N-Channel High Side driver - Page 1
Forum section: electronics / general

author: kevyk
author_rank: Contributor
timestamp: March 27, 2025, 11:51:31 pm
content: Hi AllCan anyone recommend a part that will drive an N-Channel MOSFET (the IRLML6244) as a high side switch. I am using a 3.3v supply and a 3.3v 1Mhz PWM signal. I'm trying to switch 4.2v which is the catchy part...I found the MIC5019 with perfect DC specs but it is a very low frequency driver. I can't find anything that seems like it would work.Do I have to roll my own? Or is there another way to efficiently switch 4.2v using a 3.3v PWM?Thanks for your help!

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 28, 2025, 11:06:44 am
content: Quote from: kevyk on March 27, 2025, 11:51:31 pmThere's certainly a way. But to make a good recommendation we need to know what circuitry is on the load side.At the worst you're looking at a fully isolated gate driver with its own isolated power supply, but depending on the load you may be able to use something much similar.

author: kevyk
author_rank: Contributor
timestamp: March 28, 2025, 01:18:25 pm
content: Thanks for the thoughts. I thought about a separate power supply but would like to keep this simple. I’m surprised there isn’t a single chip solution.Why would knowing the load help? My load is a 10ohm current shunt followed by an ideal diode followed by a LiPo, so essentially a LiPo. But isn’t a load just a load, or perhaps not so in this case?And if a separate power supply were used, why would it need to be isolated? Couldn’t there be a common ground?Thank you!

author: Peabody
author_rank: Super Contributor
timestamp: March 28, 2025, 04:39:55 pm
content: Can you clarify what leads to the need for 1MHz PWM to charge a battery?

author: kevyk
author_rank: Contributor
timestamp: March 28, 2025, 04:50:42 pm
content: Sure. This is an optimization to an energy harvesting application where I need to modulate the current draw on the input load. So I use a PWM to minimize the load seen by the input and not sag it down.

author: Peabody
author_rank: Super Contributor
timestamp: March 28, 2025, 05:02:15 pm
content: Thanks. Well I'm above my pay grade here, but just wondered why it needs to be so fast.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: March 28, 2025, 05:04:30 pm
content: Any reason why you settled for the 1MHz and not something lower, say 10KHz? Increasing the frequency will increase the losses, and you will need more powerful drivers to actually drive the FETs. If its just a matter of PWM, the actual switching frequency doesn't matter as much as the ratio of the on to off time.I would recommend a charge pump circuit to generate a higher voltage for the gate driving, but again using PFETs for this would be better, the gate driver circuitry will without a doubt consume a lot more than you will save by using an N-channel MOSFET

author: kevyk
author_rank: Contributor
timestamp: March 28, 2025, 06:11:47 pm
content: Quote from: BadeBhaiya on March 28, 2025, 05:04:30 pmYes, I noticed my LC LPF had a little too much ripple at 100Khz. I up'd to 1Mhz which resolved that, but, as you note, at the expense of more losses which I didn't think about 🙁Quote from: BadeBhaiya on March 28, 2025, 05:04:30 pmAlso didn't think that the gate drive would end up eating more power than just using a PFET. I choose a NFET due to its low rdsON.Perhaps I can do away with a gate driver and try to find a low rdsON PFET. Any recommendations?Thanks everyone for your kind thoughts!

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 28, 2025, 11:21:02 pm
content: What keeps you from using a P-Fet?

author: kevyk
author_rank: Contributor
timestamp: March 29, 2025, 12:34:45 am
content: Quote from: DavidAlfa on March 28, 2025, 11:21:02 pmI thought PFET's had a much higher Rds(on). Turns out that's not the case! There are PFET's down to 1 milliohm!Their switching time, however is not as fast as N-Channels so there are some losses during switching. That said, I am looking into them.Thanks!

author: temperance
author_rank: Super Contributor
timestamp: March 29, 2025, 12:42:16 am
content: QuoteI wonder about the same and with every piece of information added, an N-CH device seems like a bad idea. But I might be completely wrong.We've been fed some bits and pieces of the puzzle. LC low pass filter, 10 R resistors, LiPo battery... On resistance must be low, hence the N-CH MOSFET.Maybe you want to clarify the problem so people can better understand what you are after.

