Why 540 mosfets are unhappy!  - Page 1
Forum section: electronics / projects

author: dave_j_fan
author_rank: Regular Contributor
timestamp: October 26, 2014, 04:36:17 am
content: Building inverter where i have deployed irf540 x2 with a ferrite ee coreI am using ee ferrite transformer .core is 30mm x 6mm i hand wound the eecore withprimary 12turn ct 12turnsec 180turns...took idea from herehttp://easy-electronic-circuits.blogspot.in/2014/05/100-watt-compact-ferrite-inverter.htmli just made the stepup ckt (top ic4047 and transformer)There was an error in schematic i corrected and adjusted 4047 to give 60kHzOn the secondary load is just a 3w led floodlight with AC 85-285V driver.I am powering from a 12V SLA battery .The problem is mosfets heat EVEN ON HEATSINK(6cm x 6 cm ) ..after 60seccore makes hiss after sometimes hiss stopsplease help

author: bktemp
author_rank: Super Contributor
timestamp: October 26, 2014, 07:21:05 am
content: There is so much wrong in this circuit:- IRF540/9540 are only 100V rated. For 220V you should use at least 400V rated mosfets, but getting a 400V P channel mosfet will be hard- the upper IC1 needs some bypass caps: Everytime it switches the mosfet gates its supply voltage collapses- R3/R4 should be much lower. Better use an external mosfet driver- there is no rectifier/filter at the output of the transformerI think it is hopeless to make this circuit work.

author: planet12
author_rank: Regular Contributor
timestamp: October 26, 2014, 11:31:08 am
content: Question: if you didn't build the bottom (low frequency chopper) circuit, what is at the output of the transformer instead? Please draw up a diagram of what you've actually built.Regardless, there are a bunch of major problems:The CD4047 used as an oscillator has no concept of "dead time". Dead time is needed to stop both output transistors conducting at the same time. If they conduct at the same time, the two sides of the center-tapped primary of the transformer will look like a dead short, and high current will flow - heating and/or destroying them.The CD4047 also has very limited output current. Looking at the Fairchild datasheet for the part, the Q and not-Q outputs will only source or sink a very small amount of current - around the 1-2mA area. This will cause the 2nF input capacitance of the gates of the IRF540 MOSFETs to charge very slowly. MOSFET gates act roughly like a capacitor, and you need to charge and discharge it rapidly in order to make the MOSFET switch hard-on then hard-off. If it's charged slowly, the transistor will partially conduct, meaning it has both a decent voltage over it, and a decent current through it - hence power dissipation, so it gets hot.There are no snubbers over Q1 or Q2. Leakage inductance in your transformer and wiring will cause voltage spikes on their drains, and a snubber circuit will absorb this and protect your transistors, along with reducing the electrical noise your circuit will put out.There's no indication that the diodes used on the secondary of the transformer are a suitably rated ultrafast or Schottky type diode. If you use regular silicon diodes such as 1n400x they will do a very bad job of things, as they are far too slow.There's no output inductors, feeding in to filter capacitors. If you're then feeding the output directly into your LED floodlight, the input circuit of that is having to rectify and filter your 60KHz again, and the input components will have only been designed for doing 50/60Hz mains frequency.There may be more, but that's where I've got to so far.@dave_j_fan - if you can provide an updated schematic, I'll comment further and give you some ideas to try.

author: Seekonk
author_rank: Super Contributor
timestamp: October 26, 2014, 12:01:48 pm
content: My eyes glazed over quickly looking at this. I stopped at two oscillators not synced to each other. Burn baby burn.

author: bktemp
author_rank: Super Contributor
timestamp: October 26, 2014, 02:18:19 pm
content: Quote from: Seekonk on October 26, 2014, 12:01:48 pmThey do not need to be in sync: The upper oscillator drives the stepup transformer at a high frequency, the lower one selects the polarity of the output voltage at 50Hz.The idea behind this is great, but the implementation is a total fail.

author: mikerj
author_rank: Super Contributor
timestamp: October 26, 2014, 02:39:37 pm
content: Quote from: 3roomlab on October 26, 2014, 10:13:39 amWhy would it? Provided the switching MOSFETs are correctly driven (i.e. not directly from the output of a feeble CMOS IC) then power loss will be very small.

