The LaST upgrade Part 33 - PICO PSU REVIEW

As many know already, I am not a fan of the "Pico PSU" simply because when I purchased 4 of them a couple of years ago, from various sellers, none of them worked correctly or simply did not work at all. I am not saying I am against these PSU's, many people use them without problem. Though I have also heard stories over the past year that PICO's might have been the cause for some Falcons being killed off.

In light of such issues, and myself personally wanting to re-vist the PICO technology, I ordered 3 Pico PSU's as illustrated below. One looks to be a genuine PSU, the other 2 seem to be a cheaper clone. I brought these off evilbay (and no I am not linking evil bay links into my site pages!) and I already see that the clones do not even give any sort of amps rating per supply rail. Likely I will test in 1 amp increments until something dies a death.

Testing PSU's and reviewing parts can be time consuming. I am not going to spend huge amounts of time on these, but will do some basic tests. In order to explain what is "good and bad" I need to explain some basics about Inductors and Mosfets. I'm not going to explain everything in detail as it would make a huge article in itself. So I will try and keep things simple to understand. If you are not intetested in some small tech info, then skip the following text.

INDUCTORS - I have wrote about this somewhere else, probably on my own designed PSU. Though here is a crash course.

Inductors are used to store energy for switchmode PSU's. Good inductors will have low resistance, a good core material (A bad core can lose 60% of the energy) and must be able to hold its inductance value within a good tolerance.

RESISTANCE - Generally the larger the inductor, the larger the wire used and the lower the resistance. Higher resistance will cause heat build up and lose efficiency.

INDUCTANCE - Must maintain its value at rated current to approximately 10%. Difficult to find quality inductors.

CURRENT RATING - Must be a genuine figure. The problem is, a lot of manufactures either don't list the current, or they list the "fuse current" of the wire, not the amps the inductor is typically capable of. This is a huge trap for people who assume a 10uH 10amp inductor can delivery that, as normally they can't. The reality is, after just 1amp, the inductance can drop by 10% or more. In fact 10amps 10uH inductor, might just act like a straight piece of wire at 10amps or lose 90% of its value.

This boils down to saturation current, Which the only way to tell is to look at the graphs, assuming the manufactures list them. If they do not, then avoid that manufacture totally as they are likely going to be poor quality inductors.

As the switchmode controller is trying to deliver a voltage and maintain it under heavy loads, the controller will push more and more current into the inductor when more amps are needed. If the inductor starts to saturate, the Inductance will drop anywhere between 10% to 90%. If the inductor loses 50% of its value, then the switchmode controller will likely try and push 50% more current into it to maintain the output current. This then ends up with a huge drop in efficiency where things start to heat up to the point they melt. Of course its possible to melt a so called 10amp inductor with just 1 amp of current.

Overall, what I calls as a good inductors is one which maintains its value within 10% and has a genuine rated current. While I can test resistance to get a idea on quality, I am not going to test saturation currents as it would be very time consuming. Even so, I will be testing efficiency of each PSU, so if efficiency starts to drop down, then we can assume it has hit the saturation current of the inductors. Looking at the tiny sizes of them and considering they are switching 8amps on the 3.3V rail, it will be interesting to see how they hold up.

MOSFETS - Total nightmare to give even a simple summary, huge nightmare to find a mosfet suitable for a application as there are so many conflicting specs.

RDSon - On state resistance. Of course lower resistance is always better. But actually its not that simple. One of the PSU's quotes 4mR resistance (0.004ohms) which is great, but there are other issues at work here as well. Generally low resistance mosfets have a huge die. As a result, they are slower and need more peek amps to turn them on and off. Resistance is when the mosfet is fully turned on (this isn't the same as switching resistance which is a complex topic in itself)

CISS - Input capacitance. Large die is large input capacitance. Typically in the order of 3,000pF. (3nF) which is actually a lot. This capacitance has to be charged and discharged at a rapid rate. The faster you want the mosfets to switch, the harder it becomes. Don't forget a capacitor acts as a short circuit when at 0 volts. So the mosfet driver circuit needs to be able to deliver some serious peek amps to get things moving. Also this is a problem as several peek amps can be needed, this means the pico's PSU rail itself, needs to be stable and be able to deliver the peek amps also. This can result in a lot of noise being generated in the PSU's own circuit, which is bad.

There is also other capacitance's at work such as "reverse transfer capacitance". Where you also have a capacitor between the mosfet gate and the load you are switching. For example, if you are turning your mosfet on and off with 12 volts, and are switching a 50volt rail, then you will have a small capacitors (well several hundred pF probably) between the 12volt and 50volt parts internally in the mosfet. This means the mosfet driver also has to have good control over the mosfet gate, else it could see this 50volt rail and this could kill the driver circuit. Of course all this is a very complex topic in itself.

Td(on/off) Total delay on and off. Large mosfet dies take longer to turn on and off. The delays for turning and mixed in with the actual rise and fall times of the mosfet can really add up. Generally Mosfets can switch anywhere between a few nanoseconds to 100ns or more. The faster they can switch the better.

ID ( AMPS) - The more amps you can switch the better. Well not really. Again large dies have lower resistance, higher current ratings, but take longer to turn on and off. Great if you don't need speed. Though if your are using them in a switchmode PSU, then generally you need speed, and a large die isn't going to work.