author: temperance
author_rank: Super Contributor
timestamp: March 29, 2025, 12:50:24 am
content: QuoteThose 1mOhm devices aren't slower. The speed at which a MOSFET switches is controlled by the gate current. As such the data sheet specifies gate drive conditions for the stated switching speeds.Edit:https://www.ti.com/lit/ml/slua618a/slua618a.pdf?ts=1743131086574

author: kevyk
author_rank: Contributor
timestamp: March 29, 2025, 12:53:53 am
content: Quote from: temperance on March 29, 2025, 12:42:16 amDetails are mentioned above.Thanks.

author: temperance
author_rank: Super Contributor
timestamp: March 29, 2025, 01:13:50 am
content: QuoteNot entirely. From what you wrote I understand that you have a buck regulator. Putting all you posted together:Buck regulator input: 4.2 V maximum.Load: 10 RInductor: unknown. Do you know you can reduce the ripple by changing the inductor instead of increasing the switching frequency?Output capacitor: unknownEarlier you posted:QuoteYes it does help if you want the best performance possible without wasting power.The current supplied by the gate drive circuit equals Qg (MOSFET gate charge up to drive voltage) x switching frequency.To give you an idea.https://www.onsemi.com/pdf/datasheet/fdn304p-d.pdfFDN304P, 10 nC with Vdrive is 4 V would result in 1 mA supply current when driven at 100kHz.Some random 1 mOhm MOSFET. Probably about > 100 nC with Vdrive is 6 V would result in 10 mA when driven at 100 kHz and 100 mA at 1MHz.What you have to find is a balance between switching frequency, gate drive loss and the on resistance.Increasing the switching frequency at random isn't going to help and that's why people ask you valid questions. But your response boils down to:QuoteMy sincere apology if you find me to lazy to go and collect all the details myself.

author: kevyk
author_rank: Contributor
timestamp: March 29, 2025, 01:34:26 am
content: Quote from: temperance on March 29, 2025, 01:13:50 amAh, ok, thanks. I didn't realize you were drilling down to the specifics at that level:You are correct with the regulator outputting 4.2v, the load is about 10Ω, the LPF inductor is 4.7µH and cap is 22µF || 100nF. I do know I can change LC to lower the Fc. I am using SMT parts so can't get too high with the cap.Does that paint a little clearer picture?Quote from: temperance on March 29, 2025, 01:13:50 amAll new to me. Thanks for the details! Very helpful.I'll try to lower the LPF Fc to optimize the gate current (maybe a few hundred Khz?). Right now I'm landing on thehttps://www.mouser.com/datasheet/2/115/DIOD_S_A0009691884_1-2543442.pdfwhich has a Qg of 26nC which seems pretty reasonable. It also has an Rds(on) of 11mΩ which also seems pretty good.What do you think about this part given the above details?Thanks again!

author: temperance
author_rank: Super Contributor
timestamp: March 29, 2025, 02:26:44 am
content: With 4.7 µH and a switching freq around 100k, the regulator is probably working in discontinuous mode. In this mode, the inductor current becomes zero during every switching cycle and the output capacitor must supply the output current while the inductor current is zero.This results in very high peak current in the switching devices and a correspondingly high RMS current and high losses in the MOSFET (switching losses and on resistance losses).Optimizing this circuit:1. increase the inductance such that the inductor ripple current becomes reasonable and doesn't become zero.This might help:http://schmidt-walter-schaltnetzteile.de/smps_e/abw_smps_e.html2. With the inductor current ripple within reasonable limits you can select a MOSFET with a reasonable on resistance by optimizing on resistance loss and gate drive lossA reference containing all you need:https://www.ti.com/lit/an/slyt664/slyt664.pdf?ts=1743155931158With some effort, you can find an optimum selection for the inductor and the MOSFET.Driving a P-CH MOSFET is in this case easier and probably more efficient.The input voltage is limited to < 5 V. For this low voltage you will have some difficulty finding MOSFET drivers because they often contain an under voltage lockout and require a supply voltage > 7 V. An easy solution could be a 74HCT14 hex inverter with all 6 gates wired in parallel to obtain a high current buffer. The static output resistance of one gate is about 25 Ohm. With all outputs wired in parallel the inverters can drive a MOSFET directly.

