I got a cheap 400W 12V switching power supply in the mail the other day. I wanted to use it as replacement for an ATX power supply for my 3D-printer.
But what is inside those things?
400W / 12V Power supply from china
First of all let’s have a look at the outside of the power brick. The Frame is made from two 1.5mm aluminium sheets. There is a fan in the top and the screw terminals are covered with a plastic lid. The frame has two screw holes on each side for mounting. It does not have a switch but there is an indicator LED next to the terminal.
The fan on the top cover is connected to a pin header so it can be removed with the lid. The housing itself is pretty sturdy so it looks good so far.
The input voltage can be selected via a switch that is accessible from the outside. You can select a supply voltage of 110V and 220V. So it can be used worldwide since the frequency of the input current does not matter for a switching power supply.
After lifting the cover up we can have a look at the PCB inside. It looks like there is a single sided through hole board inside. Four high power semi-conductors are placed near the side walls and thermally connected to them.One thing I noticed was a loose screw flying around inside the supply. This can be extremely dangerous since it can cause a short in the supply. Also there where two mounting screws missing on the board. One in the middle and one in the upper left corner.
The underside of the PCB shows the different components of the circuit. The picture below shows the different parts of the power supply. First of all we can see the terminal at the right side and the high voltage AC input at the lower part of the screw terminals. The yellow area is protective earth and surrounds the hot part of the high voltage mains circuit. This is where one of the mounting screws where missing! The mains voltage is decoupled via a transformer and goes from the yellow part into the green area. There it is rectified and buffered in two 680µF caps. Those are rated 250V so I’m not sure what voltage this area has. It certainly can not be rectified mains voltage since this is nearly 400V! The pink area is control circuitry with a central controller KA7500B. It brings everything along to control the switching regulator found at the back side heat-sink. The blue square is the main transformer, that transforms the higher voltage on this side to the desired 12V on the other side. There you can see that the traces on the PCB get flooded with solder to decrease their resistivity. The PCB trace width calculator gives a rough estimation for about 97.9mm trace width. This is certainly not the case so the added solder leads to a reduced trace width. The output of the Terminals is monitored by the chip. So they have to be connected to the 12V output traces. Since the high current leads to a voltage drop over the distance from the transformer to the terminals the measurement of the output voltage should be done at the terminal point. Therefore there should be at least one sens line going back to the controller circuit. Yes, there it is. Marked with the orange arrow.
Oh wow they are high quality Rubycon Caps… Oh wait. no they aren’t
So after all I can say you get what you pay for. The power supply is not bad. But it certainly is no high end laboratory style power brick. Let’s see how it works out under load.
The output voltage at the terminal droped one volt after loading the power supply with 350W. This can be compensated at the trimmer next to the terminal. so now the output voltage is bang on 12V and the ripple is in an acceptable 0.2Vpp. The supply can now deliver 12V without dropping to 10V like the ATX did. Let’s fix up the printer to get going again.