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Anyone know this info about switching supplies?

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Tom Hammond-2

Anyone know this info about switching supplies?

Folks:

A local discussion has 'erupted' regarding switching poser supplies and whether
they can fail in an UNregulated (e.g. high) output voltage condition, or if
when they fail will they (almost) always fail in a NO OUTPUT mode?

I think this discussion may have been started as a result of a subject here on
the reflector about overvoltage protection (OVP), but it's managed to take on
a life of its own with respect to the MODES in which a switching P/S
can/will fail.

Anyone have enough real experience to express their thoughts on this subject.

Must admit... I have no clue myself. I've 'heard' that most switchers fail in
the NO OUTPUT mode, but this is strictly hearsay.

Let's try to get several responses and the let the subject drop before IT takes
on a life of its own on the reflector and Eric has to step in... he's
busy enough
right now. <G>

73,

Tom N0SS

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Brendan Minish

Re: Anyone know this info about switching supplies?

I have never ever seen a Switchmode power supply fail 'high' It's quite
possible that sometimes big over-voltage transients may be produced but
I have never had anything killed by this myself.

I have seen quite a few of these go boom, sometimes with a very big bang
and large mains breakers tripping as a result.

My experience ranges from various wall wart type SMPS, Computer
supplies, ham supplies and server room 48V supplies

I would be interested in hearing of other horror stories

73's

Brendan EI6IZ

On Tue, 2007-11-06 at 09:35 -0600, Tom Hammond wrote:

> Folks:
>
> A local discussion has 'erupted' regarding switching poser supplies and whether
> they can fail in an UNregulated (e.g. high) output voltage condition, or if
> when they fail will they (almost) always fail in a NO OUTPUT mode?
>
> I think this discussion may have been started as a result of a subject here on
> the reflector about overvoltage protection (OVP), but it's managed to take on
> a life of its own with respect to the MODES in which a switching P/S
> can/will fail.
>
> Anyone have enough real experience to express their thoughts on this subject.
>
> Must admit... I have no clue myself. I've 'heard' that most switchers fail in
> the NO OUTPUT mode, but this is strictly hearsay.
>
> Let's try to get several responses and the let the subject drop before IT takes
> on a life of its own on the reflector and Eric has to step in... he's
> busy enough
> right now. <G>
>
> 73,
>
> Tom N0SS
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
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--
Don‘t complain. Nobody will understand. Or care. And certainly don‘t try
to fix the situation yourself. It‘s dangerous. Leave it to a highly
untrained, unqualified, expendable professional.

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Dave Martin-2

Re: Anyone know this info about switching supplies?

In reply to this post by Tom Hammond-2

If a switcher had a component failure in the feedback loop it would go
full bore and probably produce twice its rated output voltage until it
totally failed, but none of these components are under much stress, so
it's unlikely to happen. The failures are usually the switching
transistors or the rectifiers, which both run hot. But these will
just kill the supply. So we're pretty safe compared to a linear
supply whose pass transistors run hot and might short, passing the
total unregulated output voltage to the radio.

Dave W5DHM

On Nov 6, 2007 10:35 AM, Tom Hammond <[hidden email]> wrote:

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Jack Brindle

Re: Anyone know this info about switching supplies?

In reply to this post by Tom Hammond-2

In the early 80s there was a video game manufacturer that had this
exact problem - their power supplies would fail, placing 18 volts on
the output, and thus frying the video logic. So, yes, it is quite
possible for them to fail "full bore."

The company is no longer in business...

As a further note, the supplies showed up in Radio Shack sometime
later where everyone who purchased one of the boards got to see the
problem for themselves. At least R.S. was smart enough to label them
as surplus...

On Nov 6, 2007, at 7:35 AM, Tom Hammond wrote:

> Folks:
>
> A local discussion has 'erupted' regarding switching poser supplies
> and whether
> they can fail in an UNregulated (e.g. high) output voltage
> condition, or if
> when they fail will they (almost) always fail in a NO OUTPUT mode?
>
> I think this discussion may have been started as a result of a
> subject here on
> the reflector about overvoltage protection (OVP), but it's managed
> to take on
> a life of its own with respect to the MODES in which a switching P/
> S can/will fail.
>
> Anyone have enough real experience to express their thoughts on
> this subject.
>
> Must admit... I have no clue myself. I've 'heard' that most
> switchers fail in
> the NO OUTPUT mode, but this is strictly hearsay.
>
> Let's try to get several responses and the let the subject drop
> before IT takes
> on a life of its own on the reflector and Eric has to step in...
> he's busy enough
> right now. <G>
>
> 73,
>
> Tom N0SS
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

- Jack Brindle, W6FB
------------------------------------------------------------------------
---------------------

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Matt Palmer-4

Re: Anyone know this info about switching supplies?

