Re: Watson Power Supply
- From: "Brian Gregory [UK]" <ng@xxxxxxxxxxx>
- Date: Tue, 23 Mar 2010 20:31:20 -0000
Your parents were pretty weird naming you after a news server, but naming
you after a news server than belongs to a lying thieving incompetent rip-off
cable company, that's really cruel. You have my sympathy.
"news.virginmedia.com" <report@xxxxxxxxxx> wrote in message
Switch mode power supplies have the advantage of being smaller and also
cost less because they do not have a transformer in them
They typically have a smaller transformer in them so that the output is
isolated from the mains input.
and thats one of the expensive parts in a linear power supply....Switch
mode power supplies work differently and do not like --->>>>Inductive
It depends what they're designed to cope with.
This could be like a electric motor or even a fluorescent lamp or
fluorescent tube....Switch mode power supplies are more energy
efficient... cost less and weigh less...I deal for many more purposes like
mobile phone charging...... I personally dont like them for running radio
equipment.. I prefer linear power supplies..
Yes many, but not all, switch mode power supplies create interferance when
used to supply radio equipment.
Comparison of a linear power supply and a switched-mode power supply
Linear power supply Switching power supply Notes
Size and weight If a transformer is used, large due to low operating
frequency (mains power frequency is at 50 or 60 Hz). Small if
transformerless. Smaller due to higher operating frequency (typically 50
kHz-1 MHz) A transformer's power handling capacity of given size and
weight increases with frequency provided that hysteresis losses can be
That's weight of the core.
You still need thick windings to carry high currents.
Therefore, higher operating frequency means either higher capacity or
Ah, so now you're admitting that switch mode power supplies have
transformers in them.
Output voltage With transformer used, any voltages available; if
transformerless, not exceeding input.
Not true. You don't need a transformer to increase the voltage.
If unregulated, voltage varies significantly with load. Any voltages
available. Voltage varies little with load. A SMPS can usually cope with
wider variation of input before the output voltage changes.
Efficiency, heat, and power dissipation If regulated, output voltage
is regulated by dissipating excess power as heat resulting in a typical
efficiency of 30-40%; if unregulated, transformer iron and copper
losses significant. Output is regulated using duty cycle control, which
draws only the power required by the load. In all SMPS topologies, the
transistors are always switched fully on or fully off. The only heat
generated is in the non-ideal aspects of the components. Switching losses
in the transistors, on-resistance of the switching transistors, equivalent
series resistance in the inductor and capacitors, core losses in the
inductor, and rectifier voltage drop contribute to a typical efficiency of
60-70%. However, by optimizing SMPS design, the amount of power loss and
heat can be minimized; a good design can have an efficiency of 95%.
Complexity Unregulated may be diode and capacitor; regulated has a
voltage regulating IC or discrete circuit and a noise filtering capacitor.
Consists of a controller IC, one or several power transistors and diodes
as well as a power transformer, inductors, and filter capacitors. Multiple
voltages can be generated by one transformer core. For this SMPSs have to
use duty cycle control. One of the outputs has to be chosen to feed the
voltage regulation feedback loop (Usually 3.3 V or 5 V loads are more
fussy about their supply voltages than the 12 V loads, so this drives the
decision as to which feeds the feedback loop. The other outputs usually
track the regulated one pretty well). Both need a careful selection of
their transformers. Due to the high operating frequencies in SMPSs, the
stray inductance and capacitance of the printed circuit board traces
Radio frequency interference Mild high-frequency interference may be
generated by AC rectifier diodes under heavy current loading, while most
other supply types produce no high-frequency interference. Some mains hum
induction into unshielded cables, problematical for low-signal audio.
EMI/RFI produced due to the current being switched on and off sharply.
Therefore, EMI filters and RF shielding are needed to reduce the
disruptive interference. Long wires between the components may reduce the
high frequency filter efficiency provided by the capacitors at the inlet
Electronic noise at the output terminals Unregulated PSUs may have a
little AC ripple superimposed upon the DC component at twice mains
frequency (100-120 Hz). Can cause audible mains hum in audio equipment or
brightness ripples or banded distortions in analog security cameras.
Noisier due to the switching frequency of the SMPS. An unfiltered output
may cause glitches in digital circuits or noise in audio circuits. This
can be suppressed with capacitors and other filtering circuitry in the
output stage. With a switched mode PSU the switching frequency can be
chosen to keep the noise out of the circuits working frequency band (e.g.,
for audio systems above the range of human hearing)
Electronic noise at the input terminals Causes harmonic distortion to
the input AC, but relatively little or no high frequency noise. Very low
cost SMPS may couple electrical switching noise back onto the mains power
line, causing interference with A/V equipment connected to the same phase.
Non power-factor-corrected SMPSs also cause harmonic distortion. This can
be prevented if a (properly earthed) EMI/RFI filter is connected between
the input terminals and the bridge rectifier.
Acoustic noise Faint, usually inaudible mains hum, usually due to
vibration of windings in the transformer and/or magnetostriction.
Inaudible to humans, unless they have a fan or are
unloaded/malfunctioning. The operating frequency of an unloaded SMPS is
sometimes in the audible human range.
Power factor Low for a regulated supply because current is drawn from
the mains at the peaks of the voltage sinusoid. Ranging from low to medium
since a simple SMPS without PFC draws current spikes at the peaks of the
AC sinusoid. Active/passive power factor correction in the SMPS can offset
this problem and are even required by some electric regulation
authorities, particularly in Europe.
Risk of electric shock Supplies with transformers allow metalwork to
be grounded, safely. Dangerous if primary/secondary insulation breaks
down, unlikely with reasonable design. Transformerless mains-operated
supply dangerous. In both linear and SM the mains, and possibly the output
voltages, are hazardous and must be well-isolated. Common rail of
equipment (including casing) is energised to half mains voltage, but at
high impedance, unless equipment is earthed/grounded or doesn't contain
EMI/RFI filtering at the input terminals. Due to regulations concerning
EMI/RFI radiation, many SMPS contain EMI/RFI filtering at the input stage
before the bridge rectifier consisting of capacitors and inductors. Two
capacitors are connected in series with the Live and Neutral rails with
the Earth connection in between the two capacitors. This forms a
capacitive divider that energises the common rail at half mains voltage.
Its high impedance current source can provide a tingling or a 'bite' to
the operator or can be exploited to light an Earth Fault LED. However,
this current may cause nuisance tripping on the most sensitive
I've known SMPSUs that apparently meet safety requirements where this
leakage current is enough to give a nasty jolt.
Risk of equipment damage Very low, unless a short occurs between the
primary and secondary windings or the regulator fails by shorting
internally. Can fail so as to make output voltage very high. Can in some
cases destroy input stages in amplifiers if floating voltage exceeds
transistor base-emitter breakdown voltage, causing the transistor's gain
to drop and noise levels to increase. Mitigated by good failsafe
design. Failure of a component in the SMPS itself can cause further damage
to other PSU components; can be difficult to troubleshoot. The floating
voltage is caused by capacitors bridging the primary and secondary sides
of the power supply. A connection to an earthed equipment will cause a
momentary (and potentially destructive) spike in current at the connector
as the voltage at the secondary side
Brian Gregory. (In the UK)
To email me remove the letter vee.
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