author: kevyk
author_rank: Contributor
timestamp: March 29, 2025, 03:54:33 am
content: Quote from: temperance on March 29, 2025, 02:26:44 amWell Temperance, it does seem like you've done this before. This is my 1st...My ultimate goal is an energy harvesting application where I modulate the load impedance the LiPo presents by way of a PWM. During the off cycles, the output does indeed draw from the caps to keep current flowing into the LiPo. This is by design because I am trying not to load down the Vin on the boost converter. If the Vin on the boost is attached to a very high impedance low voltage input, I don't want to sag that too much. So pulsing this input with a low duty cycle seems to work and not sag the boost converters Vin input. The transfer currents throughout the whole chain are only on the order of several mA's to perhaps a few hundred mA's.If that makes sense...Thanks again (again)!

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 29, 2025, 12:25:48 pm
content: Quote from: kevyk on March 28, 2025, 01:18:25 pmGenerally what you want is a circuit which takes a GND-refernced 0-3.3V logic input and produces an output with some amplitude that is referenced to the source of the FET (which is not GND). This fundamentally means that the driver (and its DC power source) cannot share a common ground with the input.One very simple approach to this is with a bootstrapping gate driver. But this approach requires that the source node return to near GND when the FET turns off. This is why the load circuitry is important, that determines whether convenient approaches like bootstrapping drivers will work for you.Quote from: kevyk on March 28, 2025, 06:11:47 pmOk so the load is a 10 ohm resistor, a diode, and a LiPo cell in series...?Quote from: kevyk on March 28, 2025, 06:11:47 pmOk and now there's a LC filter.... somewhere?

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 29, 2025, 12:34:39 pm
content: Quote from: kevyk on March 29, 2025, 03:54:33 amOk so you have an energy-harvesting boost converter, which you want to use to charge a LiPo cell. But you are adding a buck stage in between the two in order to regulate how much power is delivered to the LiPo cell?The boost converter alone should be able to do that. I don't see the point in the additional DCDC stage, unless the boost converter is also providing power to other loads, and you want to prioritize power to those loads over power to the LiPo cell. Is that the case?

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 29, 2025, 01:44:17 pm
content: Quote from: kevyk on March 29, 2025, 12:34:45 amQuote from: kevyk on March 27, 2025, 11:51:31 pmWell, 1MHz is too much for almost every mosfet in earth!Also I doubt the efficiency of the system will be any good at such frequency...Quote from: kevyk on March 29, 2025, 03:54:33 amI' use a buck converter, taking VIN to limit the feedback loop, so when VIN drops the regulator will lower the output, unloading the input.It'll probably be better with a current-mode regulator.

author: Peabody
author_rank: Super Contributor
timestamp: March 29, 2025, 02:03:04 pm
content: Quote from: DavidAlfa on March 29, 2025, 01:44:17 pmThis is confusing to me. The MT3608 boost chip datasheet says it oscillates at a fixed frequency of 1.2MHz. It says the SW output is the drain of the internal mosfet. And as boost converters go it seems to be pretty efficient. Can I get some clarification on this?Also, for us hobbyists trying to follow the thread, is there any chance that someone could post a schematic of what the OP is doing?

author: temperance
author_rank: Super Contributor
timestamp: March 29, 2025, 02:06:02 pm
content: Can you share the schematic? That would help a lot.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 29, 2025, 02:45:17 pm
content: Quote from: Peabody on March 29, 2025, 02:03:04 pmBecause it's a dedicated device, designed to be efficient, everything is contained in a little chip, thus it can work very fast, externally only the inductor receives and filters these 1MHz pulses.However you won't see 1MHz single-phase regulator using external mosfets, 1MHz is reaching RF and you'll find plenty of issues with capacitances, inductances, resonance phenomenoms... will make it very impractical and complex, also discrete power fets larger and harder to switch so quickly, they're rarely used go over 100KHz in power applications.For very high power applications which require quick response (I.E. computing, where the systems have very fast power transients), multi-phase regulators are used, it's the same but with 2,4...12 outputs working in sucession, let's say each stage works at ~100KHz, but with a small phase offset, so the power "pulses" are overlapped, working like a 200,400KHz...1.2MHz system.

author: Peabody
author_rank: Super Contributor
timestamp: March 29, 2025, 02:56:56 pm
content: Thanks very much for the explanation. Now if we can get some idea of what this is all about - harvesting from what?