In reply to this post by Dave Martin-2

Since building switchers for the military is my profession these days
i would like to think i have some authority on the subject. Failures
can occur almost anywhere in a switcher and the most typical type
depends on the topology of the supply. Typically there is a pass
transistor on the input that will turn off power to the whole supply
(at least in my designs) if anything goes "out of spec" however i
would bet in cheap commercial supplies this is not always the case. My
most common failure in lab tests for me is the primary side
transistor, what happens depends on whether the transistor fails short
or open. If it fails open you typically have no voltage on the output
(again this is dependent on topology) but if it fails closed, you can
get runaway on the output in certain topologies. I would like to think
any 'smart' design has a pass transistor and enough bit and
supervision circuitry as to shut the supply down if something goes
wrong, Component failure in the feedback loop is going to be rare, as
these are typically passive components, either a resistive divider, a
sense winding on the transformer, these parts typically don't fail,
and if they do, they fail in spectacular fashion which usually gives
you a clue something is going wrong.

Matt
KD8DAO
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wayne burdick

Re: Anyone know this info about switching supplies?

Administrator

In reply to this post by Jack Brindle

>> A local discussion has 'erupted' regarding switching poser supplies
>> and whether
>> they can fail in an UNregulated (e.g. high) output voltage condition,
>> or if
>> when they fail will they (almost) always fail in a NO OUTPUT mode?

I've never seen this happen myself, but I just added a task to my
firmware list to turn the K3 off automatically if there's a persistent
overvoltage condition (say, > 17 V for several seconds).

73,
Wayne
N6KR

---

http://www.elecraft.com

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Don Brown-4

RE: Anyone know this info about switching supplies?

In reply to this post by Tom Hammond-2

Hi All

I ran the engineering department at a semiconductor burn lab for some years and we had well over a thousand power supplies of analog, switchers and SCR pre-regulated designs. We even modified Hobart MIG welders into power supplies (0-60 volts up to 600 amps). It has been my experience that the switchers will fail in a no output mode most of the time. This makes sense because the usual failing part is the switching transistors. If one or more of them fail the primary fuse will usually blow. With the output fed from transformer secondary there will be no output voltage or low output even if the fuse does not blow because of the distorted waveform driving the transformer.

Analog power supplies usually fail by going to the rail because the series pass transistor shorts and connects the raw supply to the output. This is why most high end analog power supplies have some kind of crowbar circuit on the output that shorts the output if it exceeds some set voltage. It is always possible that the regulator could fail or even the feedback circuit that sets the output voltage could fail and cause an overvoltage condition although this is very rare. A dirty pot or loose connection at the Kelvin jumpers can cause any supply to go to the rail.

A crowbar circuit would be easy to add to any power supply. A power SCR, a few resistors, a zener and a fuse is all that is needed. Some of the power supply Mfg sold these as an outboard add on for their supplies.Don Brown

KD5NDB

> Date: Tue, 6 Nov 2007 09:35:11 -0600> To: [hidden email]> From: [hidden email]> Subject: [Elecraft] Anyone know this info about switching supplies?> > Folks:> > A local discussion has 'erupted' regarding switching poser supplies and whether> they can fail in an UNregulated (e.g. high) output voltage condition, or if> when they fail will they (almost) always fail in a NO OUTPUT mode?> > I think this discussion may have been started as a result of a subject here on> the reflector about overvoltage protection (OVP), but it's managed to take on> a life of its own with respect to the MODES in which a switching P/S > can/will fail.> > Anyone have enough real experience to express their thoughts on this subject.> > Must admit... I have no clue myself. I've 'heard' that most switchers fail in> the NO OUTPUT mode, but this is strictly hearsay.> > Let's try to get several responses and the let the subject drop before IT takes> on a life of its own on the reflector and Eric has to step in... he's > busy enough> right now. <G>> > 73,> > Tom N0SS> > _______________________________________________> Elecraft mailing list> Post to: [hidden email]> You must be a subscriber to post to the list.> Subscriber Info (Addr. Change, sub, unsub etc.):> http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm> Elecraft web page: http://www.elecraft.com_______________________________________________
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Tom Hammond-2

Re: Anyone know this info about switching supplies?

