.

I am using CDRH127 series 22uH inductors from Sumida, which are rated at
3.3Amp and has a DCR of 43mR

I use a 39K resistor to obtain approx. 400kHz switch frequency.

I use 7mR as Rsense and MBRS360 as my power diode.

My Compensation-filter consists of 100nF and 1K.

My Mos-Fet is NP55N055 from NEC which has 12.5mR on-resistance.

Since I am a novice in the field of SMPS, this has me baffled quite a bit,
and I hope that someone could lead me on right path.

7mR seems like a pretty low value sense resistor for a 3A output and a
350mV fault threshold. Shouldn't it be 15 or 30?

RL
 
H

Henrik [7182]

Jan 1, 1970
0
legg said:
7mR seems like a pretty low value sense resistor for a 3A output and a
350mV fault threshold. Shouldn't it be 15 or 30?

RL

Actually you may be right, I did not think this could have any effect on my
heating issue, so I just picked somthing so low that I didn't expect to get
any problems. Perhaps I am wrong? As I wrote earlier, I am still a novice in
the field of SMPS and SEPIC in particular, but eager to learn ;-)

Best regards
Henrik
 
H

Henrik [7182]

Jan 1, 1970
0
Joerg said:
Sorry but I cannot read this kind of format. Maybe a Linux format which I
don't have. Can you post them in png, tiff, jpeg or some other more common
format?

Hello Joerg,

Thank you for your time, and am sorry for the fileformat, I have now placed
each file separately as JPEG for easier access:

www.impc.dk/files/LM3478_fet_gate.jpg
www.impc.dk/files/LM3478_fet_drain.jpg
www.impc.dk/files/LM3478_fet_source.jpg
www.impc.dk/files/schematic.jpg

I hope they are viewable at your location.

Thanking you in advance

Best regards
Henrik
 
L

legg

Jan 1, 1970
0
Actually you may be right, I did not think this could have any effect on my
heating issue, so I just picked somthing so low that I didn't expect to get
any problems. Perhaps I am wrong? As I wrote earlier, I am still a novice in
the field of SMPS and SEPIC in particular, but eager to learn ;-)

Safest sense resistor values would be slightly higher (even double)
the estimated optimum, to begin with.

If it was large enough, you could use it to scope the current
waveform, and post this as well.

Try to scope a single cycle, or even the rising or falling edges -
with surrounding noise. It helps with visual correlation of single
trace plots, if the waveforms are not close to 50% duty cycle, as
well.

RL
 
J

Joerg

Jan 1, 1970
0
Henrik said:
Hello Joerg,

Thank you for your time, and am sorry for the fileformat, I have now placed
each file separately as JPEG for easier access:

www.impc.dk/files/LM3478_fet_gate.jpg


The ramp up looks really sluggish. I don't know the data of this FET but
maybe it's way too big for the LM3478 to drive. Looks like it may have
too much Cgd.



That must be one hell of a FET being able to hold all this down. Your
inductor must drop almost a volt at peak.



Way, way too much peak current. I don't know what you voltage levels
are, no data in the image about that.


Aha! Unless I am mistaken your current peaks exceed your inductor
rating, big time. Isense trips around 165mV and with 7mohms you'd be at
23amps by then. Try raising Rsense to something like 50mohm. You may
have the inductor sitting in saturation and there it won't have any
inductance to write home about, just resistive losses.

BTW I wouldn't use an electrolytic for the SEPIC cap (C5). I'd consider
a few ceramics in parallel. Electrolytics get stressed out there and
might explode some day. Unless it's a really, really good one and can
take the ripple at frequency. But if it gets warm to the touch don't use it.

I hope they are viewable at your location.

Yes, they are. Next time try if you can get a real screen shot from your
scope, with the setting and all.

Just FYI, before you release BOM and schematic some day: Replace LM347B
with LM3478.
 
H

Henrik [7182]

Jan 1, 1970
0
Dear Joerg,

Thank you very much for your replies. I will with a 50mR or so Rsense
tomorrow, Also I will try to get the scales and such nice info into the
shots.

Also I will try exchanging the sepic capacitor to a large 1uF ceramic
capacitor that I have in stock.

Thank you so very much for your help. I will sleep much better now that I
see a possible solution to my problems ;-)

Best regards
Henrik
 
H

Henrik [7182]

Jan 1, 1970
0
Dear Sir

Thank you for your valued reply, I will try bumping up the sense resistor
tomorrow and post a few more and better screen shots.

