Introduction to Small Switchmode Power Supplies
peers inside those small switchmode power supplies commonly used with
cellular phones and an increasing range of consumer electronic
time to time, I reuse or modify a number of those compact switchmode AC
power supplies for use with my projects. I've recovered most of these
'wall wart' power packs from discarded equipment. Relatives and friends
who know I can make use of these devices also provide me with some of
these from time to time. Some are old cellular phone chargers, while
others have come from a variety of discarded electrical equipment. As
they arrive, I toss these little power supplies into a big cardboard
After a few years, it's really amazing to see the number that have
ended up in that box. The vast majority, of course, are those little
"wall-wart" ones that seem to cluser around our power sockets like
moths around a lightbulb. A few years back, most of these small power
supplies used a transformer. More recently, ever-increasing numbers of
these power supplies are turning out to be switchmode power supplies.
The nice thing about these switchmode power supplies for me is the ease
with which the output voltage can often be adapted to suit the
requirements of a new project. Over the years, I've modified quite a
few. An Ericsson cellular phone charger was changed from 8V to 16VDC to
power my microprocessor programmer. A Motorola cellular charger was
modified to 6VDC to power a PC computer sound system. More recently, a
Chinese-made switchmode 'wall wart', actually branded 'Motorola', was
modified from 10VDC to 5VDC to serve as a USB charger for my son's
Apple iPod Shuffle. Elsewhere on this
site, there's a description about how I modified one for use with my
The photo at the top of this page shows a typical example of a small
switchmode power supply. This was found inside a small plug-in AC mains
power supply for a wireless video system (See Example #1 below for the
Switchmode power supplies are easy to identify. The first sign is often
their weight. They are considerably lighter than their older
transformer equivalents. It is possible to be misled by weight alone,
especially with many low-end manufacturers skimping on the iron content
of their transformers!
A more accurate indication is the label on the powerpack. The label
usually states that the power supply will operate across an input AC
voltage range from 110 to 250VAC. Transformer power supplies are
limited to operating from only one of two fixed AC input ranges; Either
110-120VAC or 220-250VAC.
I have modified a number of these power supplies so far, including
changing them to give a new fixed output voltage, and turning another
cellular charger into a compact variable power supply. These are
described on other pages on this site.
Taking a Closer Look at Switchmode Power Supplies
In order to modify switchmode power supplies, it is necessary to get
some idea of how they work. I decided to spend a day tracing out the
circuits of three of these power supplies from the pile I had in my
cardboard box. The three I randomly selected for this examination
seemed to be fairly typical examples. One was a cellular charger, the
second had previously powered a cordless phone, while the third was
used with a TV video/sound wireless extension device.
I don't usually bother tracing out the circuit each time I modify one
of these switchmode power supplies. They tend to follow a fairly
similar arrangement, as you will see.
There doesn't seem to be much information available on the internet
about these sorts of power supplies. There are lots of applications
notes describing the design of such power supplies from the component
manufacturers, but little exists in the way of circuit diagrams for
commercially made power supplies. My guess is that most equipment
suppliers simply buy their power supplies from a huge number of
manufacturers and only specify the performacne they want. These are not
items that would ever be worth servicing, so there's no need to provide
circuit diagrams and board layouts. But, for us, it's helpful to see
what's going on in there, so we can better tailor any modifications we
seek to make.
First, let's look at the basic structure of a switchmode power supply.
This will help to explain what I found inside these three power
supplies I traced out.
Switchmode Power Supply Basics
power supplies follow a very similar basic structure illustrated in the
following block diagram:
1. A rectifier stage which produces a smoothed DV voltage directly from
the incoming AC mains supply.
2. A regulator stage which uses a high voltage switch, usually a power
FET to generate a variable AC voltage into the transformer primary,
driven by a PWM oscillator.
3. The transformer to provide both power conversion and isolation
between the primary AC mains input side of the power supply and the
secondary side of the transformer. The secondary side is
connected to the equipment.
4. A low voltage rectifier stage to convert the secondary AC voltage to
the required secondary DV voltage.
5. A voltage detector and feedback stage to the primary-side high
voltage switch, usually via an optocoupler to preserve
primary-secondary isolation, to maintain the required output voltage
under different loads, and
6. An optional current limiting stage to limit the maximum output
current of the power supply to design limits.
The 'primary side' is the high voltage 110VAC or 230VAC mains power
supply side that feeds the rectifier and regulator stages of the
switchmode power supply. The 'secondary side' is the low voltage DC
side that connects to my circuits. It's really important to make sure
that there is absolutely no chance that the primary side voltage, which
can rise to above 340VDC, can come in contact either with the circuit
being tested, or, more importantly, the person doing the testing - you
Isolation in the older transformer-type power supply is provided by
those relatively large and heavy iron transformers. In a switchmode
power supply, or at least good ones, the isolation is provided by the
combination of the tiny switchmode transformer, the physical separation
provided by a good PCB layout, and (usually) an optocoupler which is
used to carry the voltage feedback to the primary side switching
device. While these supplies typically employ a high voltage FET driven
by a simple transistor oscillator, an increasing number are using a
single small high-voltage IC containing the switching device and many
of the associated primary-side components.
The switchmode power supply transformer is smaller because transformer
losses reduce with increasing switching frequency. The old transformers
operate at AC mains voltage frequencies of 50 or 60 Hz. Modern
switchmode power supplies operate at much higher frequencies, anywhere
from 30kHz to 300 kHz, with the result that losses are reduced. So,
transformer sizes can be smaller too.
Some cheap power supplies avoid
optoisolation, relying instead on
feedback from one of the transformer windings. These are often more
difficult to successfully modify, so I tend to avoid these in favour of
those with optocouplers.
Three Examples of Small Switchmode Power Supplies
following schematics are based on my investigation of a number of these
power supplies. Some component values are not shown here. If I couldn't
see a value, I've chosen to simply to leave the component value
blank rather than give a value based on my guess. Most of these
parts are surface mount capacitors which do not have any details
printed on them.
Since the accuracy of the circuit diagrams is based on my eyesight, you
should not rely on their absolute accuracy. Also, while your power
supply may look exactly the same as those shown here, there may be
small but critically important differences between yours and mine. So,
Careful review of these first two schematics, and the third shown
below, will show some common patterns. Firstly, all three directly
rectify the incoming AC mains supply using a bridge rectifier and
several filter capacitors. This typically generates a DV voltage
between 200 and 330VDC. This is applied by way of the transformer
primary to the high voltage switching FET.
OK. So this is probably an ideal place to add the following note.
WARNING! These schematics
clearly reveal some of the dangers associated with these power
supplies. There is no transformer or other isolating device between the
unwary fingers of the ignorant and some potentially lethal
voltages and currents. If you choose to experiment with power supplies
like these, only do so if you know what you're doing. These power
supplies look innocent but they have the potential to kill you.
On with the interesting stuff...
Two of the three supplies feature snubber circuits across the
transformer primary. This consists of a high voltage diode connected in
series with a resistor/capacitor pair which protects the FET from high
voltage switching transients. The power supply which lacks this feature
uses a FET with an internal protection diode.
This one was inside a worn plastic
case of unknown vintage. It looked quite simple from this top side view
but underneath it's a complex cluster of surface mounted components.
Example #2 shows the schematic.
In each case, a pair of small signal transistors form an oscillator
which drives the gate of the FET. This oscillator is in turn controlled
primarily by feedback from the voltage detector on the secondary side,
as well as, in some cases, a current limit detector on the secondary
side as well as a current detector on the primary side. These alter the
switching FET's duty cycle to maintain the required output voltage of
the power supply.
A further common element in all of these circuits is the presence of a
diode rectifier usually connected to a separate primary side winding.
This improves the switching speed of the FET and the power supply
The secondary side of each circuit uses a single high current low loss
rectifier to rectify the high frequency switched AC supply generated by
the switching of the primary side high voltage DC voltage by the FET.
This voltage is smoothed by a large value electrolytic capacitor. The
resulting DC voltage is detected either by a TL431, which then drives
the feedback loop to the FET via an optocoupler, or by a simple series
connected zener and optocoupler LED.
Note the capacitor which connects the primary-side ground to the
secondary-side in these two examples. In Example #1, C7 (3n3) actually
connects to the positive output on the secondary side while C3 and C4
(each 4n7) in Example #2 connect between primary-side and
These high voltage capacitors are called "bridge capacitors". They
reduce common-mode radiation from the supply which are caused by
common-mode ground loop currents. These currents occur due to the
leakage capacitance between primary and secondary windings of the
friend bought a new celphone and was about to toss out the old
This Nokia-branded power supply had a surprisingly simple
case, there is no bridge capacitor in the supply. The transformer
probably contains an extra winding to provide additional shielding
and secondary transformer windings to reduce this leakage capacitance.
examples may be added to this page over time if a new power supply
shows some interesting features. I came across one power supply about
six months ago which integrated the switching FET and the primary side
oscillator transistors and additional protection circuitry within a
single IC package. It used one of Power Integration's family of
devices, the TNY264P. This chip reduced the number of components
significantly, as well as the size of the power supply.
I didn't get a photo of that power supply. I did a quick modification
to allow it to be used as a 5V USB charger for my son's iPod Shuffle.
It's really solidly sealed up in its new case using a very large
quantity of Superglue and hot glue!
But I did find a similar
power supply in a surplus electronics shop afterwards, and that unit is
shown in the photo below.
Branded "Wan Nien" on the PCB, it also used an 8 pin controller IC as
the primary-side switcher. In this case, it's a Fairchild KA5L0165
which has an internal power FET rated at 650V/1A. The power supply
module, missing its case but with the DC cable and connector still
attached, plus the cable clamp, cost me just over $US1. Great price
when you think how much it would cost to build a regular transformer
power supply with regulator stage.
Actually, the chip was simply marked "5L0165" and it had no brand so it
may well be an Asian-made clone chip with similar (or poorer!)
specifications. It was described as a 5V 1A power supply in the shop
and the cable and connector attached suggests it was designed as some
form of OEM celphone or PDA charger. I'll need to test it to confirm
the ratings before using it.
The secondary side features a KA431 (a TL431 clone) and an
opto-isolated feedback loop using a Sharp PC817. Drive to the
optocoupler is provided by a surface mount (SMD) LM358 opamp.
All told, this looks like another good candidate for a modification
should the need arise.
More About SMPS Modules and Modifications on This Site:
Compact Variable Switchmode Power Supply
Want to go back to the main page? Click