MOSFET Rectification

Completely by accident last month when looking at options for single-cell boost converters, I came across a new Linear Technology chip called the LT4320.  This one-chip wonder simplifies a problem which has been dealt with in switched mode regulators for some time but hadn't made it across to the linear domain - the forward voltage drop of a rectifier diode bridge.

When you have a system that is pulling a large amount of current, no diode available will give a properly low voltage drop, Schottky or not.  What you essentially want is to use a solid state switch with very low on-resistance... for which the bog-standard N channel MOSFET is perfect.  The catch has always been to drive the gates, which is do-able but requires a lot of circuitry... Linear Technology has encapsulated it all onto one chip, dramatically simplifying things... four Nch MOSFETs, a couple of caps and you're away!

So I decided to do a little PCB for it... here's the schematic...

It's very simple, as you can see.  This is what the PCB ended up like... the usual 2oz copper, ENIG finish for power stuff... the 4oz used on power amp boards didn't feel necessary.

The LT4320 is rated for an absolute maximum of 80V potential so it can't be used in any application but is best suited for high current applications where the inefficiency of a rectifier diode is very obvious.

The board was designed so either SO8-like or D2PAK packages could be supported.  In any reasonable application the power dissipation should be very low from either of them, but it gives more options as to what packages to buy.

This particular board is to replace the rectifier diodes in my router UPS/PSU so went for NXP 30V MOSFETs of "79A" rating.

Here's the front side of the board... the double connections for the DC outputs mean either spades or block terminals can be installed.  There is space for a snubber on the transformer secondary but for this application it was deemed unnecessary.

Here's the old rectifier board to be pulled out...

 

And here's the new one slotted in place...

As the current draw of the PSU isn't that high, there isn't a big improvement in efficiency (less than 5% improvement, I think), but it does mean that the raw DC voltage is a bit higher which helps the battery charge properly under load.

Overall, worth it but there are projects which will benefit more, like valve filaments!

12V Power supply gets a revamp, part 2 - maximum overdrive...

So yes... a bit carried away.  It all starts with buying some large chokes... then a big case... then a big chunky 2x12V 225VA encapsulated toroid.

And before you know it you have a pretty silly brute force unregulated supply that could deliver something like 15A.  Yes, ideally you wouldn't have this iron in such close proximity, but this is all I could do in the case... these chokes are pretty large!

The supply has an off-load voltage of around 19VDC though it doesn't take much to drop it down to 17VDC.   This is much higher than I need plus the off-load losses are significant - I was seeing 4W dissipation with nothing connected.  Ouch... some of that will be in the 17V zeners, but this all seems like gross overkill.

It will make the beginnings of a good audio PSU but this perhaps isn't what I want for a network supply.

Now I had the NAS, I wanted to try running it with the existing 12V PSU I built early last year, but found that it just didn't have the grunt to cope with the spin-up of the HDs in the NAS.  Sticking a big cap on the output only made it hiccup, so figured a better solution would simply be a beefier DC/DC conversion stage.

To keep efficiency, it had to be a synchronous buck topology, and a bit of research suggested that the LM3150 would be a good option.  TI sell evaluation boards for these, so a bit of a play in their Web Bench suggested what parts I'd need to reconfigure the boards to do my bidding - they are made on beefy 2oz copper with wide tracks so should have no problem in delivering >10A of current.

Thankfully pretty much everything was available from Mouser.  To handle the increased current, the N-channel MOSFETs needed to be changed.  As it so happens, TI make some pretty good ones.. the catchily-named CSD16321Q5C is a 25V, 100A (okay, 31A in most real world cases) part which has more than enough current capability, and is also designed to be heatsinkable from the top if you need more current handling!

A literally big component change was replacing the inductor with a much beefier one - a SER2915L-682KL... it barely fitted on the board!

The aim was to try and keep copper losses as low as possible... this inductor is a >=20A part so should have no trouble at all delivering 5A, even in a box with very little ventilation.


To swap over the FETs, I used my new hot air gun, an Atten 858D.  I saw the review on the EEVBlog a while back and it did seem to be the best tool for the job for a fairly small amount of money.  When it comes to 2oz copper board with wide traces, even an 80W soldering iron will struggle to get enough heat in to remove parts - this is where a hot air gun really comes in handy.   I don't think there's any way I would have gotten the solid caps off with my iron.

So I tested up the boards with crocodile clips - success - a clean 12V output!  Although the regulation seemed a little poor... I later found out that the crocodile clips had a whopping 0.2 ohms resistance - horrendous for any kind of significant current testing!  Need to make some fresh ones with thick copper cable...

 The new DC/DC seemed to be working okay, but the existing 50VA transformer in the PSU box just wasn't going to have the oomph required.  Even with a single HD, the cable modem, router and NAS were going to peak at 35W or so consumption when the drive spins up... the 50VA will be well out of its comfort zone.

Fortunately I had an 80VA handy... the catch being that it's a 2x9V rather than a 2x12V.  Standard transformer voltages are a little bit annoying... the standard windings go along the lines of 6V, 9V, 12V, 15V... thing is that no transformer offers perfect regulation so there can be a significant disparity in off-load vs. full-load voltage.  A 12V transformer after rectification and smoothing is likely to be something like 17VDC with only light loading, unless you put a big choke in before the cap.

