The End of the Full Bridge Rectifier? (Sorry ElectroBOOM) Active Rectifier is here!

This little component

here

is a FULL BRIDGE RECTIFIER that converts the AC mains voltage we get from our outlets into a fairly irregular DC voltage. This may seem unspectacular, but if we add a capacitor to the output of the

rectifier

we get a smooth DC voltage that is basically necessary for any modern power supply to work. So yes,

full

bridge

rectifier

s are very important and make our modern electronic world possible, but that doesn't mean they are perfect. In fact, they come with a noticeable voltage drop that, multiplied by the current flowing, is equivalent to a loss of power that the rectifier dissipates in the form of heat.

Needless to say, we want to keep this power loss as small as possible to increase efficiency and that's why I was very excited to find this

active

rectifier board on the internet that apparently replaces the old school diodes of a

full

bridge

rectifier with MOSFETs. in order to reduce the voltage drop and, t

here

fore, the power losses. So in this video we will not only put this pre-made

active

rectifier board to the test, but we will also try to create our own DIY version to find out if they are the future of rectification or if we should stick with the old school solution. .

Let us begin! This video is sponsored by Altium! You may already be familiar with Altium Designer for creating schematics and PCBs, as I've mentioned it once or twice before. But today I want to talk to you about Altium 365, which is already integrated into the design software and allows you to easily share your designs not only with your colleagues but with basically everyone. This way you can easily get feedback and speed up your design process. So why not try it for free by clicking the link in the description? First, we need to understand how a traditional bridge rectifier works.

As you can see from this schematic, it only consists of 4 diodes placed in a particular arrangement which I personally like to draw like this because it is easier to understand that way. And let's imagine that we have a resistive load at the output through which we should then get the irregular DC voltage that we saw before and at the input we get the AC voltage from the mains. So the question is how exactly we get from this to this. Well, the answer lies in the behavior of a diode that lets current flow only in one direction and not the other.

That means that when the AC mains voltage is in the positive region, we have a voltage potential here that allows current to flow this way. And when the AC mains voltage is in the negative region, we have the opposite voltage potential here, which this time allows current to flow in this way. So in both cases a flow of current through the load was possible and what was notable is that the direction of the current was always the same, which basically means that we doubled half a wave of the AC voltage and thus created DC. It's pretty simple in my opinion, but the bad news is that each diode comes with a certain voltage drop, which causes the power loss I talked about at the beginning.

The active rectifier idea wants to change that by replacing diodes with MOSFET switches that come with super low resistances and should therefore decrease power losses. The only problem is that MOSFETs are not exactly diodes, although they do come with a body diode that also allows only one direction of current path. But the goal is not to use the body diode, since its voltage drop is also quite high. No; We want to apply a DC voltage to the MOSFET gate so we can turn it on properly. So for a full bridge rectifier we would need some sort of circuit that detects when the AC voltage is positive or negative and depending on that just turns on the 2 MOSFETs diagonally from each other to mimic the functionality of a full bridge rectifier.

Good timing is also key here because if just one extra MOSFET is turned on at any point, we basically have a short circuit. And that's exactly why I was quite happy that I didn't have to design such a circuit myself because I found the IC TEA2206 and TEA2208 on the Internet. They are active bridge rectifier drivers and can do what I just described using their built-in comparators and MOSFET drivers. So according to your typical application schematic, I don't have much to add to the ICs to create an active rectifier, just some MOSFETs and capacitors. And in case you're wondering, the 2206 only replaces the bottom side diodes with MOSFETs, while the 2208 replaces them all, which begs the question why I bought the 2206 version if it only does half the job. to reduce energy losses.

Well, the reason why I have my hands on a development board that I can use to get my feet wet is that it should be fully functional, right? To test it safely, I used my small autotransformer to step down the mains voltage to 25 VAC RMS, which I connected directly to the active rectifier. And as you can see on the oscilloscope, it seems to work very well, fantastic. That means it was time for the first efficiency tests/comparisons which I performed by adding a bank of capacitors to the output of the rectifier as well as a constant load and then drawing variable constant current while measuring how much input power it required.

Of course, to get accurate values ​​for the output side, I also used my multimeter to accurately measure the current and my oscilloscope to get the RMS voltage value. Once this was done, I replaced the active rectifier with a normal full bridge rectifier and basically repeated this test to find that the average efficiency increased by 2-4% with the active version, not bad. But of course next I wanted to replace all the diodes to increase efficiency even more. To do that, I soldered the TEA2208 IC to a breakout board and then to a perfboard to which I then added the 4 MOSFETs and some capacitors, pretty much as the typical application schematic recommended.

The result didn't seem that bad, but I was still very nervous while doing the first test with the mains voltage low. But as you can see on the oscilloscope, this active rectifier also seems to work well, at least with resistive loads. Because while I was trying to charge the capacitor bank for the power test, there seemed to be some kind of short circuit problem with the rectifier. I'm not sure if my design with low-cost, high-resistance MOSFETs is the problem or if the topology and IC generally can't handle capacitive loads because the datasheet also states that a step-up type power factor circuit should follow the application that more or less prevents power surges. of capacitors.

However, the circuit works well with restless loads, as I said. But due to obvious safety concerns, I didn't really want to include it in the following mains voltage power supply test where I basically measure the efficiency of a common switched mode power supply before and after implementing an active rectifier. The only thing that stopped me once again was a familiar phrase in the TEA2206 datasheet indicating that it is intended for applications followed by a PFC. And since said power factor circuit was mentioned twice, let me tell you that it basically prevents the mains current from only being consumed near the peak of the AC mains voltage.

Instead, it distributes the current draw evenly across the entire sinusoidal voltage, but we'll talk more about that in another video. The only problem was that almost all of my power supplies did not come with this feature, except of course for one that I need to charge my newer laptop. And I decided not to sacrifice it for this video, so I tried my luck and tested it with this slightly better power supply that not only comes with less abrupt current spikes but also some filters on its input. So, after performing the usual power measurement test with it, I desoldered its original bridge rectifier and simply replaced it with the leads from the active rectifier board.

And luckily nothing exploded after power on and everything worked perfectly fine so I could do all the measurements. Now the final results tell us that the active rectifier is again more efficient but this time only with a difference of around 0.2 up to a maximum of 0.9%. The reason is that due to the higher mains voltage level of 230V being down transformed, only a small current flows through the rectifier which does not create such a large power loss and is therefore not that important. for efficiency. But if we have a low voltage, high current system without transformation, then a better rectifier is definitely recommended, as we demonstrated in test 1 of this video.

So in conclusion, the old school full bridge rectifier is definitely good enough for most mains voltage rectification because it is cheap, rigid and efficient enough for this task. That said, I hope you learned something new through this video and now understand the advantages and disadvantages of active rectifiers. If so, please consider supporting me through Patreon. Don't forget to like, share, subscribe and hit the notification bell. Stay creative and see you next time.