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2014, 03:37:18 pm
content: Why bother with the second stage? Rectify the high voltage, filter cap ( 10uF 450V will work fine here) and feed direct into the floodlight. There is no reason to convert DC to AC only to use a simple diode bridge to make it DC again directly inside the lamp. Do a better first stage, look at any simple SMPS controller and see how they do dead time, snubbing and driving MOSFETS, and do a better driver.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: October 26, 2014, 09:52:07 pm
content: Added the circuit i builtI am very indebted to get many expert suggestions.PFA actual circuit used ...Suspect the dead time and bad designis killer herejust to clarify1) Chopper is not needed, HF inverter is fine as i am planning only to light 4 LED bulbs2)seems the no dead time is causing iti tried another trafo with 20T primary and 60T secy ferrite with just one mosfet and foundit was lightly warm @3Watt flood light but ok.But cant take 5 W BULB gets very warm in 30 sec!once gets hot even though i remove load of 5w it keeps getting hotter ...and i remve power

author: planet12
author_rank: Regular Contributor
timestamp: October 27, 2014, 02:35:55 am
content: The thermal runaway you describe (once hot, keeps getting hotter even with load disconnected) is a strong indication that you're not driving the gates of the MOSFETs hard enough.The lack of dead-time will be exacerbating the issue, however the inadequate gate drive may actually be saving your FETs from total destruction due to simultaneous conduction at this point.Trying to make this circuit work properly is, in my opinion, a lost cause. You're better off finding a chip that's designed for the job and using that instead.Are you able to get your hands on a SG3525 / KA3525 chip? They can drive the FETs directly, and have a dead-time function, as they're made for SMPS operation. A slightly worse (for this application) but still usable option is a TL494 / KA7500 chip - they'll do the dead time, but you'll need to add a bit of circuitry to drive the FET gates properly.If buying them is an issue, you'll probably be able to find them in a old PC power supply - the TL494/KA7500 and to a lesser extent the SG3525 are commonly used in these.A good start for a circuit is figure 9 fromhttp://sound.westhost.com/project89.htm- however you won't need the doubled-up MOSFETs with your power requirements, so you can remove M2/M4, R4/R6, and you won't need the shutdown circuit either - remove Q1, R11, C18, R12 and connect pin 10 of the SG3525 to ground.Do you have access to an oscilloscope? It's pretty much a requirement for working on something like this. You want to get things set up with the drains of the FETs disconnected and check that you're getting a good drive signal on the gates with no overlap between the two. Only once you're happy should you connect the FET drains to the transformer.Additionally, how was your transformer designed? Did you go through a process with a known core type and area, getting the right number of turns to support a specific flux density given your primary voltage and frequency, or was primary turn count just... made up?If the primary turns are too low, you'll saturate the core, and it will act as a short circuit after the saturation point, causing overheating/failure of the FETs, and overheating of the transformer due to resistive losses. Too high a turn count will reduce efficiency (you won't be making optimum use of the core, and you'll end up using thinner wire than you need in order to make it fit in the winding window), but is better than too few. A good introduction to transformers and inductors ishttp://ludens.cl/Electron/Magnet.html

author: dave_j_fan
author_rank: Regular Contributor
timestamp: October 27, 2014, 05:36:00 am
content: Thank you planet 12I will line up some goals* inverter has, to be compact and low cost*EI core costs much money for the copper that is why ferrite core is selected*preferably the circuit has to be minimalYes I have a 100MHz DSO (scope) please let me know where to pull traces for your reference1)why the IC4047 in 120Hz or 50Hz work fine is it because the Mosfets getting enough time to turn off ?many people have made inverters with 50HZ and EI coreif there is gate signal over lapping at 50Hz also mosfets might have heated heavily2) i observed thermal runaway with single MOSFET while on load is 5Watt3Watt flood it is driving fine with a little warm means this has some other issue may be the drive current is very lowyes when on 2 MOSFETS and 5W load they heat fast in 20 secondSo some thoughts* will 2 small BJT like BC547 /557help in drive currnt like the circuit you gave*I have seen 4047 driving BD 699http://www.eleccircuit.com/wp-content/uploads/2009/12/power-inverter-12vdc-to-220vac-using-cmos-4047.gifso elminating MOSFET driving stage may help ?

author: wiss
author_rank: Frequent Contributor
timestamp: October 27, 2014, 08:59:23 am
content: Maybe you could use a forward topology to get around the overlap problem?http://en.wikipedia.org/wiki/Forward_converter

author: dave_j_fan
author_rank: Regular Contributor
timestamp: October 27, 2014, 07:35:47 pm
content: Partial good news !changed the 220ohm to 68ohms.The MOSFETS warm verylightlyyey! the the smally-inverter driving a 5watt bulb and 3watt flood well .One issue is the transformer is little warm now , may because i am using 2 of the transformers (they were ordered for another LED driver circuit ) and primaries dont have CT in theselike in the picSo task just completed a bigger transformer with pretty thick primary and CT12T-ct-12T to 250T secyKeeping fingers crossedWiss..i will think of that is it complicated ?

author: wiss
author_rank: Frequent Contributor
timestamp: October 27, 2014, 08:16:29 pm
content: Quote from: dave_j_fan on October 27, 2014, 07:35:47 pmYou replace one of your drive-transistors with a diode, to (magnetically) reset the core, the volt-turn-time has to be the same, so if you drive with 50% duty-cycle you'd need one less turn on the reset-winding to guarantee a reset. The will on the other hand produce a higher unloaded voltage on the output, but I do not think that will be a problem here.You should definitely use a choke on the output, or anywhere in the main current-path, without one the turn-on current will quickly go from zero to max while the MOSFET(S ) are still turning on (= power-loss).

author: planet12
author_rank: Regular Contributor
timestamp: October 28, 2014, 01:11:54 am
content: Traces from the gates of the two MOSFETs are of particular interest. If you can get a screenshot of both gates at the same time (ie. dual trace scope) that would be very helpful.(1) Why do you not have the same problem at 50Hz? At that frequency, switching slowly is still a problem - but it's a much lower proportion of the overall time: the FET is on for 10ms then off for 10ms, so a couple of microseconds - lets say 2us for arguments sake - switching time 100 times a second is only 200us per second.When you take this up to 60KHz, you're switching the FETs on and off every 8.3us - and 2us switching time now adds up to 240ms - so you're spending 1/4 of your time in switching, dissipating lots of power during it. That's 1,200 times the switching losses for the 50Hz case.(2) Thermal runaway - Rds(on) goes up with temperature, Vgs(th) goes down - meaning, when slow switching, you start switching earlier, and I2R losses go up due to Rds(on) increase. The design page I linked earlier has a discussion on thermal runaway in MOSFETs used in switching applications.Using a totem-pole of BJTs will help significantly - this is one of the reasons I suggested the SG3525 chip, as it has a PNP/NPN totem pole on each output, allowing direct drive of MOSFET gates. Instead of BC547/557 however I would suggest the higher-current BC327/337's. As mentioned before, faster switching then runs the risk of both MOSFETs being on at the same time due to no dead-time.In the case of the 4047 driving the BD699, firstly the circuit you've linked is running around the 50/60Hz range using an iron core transformer. Secondly, it can achieve the required drive current as the BJT is (a) a darlington with a minimum gain of 750, (b) effectively current driven.With a 12V supply, and Vbe of the darlington of 1.2V, you can drive (12 - 1.2) / 470 = 22mA into the base of the BD699, easily allowing several amps of collector current while remaining in saturation. If you tried to run it faster you'd run into problems here too.Dropping the gate resistor to 68ohm will be helping your switching times, but possibly overloading the 4047's output. You'll be much better off with a BJT totem pole on each output of the 4047, then a low-value - eg. 22ohm - resistor to the IRF540's gate.Your transformer getting warm will be a combination of resistive and core losses. This comes back to your transformer design - (a) making sure you run the core at a suitable flux level, probably around the 100-200mT range (got the datasheet for your transformer core? The core isn't gapped is it? If so it's not suitable for the type of converter you're making here), and (b) making sure your wire is suitable.At 60KHz, skin effect starts having a pretty big effect, and simply making your primary wire thicker may not actually help you at all. Skin depth at 60KHz is about 300 micrometers, translating into a maximum useful wire diameter of double that - 0.6mm or AWG #23. If that wire is not going to handle the current you need, you have to double it up - wind the primary using two or more strands in parallel, connected only at the ends.

