Are inductors polarised?

BrendanP

Senior Member
I got the board backs for the switch mode battery charger using the MAX713. The charger uses a inductor cooper-bussman DR127_221_R. The data sheet shows the two connections shown on the package marked as one and two.

Looking at the MAX713 suggested circuit app it shows current passing through the inductor to the battery. Does it matter which way around the inductor goes?

As as far as I can understand the inductor is just a coil?
 

moxhamj

New Member
Correct, an inductor is just a coil of wire. And they are not polarised, so you can install it either way.
 

leftyretro

New Member
I got the board backs for the switch mode battery charger using the MAX713. The charger uses a inductor cooper-bussman DR127_221_R. The data sheet shows the two connections shown on the package marked as one and two.

Looking at the MAX713 suggested circuit app it shows current passing through the inductor to the battery. Does it matter which way around the inductor goes?

As as far as I can understand the inductor is just a coil?
As as far as I can understand the inductor is just a coil?

That is correct, an inductor does not have a polarity. However in transformers that may have several windings there is sometimes polarity marks to show the phase relationship between the windings. A secondary winding can be connected such that it is in zero degrees or 180 degrees related to the primary (driven) winding.

Lefty
 

BrendanP

Senior Member
Thanks guys, I can cross that off the list, something isn't right, the mosfet that does the switching (IRFR/U9024) keeps getting hot and expelling the magic smoke and going belly up. Ill keep chipping away at it.

I'll load a new pcb , I might of damaged the MAX713.
 

Dippy

Moderator
"I got the board backs for the..." - in simple English, what does this mean?

Does the MOSFET get red hot under load or no load?

What does the Gate drive look like on your 'scope? (with and without MOSFET in place). This will show if the complementary bipolars are working properly.

What does the drive output (DRV pin) look like on scope?

Please say you've got a scope :)

I take it you've checked for short circuits, bad soldering etc.

Have you got Maxim's eval. board or is it homebrew? These are quite expensive chips to blow up.

You've got quite a lot of chipping to do. Good luck.
 

moxhamj

New Member
Yes I'll second Dippy on that. Have you got a scope?

You can build picaxes without a scope. Digital and microprocessors are also possible without a scope. Analog and audio is possible but a bit harder. But switch mode power supplies are not possible, in my opinion, without the information a scope gives you. A scope may end up costing you less than your hours spent debugging and zapped components :)

Oh, and re the new pcb, I presume you breadboarded this? You did, right? Cool. Coz a shiny new pcb with the incorrect circuit will still let the smoke out.
 
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BrendanP

Senior Member
Yes boys I have a 'scope.

Sorry for the typos Dippy. I meant to say I got the pcbs back from the maker. Yep the MOSFET gets hot! damn hot! Til it finally dies within a few seconds. This only happens with the battery pack connected (under load).

I discovered I had one of the SMD diodes soldered on around the wrong way, fixed that that it still gets hot.

I've got about forty of the MAX713 sitting here, they're about $5 each. A few fried in the course of R&D is no big deal. Im down $25,000 on this thing, Im passed the point of worrying about 5 buck IC's. Time is what Im worried about.

I don't think I've fried any of them.

The problem seems to be in the layout of the pair of transistors. Ive posted a query concerning component pinouts.

No I didn't bread board the design I drew a pcb based on the MAX 713 data sheet . I dont have the time to scratch design a circuit.
 
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moxhamj

New Member
Is the circuit the one on page 16 of the datasheet?

If so, can you post a photo of your board - maybe a component is round the wrong way etc.

And is it possible to scope the gate of the mosfet and snap a quick picture of the scope trace? If the driver transistors are not working then the gate will not be going up and down properly. If the mosfet is hot, I'd be suspicious that the gate is sitting at a half way point for too long and that could suggest the driver transistors not working.
 

BrendanP

Senior Member
Thanks Doc. yep the circuit is drawn up on the schematic on page 16. I'm about to bread board the transistor set up and make sure Ive got things the right way around. I'll get a pic up if I cant make progress.

I too am suspect on the driver transistor set up.

Give me call on 0429 336 867 if you want Doc and Ill call you back straight away. Ill pick your brains on this thing.
 
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BCJKiwi

Senior Member
The data sheet for the part you are using will give you it's pinout. Compare that with the Board design.