In reply to this post by wayne burdick

WB:

We're discussing it here right now... what would the 'damages' be if
a supply (ANY supply) could/would fail placing 24VDC at up to 30A on
the K3's DC line?

I'm thinking about designing an external plug-in OVP kit. Unless YOU
want to do it.

T

At 10:45 11/06/2007, wayne burdick wrote:

>>>A local discussion has 'erupted' regarding switching poser
>>>supplies and whether
>>>they can fail in an UNregulated (e.g. high) output voltage condition, or if
>>>when they fail will they (almost) always fail in a NO OUTPUT mode?
>
>I've never seen this happen myself, but I just added a task to my
>firmware list to turn the K3 off automatically if there's a
>persistent overvoltage condition (say, > 17 V for several seconds).
>
>73,
>Wayne
>N6KR
>
>
>---
>
>http://www.elecraft.com

wayne burdick

Re: Anyone know this info about switching supplies?

Administrator

Tom Hammond wrote:

> WB:
>
> We're discussing it here right now... what would the 'damages' be if a
> supply (ANY supply) could/would fail placing 24VDC at up to 30A on the
> K3's DC line?
>
> I'm thinking about designing an external plug-in OVP kit. Unless YOU
> want to do it.
>

The K3 already has OVP built-in, in the form of high-current,
high-power zener clamp that kicks in at around 16 V. It can handle
hundreds of amps for long enough to blow the self-resetting polyfuse
(F2 on the RF board schematic). An identical device is used on the
KPA3, which has its own circuit breaker. So the OVP is very rugged and
reliable.

In addition, as a backup, the K3 will turn itself off if it senses an
over-voltage condition.

73,
Wayne
N6KR

---

http://www.elecraft.com

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Tom Hammond-2

Re: Anyone know this info about switching supplies?

Damn Wayne... you DO think of everything!

73,

Tom

At 11:09 11/06/2007, wayne burdick wrote:

>Tom Hammond wrote:
>
>>WB:
>>
>>We're discussing it here right now... what would the 'damages' be
>>if a supply (ANY supply) could/would fail placing 24VDC at up to
>>30A on the K3's DC line?
>>
>>I'm thinking about designing an external plug-in OVP kit. Unless
>>YOU want to do it.
>
>The K3 already has OVP built-in, in the form of high-current,
>high-power zener clamp that kicks in at around 16 V. It can handle
>hundreds of amps for long enough to blow the self-resetting polyfuse
>(F2 on the RF board schematic). An identical device is used on the
>KPA3, which has its own circuit breaker. So the OVP is very rugged
>and reliable.
>
>In addition, as a backup, the K3 will turn itself off if it senses
>an over-voltage condition.
>
>73,
>Wayne
>N6KR
>
>
>---
>
>http://www.elecraft.com

Brett Gazdzinski

Re: Anyone know this info about switching supplies?

In reply to this post by Tom Hammond-2

We use nothing but switching supplies in my industry, and I have NEVER
had one fail and take out what it powers.
Talking about 10,000 or so supplies.

I dont know much about the ham type stuff, except to stay away from anything
made by mfj...

Brett
N2DTS

----- Original Message -----
From: "Tom Hammond" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, November 06, 2007 10:35 AM
Subject: [Elecraft] Anyone know this info about switching supplies?

> Folks:
>
> A local discussion has 'erupted' regarding switching poser supplies and
whether
> they can fail in an UNregulated (e.g. high) output voltage condition, or
if
> when they fail will they (almost) always fail in a NO OUTPUT mode?
>
> I think this discussion may have been started as a result of a subject
here on
> the reflector about overvoltage protection (OVP), but it's managed to take
on
> a life of its own with respect to the MODES in which a switching P/S
> can/will fail.
>
> Anyone have enough real experience to express their thoughts on this
subject.
>
> Must admit... I have no clue myself. I've 'heard' that most switchers
fail in
> the NO OUTPUT mode, but this is strictly hearsay.
>
> Let's try to get several responses and the let the subject drop before IT
takes

> on a life of its own on the reflector and Eric has to step in... he's
> busy enough
> right now. <G>
>
> 73,
>
> Tom N0SS
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
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>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com