author: planet12
author_rank: Regular Contributor
timestamp: October 29, 2014, 08:33:59 am
content: @3roomlab - short answer, since you're hijacking - get your own thread!The IRF540's we're talking about here have about 2nF of gate capacitance. That's going to add enough phase shift to make the AD8010 oscillate like mad - datasheet says a max load capacitance of 40pF. Also, current limit is 200mA - how often can that empty and full the 71nC of gate charge of the IRF540? Short answer: a lot more slowly than the ~13MHz required for your Metcal.There's a time and place for an opamp driving a FET, but using it to hard-switch it isn't one of them.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 01, 2014, 08:33:04 pm
content: many thanks to planet12 aand othersi learnt a lot ... getting a special chip is 30day gameso i did some trialsgrabbed a 20T into 60 turn ferrite transformer that is made by pro companydisabled one mosfetand final one added uf4007 after secythis is nicely working and lighting 5W x2 lamps!good for small shops and low height street lights ..i willpost pics and nice cabinet in 10 minsmay nt be super efficient.I will discuss..thank you all...Here I am trying to manage with minimal components ,This will be given to poor and needy people , small shopkeepersAs there is just freq needed to be fed to transformerhow about using ccfl circuithttp://danyk.cz/ccfl.pngkeeping secy turns less we will get 200v-240v rangecoz BD139 is 0.1$ hereone BD139 can go 1.2amp or morei can parallel 4 of bd139 to raise the handling power to 80wattThis inverter will light 4 rooms with 7 watt led bulbs .

author: wiss
author_rank: Frequent Contributor
timestamp: November 01, 2014, 08:52:11 pm
content: Quote from: dave_j_fan on November 01, 2014, 08:33:04 pmIt is still in circuit with gate grounded?You'd want a very fast diode in it's place, the body diode of a mosfet is not slow but there are better options.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 01, 2014, 09:53:24 pm
content: Quote from: wiss on November 01, 2014, 08:52:11 pmgate open and just one leg on gnd actually unconnected .. this transformer is not ct so just primary fed from the Mosfet that is activeas promised added PIC of the inverter ....this has given me confidance ... as a start=================Adding bells and whistles of inverter UPS1)want to add a overload tripper circuitwill circuit like this help ..this i will put in series with main MOSFEThttp://www.eleccircuit.com/fet-overload-current-trip/[preferably no relays ]2)auto on UPS on powercutsome opto i will drive that keeps the inverter off(i hope i can do this)3) elcheapo led drivers as as SLA charger will be used(my concept ...please comment)

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 01, 2014, 10:08:22 pm
content: some thing to show the general people as outer ..the indicator dacals were made of butter paper 2 layersi will redo the text and will file the box for better fit

author: wiss
author_rank: Frequent Contributor
timestamp: November 01, 2014, 10:29:30 pm
content: Quote from: dave_j_fan on November 01, 2014, 09:53:24 pmFor gods sake, ground the gate!What will hapen when enough charge accumulate on it?

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 01, 2014, 10:37:31 pm
content: whr from it will get charge? only 1pin connected if i can do away 80w with just 1mosfet ..will remove other

author: wiss
author_rank: Frequent Contributor
timestamp: November 01, 2014, 10:46:12 pm
content: Any isolated thingie can and will accumulate charge eventually, in this case that charge will turn on a switch between supply and gnd, not good...And, when the driven mosfet turns off there will be a significant back-kick from the transformer, now that is taken by the second primary winding and the body-diode of the inactive mosfet, without that diode the first mosfet will get very hot.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 02, 2014, 06:51:19 am
content: Quote from: wiss on November 01, 2014, 10:46:12 pmfor now it is there just like a Rnd pcb .... surely will go in final board ..or will parallel with another mosfet.. see the diagram ..according to you this guy will grab charge inductively and wirelssly short some vcc to Gnd ..i doubt that heavily .currently he is passive .. later will pull gnd line away

author: wiss
author_rank: Frequent Contributor
timestamp: November 02, 2014, 11:32:09 am
content: Ok, so that mosfet is out of the circuit. You only use one primary winding now?You have full wave rectification of the output? From what I understand that is built into the LED-lamps? In that case the lamps will take the inductive kick, do not run this without a load.Will the rectifiers in the lamps be fast enough?

author: mikerj
author_rank: Super Contributor
timestamp: November 02, 2014, 04:53:17 pm
content: Quote from: dave_j_fan on November 02, 2014, 06:51:19 amKeep doubting it until it goes wrong then. A floating MOSFET gate WILL cause problems sooner or later.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 03, 2014, 05:18:33 pm
content: that is just a temporary place..got the extech210 meter.. will work on the overload circuitfinallyexperts do provide guidance@wiss* yes the trafos hav single primary , i ordered them to make a stepdown smpsnow suitable for the reverse* Most drivers have rectifiers i suppose the bulb too has fast enough diodes , seen the flood led driver having bridgei also have UF4007 in the HV line , adding this stopped the heating of transformer* inductive kick should be within limits (guess work is the voltage is <200V) i will measure with this trueRMS DMMwiss ,can i combine 4 tiny transformers(max 15Watts) and make a 60watt inverter?each being fed with a mosfet and final voltages somehow clubbed with diode or some arrangementif possible i can make a scalable inverter that can give many driving capacity based on the number of MOSFET and trafo pairs

