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From: Henrik [7182] on 29 Jul 2008 08:35
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


From: Joerg on 29 Jul 2008 12:39
Henrik [7182] wrote:
> 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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Henrik [7182] on 31 Jul 2008 06:25

"Joerg" <notthisjoergsch(a)removethispacbell.net> skrev i en meddelelse
news:eJHjk.16580$uE5.15768(a)flpi144.ffdc.sbc.com...
>
> 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.
>
> --
> Regards, Joerg
>
> http://www.analogconsultants.com/
>
> "gmail" domain blocked because of excessive spam.
> Use another domain or send PM.

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


From: Joerg on 31 Jul 2008 11:49
Henrik [7182] wrote:
> "Joerg" <notthisjoergsch(a)removethispacbell.net> skrev i en meddelelse
> news:eJHjk.16580$uE5.15768(a)flpi144.ffdc.sbc.com...
>> 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.
>>
>> --
>> Regards, Joerg
>>
>> http://www.analogconsultants.com/
>>
>> "gmail" domain blocked because of excessive spam.
>> Use another domain or send PM.
>
> 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?

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: legg on 31 Jul 2008 11:51
On Tue, 29 Jul 2008 14:35:36 +0200, "Henrik [7182]"
<not.valid(a)nowhere.dk> wrote:

>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
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