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 29, 2025, 03:53:47 pm
content: Probably some renewable source like solar panels, windmill...

author: kevyk
author_rank: Contributor
timestamp: March 29, 2025, 09:48:36 pm
content: Thanks everyone for the feedback and input.Quote from: Peabody on March 29, 2025, 02:56:56 pmThis test hardware is tapping off small solar panels without loading them down, thus the need for dynamic input current regulation. But I want to do this without using the IC's that are dedicated for this purpose that have built in MPPT. MPPT, btw seems is more for dynamically changing the input impedance (what I want to do) based on the amount of light shining on the solar cell - a little or a lot (not what I'm looking to do).I think I have enough info to work from now. Learned a lot about DC converter switching frequencies and efficiencies.Thanks to all who provided knowledge! You guys (& gals?) ROCK!

author: Peabody
author_rank: Super Contributor
timestamp: March 30, 2025, 02:44:53 pm
content: So it's a single-cell solar charger?Well for consideration by hobbyists reading this in the future, I'd just like to put in a plug for a linear alternative. A linear charger will supply as much current as it can at or slightly above the battery voltage. The panel won't be drawn down below that because no charge current will flow below the battery voltage. Of course a switching MPPT charger will do better, particularly in bad light, but it has to be enough better to overcome the 15-20% switching losses, and then enough better after that to make any material difference.TI makes several linear charging ICs that are almost too small to be viewed by the naked eye, so not particularly breadboard-friendly. But Adafruit makes modules featuring those chips. Or if you want to roll your own, the circuit shown below works quite well. No inductors in any of these.I say for the benefit of hobbyists because it seems many engineers wouldn't be caught dead designing a linear charger. For big auto/marine stuff, I understand that MPPT is substantially better, but for small low-current single-cell electronics projects I believe that a linear charger will perform very nearly as well, with less cost and complexity.

author: TSOL
author_rank: Contributor
timestamp: April 17, 2025, 06:39:25 pm
content: Quote from: Peabody on March 30, 2025, 02:44:53 pmVery true. Linear has terrible efficiency which isn't a good match with the low power output that hobbyist solar typically provides.Quote from: kevyk on March 29, 2025, 01:34:26 amI still can't tell what you're doing with this circuit with all the data you have provided. Are you trying to *charge* a battery? Temperance said 4.2V input, which is not 4.2V output.If you are trying to use a solar panel to charge a battery, a switching converter is a good idea. Those switching frequencies in the Megahertz region will accrue losses quickly, but GaN transistors exist to do switching well into the double digit megahertz region. Power GaN devices hardly draw any current when switching in comparison to silicon devices, at least. However, as David Alfa said,Quote from: DavidAlfa on March 29, 2025, 02:45:17 pmParasitics will make this job very difficult very quickly, and at the skill level that you are portraying yourself to be at, no offense but it is much better to start much slower.Switching at 30khz-50khz with a 100uH inductor will cause your current to run in "continuous mode" where it doesn't drop to 0 right away. Your inductor is so small that it is little more than a short circuit at low frequencies. You can have an output capacitance of whatever you want, but I'd probably go something in the 100uF to 630uF range. Throw some ceramics in parallel too (looks like you've done that in your current setup, great!). If you are making a buck converter like it seems you would be (to regulate ~20V at the panel down to 2.8-4.2), you can use a buck controller IC to monitor the current through the inductor and switch accordingly. There are some great ICs out there from Texas instruments et. al., that can get the job done well, or you can try to design the control circuit yourself, but this will be VERY difficult, and for the time required to learn, 2$ is not so expensive for an IC.Whatever IC you do choose will need to do current control for that to work, just a heads up.

author: mzzj
author_rank: Super Contributor
timestamp: April 18, 2025, 06:25:31 pm
content: Quote from: DavidAlfa on March 29, 2025, 02:45:17 pmWhile certainly not best options for beginners 1Mhz switching regulators are nothing exotic today.MIC2168 1Mhz external fet single-phaseLTC3833 200khz to 2MhzLTC7831 up to 3Mhz (GAN fet special)