Zero Forward-Voltage Diode, Would this work? - Page 1
Forum section: electronics / beginners

author: bruce965
author_rank: Newbie
timestamp: September 01, 2016, 01:58:29 pm
content: Hello, everyone!I was thinking that a circuit like this should behave like a zero forward-voltage diode, but I'm a total newbie in electronics. I might be missing something essential.The idea is to use an op-amp to drive a (normally close) solid-state relay.VO14642AT Datasheet:http://www.mouser.com/ds/2/427/vo14642a-279692.pdfWhat do you think, would a circuit like this make sense? Could it work?

author: tszaboo
author_rank: Super Contributor
timestamp: September 01, 2016, 02:07:41 pm
content: What is powering the opamp? If you have a secondary supply, then still, it would not be a diode, because it would conduct if the voltage is positive, not when the input voltage is higher than the output voltage.If you want an almost ideal diode, check Linear technology's products for that. Like LTC4357.

author: klr5205
author_rank: Regular Contributor
timestamp: September 01, 2016, 02:35:00 pm
content: Also, depending on your current sourcing needs, google "super diode" or "op amp precision rectifier".Even if it doesn't fit your requirements for this project, its another building block for your bag of tricks.

author: CJay
author_rank: Super Contributor
timestamp: September 01, 2016, 02:40:39 pm
content: What you need to look at is synchronous rectification or active diodes, it's a technique used to minimise diode drop in power rectifiers, there's lots to be gained from using it if you're dealing with lots of amps.https://en.wikipedia.org/wiki/Active_rectification

author: bruce965
author_rank: Newbie
timestamp: September 01, 2016, 05:13:42 pm
content: Quote from: NANDBlog on September 01, 2016, 02:07:41 pmRight, I was thinking about this as a block where load would always be pulling down... so yeah, I should have linked inverting input to the relay's output, sorry.The LTC4357 seems to behave exactly like what I was thinking. I have seen other similar products as well, but I noticed that the price is higher than an op-amp + a solid state relay with the same range. Odd.Quote from: klr5205 on September 01, 2016, 02:35:00 pmCool, I'm actually interested in power and not signals with this experiment, but I like this configuration. I will remember it when the day will come!Quote from: CJay on September 01, 2016, 02:40:39 pmYeah, there is a lot to read on this topic... I have seen this kind of devices before as replacements for solar panel bypass diodes. To be honest I didn't investigate on the inner working, I had no idea they used a MOSFET, but indeed there is much in common with the circuit I posted. This is definitely a better solution.So from what I understand from CJay's Wikipedia link: this circuit cound work.Really thanks a lot for replying!

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2016, 05:03:25 am
content: That is about it. For solar and many other applications only one MOSFET is needed. The comparator can be replaced with a pair of bipolar transistors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2016, 07:45:48 pm
content: Note that the reaction time is ponderously slow, barely enough to handle mains frequency rectification... those photo-MOS SSRs are quite slow indeed!There are always two costs to a diode: the DC characteristics of forward drop and reverse leakage, and the dynamic characteristics of forward/reverse recovery.You can have an extremely fast diode (no recovery), with low voltage drop, but you'll pay for it in reverse leakage and low breakdown voltage: schottky diodes.You can have a low voltage drop (a "precision" rectifier), but you'll pay for it in recovery time, or auxiliary power (both as shown above!).Note that a comparator takes time to decide, approximately inversely proportional to the magnitude of the input signal (plus a built-in minimum delay). This matters for very small inputs, on the order of a few mV. Most comparators go equally fast when the input changes by >50mV, whether it's 100mV or 10V: that's the minimum delay. For smaller signals, though, it takes extra time to... "think about it".When using op-amps only (in a precision rectifier for signal purposes), the delay manifests as the amount of time required for the output to slew from saturation (or from one diode, if using the diode-wraparound-to-neg-input trick) up to the output voltage, plus a diode drop. An op-amp is more fundamentally an integrator, so that the output won't simply transition instantly: it always follows a slope, and that slope's maximum rate is limited by the input stage (which tops out when the input is over about +/-50mV).Even very slow comparators or op-amps are faster than MOS SSRs, though. The SSRs turn on and off in milliseconds; comparators in microseconds.What's worst is, because the comparator ideally switches at zero, it can only turn off the SSR once the voltage has fully reversed: and therefore, you're drawing negative current. Even if the SSR were instantaneous, the comparator still takes time to decide, and therefore limits your turn-off time.This isn't meant to be an overly elaborate and harsh criticism of your circuit -- just to say that, there are many places where you can lose, and if you think about conventional diodes as having these elements inside them (how does an ordinary silicon P-N junction know to slew like a comparator?!), you should realize, it'sdamned hardto beat them at their game! You can very easily come up with a circuit that wins in some respects (DC is easy!), and loses greatly in others (recovery time, voltage/current range, auxiliary power requirements..).Active rectification is definitely a thing, today, but it's largely special-case: switching converters, where they can anticipate delays by generating a drive signal in advance; or low frequency rectifiers where the control circuitry doesn't need to draw much power (likehttp://www.linear.com/docs/43273). With all our technology today, it's simply not possible to beat a UF4007 at its job!Tim

author: bruce965
author_rank: Newbie
timestamp: September 05, 2016, 10:44:28 am
content: Terrific. I couldn't ask for a better explanation. I didn't consider all these factors.Thank you very much for the precious lesson!