Best regards
Henrik
 
J

Joerg

Jan 1, 1970
0
Henrik said:
Dear Joerg,

Thank you very much for your replies. I will with a 50mR or so Rsense
tomorrow, Also I will try to get the scales and such nice info into the
shots.

BTW it makes it easier for others to follow and contribute ideas if you
write below the quoted text.

Also I will try exchanging the sepic capacitor to a large 1uF ceramic
capacitor that I have in stock.

Stack a few of those for the test. 1uF may be a tad too small for a 1A
switcher (tho it'll work). Nowadays you can get pretty good 10uF or at
least 4.7uF caps but even there you may have to parallel in order to
ease and share the current load. They have become quite cheap.

With switchers it is really important to carefully study the datasheets
of even the most mundane parts. We take those caps for granted in bypass
situations but here they'll have to slosh around a lot of energy. I
leaned that the hard way. Everything was fine for an hour or so. Then a
very loud bang, pieces flying around, and what was left from the
ceramic material of a capacitor had turned into green bubbly glass.
Quite pretty actually but it could have been a bad situation if I hadn't
worn eye protection glasses. Best to wear those when you work on
switchers and similar gear that can go "exotherm".

Thank you so very much for your help. I will sleep much better now that I
see a possible solution to my problems ;-)

Since you are in Denmark this would be the perfect time for an ice-cold
shotglass of Aquavit. Good stuff :)

[...]
 
H

Henrik [7182]

Jan 1, 1970
0
Joerg said:
BTW it makes it easier for others to follow and contribute ideas if you
write below the quoted text.

Yes, mea culpa. I actually know this, it was just plain sloppiness on my
part.
Stack a few of those for the test. 1uF may be a tad too small for a 1A
switcher (tho it'll work). Nowadays you can get pretty good 10uF or at
least 4.7uF caps but even there you may have to parallel in order to ease
and share the current load. They have become quite cheap.

I stacked a few of my 1uF 50V on top of each other.

With switchers it is really important to carefully study the datasheets of
even the most mundane parts. We take those caps for granted in bypass
situations but here they'll have to slosh around a lot of energy. I leaned
that the hard way. Everything was fine for an hour or so. Then a very loud
bang, pieces flying around, and what was left from the ceramic material of
a capacitor had turned into green bubbly glass. Quite pretty actually but
it could have been a bad situation if I hadn't worn eye protection
glasses. Best to wear those when you work on switchers and similar gear
that can go "exotherm".

A chilling history indeed, I once experienced something similar. A colleague
of mine got a small part of his earlope shot away by and exploding tantalum
drop-type capacitor. He bled quite a lot and was really shocked.
Since you are in Denmark this would be the perfect time for an ice-cold
shotglass of Aquavit. Good stuff :)

Aquavit is good stuff indeed! An I soon might need some of it to calm myself
down ;-) It is really really hot in Denmark these days and this switcher
design of mine, does not exactly cool anything down. The bugger keeps
getting hot beyond my wildest fears.

I tried experimeting with the Rsense and I have found out that anything
above 20mR in value makes my output not stable when input is below 24V.

I have tried various types of mosfet that I have stocked, but no
considerable change in the gate waveform came from this.

I tried changing the inductors to 33uH, but still stuff gets really hot when
I draw 1A.

I have made a few new screenshots, where the scaling information is present,
perhaps this could be of some help in the fine assistance I have already
received.

The shots are made from a running system with 20mR Rsense and 33uH inductors
of the CDRH127 series.

www.impc.dk/files/drain.jpg
www.impc.dk/files/source.jpg
www.impc.dk/files/gate.jpg
www.impc.dk/files/fb_pin.jpg

Thanking you in advance!

Best regards
Henrik
 
J

Joerg

Jan 1, 1970
0
Henrik said:
"Joerg" <[email protected]> skrev i en meddelelse

[...]
With switchers it is really important to carefully study the datasheets of
even the most mundane parts. We take those caps for granted in bypass
situations but here they'll have to slosh around a lot of energy. I leaned
that the hard way. Everything was fine for an hour or so. Then a very loud
bang, pieces flying around, and what was left from the ceramic material of
a capacitor had turned into green bubbly glass. Quite pretty actually but
it could have been a bad situation if I hadn't worn eye protection
glasses. Best to wear those when you work on switchers and similar gear
that can go "exotherm".