While one would expect the same numbered part to be interchangeable, why make such an assumption when the datasheet from the maker of the part you have can be compared to the one specified.
 
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BrendanP

Senior Member
MAX 713 data sheet is here

http://datasheets.maxim-ic.com/en/ds/MAX712-MAX713.pdf
page 16

I drew up the pcb according to figure 19 page 16.

Update:

I cut Q1 and Q2 off the board ( I have ten pcbs so i can load more boards as I need).

I connected the gate of M1 to a 08M pwm pin and am pulsing the gate with pwmout 2 , 99, 20 which works out at a 5% duty cycle.

I'm powering the 12V going into the pcb with my bench top power supply, it shows the circuit drawing 690 milli amps. My multi meter shows 4.2V across the battery pack.

So from this it seems that the MOSFET/ inductor side of things is functioning properly?

I notice the indcutor is hottish to touch. The MOSFET is warm. I presume this is normal?

Im going to load another pcb without the MOSFET and see what the pair of transistors are doing with my scope.
 

moxhamj

New Member
Inductors ring because the wires are moving due to the magnetic field. Over a long period of time, the wires can rub and wear out the insulation and short turns, and ideally an inductor makes very little noise. I found dipping inductors in varnish fixes the problem, but yours may already have had that done. They all do make a slight noise.

That inductor is 0.376 ohms and is good for 1.29 amps. Ideally an inductor runs warm but not hot as heat will decompose the varnish/insulation. You could swap in an inductor with a lower resistance. It will be bigger. Maybe a 4 amp one.
 

Dippy

Moderator
I hate to say it but a few days+prototyping would have saved money and all the time you 'saved' (!!) has now been blown. Unless you know exactly what you're doing, you should Breadboard > Prototype(s) > Production.
But I guess you realise that now. I'm not being smug and I don't mean to crticize as you are obv feeling a little blue but this is a lesson to others.

The FIRST things you should be checking with a scope are the FET and drivers, this was suggested days ago.

The data I'm looking at for that inductor is different to DrAcs.
http://www.a51magnetics.com/productexd.asp?id=DO3340P-224

Isat = 1.6A
Irms = 0.7A

You say, during your MOSFET/PICAXE-PWM test that the circuit was drawing 690mA, but did you chck the current being thumped into the battery pack? A good switched-mode converter could be bangng in quite a lot: 12Vx0.69 >> 4.2Vx? Amps. That could be why things where getting a little warm.

Have you even checked the DRV pin yet? With your scope.
 

moxhamj

New Member
The specs I was looking at are http://parts.digikey.com/1/parts/647001-inductor-shield-pwr-220uh-smd-dr127-221-r.html

The whole point of switchmode is that things are more efficient and don't get hot. Coils should not be hot - I'd be suspecting the coil is going into saturation, where the coil stops working as an inductor and the current rises very quickly and it effectively just becomes a resistor.

I appreciate you might not want to take a photo of the board in case we say that a component is in wrong and then you have to remake the boards. Even if the board was wrong though, there might still be a fudge - eg bending a few transistor pins in a different way so it fits correctly. Photo please, please! And some scope readings as Dippy suggests.

Re "I connected the gate of M1 to a 08M pwm pin and am pulsing the gate with pwmout 2 , 99, 20 which works out at a 5% duty cycle."

I'm not sure that will work. Looking at the circuit diagram, I am pretty sure the gate is not swinging 0-5V. Indeed, I suspect it is going between 0V and the supply volts. Driving it from a 5V picaxe will only be half turning on the mosfet. The mosfet datasheet show a test signal of 0-10V.
 
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BrendanP

Senior Member
"....I hate to say it but a few days+prototyping would have saved money and all the time you 'saved' (!!) has now been blown. Unless you know exactly what you're doing, you should Breadboard > Prototype(s) > Production...."

I freely admit I dont know what Im doing, thats why I just drew the board up copied from the schematic that MAXIM provide. I dont think its unreasonable expectation to presume the MAXIM engineers know what they're doing and to go ahead and act on that presumption. Even if I bread boarded the Maxim design and it worked what would that prove? That Maxim engineers know what theyre doing? Do any of us really doubt that?

If there are errors in the board due to my inexperience then they're going to be be there no matter how many times I breadboard the thing.

The problem is in the execution not in the design.

I made a linear charger board drawn from their schematic in the MAX712/713 data sheet and it worked fine.