2x9V would have been perfect for a 12V supply, but I need more than that to charge a battery... ideally between 13.5V and 13.8V for a 12V, depending on temperature.

To handle UPS functionality, I decided to go with a PicoUPS board.

This is quite a neat little board which is based around a couple of ORing controllers that let it use beefy NMOS as diodes, essentially... the controllers have a comparator which decides whether to switch battery or mains sourced DC to the output... when the mains voltage drops below the battery, it should seamlessly switch to the battery.

In order to charge the battery, the board uses a P-channel MOSFET with a little controller IC that appears to provide constant current.  As far as I know, it doesn't apply any continuous voltage limiting, although the board itself should not be run above 17VDC input.

While I had a nice efficient UPS solution now, the transformer was a worry.  9VAC would barely rectify to above 14VDC, even off load... ideally this needs a 10VAC output transformer but such things are custom jobs and as such very expensive.  The fact that the transformer is epoxy-sealed eliminates the possibility of adding extra turns... so let's try to make things as efficient as possible...

After some searching, I found what appear to be the very best diodes you can buy short of doing it all with MOSFETs and a controller.. the "Super Barrier" range from Diodes, Inc.  The forward voltage drop is considerably less than any other diode I encountered, so combined with thick 30A copper cabling, should help to minimise losses.  The particular model is the SBR10U40CT - a 2x5A rectifier in TO220 format.

Technically it is a dual diode, but as far as I can tell, they are fabricated on the same die and thermally connected so providing the resistances are well matched, should be okay to wire in parallel.  I wouldn't recommend running them up to 10A in this configuration but it should be plenty good enough for 5A continuous, I'd imagine.

A simple means of mounting them was to use the mounting hole for the toroid and drill/cut out a piece of stripboard to mount them on.  These things run very cool - not surprising at all given their very high efficiency.  You do have to wonder why they even bothered putting them in a TO220 package as it's kind of self-defeating... the heatsinking is so good that they don't get warm enough to give anywhere near their best voltage drop performance.

The thing about diodes is that the voltage drop tends to decrease as the temperature goes up - so you'll get the worst performance Vf-wise at room temperature or less... at 85C, you'll get a considerably lower drop.  On the minus side, the leakage is much higher and the devices lifespan will be impeded.  Swings and roundabouts.

In any case, when I get around to buying one, cutting off the heatsink tabs with a high speed disc will probably help save a fraction of a volt - before doing this, will need to check the part temperatures in a closed box after sustained use as they may well be getting much warmer now!

Putting it all together... in goes the new DC/DC converter board, face up.  Plenty of mylar tape to prevent shorts with the bottom of the case, lots of insulation on the mains wiring.  There's a switch on the front to give the option of disconnecting the PicoUPS from the DC/DC converter, just for when you *really* want to cut the power.

So does it work?  Indeed, yes!  Voltage to the battery with the NAS powered down is about 13.9V, within acceptable limits.  When the NAS is operational, voltage to the battery decreases to about 13.5V, which should just about enough to float charge it but probably marginal.

Switching off the mains seems to give a seamless switch to the battery, and it seems to be able to power the router, cable modem and NAS with no problems... I think it would have a tougher time starting them from battery, but definitely okay for switchover!

Efficiency seems to be about the same as before.  The new gigabit router uses about a watt more than the old Linksys, so now up to about 8W with the NAS switched off.  Not bad, though, given that now includes trickle charging the battery.

I should note that I haven't put in undervoltage protection - this was present on the old DC/DC converter board but didn't have a chance to figure out a good solution for the LM3150... this essentially means that in an extreme situation, the battery would discharge to the point of damage.  However, this would be a very extended run on the battery (without NAS, I figure at least 4 hours) and the equipment attached is likely to stop drawing much current before things get really bad for the battery.  Hopefully the desulphator won't get called into action too often!

12V Power supply gets a revamp, part 1...

Doesn't time fly... I have been doing stuff at home... just not as much as I'd like!

I decided it was time to get a NAS and get a proper storage solution sorted out for 2013... so ended up going for one of the Synology boxes in January.  I wanted highish performance (>50MB/sec) and lowish power consumption (<10W for the CPU idle) in the same box... I was hoping to hold out for the Atom SoC based stuff but it seemed to be taking forever to materialise.  Of course, now they have been announced.

While it is relatively power efficient (I measure it at about 9W before the HD spins up, at which point it sits at around 14W) for the speed of it, I was concerned out what would happen in the advent of a power cut.  They are not renowned for coping well in such a scenario.

So a UPS seemed like a good idea.  An off the shelf inverter-style one would be not very efficient, so wanted to go for something that could output 12V cleanly and easily.  Something that uses a cheap 12V lead acid battery and is designed for efficiency.

The first approach was to try modifying the switching supply that the Synology came with to output 13.8V.  It's a standard block power supply, efficiency rating V so not bad at all in terms of waste power.

After finally breaking my way into the case (they really don't want you to open these things!) and removing some extra metal shielding, I was greeted with this...