A chilling history indeed, I once experienced something similar. A colleague
of mine got a small part of his earlope shot away by and exploding tantalum
drop-type capacitor. He bled quite a lot and was really shocked.

Haven't seen it this bad but one guy had a shirt full of holes after a
dozen tantalums exploded. I do not use tantalums in designs, except in
low current timing circuits. And even there only once in a blue moon.

Aquavit is good stuff indeed! An I soon might need some of it to calm myself
down ;-) It is really really hot in Denmark these days and this switcher
design of mine, does not exactly cool anything down. The bugger keeps
getting hot beyond my wildest fears.

Look at the bright side: Once you've mastered a few designs like this
you'll become quite the expert in SMPS design. There certainly aren't
many of those engineers left in the industry. Many younger engineers
just take an app note or sample design from a datasheet and when it
doesn't work they give up. Others are so afraid of SMPS that they always
buy "modules in a can".

The high art later will be to build switchers without special PWM chips,
just from logic chips and discrete parts. Can be real fun.

I tried experimeting with the Rsense and I have found out that anything
above 20mR in value makes my output not stable when input is below 24V.

20mohm still allows >8 amps. That is way too much for the inductors you
chose.

Instability for higher values: The switcher then goes into what's called
continuous conduction mode or CCM. This can lead to right half plane
zero instability, often called RHP-zero. If you really have to live with
such small inductors you'll have to read up on the topic and change the
feedback compensation. Read under slope compensation, page 10 of the
LM3478 datasheet.

Bottomline your 3.6A inductors are rather marginal for the power level
you need. If you want to avoid CCM altogether you need much bigger ones.
But you cannot use 20mohms with the higher value CDRH127 inductors,
they'll saturate.

I have tried various types of mosfet that I have stocked, but no
considerable change in the gate waveform came from this.

This usually happens when the FET has really high Cgd. The LM3478 is not
a very strong driver so you can't use FETs with Rdson in the range of a
few milliohms, the are too large.

I tried changing the inductors to 33uH, but still stuff gets really hot when
I draw 1A.

I have made a few new screenshots, where the scaling information is present,
perhaps this could be of some help in the fine assistance I have already
received.

The shots are made from a running system with 20mR Rsense and 33uH inductors
of the CDRH127 series.

Those inductors cannot take 8A or current peaks.



This you need to crank up a lot more. It's hard to see the slope at
Isense. Either ignore the spikes or connect to Rsense via a coax to get
rid of them. Best to connect it where that 1000pF muffling capacitor on
the sense line is.



Looks like you are getting a lot of noise into that node. Do you have a
full ground plane? The LM3478 needs that.
 
H

Henrik [7182]

Jan 1, 1970
0
Joerg said:
Look at the bright side: Once you've mastered a few designs like this
you'll become quite the expert in SMPS design. There certainly aren't many
of those engineers left in the industry. Many younger engineers just take
an app note or sample design from a datasheet and when it doesn't work
they give up. Others are so afraid of SMPS that they always buy "modules
in a can".

Well, I try and keep myself focused on the silver lining, even when burning
my fingers on various inductors ;-) The app-notes and sample designs are
fine for ideas, but the true art of our trade is in understading, so only
one way to go. Into the quagmire. I NEVER give up. being the youngest of 4
boys taught me that. I get beaten sometimes, but I never give up, neither on
this matter ;-) Aquavit to the rescue, though. As Mr. Sean Connery so
brilliantly points out in "the Rock" : "Loosers always whine about [giving
their] best, winners go home and **** the prom queen". Well, I am no Nicolas
Cage, but I certainly did marry the prom queen, so no caving in.
The high art later will be to build switchers without special PWM chips,
just from logic chips and discrete parts. Can be real fun.

Well that sound like a job for the Picaso's of our esteemed trade ;-)
20mohm still allows >8 amps. That is way too much for the inductors you
chose.

Instability for higher values: The switcher then goes into what's called
continuous conduction mode or CCM. This can lead to right half plane zero
instability, often called RHP-zero. If you really have to live with such
small inductors you'll have to read up on the topic and change the
feedback compensation. Read under slope compensation, page 10 of the
LM3478 datasheet.

