[SI-LIST] Re: Inductance of VRM
- From: steve weir <weirsp@xxxxxxxxxx>
- To: cchalmers@xxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 22 Mar 2004 01:43:47 -0800
Chris, the closed loop transfer function gives the VRM its phase shift
versus frequency.
The VRM typically has multiple poles. One of the poles is to effect an
integrator transfer function for very high DC gain, and another is either
the open-loop transfer function of the error amplifier in a linear VRM, a
single effective pole from a current mode SMPS, or two poles from the LC
output filter in a voltage mode SMPS. Most SMPS have operated in current
mode rather than voltage mode for the past twenty three years or so. The
error amplifier does not have to be set-up this way, but this has proven
very cheap and effective.
The feedback loop must close the gain at 0dB while there is still phase
margin to 180 degrees to prevent oscillation. A VRM usually approaches 0
dB to within 5-10dB on a -2 slope, and then cuts back to a -1 slope to
stably cross 0dB. At some point beyond 0 dB the phase can rapidly fall
away to 180 degrees or more. This means that the impedance is on the
complementary slope of +2, and then goes to +1 near 0dB where the phase
will actually be more like 135 degrees than 90 degrees lagging as with a
simple inductive characteristic. Where the decoupling network needs to
transition from the VRM depends on how much greater the VRM open loop
impedance is than the system target impedance.
Many modern VRMs have bandwidths into the 100's of kHz, and some linear
regulators even into the low MHz, particularly if they have to support the
current generation of power thirsty processors. In an SMPS, the switching
frequency sets the upper limit on frequency response. Clearly, the loop
cannot effect average voltage changes in a single cycle. If we apply
Nyquist, we are limited in the best case to Fsw/2. However, typically, the
feedback loop closes at 1/5th the switching frequency or less.
Steve.
At 09:10 AM 3/22/2004 +0000, Chris Chalmers wrote:
>Si List,
>
>Here's a question that has been burning my brain cells.
>
>The effective operating frequency for a Voltage Regulator
>module in the PDS is from DC to a few hundred hertz. Then the
> bulk capacitors take over from there to 1 MHz. The VRM becomes
> inductive above a few hundred hertz. My question is, what makes
> the VRM look inductive? Is it the package of the device that
> is so inductive at 1MHz that the impedance begins to go above the
> target system impedance or is it that the bandwidth of the error
> amplifier inside is only in the hundred of kilohertz.
>
>If it is the error amplifier then why don't they increase the
> bandwidth of it and reduce the inductance of the package to
> reduce the need for bulk capacitance.
>
>Also if it is just the inductance of the package, if you put a
> 1MHz sine wave through it (from the die to the point of load
> would the 1MHz signal rise be slewed due to the inductance that
> much?
>
>Thanks in advance
>
>Chris
>
>
>**********************************************************************
>This communication contains information which is confidential
>and may also be privileged. It is for the exclusive use of the
>intended recipient(s). Please note that any unauthorised
>distribution, copying or use of this communication, or the
>information in it, is strictly prohibited. If you have received
>this communication in error, please notify us by email, by
>telephone: +44 1383 828800, or fax: +44 1383 828801, then
>delete the email and any copies of it.
>
>This communication is from CRL Opto Ltd., whose registered
>office is at 1 St David's Business Park, Dalgety Bay,
>Dunfermline, KY11 9PF, Scotland.
>
>This footnote also confirms that this email message has been
>checked for the presence of computer viruses.
>**********************************************************************
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts: