noise in SEPIC controller

T

Tom Penick

Guest
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP
 
On Wed, 5 Nov 2003 10:29:25 -0800, "iamwatchingyou"
<hitide@hotmail.com> wrote:

"Tom Penick" <tom@tomzap.com> wrote in message
news:t8bfqv8bk0ssupiahfkdqa3epe7vp4e974@4ax.com...
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP

Not sure what your problem is?
Namely, do you have a load problem with spikes or ringing or both? It is
inherent to all switchers.
Ringing f could be from FET not swithcing off properly. Try complementary
gate drive.
Turn on spike must be snubbed, but first determine what is the cause. Maybe
your FET has high on R?
Click to expand...
Thank you for your remarks.

I am more concerned about the spikes than the ringing; the ringing
decays quickly. The spikes occur on both turn-on and turn-off, with
the turn-on spike having the greater amplitude. I am using an IXYS
IXDD408SI MOSFET driver that is switching 15 volts. I have tried
driving the FET directly from the DC-DC controller, which is designed
to drive a logic-level gate with 5 volts. Driven by either device,
the circuit exhibits the same spike behavior.

The FET is IRFB61N15D, which has a .032 ohm on resistance,
18 ns on delay, 110 ns rise time, 28 ns off delay, 51 ns fall time
according to data sheet. This is slowed down by gate series
resistance and snubber.

TP
 
"Tom Penick" <tom@tomzap.com> wrote in message
news:t8bfqv8bk0ssupiahfkdqa3epe7vp4e974@4ax.com...
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP
Click to expand...
Not sure what your problem is?
Namely, do you have a load problem with spikes or ringing or both? It is
inherent to all switchers.
Ringing f could be from FET not swithcing off properly. Try complementary
gate drive.
Turn on spike must be snubbed, but first determine what is the cause. Maybe
your FET has high on R?
 
Tom, is it a spike or pulse?
What can you see on the lowest time base? Sorry for simpleton but I have
battled many switchers, and most of the time pulse did not matter for
application at all.

"Tom Penick" <tom@tomzap.com> wrote in message
news:19ngqv4gvib4ko6tdppcu7thhrcipeac49@4ax.com...
On Wed, 5 Nov 2003 10:29:25 -0800, "iamwatchingyou"
hitide@hotmail.com> wrote:


"Tom Penick" <tom@tomzap.com> wrote in message
news:t8bfqv8bk0ssupiahfkdqa3epe7vp4e974@4ax.com...
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP

Not sure what your problem is?
Namely, do you have a load problem with spikes or ringing or both? It is
inherent to all switchers.
Ringing f could be from FET not swithcing off properly. Try complementary
gate drive.
Turn on spike must be snubbed, but first determine what is the cause.
Maybe
your FET has high on R?


Thank you for your remarks.

I am more concerned about the spikes than the ringing; the ringing
decays quickly. The spikes occur on both turn-on and turn-off, with
the turn-on spike having the greater amplitude. I am using an IXYS
IXDD408SI MOSFET driver that is switching 15 volts. I have tried
driving the FET directly from the DC-DC controller, which is designed
to drive a logic-level gate with 5 volts. Driven by either device,
the circuit exhibits the same spike behavior.

The FET is IRFB61N15D, which has a .032 ohm on resistance,
18 ns on delay, 110 ns rise time, 28 ns off delay, 51 ns fall time
according to data sheet. This is slowed down by gate series
resistance and snubber.

TP
Click to expand...
 
At the lowest time base I see a spike that rings for about 5 cycles at
45 MHz. I think there are also 20 MHz and 60 MHz components that
have been reduced beyond recognition by snubbing. The specification
requires that it be less than 130 mVp-p, but it's still about 800 mV.

TP


On Wed, 5 Nov 2003 12:46:54 -0800, "iamwatchingyou"
<hitide@hotmail.com> wrote:

Tom, is it a spike or pulse?
What can you see on the lowest time base? Sorry for simpleton but I have
battled many switchers, and most of the time pulse did not matter for
application at all.

"Tom Penick" <tom@tomzap.com> wrote in message
news:19ngqv4gvib4ko6tdppcu7thhrcipeac49@4ax.com...
On Wed, 5 Nov 2003 10:29:25 -0800, "iamwatchingyou"
hitide@hotmail.com> wrote:


"Tom Penick" <tom@tomzap.com> wrote in message
news:t8bfqv8bk0ssupiahfkdqa3epe7vp4e974@4ax.com...
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP

Not sure what your problem is?
Namely, do you have a load problem with spikes or ringing or both? It is
inherent to all switchers.
Ringing f could be from FET not swithcing off properly. Try complementary
gate drive.
Turn on spike must be snubbed, but first determine what is the cause.
Maybe
your FET has high on R?


Thank you for your remarks.

I am more concerned about the spikes than the ringing; the ringing
decays quickly. The spikes occur on both turn-on and turn-off, with
the turn-on spike having the greater amplitude. I am using an IXYS
IXDD408SI MOSFET driver that is switching 15 volts. I have tried
driving the FET directly from the DC-DC controller, which is designed
to drive a logic-level gate with 5 volts. Driven by either device,
the circuit exhibits the same spike behavior.

The FET is IRFB61N15D, which has a .032 ohm on resistance,
18 ns on delay, 110 ns rise time, 28 ns off delay, 51 ns fall time
according to data sheet. This is slowed down by gate series
resistance and snubber.

TP
Click to expand...
 
If you are switching an external FET try addign a small resistor in
series with the GATE. I had similar problem on an injector driver, and
aSEPIC PSU for a transceiver. In both cases the R combined with Cgs
redcued some of the high frequency switching components.

Al

Tom Penick wrote:
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP
Click to expand...
--
Please remove capitalised letters to reply
My apologies for the inconvenience
Blame it on the morons that spam the net
 
I have 5 ohms gate resistance, which is about all I can add at the
200 kHz switching frequency.

TP


On Thu, 06 Nov 2003 10:18:39 GMT, onestone
<onestoneXYZ@ABCbigpond.net.au> wrote:

If you are switching an external FET try addign a small resistor in
series with the GATE. I had similar problem on an injector driver, and
aSEPIC PSU for a transceiver. In both cases the R combined with Cgs
redcued some of the high frequency switching components.

Al

Tom Penick wrote:
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP
Click to expand...
 
Tom Penick <tom@tomzap.com> wrote in message news:<t8bfqv8bk0ssupiahfkdqa3epe7vp4e974@4ax.com>...
I'm designing a 50W SEPIC power supply with 12V output.
I'm having trouble getting rid of the switching noise, especially
the turn-on spike. I have tried the following:

Oscon caps on output.

Ceramic caps on input and output. Large electrolytics.

Snubbers on FET and diode.

Increasing FET slew rate with gate series resistance.

Differential and common mode chokes on input and output.

Isolation of the FET heatsink from the case.

Grounding the case.

Layout improvements.

All of these help, but the spike noise is still about 1V peak to peak
and I would like to get down to about 100mV. The spikes ring at
frequencies from 20 MHz to 60 MHz. Switching frequency is
200 kHz.

Any insights?

TP
Click to expand...

G'day Tom,

Try complimentary gate drive and series gate resistor (try 10 ohms as
a starting point) as previous posters have mentioned.


Andy
 
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