Everfine WY305 DC supply teardown (w/unusual power architecture) - Page 1
Forum section: electronics / general

author: D Straney
author_rank: Frequent Contributor
timestamp: June 10, 2019, 05:34:48 pm
content: Here's another candidate for the "was this really designed in this millennium?" award. I had the chance last week to open a large Everfine WY305 DC source. Everfine normally makes lighting-related test equipment (integrating spheres, flicker meters, etc.), but they've also got a limited range of power supplies as well.Interestingly, they advertise it as a 150W (50V, 5A)linearpower supply ontheir website. My first impression, after lifting it, is that it's seriously oversized for only 150W - it's both larger and about the same weight as the 1.2kW Chroma (almost 10x the output power!)which I took apart before.The underside of the top cover has dust filters over the vent holes, which is a nice touch. I hate seeing the top vents which just deposit dust all over the places where you really don't want it (and which no one ever cleans because even most engineers don't like maintaining things)....and here's the inside:There's some transformers at the top-left, a control board at the bottom-left, and a power section at the right consisting of a couple boards and an inductor.Let's take a closer look at those transformers:That's an unusual toroid-but-in-rounded-square-shape core, never seen anything like that before. Definitely 50/60 Hz ones, as they're fed directly from the AC input (which goes through a comically small EMI filter, compared to the rest of the components:)Anyways, from the wire routing my guess is that the larger bottom transformer feeds the power section, while the top transformer generates a few different voltages for the control board.Here's the control board:It's got some linear regulators at the left, a microcontroller in the middle, some analog (op-amps and analog switches, with part numbers conveniently on the silkscreen, plus what seem to be matched resistor arrays) at right probably for sensing and feedback, and a large hack-ish-looking 20 mOhm current shunt at top-right:The power section is where it gets interesting, though:It turns out that those big stud-mount devices are actually SCRs (according to both part number and refdes on silkscreen):That heatsinked device in between the two SCRs is a bridge rectifier:So where's the linear regulator part? Turns out that's on the other side of that massive heatsink next to the SCRs:There's 4 transistors paralleled here, with an empty spot for a 5th, and an additional one mounted further up on wires for some unknown reason.Two giant 10,000uF 80V aluminum caps sit on this board as well, one on the output of the SCR stage, and the other on the output of the linear regulator:You can also see from that photo that all the power connections are made to and from that board with wire-to-board spring terminals (like screw terminals but you push a button to release the wire), which has actually got to be great for assembly and servicing.Let's also not forget to give that giant inductor in the power section its 15 minutes of fame:So based on the SCRs and inductor, plus following a few traces and doing a few continuity checks, here's what I think is going on. The SCRs + inductor seem to be a pre-regulator, which steps down the line voltage by chopping the AC line waveform (similar to a phase-cut dimmer; Wikipediahas a good scope shot) and then filtering it heavily with a large inductor and output capacitor to remove most of the 120 Hz ripple. It actually works a little differently than a phase-cut lamp dimmer because if you use a large-enough inductor that it keeps the output current pretty constant, then the SCRs keep conducting through the zero-crossing of the AC line, and each SCR turns off only when the opposite SCR turns on. Here's the normal circuit arrangement for that, and some waveforms to show what I mean:In this case, the unfiltered output voltage "Vrect" averages toless thanit would with no SCR control, because of the way that the SCRs intentionally conduct partway into the opposite half-cycle and get some negative voltage in there. With this kind of regulator, you can vary the output from full positive (if you swap SCRs right at the zero-crossings) to 0V (if you swap SCRs right at the peaks of the line, so that "Vrect" is half-positive and half-negative) to full negative (if you turn on each SCR only during its negative-output half-cycle) where the supply is actually pushing power back to the AC line. I couldn't find a good diagram online to illustrate this, butthis imageshows it pretty well for a 3-phase system.However, in the WY305, the secondary winding on the power transformer isn't center-tapped, and there's also not a 3rd and 4th SCR which you'd need to make a full-wave rectifier, but there is a "normal" uncontrolled-diode bridge rectifier. I think they used a clever scheme where they avoided the extra cost, size, and control difficulties of using an extra 2 SCRs, while also avoiding the worse winding utilization of a center-tapped transformer secondary, by using half of the bridge rectifier for return currents like this:So essentially, we have a 60 Hz switch-mode step-down regulator at the input, so that the linear regulator's power dissipation stays manageable. The "phase-controlled rectifier" SCR pre-regulator is a pretty well-known circuit, but it's unusual to see it in something like this: it made much more sense either a long time ago (before 50 kHz+ switch-mode power supplies were so good/common) or in insanely-high-current industrial DC motor drives or electrochemistry applications where slow SCRs are still the only things that can handle the currents involved. It's just bizarre to me to see a circuit that usually turns up in 70's electronics or 100A+ applications being used in a pretty standard 150W bench supply where a pretty standard off-line flyback or half-bridge would work great. My guess is that the supply is designed this way either because (1) there's just one 70-year-old guy at Everfine who has been designing controlled-rectifier supplies his whole life and isn't about to change that now, or (2) they don't do much actual electronics R&D and this is one of those "standard" designs that has just been endlessly copied for decades because it works well enough as power supplies aren't their main business. The downsides are, obviously, some pretty serious weight and size for the power level (which can't be cheap either, between sheer amount of copper+steel and shipping) and pretty bad AC power factor if it's not near full-scale...but if you don't care about those things...Anyways, ramble over: here's what seems to be the isolated gate drive circuitry on the control board, to send turn-on pulses to the SCRs:After the main power supply, there's also an output board with a bunch of relays and power resistors of different sizes, including some messy-hand-wound-on-a-pencil-looking 0.2-ohm guys:One of them, a 100-ohm 50W monster, is large enough that it has to live off-board and stand up vertically:The value is definitely too large for a current sense (and it has the 20-mOhm shunt for that anyways), so it's probably either there to bleed down the bulk caps, or for some kind of weird series-current-limiting thing in constant-current mode.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 12, 2019, 01:12:27 pm
content: There's the German manufacturer "Heinzinger" (well known for their variety of power supplies) - they used this kind of SCR pre-regulator in a lot of their power supplies, ranging from 32V / 2A to whatever kilowatts. I believe their main reason was to reduce power dissipation in the linear pass transistors, and also reduce the number of pass transistors, as some of these supplies have a quite nasty behaviour: If you "short" the output, an internal SOA protection circuit kicks in and shuts down the output for a short time, allowing the pre-regulator to discharge. This is particularly irritating when testing circuits that drive the supply from CV to CC on purpose.Find some manuals and schematics here:https://cb.wunderkis.de/wk-pub/heinzinger/Anyway, a 10000uF electrolytic at the output side of the linear regulator looks pretty insane to me. The transition from CV to CC will take ages.