author: wiss
author_rank: Frequent Contributor
timestamp: November 03, 2014, 06:12:04 pm
content: Quote from: dave_j_fan on November 03, 2014, 05:18:33 pmI've only seen flybacks use single-primary, could be made to work here..._NEVER_ run that without a load!Make sure that you have fast full wave rectification on the secondary!It can produce very high voltages!QuoteIf they are for 50/60 Hz they will not be fast enough, add switching diodes, fast recovery or schottky diodes.QuoteThe DMM will be way too slow to measure anything. The peak will be as high as the breakdown of whatever is in front of it!QuoteDrivers and primaries separate and the combined after secondary rectification? probably...Post a schematic!

author: wiss
author_rank: Frequent Contributor
timestamp: November 03, 2014, 09:54:22 pm
content: BTW, this circuit could very well kill your brand new multimeter in a few microseconds...

author: planet12
author_rank: Regular Contributor
timestamp: November 04, 2014, 03:32:50 am
content: I second @wiss on the updated schematic request. Given what's been said, I'm not sure whether it's now a flyback or some kind of mutant one-switch forward converter (without a reset winding?).Details on the transformer would also be good - anything you have from the manufacturer about it would help.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 04, 2014, 09:38:15 pm
content: Quote from: wiss on November 03, 2014, 06:12:04 pmwell scary voltage spikes i came to know from you..but why single primary makes highvoltages?regarding rectifiers @ bulbs and floodlightsi dont want to modify the load items .. that end user will not b able to do .given the schematic ..i n a way this is way easy solution wrt Joule ringers that people have reasearched months ..efficiency part yet to checkdid trialswith a few 5630SMD LEds for a bulb idea! to accompany this inverterThe bulb is not done yet ,but 7 leds gave pretty good lightschematic attached

author: tom66
author_rank: Super Contributor
timestamp: November 04, 2014, 10:17:40 pm
content: Where's your snubber? You'll kill that MOSFET pretty quickly. If you've put the transformer windings in opposed directions all you've made is a flyback, a very inefficient one at that.If you put them in the same direction and add a reset winding in reverse on the primary, with one end to supply and diode to ground, you'll make a fairly nice forward converter (you'll also have to change the output topology)http://www.powerelectronicsconsultant.com/uploads/3/6/3/8/3638391/2592511.jpg

author: wiss
author_rank: Frequent Contributor
timestamp: November 05, 2014, 09:59:46 am
content: There is a thin explanation of the turn-off spike here:http://www.maximintegrated.com/en/app-notes/index.mvp/id/3983This page is more about an alternative method to reset the core, which might be useful in your application.You do have an oscilloscope?

author: planet12
author_rank: Regular Contributor
timestamp: November 05, 2014, 10:10:11 am
content: @dave_j_fan I think it would be to your great advantage to have a good read through the following two app notes from Microchip - "Switch Mode Power Supply (SMPS) Topologies" part one and two:AN1114 - SMPS part 1:http://ww1.microchip.com/downloads/en/AppNotes/01114A.pdfAN1207 - SMPS part 2:http://ww1.microchip.com/downloads/en/AppNotes/01207B.pdfIt's great that you're willing to jump in and give it a go building things up, but these documents will help you greatly in your understanding - and also allow you to talk to us in the standard terms, eg. "one switch/two switch forward converter" / "flyback converter" and know what is meant by them.@wiss - I hadn't seen that resonant reset technique, thanks for the pointer! *scurries off to the lab to play with it*

author: wiss
author_rank: Frequent Contributor
timestamp: November 05, 2014, 10:47:14 am
content: Quote from: planet12 on November 05, 2014, 10:10:11 amI just googled for an explanation of forward converters and stumbled over it just nowIf my bench weren't full of other things I would too. I do suspect that it might not behave under no-load...Please report @planet12 !

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 06, 2014, 06:18:35 pm
content: 3roomlabthere are hours on inverter videos good post ..planet12/wissmany thanks ..links helping a lotreading mosfet protectionhttp://www.circuitstoday.com/how-to-protect-mosfet-devicesyes the mOSfets may take charge build up and kill itself , i will short the gate---i will come back after a while testing a dumb touch fan control --