Small Switchmode Power Supplies
After a few years, it's really amazing to see the number that end up in that cardboard box. The majority, of course, are those little wall-mount ones that seem to cluser around our power sockets like moths around a lightbulb. A few years back, most of these small wall power packs used a transformer. More recently, increasing numbers of these power supplies are turning out to be switchmode power supplies.
The really useful thing for me with these switchmode power supplies 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 battery charger for my son's Apple iPod Shuffle.
Elsewhere on this website, I have described several specific modifications to these power supplies including modifying a power supply for a new fixed voltage, and another modification to turn a cellular charger into a compact variable power supply.
I decided to spend a day tracing out the circuits of three of these power supplies. The three I chose pretty much at random seemed to be typical examples of power supplies which I've modified over the years. 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. My assumption had been that they tended to follow a fairly similar arrangement, but I felt it was time I put this to the test.
Also, there doesn't seem to be much information about on the web about these sorts of power supplies. There are many 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. Most equipment suppliers buy the power supply from a huge number of small manufacturers, and a common vendor power supply part number can actually cover a large number of different designs. I've seldom seen any equipment service manual which actually describes the circuits of the power supply. Hence my curiosity.
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.
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.
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, comes 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 that large transformer. 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, some now use a small high voltage IC containing the switching device and associated 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.
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.
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.
Onwards 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.
(See Example #2 above for the schematic)
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.
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 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.
Now onto the third example.
I can't get a photo of this modified unit now. This chip was used in the Motorola charger I modified for my son's iPod Shuffle USB charger. Not only is it hard to prize away from my son who uses it every other day, but it's also really solidly sealed up in its new case using a very large quantity of Superglue and hot glue!
(See Example #3 above for the schematic)
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. (I'll try to improve on the picture below at some stage)
All told, this looks like another good candidate for a modification should the need arise.
