Intro
Linear power supplies were the mainstay of power conversion
until the late 1970’s when the first
commercial switch-mode became available. Now apart from very
low power wall mount linear power
supplies used for powering consumer items like cell phones
and toys, switch-mode power supplies are
dominant.
So what is the difference in how they work?
Linear power supplies have a bulky steel or iron laminated
transformer. This transformer has two purposes
- It provides a safety barrier for the low voltage output
from the AC input and reduces and the input from
typically 115V or 230VAC to a much lower voltage around say
30VAC.
The low voltage AC output from the transformer is then
rectified by two or four diodes and smoothed into
low voltage DC by large electrolytic capacitors.
That low voltage DC is then regulated into the output
voltage of choice by a dropping the difference in
voltage across transistor or IC (the shunt regulator).
Switch-mode supplies are a lot more complicated. The 115V or
230VAC voltage is rectified and smoothed
by diodes and capacitors resulting in a high voltage DC.
That DC is then converted into a safe, low
voltage, high frequency (typically switching at 100kHz to
500kHz) voltage using a much smaller ferrite
transformer and FETs or transistors. That voltage is then
converted into the DC output voltage of choice by
another set of diodes, capacitors and inductors. Corrections
to the output voltage due to load or input
changes are achieved by adjusting the pulse width of the
high frequency waveform.
Sounds complicated? Yes, but the payoff is worth it!
The advantages and drawbacks of both technologies
Size: - A 50W
linear power supply is typically 3 x 5 x 5.5”, whereas a 50W switch-mode can be
as small as
3 x 5 x 1”. That’s a size reduction of 80%.
Weight: - A 50W
linear weighs 4lbs, a corresponding switcher is 0.75lb. As the power level
increases, so
does the weight. I personally remember a two-man lift needed
for a 1000W linear. Today I carry a 2000W
in my carry-on luggage when I fly!
Input Voltage Range:
- A linear has a very limited input range requiring that the transformer taps
be
changed between different countries. Normally on the
specification you will see
100/120/220/230/240VAC. This is because when input voltage
drops more than 10%, the DC voltage to
the shunt regulator drops too low & the power supply
cannot deliver the required output voltage. At input
voltages greater than 10%, too much voltage is delivered to
the regulator resulting in overheating.
If a piece of equipment is tested in the US and shipped to
Europe, or even to Mexico in some cases, the
transformer “taps” have to be manually changed. Forget to
set the taps? The power supply will most
certainly blow the fuse, or may well be damaged.
Most switch-mode supplies will operate anywhere in the world
(85 to 264VAC), from industrial areas in
Japan to the outback of Australia without any adjustment.
The switch-mode supply will also be able to withstand small
losses of AC power in the range of 10-20ms
without affecting the outputs. A linear will not. No one
will care if the AC goes missing for 1/100th of a
second when charging your phone, it will take 100 of these
interruptions to delay the charge by one second!
Having a piece of equipment reboot 100 times a day will
cause some heartbreak!
Efficiency: - A
linear power supply because of its design will normally operate at around 60%
efficiency
for 24V outputs, whereas a switch-mode is normally 80% or
more. Efficiency is a measure of how much
energy the power supply wastes. This has to be removed with
fans or heat sinks from the system.
For a 100W output linear, that waste would be 67W. A 100W
switch-mode would be just 25W.
67W – 25W = 42W is
the extra power lost
Doesn’t sound much, but don’t try touching a 40W light bulb!
If the equipment were running 24 hours a
day, then the extra losses would be 367kW hours, even at
$0.1 per kW hour, that’s an extra $37 a year for a
power supply that costs around $80.
As a quick note, in Europe, they are trying to limit those
losses of all power supplies used by consumers
particularly when operating off load (as many products are
left plugged in 24 hours a day). Imagine 250
million power supplies eating up a couple Watts. That
equates to the output of a whole power station!
Reliability: - If
reliability is calculated using a part count method, then the linear power
supply will win.
With the design & quality improvements made in the last
few years with switch-mode parts & technology,
in reality this advantage has been negated. I have
demonstrated life testing data showing no failures after
over 1,000,000 hours on some Lambda products.
Electrical Ripple and
Noise: - This is where the linear really scores!
5v Linear 5v Switch-mode
The linear obviously is a lot “quieter”, by up to a 10,000
times. The topology of the switch-mode supply
with its high frequency switching technology had to have a
downside right? So if the noise is 10,000 times
worse, how can anyone use it? Sounds so bad.
In truth, there are some applications (studio mixers and
very sensitive test equipment) where low electrical
noise is critical. The others? One of my first sales calls
in the USA was to a manufacturer who built
semiconductor fabrication equipment. They used 8 really big
linear units in a large box measuring 2x3x4
feet, it was heavy & actually being dictating the size
of their end equipment. I told the engineer that I could
replace all eight units with two modular products measuring
5x5x10”. He laughed and said the noise
would be too great. I sent him samples and went to visit
three weeks later. He was delighted with the
performance and has been a long term Lambda customer ever
since.
Transient Response:
Transient response is how a power supply reacts to a (fast)
change in load.
If the output load quickly changes from say full load to
half load, the output voltage of the power supply
will rise (overshoot) before the internal control circuit
has time to compensate, and then undershoot a little
less as the circuit over compensates. The length of time is
takes from the instant of the load change to the
time the output voltage settles back into the load
regulation limits can be critical to some loads. Here the
linear again outperforms the switch-mode.
For a 50% change in load the switch-mode will often take
3000us to recover. A linear supply will recover
in 50us.
Is this critical for all applications? There are a few
specialized technologies where this is important and
most engineers will advise you if this is critical. For the
other instances on board capacitors at the end load
& the inductance of cables is enough to reduce overshoot
down ten-fold to where it no longer is a concern.
Low leakage currents
and Conducted EMI:
A widely used technique in the design of switch-mode power
supplies is to connect special capacitors from
the AC input terminals to Earth. This is a cost effective
method to reduce noise from being fed back
through the input wires and potentially affecting other
equipment.
These capacitors have a side effect of allowing a “leakage
current” to be passed through the Earth or
ground cable. Many safety specifications have limits on the
amount of this current that is allowed.
UL1950 allows 3mA, medical industrials less than a tenth of
that. The gaming industry is even tighter.
As linear power supplies are “quieter” and do not need these
capacitors, they simplify the system filtering,
and allow more of the system leakage current “budget” to be
used for other parts like monitors. The overall
size of the system filter can also be reduced. How much that
impacts cost & performance varies from
customer to customer.
Some switch-mode power supplies (like Lambda’s Vega series)
are now available with increased internal
filtering that allows for low leakage versions to be offered
to meet medical specifications.
In Summary:
|
|
Linear
|
Switch-mode
|
Comments
|
|
Size
|
x
|
OK
|
Typically, 80% smaller
|
|
Weight
|
x
|
OK
|
Typically, 80% lighter
|
|
Input Voltage Range
|
x
|
OK
|
10% vs. up to 300% range
|
|
Efficiency
|
x
|
OK
|
Calculate it long term!
|
|
Reliability
|
OK
|
x / OK
|
Component count method, demonstrated probably equal
|
|
Ripple & Noise
|
OK
|
x / OK
|
Up to 10,000 times - often possible to overcome though
|
|
Transient Response
|
OK
|
x
|
Up to 100 times - necessary in specialized areas
|
|
Low leakage Current
|
OK
|
x / OK
|
Often used in medical systems, switch-mode gaining share
|
Hope that is useful all those aspects presented here.
In this section, below I will share all tests and additional
research made so far.
