High Side  Load Switch  Design - Page 1
Forum section: electronics / projects

author: juanfermed
author_rank: Regular Contributor
timestamp: June 15, 2015, 04:23:24 pm
content: Hi everybody,I am currently designing a 5V 3A and a 12V 5A power supply for a device that will go mass production, and I need to switch the output on/off electronically. So at first I decided to use a P-channel mosfet as the switch driven by a simple bjt transistor circuit. See attachment. The P-channel I selected is the IRF7726PbF in micro8 package (http://www.irf.com/product-info/datasheets/data/irf7726pbf.pdf), for the reasons that I list following. But I would like to have the input of the experienced people over here whether or not my selection was correct, because I have serious doubts. This is my very first professional design and reliability is a must, and despite the investigation and testing I have done, I would like to be 100% sure with your input.Power Dissipation, Rds ON:- The IRF7726 has a very low Rds_on: max of 40mOhm @ Vgs = 5V, Id = 3A, according to Fig. 13 in the datasheet (it is probably really around 37mOhm).max of 20mOhm @ Vgs = 12V, Id = 5A, according to Fig.13 in the datasheetCalculating power dissipation based on this specifications would be: For 5V, 3A source: Pd = (Id_max)^2 * (Rds_on) = (3A)^2*40mOhm = 0.36WFor 12V, 5A source: Pd = (Id_max)^2 * (Rds_on) ) (5A)^2 * 20mOhm = 0.5WIn the Absolute Maximum Ratings, it is stated Pd_max @ Ta = 25°C : 1.79W. My application will be working at 24°C typically but for sure at 40°C maximum.So for Ta = 25°C, my margins would be:For 5V, 3A source: Pd_max - Pd = 1.79W - 0.36W = 1.43W ( a factor of 1.79W/0.36W = 4.97)For 12V, 5A source: Pd_max - Pd = 1.79W - 0.5W = 1.29W ( a factor of 1.79W / 0.5W = 3.58)For Ta = 40°C: The Linear power derating factor stated in Absolute Maximum Ratings is 0.01W/°C, so I would say Pd_max = (Pd_max @ 25°C - (0.01W/°C)*(40°C-25°C)) = 1.79W - 0.15W = 1.64W. The margins would be:For 5V, 3A source = 1.28W (factor of 4.55)For 12V, 5A source = 1.14W (factor of 2.28)With this margins, I would say the selection is appropiate, because they are very well below the absolut maximum ratings.- Added to this, always cost is an important parameter, and this was the lowest cost P-channel mosfet I could find (digikey) and that apparently was able to handle my current/voltage requirements at $0.33950 for 1000 units.Real testing:- So far I have only measured Pd and Tc for 5V, 3A source. The Power dissipation I calculated based on measured Vds and Id:Pd = (Vds)*(Id) = (0.114V)(3A)= 0.342WRds_on = Vds/Id = 0.114/3A = 38mOhmThe case reaches a temperature Tc = 48°C, which is really hot I would say for a small micro8 package.But I have doubts about this calculations because of the following, which I dont know is correct or not:- I dont know if/how the following rating applies: In page 2, it is stated an "Is"continuous source current rating of only 1.8A. By this paremeter I understand I should not pass more than 1.8A through the device in continuous manner, which is the way I will be using it.Is this correct? Would then this parameter completely kill my selection?-Fig 8. of the datasheet shows the Safe Operating Area. Unluckily there is no line for DC, so I do not have a "graphic" parameter. But I guess it would be a line under the 10ms pulse line. Since my operating Vds would be around 0.1V, I would be under the DC line (assumption). Taking the upper limit as the Rds_ON_limit, I can safely pass through the device up to 4A @ Vds = 0.1V, and probably up to 5A @ Vds = 0.13V (which would be the theorethical Vds for 5A: Rds_on * Id = 26mOhm * 5A = 0.13V).- The high case temperature of 48°C is something I am not really sure abouth. Even though I am calculating a "safe" power dissipation, this temperature seems too high to me.Other options:- I would consider using any other mosfets if anyone has a suggestion. I would prefer to use one only mosfet for both switches because I would get volume discount more easily.- I was consider changin to a low-side N-Channel mosfet: STP75N75F4, from ST. It has a low Rds_on as well, but a bigger case which would be able to dissipate safely more power, but also a much higher cost, not really great ($0.97100 for 1000 units).Thanks a lot to everybody for reading my post and prividing your feedback. I really appreciate any comments/questions/suggestions and feedback you can provide

