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Introduction to Small Switchmode Power Supplies

ZL2PD peers inside those small switchmode power supplies commonly used with cellular phones and an increasing range of consumer electronic equipment.

Introduction

From 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 box.

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 microprocessor emulator.

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 schematic)    

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    

These 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 or me!

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    

The 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, take care.
   
Example #1:    











Example #2:    


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 efficiency.

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 secondary-side grounds.

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 switching transformer.

Example #3:
A friend bought a new celphone and was about to toss out the old celphone's charger.









This Nokia-branded power supply had a surprisingly simple layout:


In this case, there is no bridge capacitor in the supply. The transformer probably contains an extra winding to provide additional shielding between primary and secondary transformer windings to reduce this leakage capacitance.

More Examples    

Further 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:    

Fixed SMPS Modifications    
Building a Compact Variable Switchmode Power Supply    

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