I hope that I am not way off in my current understanding of the matters, as
described in the following:

I rechecked all my calculations once again and found out that 33uH should be
sufficient with my chosen frequency. I then glued in a couple of TDH series
inductors from Chilisin I had laying around. Lo and behold, they do not get
hot at all! (of course, a little, but nowhere near "hot" like I am used to).
However, the downside is that it it still only possible for me to get 24V
out using no more than 20mR as Rsense. As soon as I crank up this to 50mR
the output falls and varies with the input voltage. According to my
calculations I have duty limits of respectively 0.44 and 0.62 for my input
voltage limits, which is close to or above the 0.5 "risklimit" mentioned in
the datasheet regarding feedback compensation.

So if I understand correctly, I need bigger current capaple inductors, and i
order to make my Rsense work, I need to determine a sufficient resistor to
put into my current feedback.

Maybe even if I get this feedback issue working, my CDRH127 serie would be
alright afterall for the 1 amp output. I calculated my Iavg maximum to 3,1A
which is still below the limit for these inductors.

I can see that bumping the frequency reduces the ripple current, but I guess
there might be some problems with e.g just bumping the frequency to 500kHz?
I use 400KHz in the current setup.
This you need to crank up a lot more. It's hard to see the slope at
Isense. Either ignore the spikes or connect to Rsense via a coax to get
rid of them. Best to connect it where that 1000pF muffling capacitor on
the sense line is.

I will try a produce better screenshots. Hopefully my next shots will be of
a better running system :)

Thank you for your valued input!

Best regards
Henrik
 
J

Joerg

Jan 1, 1970
0
Henrik [7182] wrote:

[...]
Well that sound like a job for the Picaso's of our esteemed trade ;-)

The most memorable comment at the beginning of a design review when this
slide came up: "What the hell does that circuit do?"

I hope that I am not way off in my current understanding of the matters, as
described in the following:

I rechecked all my calculations once again and found out that 33uH should be
sufficient with my chosen frequency. I then glued in a couple of TDH series
inductors from Chilisin I had laying around. Lo and behold, they do not get
hot at all! (of course, a little, but nowhere near "hot" like I am used to).
However, the downside is that it it still only possible for me to get 24V
out using no more than 20mR as Rsense. As soon as I crank up this to 50mR
the output falls and varies with the input voltage. According to my
calculations I have duty limits of respectively 0.44 and 0.62 for my input
voltage limits, which is close to or above the 0.5 "risklimit" mentioned in
the datasheet regarding feedback compensation.

I haven't run your numbers but you may be right in that you really do
need less than 50mOhms. That was just a suggestion to see whether it
will run the CDRH127 without them becoming hot. Since the LM3478 is a
current mode switcher and Vsense is 165mV your 20mOhms means that the
inductor must withstand 8.25A plus some margin. No matter how you toss
and turn that you'll need a 10A inductor there for peace of mind.

Still I think something isn't quite right here. Just key the numbers
into the boost model below, for example 15V in and 24V/1A out. The
inductor shouldn't go beyond 2A.

http://schmidt-walter.eit.h-da.de/smps_e/aww_smps_e.html

So if I understand correctly, I need bigger current capaple inductors, and i
order to make my Rsense work, I need to determine a sufficient resistor to
put into my current feedback.

Maybe even if I get this feedback issue working, my CDRH127 serie would be
alright afterall for the 1 amp output. I calculated my Iavg maximum to 3,1A
which is still below the limit for these inductors.

AVG doesn't matter. What matters is Ipeak. The instant the current
exceeds the limit for core saturation the inductance collapses. Still
puzzled why you need this much when your lowest input voltage is 15V.

I can see that bumping the frequency reduces the ripple current, but I guess
there might be some problems with e.g just bumping the frequency to 500kHz?
I use 400KHz in the current setup.

400kHz should be ok here. Your FET switching losses are quite high,
probably too big a FET. With higher frequency that'll become worse.

I will try a produce better screenshots. Hopefully my next shots will be of
a better running system :)

Oh, they were ok this time, just crank up the scope gain for the Vsense
node.
 
L

legg

Jan 1, 1970
0
Henrik [7182] wrote:

[...]
I rechecked all my calculations once again and found out that 33uH should be
sufficient with my chosen frequency. I then glued in a couple of TDH series
inductors from Chilisin I had laying around. Lo and behold, they do not get
hot at all! (of course, a little, but nowhere near "hot" like I am used to).

The smallest of the TDH series (open form) bobbin inductors has double
the surface area and two or three times the magnetics x-section of the
low profile closed parts originally used. Neither parts give much core
material loss information. Losses with accumulate from both features,
with core loss having a high positive temp coefficient above 60-80degC
(depending on material type and grade).