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 15, 2015, 05:40:07 pm
content: The 1.8A is continuous current through the body diode. Which means flowing from drain to source with the MOSFEToff. It's only rated at 1.8A because the power dissipation would be Is*Vf, where Vf is likely around 0.7V.Your temperature is higher than you expect because you never looked (at least you never mentioned) the thermal resistance. That package has a thermal resistance of 70 C/W, which means every watt it has to dissipate would push the junction 70 C higher than the ambient. You're burning 0.36W, which means the junction will be ~25 C above ambient. You need to keep the junction below 150 C, which means you need to keep the ambient temp below 125 C, plus some safety margin.I think it seems like a fine choice, but if you're worried about the temperature the easiest way to lower it would be to choose a device with a lower Rds(on). Ineverlook at Pd_max, it's always way to aggressive for my tastes. I just look at the expected voltage drop and the expected junction temp given my operating conditions and decide if it's acceptable. For my applications, usually voltage drop dominates and power dissipation is nil.FYI - All Pd_max is is the power dissipation that will cause you to hit the max junction temp at 25 C ambient. 25 + x*70 = 150, solve for x.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 15, 2015, 05:44:57 pm
content: You can save yourself any thermal issues by going with N-FETs that have ultra-low Rds(on). Of course they need a driver, but those are pre-packaged in very easy to use devices. Essentially you can get a dual charge-pump gate driver that is logic controlled and it will open up a far larger selection of MOSFETs that are better suited for use as a DC switch.Your P-FET: 26m?5A = 625mW dissipationTypical N-FET: 2.5m?IRF8734 (randomly chosen, probably a little over the requirement)5A = 62.5mW dissipationHuge difference. Order of magnitude.

author: max_torque
author_rank: Super Contributor
timestamp: June 15, 2015, 06:03:19 pm
content: If you have the budget, and want to:a) make you life easy as a designerandb) Provide some form of output protectionyou can get "smart" high side switches from people like Infineon, that have an N-Fet, all the necessary charge pump stuff, and things like over voltage, over current, and over temp shut down, all in one single easy to use package.The downside is the cost!

author: rx8pilot
author_rank: Super Contributor
timestamp: June 15, 2015, 06:08:21 pm
content: Quote from: max_torque on June 15, 2015, 06:03:19 pmI use devices like that from Linear. Very convenient with internal or external FET's. The cost seems high until you factor the discreet components needed to do the same job of covering OV/UV and OC in addition to a charge pump gate driver. Logic interface and fault output too. Your PCB is smaller and the BOM count is much lower, probably a cost wash in many cases.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 15, 2015, 07:53:58 pm
content: Quote from: rx8pilot on June 15, 2015, 05:44:57 pmHe doesn't need an Nch to get single digit milliohm Rds(on).Here's a Pch that's 3m at 4.5V:http://www.mouser.com/ds/2/427/SI7141DP-223663.pdfIt is a little bit more expensive than the Nch, but once you add in the driver circuitry required to get the Nch to work it's mostly a wash.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 15, 2015, 08:26:21 pm
content: Quote from: suicidaleggroll on June 15, 2015, 07:53:58 pmYou are correct, I mixed specs. Although with those FET's, I always use a +10v drive which puts them pretty close to 3m. They are never connected directly to logic in a high-side application. Splitting hairs....Quote from: suicidaleggroll on June 15, 2015, 07:53:58 pmThose are definitely good, I have been using the si7145's in a 10A DC switch application for about a year with success. They are used with a controller that can only have P-FET. As far as I know, that is about the lowest Rds(on) in a P-ch FET I have seen - 2.6m is pretty low for sure at 5A.N-Ch are down to .5m? now where you will be dissipating more heat from the PCB traces than the transistor. Crazy how far they have gone.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 15, 2015, 08:44:13 pm
content: Quote from: rx8pilot on June 15, 2015, 08:26:21 pmIt is crazy, that's lower than the contact resistance of many mechanical relays.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 15, 2015, 08:51:26 pm
content: The trade off is that the gate capacitance is generally very high so they cannot be used (effectively) for any fast switching.

author: juanfermed
author_rank: Regular Contributor
timestamp: June 15, 2015, 09:56:27 pm
content: Hi guys,Thank you very much for all your answers and time. I am really overwhelmed and amazed by you, I will analyze all these options and post back. (...I feel like such a novice)

author: rx8pilot
author_rank: Super Contributor
timestamp: June 15, 2015, 10:59:34 pm
content: 18 months ago, I could't spell MOSFET, and just completed a project with over 400 components on 7 PCB's. This forum provided a LOT of guidance to make that happen. If I continue on this learning curve, in 10 years, I may even be considered smartGood luck, hope it works out.

author: Ton
author_rank: Regular Contributor
timestamp: June 16, 2015, 12:02:56 am
content: A ?-mosfet as a load switch on a powersupply !What kind of loads are you expecting ?And what kind of loads might your users actually use ?You state reliability as a must !if you have even small amounts of capacity as parts of the load you are going to switch on, and since it is a powersupply the LOAD most probably contains some amount of capacity.Then you need to be concerned with possible huge inrush current.try to simulate you your 12V 5A output when turned on (by a micro controller) into a 10uF low Ceramic Capacitor parallel with a suitable load resistor.The current levels is scary (45Amp), and can kill your 5A pfet - maybe not the first time maybe first after 100 to 1000 cycles, this might not be reliable enough.with proper design it can be much more reliable - take a look in this Onsemi application note for detailshttp://www.onsemi.com/pub_link/Collateral/AND9093-D.PDFif you can keep the chosen FET with in it SOA based worst xcase capacitive load and above applicationnote, then you are on your way towards reliable switching with a high side FET.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 16, 2015, 12:19:05 am
content: If inrush currents are expected just size the MOSFET properly and drive the gate slower. The SI7141 mentioned in the earlier example can handle a substantial inrush current over the expected 5A. The SOA (safe operating area) is reasonable so you can slow the voltage change on the gate to keep the inrush at a reasonable level and stay within the temperature limits while transiting the linear region of the device. Can be a delicate balance and like you mentioned - it may work for a while and then self-destruct.My circuits have to start 15v/10A/5000uF loads with rather low ESR. The only practical option is to limit the rise time of the voltage while being cautious and mindful of the mosfet dissipation for the brief period it is in it's high resistance range. I destroyed a few FET's while learning the lesson.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 16, 2015, 01:50:28 am
content: It's easy to limit inrush current, just takes a cap between the gate and drain and a resistor on the gate driver. See figure 7:http://www.onsemi.com/pub_link/Collateral/AND9093-D.PDFSize it to turn the fet on as quickly as you can while keeping inrush current below the limit

author: rx8pilot
author_rank: Super Contributor
timestamp: June 16, 2015, 02:46:16 am
content: True, its easy to limit the inrush. The harder part is making sure you don't cook the transistor by leaving it in the linear region too long.