Confused ? it gets better. What if we want a high amps mofets to switch fast ? Well, basically you can't. Higher amps mosfets need a larger die and the larger the die, the slower they switch. You can't have it both ways. If we want low resistance then we need a larger die again. A smaller die will switch faster but will have a larger resistance, which we don't want either. To add more confusion, there isn't only on state resistance (which is the resistance when the mosfet is fully turned on) but also you get switching losses as well. The time it takes the mosfet to turn on or off is again depending on the die size. You can have a mosfet which can switch 100amps and have near zero on state resistance, but may take 500ns to turn on and off. This is a huge loss (called switching losses) where the mosfet will heat up a fair amount and will get hotter the longer it takes to turn on or off. So while higher amps and lower resistance is what at first appears a good thing, it depends on the application. If you are needing to switch 5amps, you don't use a 100amps mosfet. That would be a very dumb thing to do.

Now take into account the switching speed of the PSU controller. It can vary from a few KHz to even low end MHz range. If the total switching delays of the mosfet are 50ns ON and 50ns off (which is pretty typical) and the actual ON time is 100ns, then the total cycle length is 50+50+100 = 200ns. In fact This means the mosfet will spend 50% of its time in switching losses. Which is bad. To give a idea, assume you lose 50% efficiency of the mosfet (again I am not going into this in depth) and spend 50% of time fully turn on. You could have a zero resistance mosfet (RDSon) and the mosfet can catch fire due to the heat build up in switching losses.

ironically , I have seen 200mR mosfets (0.200ohms) switch a few amps and remain stone cold. Their switching time was below 5ns so the switching losses were very small, even though the on resistance is very high. Such mosfets were running around 200KHz so they spend a lot of time turning on and off, so fast switching is a must.

You could have a higher amps mosfet with zero resistance, but forget trying to turn it on and off fast. Try to drive such a device in the upper KHz ranges and the mosfet can spend 100% of its time in switching losses and in that case, you should probably find a easier hobby :)

Mosfets are a conflicting nightmare of chaos. Its enough to give any professional design engineer a headache, nevermind people just venturing out in mosfet technology. I will look at the spec sheets for the mosfets used to see what ratings they are, but without knowing exactly how the circuit is working, it probably wouldn't be fair to say any mosfet is good or bad as they are very specific on the application. Of course if we have a 1amp mosfet switching a 10amp rail, then I will probably frown upon it somewhat :)

Overall, I just wanted to make the point that just because a manufacture claims lower resistance mosfets, or high current mosfets, not to assume that is the good thing to look for as it is not so simple.

CAPACITORS - We all know what they are. Generally tantalum, ceramic, electrolytic. Problem is, SMT capacitors are generally small, and have poor ESR ratings. Larger capacitors have higher ESR ratings. What is ESR ? Basically to been seen as a resistance in series with a capacitor. For example, if you had a capacitor in a supply rail, if you run that capacitor via a 10ohm resistance, then you would think, why bother having the capacitor there in the first place ? You would be right to question exactly that!

Ceramic capacitors are my type of capacitor, You can't really go wrong with them. Though once you get into the 100's of uF, then you are pretty much forced to look at electrolytic types. Though that in itself isn't exactly true. Generally larger uF values are used to improve stability. Though as a side effect of higher uF, is physically larger capacitors, which have lower resistance. So its not fair to simply say more uF is better. For example, you could have a 1,000uF electrolytic capacitor which may give 10mR resistance. Or a 100uF ceramic which will have near zero resistance. Again, depending on application, I prefer to use ceramic where ever possible. Larger values and voltages of ceramics are on the market more lately than ever. Why bother using a electrolytic when there are ceramics available. Well, maybe cost. But if a ceramic is available in the right voltage and value, then use it! That is one thing of "quality" that I will be keeping a eye on.

AND THE REST - There as so many aspects of a PSU that it would be almost impossible to go though it all. PCB copper thickness for example. I myself use thicker copper as its improves efficiency and regulation.

There are many types of switchmode designs also. We have not mentioned P&N type mosfets. Generally P-channel mosfets sux, nobody uses them (well ok some do) but I personally just like using N-channel only. There isn't any need for P-channel unless your looking for a cheap circuit design. In my view, if you want to use 2 mosfet switching design, then use a driver capable of driving 2 N-channel mosfets!! While there are always "pro's and con's" for any design, I'm not going to go into it all here. There are many guides on the Internet about switching technologies already.

Tests are only for guidance only. The 3.3V rail was tested using a rheostat resistor where I cannot set 100% correctly.Unfortunatly I don't have the equipment to accurately set resistance values. Also I made the assumption that the 3.3V rail was actually 3.3V. So tests will have some small percent error. I think overall tests give a really good indication of operation anyway.

I am only quickly testing each PSU out. Testing methods are not ideal, though to test things out better would need a great deal of time and better testing equipment. Both things I do not have! My rheostat is a variable 20ohm wire wound rated at 200watts. Changing its value is a nightmare of trial and error. Its value was measured with a proper low resistance meter and pretty close to the values I needed. Though the 3.3V amps testing are not totally accurate but as close as I could get. Overall, the "error" factor in testing is across all PSU's and I was more interested in comparison of these PSU's rather than rock solid diagnostics of each one. Even so, I think my results are pretty close and good enough for the series of tests I wanted to do.