author: D Straney
author_rank: Frequent Contributor
timestamp: June 15, 2019, 08:02:29 pm
content: Interesting schematics, thanks. That's the kind of power supply where I would've expected to see an SCR pre-regulator like this - one from the 70's! (instead of 2011)Yeah, knowing now what I know, I wouldn't want to put any sensitive constant-current loads on the output of this! Seems like depending on the transient conditions they could easily get a huge blast of over-current before settling down... That 20 mOhm current sense resistor looks like it's after the 10,000uF output cap, but if the stored energy is there the control circuit can't do much about it even if it can sense the too-large current.For what it's worth, this supply is terrible in other ways too: the rotary encoder (for adjusting setpoint) sampling seems to be done badly, so that if you turn the knob at more than a half-asleep stoned snail's pace, it misses transitions and the setpoint starts moving the wrong direction at random rates.

author: tooki
author_rank: Super Contributor
timestamp: June 21, 2019, 11:01:55 am
content: Quote from: D Straney on June 10, 2019, 05:34:48 pmThat’s called an R-core transformer. Japanese electronics manufacturers like Sony use them in many higher-end audio products. For example, my Sony MDS-JB930 MiniDisc deck uses one, and my Tascam MH-40mkII headphone amp even uses two. I think they were trendy in the late 90s in particular.They basically match or exceed the performance of a toroidal transformer, but are also easier to make accurately and I guess weigh less, and they can be thinner than a regular EI transformer. In particular, they apparently are good at decoupling mains noise. I’ve seen conflicting claims about whether they radiate more or less mains hum than other transformer types.Here’s a few links about them:https://www.custommag.com/r-core-transformershttp://www.shilchar.com/electronics-telecom/r-core.aspx?pg=pr&sid=rchttp://jamestransformer.com/en/transformer/R_core_transformer.htmlHere’s a teardown: