Power mosfet to drive high side of the load. Does it exist? - Page 1
Forum section: electronics / beginners

author: autotel
author_rank: Contributor
timestamp: June 26, 2019, 07:59:07 am
content: In shortI did a simulation in the Falstad circuit simulator. I know that components there might be ideal, but I am looking for the closest version of it. The link is at the bottom. Basically it would be a MOSFET that gets triggered with ~1 volt in comparison to the source, and can source high voltage from the drain.The context is this: I want to drive the high side of an inductive load (different motors & solenoids) using a microcontroller. I want to make it compatible with a wide range of loads, and I want to use few, easily found parts.Side notesI noted that by having a higher "beta" value, the transistor in the simulation let's more current. I guess that beta is the slope "angle" of the Vgs/Ids line in the graph. I could find some N type power Mosfets with these characteristics. The thing that doesn't fit in my mind, is that a TIP120, which is what I was using before, did not work as a high-side switch, hence, the tip120 has something different than the transistor in the simulation.Additionally, I visited Gammon's blog, and he shows how to drive the high side of a load, and it requires many components beside the N type MOSFET. I find the same being said in many other places. However, I am pretty sure there have to be components that integrate all the part Gammon is suggesting to add as a high-side switch, in an enclosed package. Furthermore, according to various electronic books/ articles, the idea of a transistor is to let power flow between drain and source, when gate has a voltage.Should I keep looking for generic parts (and which?) or should I go for a proprietary load controller, such as those from Pololu?Link(I didn't shorten it so that it doesn't break later)http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.20027730826997+50+5+50%0Af+304+240+352+240+32+1.5+1000%0A172+224+240+192+240+0+7+2+20+0+0+0.5+Gate+Voltage%0Aw+352+256+352+304+0%0Aw+352+224+352+176+1%0A172+352+176+352+144+0+7+12+12+0+0+0.5+Drain+Voltage%0Ag+352+384+352+400+0%0Al+272+304+272+384+0+1+0%0As+224+240+304+240+0+0+false%0Ac+480+304+480+384+0+1+0.2029133990689438%0Ad+400+384+400+304+2+default%0Aw+352+384+400+384+0%0Aw+400+304+352+304+0%0Ar+352+304+352+384+0+0.2%0Ao+0+64+0+4099+20+3.2+0+2+0+3%0Ao+6+64+0+4097+5+0.00009765625+1+2+6+3%0A38+0+1+1+1000+%3Ca%5Cshref%5Cq%22mosfet-beta.html%22%5Cstarget%5Cq%22_blank%22%3EBeta%3C/a%3E%0A

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 26, 2019, 08:40:41 am
content: Look for high side mosfet driver ic, simplify alot of component esp charge pump. or just use p channel mosfet

author: garethw
author_rank: Regular Contributor
timestamp: June 26, 2019, 08:50:59 am
content: P-Channel MOSFETs are easier to implement but typically have higher RDSon values which basically means they have a higher resistance when fully conducting.Using a charge pump design enables the use of more efficient N-Channel devices but requires more components.Sent from my iPhone using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2019, 08:59:26 am
content: Quote from: autotel on June 26, 2019, 07:59:07 amThat circuit is a source follower. The voltage on the output will always be lower than the gate, by the threshold voltage.As mentioned above, the gate voltage needs to be boosted above the positive supply voltage to fully turn the transistor on. A P-channel device could be used but they have a much higher on resistance, with everything else being equal.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 26, 2019, 09:00:35 am
content: Yeah i heard that alot.. when nfet is say 14mohm, its complimentary pfet is 28mohm give or take. How bad can it be? Before mosfet age, people used power bjt which is alot more mediocre. But when that extra 14mohm matters esp in very high current app, then the best is nfet w hi side driver no doubt.

author: mc172
author_rank: Frequent Contributor
timestamp: June 26, 2019, 09:01:22 am
content: Quote from: garethw on June 26, 2019, 08:50:59 amThis was true of the 90s but now you can get P-channel FETs with Rds(on) in the single milliOhms for not much money at all.The charge pump and N-ch combination will likely cost more than the P-ch device itself.

author: autotel
author_rank: Contributor
timestamp: June 26, 2019, 09:05:49 am
content: this means that the trigger voltage is always relative to the drain an never to the source? I think the most important was clearing that about the perfect transistor of the simulation. At the end it seems that the best way to go, is with an integrated circuit.

author: ledtester
author_rank: Super Contributor
timestamp: June 26, 2019, 03:31:30 pm
content: This video might help (link starts video at the 5:00 minute mark):

author: autotel
author_rank: Contributor
timestamp: June 27, 2019, 08:04:15 am
content: that one was useful!I found the boostrap circuit somewhere else, but I sort of got super overwhelmed; this video has a bit of a better explanation.

author: temperance
author_rank: Super Contributor
timestamp: June 27, 2019, 12:31:09 pm
content: Quote from: garethw on June 26, 2019, 08:50:59 amThis was true about 10 years ago. Modern P MOSFET's feature an extremely low on resistance. 1...10mOhm in small packages (DFN) up to 40V is not an issue. 60V...100V rated DPAK sized PFET's with an on resistance as low as 4mOhm and rated for 60A DC and 300A peak can be bought less than a euro in quantities. For a high side switch which doesn't require over current protection you drive those with a current sink (not an ordinary open collector driver) and they can be made to behave extremely well. (current drive solves problems with inductance in cables, specially parasitic inductance located in the source)

author: David Hess
author_rank: Super Contributor
timestamp: June 27, 2019, 03:53:11 pm
content: Quote from: mc172 on June 26, 2019, 09:01:22 amYou could always get p-channel power MOSFETs with low on resistance. The issue was and still is economics because to get the same on resistance as an n-channel power MOSFET, a larger die is required which makes the part more expensive. The larger die also has higher capacitance.

author: temperance
author_rank: Super Contributor
timestamp: June 27, 2019, 04:43:27 pm
content: Discussions about modern FET's, are not really going to help the OP. (I'm guilty too for getting into the debate about modern FET's)What do you want to drive?-Inductive, capacitive, resistive loads or just any type of load?-current rating?-voltage ratings?-How fast?-PWM capability?-You want to have diagnostics? Current sensing and what not?Depending on those answers you have a few options.-Resistive loads: any PFET+fuse driven by an open collector will work.-Capacitive/Inductive: this requires more effort and with you're limited knowledge you're better of buying an off the shelf high side switch. TI, Infineon, ST,... They come in single, dual, quad, octal,... With a lot of options. (over-voltage, over-temperature, over-current,...)Regards

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 27, 2019, 04:44:31 pm
content: Consider an integrated switch like TPS1H100. Basically no support parts needed. Very robust.You can make parts of one yourself, but it will take a fair number of components. To run an N-channel MOSFET at DC, you need at least some microamperes of supply above the input voltage -- because Vgs(on) is always gate-to-source, and the source pulls up to input voltage, so the gate voltage is above it, and so too your gate driver/controller.Easiest way to generate this, is a little oscillator like a 7555, driving a charge pump (basically a half-wave voltage doubler) stacked on top of the input voltage.Then you need a level shifter, to get the ground-referenced logic input translated up to the source-referenced voltage (which is near ground when off, and Vin when on). This is usually done with current sources (so the signal doesn't vary with voltage), preferably balanced current sources so that capacitance is canceled out. There need not be much gate drive (peak) current delivered -- the gate is a capacitor, and it switches only as fast as you can dis/charge that capacitor; but we aren't talking switching power supplies here, so the switching speed can be slower, and this also helps deal with capacitive loads by bringing them up slowly, and inductive loads by letting them down slowly.As mentioned, the extra voltage is only required for NMOS. PMOS, the source is referenced to Vin, and Vgs(on) hangs "below", so you don't need an extra supply. You still need the level shifter though. NMOS is preferred in general, because it has about 2.5 times better performance than PMOS -- if you just want a switch, it doesn't matter much, but if you need a switch that's faster, lower power, and lower cost, the NMOS is preferred.So this gives you a switch, but it has no protection whatsoever. The slightest short circuit and, kaboom! Transistors die in say 1/10,000 of a second. Fuses are no use: they blow in 1/100 of a second or thereabouts.Current limiting can be provided by adding a sense resistor, and a transistor that pulls down the gate voltage when source current rises too high. Downside, this has a relatively high voltage drop (0.6V at the onset of limiting, with a single BJT sense circuit). Current limiting isn't all that useful anyway, because trying to switch, say, a 20V supply with 1A current limit and a short-circuited load, that's 20W continuous you're dissipating; tolerable with a TO-220 and heatsink, but that's a lot of bulk, and anything without a heatsink will quickly burn up at this power level.So you might consider adding thermal limiting, but mind that this acts very slowly (maybe 1/10 of a second, at best), so you can't handle too much power dissipation -- this isn't a scalable solution. Say if you want to handle 60V and 30A with a single switch -- easily within the switching rating of a single transistor, but that's 1800W short circuit, enough to pop it in a 1/1000 of a second.Alternately, you might detect that current limiting is active, and start a timer; when the timer runs out, a flip-flop is toggled off, keeping the transistor off until the input is turned back on again. But this has a number of limitations.So the design of one of these things is actually quite involved, and you are getting an excellent deal from the, probably hundreds of transistors that are integrated into one of those protected switches! The main downside is that those integrated switches don't absorb much energy, and they aren't made very large; you can get 24V at 30A, but you aren't going to find 120V at 10A, or 400V at 30A, or more. (Or actually, much of anything above 80V I think? They're mostly for automotive purposes, so 30-60V ratings are super common. Or for USB port power, where 5-20V ratings are super common.)So, if you need higher voltages or currents, or higher dissipation (more short-circuit capability, say to charge bigger capacitors?), you're pretty much on your own. There are a few controller chips out there, so you can use bigger external transistors, but I'm not aware of any that deliver very high performance (especially high voltages).Tim

author: temperance
author_rank: Super Contributor
timestamp: June 27, 2019, 05:12:31 pm
content: An advantage of the TPS1H100 is it's accurate current sensing if you need that.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: June 27, 2019, 11:34:32 pm
content: Quote from: T3sl4co1l on June 27, 2019, 04:44:31 pmA while back I repaired some automotive equipment that used the InfineonBTS432E2. It seemed like a pretty decent part but there's lots of others in that category.

author: Psi
author_rank: Super Contributor
timestamp: June 28, 2019, 12:31:44 am
content: +1 for automotive high side driver.They have a lot going for them.- They are high side because cars are common ground- Rated for driving inductive/capacitive loads- Most use N-channel with tiny built in boost circuit to drive it so have good Rds on- Logic level digital control, so you don't have to worry about gate drive current.- They often have extra features that are useful like- current sense voltage output- over current shutdown,- over temp shutdown.- built in transient protection- built in reverse polarity protection- detection of open circuit load.Example VN5E010AHTR-Ehttps://www.st.com/content/ccc/resource/technical/document/datasheet/1e/52/ef/e0/cc/90/4e/2e/CD00240716.pdf/files/CD00240716.pdf/jcr:content/translations/en.CD00240716.pdfThere are lots of chips like that to look through.Check out digikey Product Index > Integrated Circuits (ICs) > PMIC - Power Distribution Switches, Load Drivers

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2019, 01:12:45 am
content: Low side protected switches are of course very common as well, for when you don't need high side switching. The simplest kind is a self-powered switch with thermal protection, and, not much else really. (Mind, since these are monolithic, theycanrespond quick enough, thermally -- within fractions of a millisecond. It's sensing externally that kills the response time, waiting for heat to conduct through whole mms of materials.)Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 28, 2019, 02:05:31 am
content: At my company, we manufacture products that go into vehicle applications. For the last decade, we must have sold many hundreds of thousands of products employing those devices.I can also recommend that you use them.

author: autotel
author_rank: Contributor
timestamp: June 28, 2019, 08:21:41 am
content: I feel that my question now is very thoroughly answered. Many thanks!I agree. After reading some of the posts, I decided that it'd become too expensive to make a "custom switch" and not worth it, since my chances of messing it up are high. In a way it makes sense. I know how to program but not too much about the electronics, hence I should think myself as a systems integrator more than as a electronics developer...I found some really nice options (here in Helsinki there are only so many components I can get on the same day without paying 20 eur. shipping!) such as the VN750PS-E or VN920B5-E, and those that you guys suggested me. They have amazing specs! Lots of protection against things I was not even dreaming.Then on the other hand, someone sent me these relay boards which you can buy for ~9 euros, and they come with 8 solenoids. It was a bit frustrating that in reality, whatever effort I did, it still would come out as more expensive; the integrated drivers are roughly 0.5 to 2 euro, even in quantities. I know that massive factory stuff like that might not be the best option, however, I am aiming at making the thing as repairable as possible, and relays are easier to find in general than ic load switches. Now I will discuss with the other people involved about the options.By the way, most my loads are inductive and I am using the diode against inverse current, of course. Most these switch devices have the protection against this stuff any ways.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2019, 08:37:07 am
content: Quote from: T3sl4co1l on June 28, 2019, 01:12:45 amI have always wished that Franklin had gotten polarity correct making positive ground the standard and therefor high side switches NPN or n-channel.