I couldnt source the DO3340 coil shown in the schemtic so I used the Copper-Bussman part DR127-221-R. Before doing so I phoned their tech support in the US and asked them, they said it will work fine in place of the other part.

My board is double sided and very small, Ill put pictures up latter today.

Do you really believe I give a @#$% if someone says I have part the wrong way? If I have to get more boards made? Im here to get he job done, ego/feelings dont come into it.

I have noticed that SMPS supplies like the one for my notebook pc get quite hot in opperation.
 

westaust55

Moderator
Inductors ring because the wires are moving due to the magnetic field. Over a long period of time, the wires can rub and wear out the insulation and short turns, and ideally an inductor makes very little noise. I found dipping inductors in varnish fixes the problem, but yours may already have had that done. They all do make a slight noise.

That inductor is 0.376 ohms and is good for 1.29 amps. Ideally an inductor runs warm but not hot as heat will decompose the varnish/insulation. You could swap in an inductor with a lower resistance. It will be bigger. Maybe a 4 amp one.
Not so sure about small inductors which may be more cheaply made, but with high quality and really large inductors (physically large like up to 2m diameter), if they are air cored there is absolutely no noise. Iron-cored inductors and transformers do make a humming sound. This is not the winding moving but the steel laminations vibrating due to the magnetic field. The vibration is twice the electrical frequency so at 50Hz the hum is 100Hz.
 

Ralpht

New Member
BrendanP - I freely admit I dont know what Im doing, thats why I just drew the board up copied from the schematic that MAXIM provide. I dont think its unreasonable expectation to presume the MAXIM engineers know what they're doing and to go ahead and act on that presumption. Even if I bread boarded the Maxim design and it worked what would that prove? That Maxim engineers know what theyre doing? Do any of us really doubt that?


No offence Brendan but your comment highlighted in blue is the cause of the problem.

Most often circuits in a datasheet are examples only and are given as an indication of how the component can be used in a design. The Maxim engineers do know what they are doing but cannot foresee every use a component is going to be put to. Often the circuit will only show important parts of a circuit to demonstrate the use of the component and leave out the details because they assume the user knows what he is doing.

In this case their circuit is an indication of the simplest way to do it only and will still depend on good design by the end user engineer to get it to work properly. You did modify the design which implies potential problems if you don't know what is happening in the cct.
I agree with Dr_A, I think you are driving the Mosfet somewhere in its linear mode which will toast things quite nicely with the circuit shown. Insufficient switching is also saturating the coil and it is acting as a heater element.

Those two trannies you removed are fully driving the mosfet to the minimum required 10V. There is no way the Picaxe will do that unless you add a handful of other components to help the Picaxe deliver at least a 10V output and preferably more.

Get it working with the original components only and then work back from there one step at a time, and beef up the Picaxe output when you install that again.
 

BrendanP

Senior Member
Thanks for the input Ralph, I haven't changed the design at all, the picaxe hack was just a experiment to see what would happen. A 08M on a bread board and some hook up wire over to the pcb.

I find it hard to comprehend why a company would produce a circuit in a data sheet along with specifics of which components to use in that circuit and how to place them relative to each other if there was not a intention that people should implement that circuit.


I have already made, loaded and used a pcb according to the linear charger depicted in the fig. entitled "typical opperating circuit" on page 1. It works fine. I can't use a heat sink however so that why I've gone switch mode.

I spoke to Maxim engineers in the US today. I told them I had drawn a board up based on their schematic, that isn't the problem. I've sent through a copy of my pcb to them along with V/I readings around the board. I'll hear back from them tommorrow.
 

Dippy

Moderator
Well I think poor old Brendan has probably had enough of 'look before you leap' comments now...

I generally agree that if a Data Sheet shows an example with such exact specifications then you'd expect it to work.
BUT, Switched Mode circuits can be VERY sensitive to layout; track lengths, proximities and thicknesses can introduce unwanted inductances causing unwanted oscillations. I've seen it before when I've screwed up.
Physical proximities of 'feedback' tracks or sensitive signal tracks to power tracks (or swiching components) can really mess things up. Sometimes a screening ground plane is a solution.

The main areas I would check WITH AN OSCILLOSCOPE would be gate drives, the chip Drive output, the MOSFET to Inductor junction wave shape, behaviour at different loads (is inductor saturating), check for any nasty resonances and then repeat it all with an inductor of half the henris and ohms.