 

I took a look at the electrolytic caps inside and didn't like what I saw.  These might be a perfectly reputable brand (indeed, I was able to find a datasheet) as far as Korean caps go but felt more comfortable replacing them with more familiar Panasonic units of a 25V rating... particularly as I intended to up the voltage output.

 

A look at the underside reveals that the supply is based on an LD7578 PWM controller.  The output section is based around a neat UM603 which contains a dual op-amp and a 2.5V reference wired to one of them.  Logically, there had to be a voltage divider near the chip... indeed there was.  As an added bonus there was an unpopulated part which had pins in parallel with the resistor in question, so I could do the mod quite neatly - you'll see a diagonally placed resistor near the main chip in the picture below.

 I now had a perfect 13.8V output.  The problem was that the supply was now out of kilter.  While it will happily provide 13.8V at very light load, any significant loading of the supply was triggering protection circuitry and the supply would go into "hiccup" mode, which was audible through the iron.

I'd had enough of this hassle before with switched mode supplies, so decided to go back to the original 12V supply design with a linear first stage, though I went a bit over the top...

Building a better 12V supply

I've recently been on a bit of an efficiency drive, replacing more light bulbs with LEDs in an attempt to reduce the ever-growing electricity bill, and it occurred to me that one of the most important things to focus on is appliances which are on 24/7.

We're generally pretty good at switching stuff off, as I've put "kill switches" on all the main groups of stuff like the telly and computers, but the cable modem and router tend to be left on all the time.  I'd never bothered to check what kind of power they were using, but saw that they were linear-style "wall warts" and as a result probably not very efficient.

Measuring the two together with the little "Energy Monitor" block brought a figure of 12W (16VA, 0.61PF)... which isn't awful although could probably be a lot better.  As they both use 12V,  I thought I'd try a few of the switching supplies I had lying around to see if any could do better, only to find they had a horrid power factor and were not much better at around 9W.

So I decided it was time to have a go at building my own.  A fully switched supply can be efficient and operate at a near 1.0 Power Factor with suitable correction, but they are not commonly (yet) efficient at lowish powers, as the cheap adapters I tried demonstrated, though are likely to become so in the next few years.  A good compromise is to use a fat toroid (a house staple!) and high efficiency rectifier bridge followed by a DC/DC converter.

Looking on eBay, I chanced across this unit here.  It's a wide input range (16-40V) buck converter board with good efficiency that looked ideal for the job... so for less than eleven quid, I bought one.  When it arrived, I was slightly disappointed at the lack of the shown Oscons but nevertheless worked fine when I added a wodge of input decoupling.  With a test load of 726mA and trimmed to 12V, I calculated an efficiency of 96.4% for the unit for an input of around 17V, which is very impressive!

The unit is based around a TI TPS40057 synchronous buck controller driving a pair of Toshiba TPCA8016 60V 25A N MOSFETs... good quality parts... you'd struggle to build it yourself for the UK parts cost alone.

Building the supply

So with the converter in place, I needed to pull the other items together.  Main thing I was missing was a case, which was dutifully purchased so it could be put into something!  First thing to be installed was the toroid which is an encapsulated 2x6V 50VA unit... ideally go as big as you reasonably can for this kind of thing as power as wasted in the copper resistance, but for the low drain on the supply, this wasn't a major issue.

For the rectifier bridge, I went with a favourite choice of mine, NXP PMEG3050 - these are very efficient, very small 30V 5A rated Schottky diodes which have a commendably low voltage drop at an amp or two.  They were mounted to a little piece of vero board using copper tape, and screwed to the top.

As I wanted the unit to be able to power at least four 12V items, I went for four sockets on the back, all individually polyfused.  The appropriate LEDs on the front will dim if a fuse trips, indicating a problem - there are far more sophisticated things you can do but this would be enough to stop one device turning off the whole lot.  Even though the LEDs only run at ~1mA, they are plenty bright!

I added a few ferrite clamps to suppress some of the inevitable switching noise, and with some additional decoupling I was pretty much there, short of a few hours with the hand drill cutting out the holes!

And ended up with this...

The module looks so small in comparison to everything else!  In retrospect, I really should have put the toroid further over to the right as it would have freed up some more space for decoupling but too much on the input will hurt the power factor.

Here it is in situ, next to the cable modem and router..

The LED lights are a lot brighter than it looks here!  I think blue LEDs are a bit noughties now, but I had a bunch doing nothing, so why not... to prevent them from being eye-piercingly bright and to not stick out, they were sawn down and sanded to near-flat to give a more diffuse effect. 

Any good?

So... the point was to save some power - did all this work pay off?  Well, the consumption of the new 12V supply running the same gear came out at 7W (9VA, 0.76PF) which is a saving of 5 watts.  Not bad!  I could have increased the PF to around 0.8 by fitting a choke but couldn't find a satisfactory way to mount it... maybe at a later point.

Bigger gains will be had when more equipment is added to the supply - in particular a NAS is likely to feature at some point... have yet to decide which one best suits our requirements although the Silverstone DC01 is looking strong for good performance and low power consumption.

Probably time to consider a Gb router too... :)