author: juanfermed
author_rank: Regular Contributor
timestamp: September 14, 2015, 08:12:48 pm
content: Hello to everybody,First of all, I want to sincerely apologize for replying back so late. There were some issues and I was sent to the field for several weeks. I was able keep working on my design but not to write back until now. I hope not to make this long. I want to share some very insightful waveforms I was able to get with your support; I dont want to only ask questions without actually sharing the results and share something usefull here. .I did went through the ON Semi appnote and included inrush limiting in my design (although it is maybe not necessary).First about the design changes:1) I changed from IRF7726PBF to the IRF9321. In my attempt to make an objective and best engineering decision, I made the table shown in attachment "Mosfet Selection Table" putting together all P-channel mosfets suggested and investigated. The winner was of course IRF9321 becuase has 2x decrease in price, 4x decrease in power dissipation and around 5x decrease in temperature rise. It would also improve input-output efficiency of the complete system.2) The IRF7726PBF was not necessarily a bad decision in terms of performance. It does have a higher voltage drop and power dissipation compared to the IRF9321, but can handle very well the voltages and currents that I explained the first post. Obviously, in the presence of a better option (more importantly in terms of price), the change was an all win.About the actual measurements:1) First I wanted to identify the Turn ON regions and characteristic of the MOSFET. Something like getting the equivalent Figure 6 of the ONSemi appnote, but for my mosfet. See image "No Inrush Limiting Vds Id P Turn On Waveforms into Kindle.png". It is not as clear as in the appnote, but it is possible to see the different Vgs stages and Vds transition.2) Inrush current limiting: The possible devices that will be connected to the outputs of my circuit are primarily cellphones (smart and not smartphones), 12V LED light bulbs and possibly 12V TVs. In figure "2.35A Load No Inrush Limiting Vds Id Pd 1.1uF Load.png" I show the mosfet's drain-source voltage in blue {Vds}, Drain current in orange {Id} and Instantenous Power in light blue {Pd} for a 2.35A resistive load in parallel with a 1.1uF capacitive load connected to the 12V output WITHOUT inrush current limiting circuit. You can see the following characteristics:- Turn ON time aprox 1us.- Peak current: aprox 26A.- Peak instantaneous power: aprox 33W- Power pulse duration: aprox 1.2usThen I connected the resistor-capacitor network suggested in the appnote to limit inrush current and, in fact, the inrush current was significantly diminished with the expense of increased power pulse peak and duration. See image "2.35A Load Inrush Limited 330Ohm 1nF Vds Id Pd 1.1uF load.png". Specifically you can see the following characteristics:- Turn ON time: aprox 1.5us- Peak current: aprox 15A- Peak instantaneous power: aprox 68W.- Power pulse duration: aprox 1.5usThe power pulse more than doubled with the sligthly enlarged Turn On Time and reduced inrush current. In both cases the turn ON Power pulse and Vds, Id characteristics are tolerable by the IRF9321 as are under the SOA Curve. See image "IRF9321 SOA".At this stage of design, there was still pending the actual implementation of active shorcircuit, overvoltage and ESD protection for these outputs. I investigated several methods to achieve this, looking simultaneously for "Smart Switches" as was suggested by several people. The short answer for the system I was designing is that going for the "Smart Switches" was the best option. I was completely amazed that those devices include ALL the protection characteristics I needed and even more. Now, for my application, when doing the comparison between a discrete solution and the IC, in terms of price and time of implementation, the IC solution was a clear winner. But in these sense I got to know that viable solutions for this were available for relatively low power requirements as mine. But for higher voltage and current requirements (maybe 3x-5x of mine) the IC solution might not be available or might be more expensive than a discrete solution.That being said, these are the final devices I selected as output pass device and output protection:12V, 5A output: TPS259105V, 3A output: TPS2592BABoth devices are amazing in the rich feature set they offer and their robustness. I did not use the IRF9321 in the actual implementation, but it was a very insightful process that gave me real experience.Beyond that, If anyone else reads this in the future for a similar application, and have different voltage/current requirements, I will recommend to look a the following:1) Search in google for "Smart Switch" and "eFuse". Most manufacturers refer to these devices (pass transistor with a lot of protections) as "eFuses". Also can look for "Hot Swap Controllers" whose intended end application might not be only output protection, but can fill the requirements and price very well.2) Look at the Texas Instruments range of "Hot Swap Controllers" and "eFuses" and "Load Switches". You can look for example at the TPS25910 or TPS2592 I selected, and the page will recommend other devices as well.3) Look at the Infineon ProFET, ProFET+ or HiC ProFET families. These devices provide a lot of protection features and a wide range of RdsON devices. Some are "automotive" grade and designed for that end market, but that does not mean at all that they cannot fit other non-related applications.http://www.infineon.com/cms/en/product/channel.html?channel=ff80808112ab681d0112ab69e2d403574) Look at the ST Family of "VI-Power" devices. At some point I tested the vnn7nv04ptr-e which in the end was not suitable, but they have several other devices. They also have some "STEF12" and "STEF5" eFUSES, intended for 12V and 5V power rails, respectively.5) After investigating about those Smart Switches, I think it is highly recommended to analyze the use of these devices when reliable output protection is required. If you need short circuit, overcurrent, output overvoltage, thermal shutdown and/or foldback, ESD output protection, many of these devices are designed and intended for that kind of requirements.I would like to thank everyones recommendations and support here. Also for pointing out the existence of the smart switches.