You should find that the duty cycle is more highly dependant on load
variations.

Scoping with single traces at close to 50% duty makes visual
correlation a problem. Try to trigger on the same portion of the
operating period or wave edge, regardless of the display; if neccesary
by taking and uncalibrated signal from the pwm oscillator to the
direct trigger input.
I haven't run your numbers but you may be right in that you really do
need less than 50mOhms. That was just a suggestion to see whether it
will run the CDRH127 without them becoming hot. Since the LM3478 is a
current mode switcher and Vsense is 165mV your 20mOhms means that the
inductor must withstand 8.25A plus some margin. No matter how you toss
and turn that you'll need a 10A inductor there for peace of mind.

Still I think something isn't quite right here. Just key the numbers
into the boost model below, for example 15V in and 24V/1A out. The
inductor shouldn't go beyond 2A.

http://schmidt-walter.eit.h-da.de/smps_e/aww_smps_e.html
The switch carries the current of both inductors.


The apparent lengthy (250nSec?) negative-going excursion in the
current signal (at the source)during turn-off may be an artifact of
the scope digitization. You need to look at it with a shorter
time-base and a shorter probe ground clip.

EL
 
J

JosephKK

Jan 1, 1970
0
Henrik said:
Dear Joerg,

Thank you very much for your replies. I will with a 50mR or so Rsense
tomorrow, Also I will try to get the scales and such nice info into the
shots.

BTW it makes it easier for others to follow and contribute ideas if you
write below the quoted text.

Also I will try exchanging the sepic capacitor to a large 1uF ceramic
capacitor that I have in stock.

Stack a few of those for the test. 1uF may be a tad too small for a 1A
switcher (tho it'll work). Nowadays you can get pretty good 10uF or at
least 4.7uF caps but even there you may have to parallel in order to
ease and share the current load. They have become quite cheap.

With switchers it is really important to carefully study the datasheets
of even the most mundane parts. We take those caps for granted in bypass
situations but here they'll have to slosh around a lot of energy. I
leaned that the hard way. Everything was fine for an hour or so. Then a
very loud bang, pieces flying around, and what was left from the
ceramic material of a capacitor had turned into green bubbly glass.
Quite pretty actually but it could have been a bad situation if I hadn't
worn eye protection glasses. Best to wear those when you work on
switchers and similar gear that can go "exotherm".

Thank you so very much for your help. I will sleep much better now that I
see a possible solution to my problems ;-)

Since you are in Denmark this would be the perfect time for an ice-cold
shotglass of Aquavit. Good stuff :)

[...]

Mmmm. Aquavit. Once in a while i get some Aalborg Jubilamus variety
aperitif, sugary but very tasty.
 
J

Joerg

Jan 1, 1970
0
legg said:
]
I haven't run your numbers but you may be right in that you really do
need less than 50mOhms. That was just a suggestion to see whether it
will run the CDRH127 without them becoming hot. Since the LM3478 is a
current mode switcher and Vsense is 165mV your 20mOhms means that the
inductor must withstand 8.25A plus some margin. No matter how you toss
and turn that you'll need a 10A inductor there for peace of mind.

Still I think something isn't quite right here. Just key the numbers
into the boost model below, for example 15V in and 24V/1A out. The
inductor shouldn't go beyond 2A.

http://schmidt-walter.eit.h-da.de/smps_e/aww_smps_e.html
The switch carries the current of both inductors.

True, they don't have a SEPIC sim. But the 2nd inductor doesn't carry
that much. I really don't see where 8A peaks should be necessary. I had
to swing a few designs from boost to SEPIC because clients revised the
specs in mid flight. That has not caused any serious changes in peak load.

The apparent lengthy (250nSec?) negative-going excursion in the
current signal (at the source)during turn-off may be an artifact of
the scope digitization. You need to look at it with a shorter
time-base and a shorter probe ground clip.

Or with a real scope. SCNR :)

I am not all that happy with those Tek lunch-box size scopes. Often I
advise clients to go on EBay and bid on a Tek 2465. When it arrives it's
like as if someone had turned the lights on.

The other downside that hits in cases like SMPS designs is that many
Teks only have 2.5K memory. Not enough. My scope has 25K so I can
trigger, digitize at full bore and then blow up individual regions to
look at a spike, zoom back out to see whether my RHP-zero situation is
kosher, and so on.
 
Top