Switching power supply failure under no-load conditions

[Turntable]

I am aware of this failure mode, however, I am curious to know what causes it.

This problem seems to be more and more common as more and more "wall-wart" type chargers for devices such as cellular phones, digital cameras, and I-Pod devices begin to use switching supply technology, instead of the transformer to square-bridge rectifier/filter cap type circuits of old. If the unit is left plugged in without a load it will fail prematurely.

I first noticed the cause/effect scenario after hearing a high-pitched whine when the units are not connected to a load- apply a load and the noise goes away. Is there some sort of oscillation that builds up inside the unit when there is no load - feedback loop runaway?

Thanks in advance for any insights...

-A<P ID="signature">______________
"...How can you be deaf, with ears like that??"</P>

 

[boiseengineer]

A lot of them seem to shut downif there's no load at power up. Seems to be normal.

 

[Turntable]

I'm not talking about quality power supplies with protection circuits. I'm talking about el-cheapo chinese made wall-warts. I'm talking about a complete, throw-it-away failure.

> A lot of them seem to shut downif there's no load at power
> up. Seems to be normal.
> <P ID="signature">______________
"...How can you be deaf, with ears like that??"</P>

 

[Andy_Brown]

why? Cheepnis

> Is there some
> sort of oscillation that builds up inside the unit when
> there is no load - feedback loop runaway?

Not runaway, more like pedal to the metal (saturated} and unable to avoid burning up without the internal load resistor. It's just cheepnis in manufacturing, since switching supplies must have a load to operate correctly.

At no load, the voltage will rise to the peak value of the switching waveform. The feedback loop may be able to compensate for some of this but not for long(especially if multiple levels are controlled by one feedback loop). It is customary to include overvoltage (crowbar) sensing or protection on all outputs (saturation) and if the voltage rises on any output, these may be triggered. Without protection circuitry, no load conditions will cause overcurrent in the lighter smaller transformers that switching supplies allow you to incorporate.

<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>

 

[Turntable]

Re: why? Cheepnis

Thanks, that's about what I figured. Actually that was my first thought... no load resistor? heh.

-A

> > Is there some
> > sort of oscillation that builds up inside the unit when
> > there is no load - feedback loop runaway?
>
> Not runaway, more like pedal to the metal (saturated} and
> unable to avoid burning up without the internal load
> resistor. It's just cheepnis in manufacturing, since
> switching supplies must have a load to operate correctly.
>
> At no load, the voltage will rise to the peak value of the
> switching waveform. The feedback loop may be able to
> compensate for some of this but not for long(especially if
> multiple levels are controlled by one feedback loop). It is
> customary to include overvoltage (crowbar) sensing or
> protection on all outputs (saturation) and if the voltage
> rises on any output, these may be triggered. Without
> protection circuitry, no load conditions will cause
> overcurrent in the lighter smaller transformers that
> switching supplies allow you to incorporate.
>
<P ID="signature">______________
"...How can you be deaf, with ears like that??"</P>

 

[_Jim]

N-channel MOSFET failures my bet ...

> I'm not talking about quality power supplies with protection
> circuits. I'm talking about el-cheapo chinese made
> wall-warts. I'm talking about a complete, throw-it-away
> failure.
>
> > A lot of them seem to shut downif there's no load at power
>
> > up. Seems to be normal.
> >
>

My bet about what's failing?

Gate 'punch through' on the switch, usually an N-channel
enhancement-mode MOSFET ... the gate to drain/source
breakdown voltage is usually only spec'd to about 20 volts
on this type of part, and there is no telling what an individual
FET may break down at, and once it does (this is not a PN
junction device, but rather a MOSFET type device that uses a
thin layer of Silicon Nitride as an insulator under the
gate) it is toast ...

Regards, _Jim