As suggested (maybe?) above if your MOSFET isn't being driven on/off fully or fast enough it may be feeding your inductor with a pathetic wave or even mostly DC with would give a hot FET and Inductor.
I don't think you've indicated any 'scope results yet?
 

moxhamj

New Member
The inductor should be ok. A bit strange as the D03340 lists a high resistance (0.44R) but a higher current (2.1A) than the one you used. (0.376 ohms 1.29 amps). I'd probably trust the resistance value and thus suggest the inductor you have is slightly better. But others may have a comment about that maximum current value and how it relates to the properties of the coil former.

So, why isn't it working? Why is it getting hot?

Three components are critical - the mosfet, and the two driver transistors.

A quick thought given the inductor is a different part - I presume the diodes are the correct parts?

I know a photo can be tricky to get the lighting right etc. If it is too hard, can you pls do a screen capture of the schematic (paint shop pro etc) and upload that? It will be very helpful as I think you are very close to getting this working.
 
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BrendanP

Senior Member
Thanks for your help guys. I've pasted in below my emails to and from MAXIM. As you guys have already stated it appears the problem is the inductor is saturating.

Ive got a old glass screen scope and so I cant get a screen shot of it.

The device charges the battery OK and terminates fast charge and goes to trickle charge OK. It does not appear to be heating the coil on trickle. Of course it could still be misfunctioning on trickle I suppose but with the small current flow (100 milliamps it cant generate any heat).

So I was thinking to look at the coil wave form when the device is trickle charging and then compare that with how it looks on fats charge when it is over heating or running hot and, presumably, saturating.

What differences am I looking for? When saturated and not longer switching I suppose the coil wave form will be a flat line? where as when its working OK I should see a square wave?

Where do I measure the coil wave form from?

I am using the exact diodes specified in the data sheet MBRS340T3.



Regards,

Brendan.




Hi Brenden,

This is regarding our recent conversation on the MAX713. Up on further investigation, it looks like the part numbers you gave me for the MOSFET and inductor should be adequate for your application without significant heating. I suspect the excessive heating may be due to parasitic inductance causing more than expected voltage levels in your circuit. Please send me the schematic and layout of your board for further investigation. Also, if possible please measure the voltage across and current through the MOSFET and inductor using an oscilloscope and let me know what you observe.

Regards,
Ben Wolde

MTS Customer Applications Engineering

Maxim Integrated Products, Inc.

120 San Gabriel Dr. MS 165

Sunnyvale, CA 94086

New email: ben.wolde@maxim-ic.com



Hi Ben,
Thanks for your help. Im under the pump with this thing, I originally wanted to use the MAX712 but couldn't because of heat dissipation problems.

I have attached a jpeg scan of a print out of the pcb design. My pcb program wont let me export a pdf it seems. Ive emailed them and if they get back to me I'll send on a color view. Which will make things clearer.

There are four views depicted.
Top left is tracks only.
Top right shows both top and bottom layers.
Bottom left is component layer only.
Bottom right is copper layer only.

I drew the pcb according to the schematic depicted on the in fig 19 page 16 of the MAX712-MAX713 data sheet. I am using a R25 as a current sense R. (R3)

There are couple of misreprensentations in the print out. I use a MPSA06 in a TO-92 package in place of the CMPTA06.(Q1 on the schematic) The print out shows the MPSA06 the wrong way around, I have the part the correct way around on the board.
The 10uf electrolytic on the bottom left of the board (C3 on the schematic) is shown polarity reversed. This is corrected on the pcb.

The battery pack I'm testing with is composed of three 3700 Gold Peak mAh NiMH batteries in series.

I took the following readings:

With DC in being 9V:

There is 1.15 amps going into the battery on fast charge
49 milliamps on trickle
charge terminates when cell pack reaches 4.7V

MOSFET
gate 3.19V
drain 5.59V
source 8.99V

MAX713 pin levels are as follows during fast charge:

Vlimit 2.28V
Batt+ 5.22V
PGM0 5.22V
PGM1 2.27V
THI 5.22V
TLO .6V
TEMP .77V
(fast charge indicator is operating correctly, I have a LED hooked up to it via a 470R and it lights when fast charging and goes out on trickle)
PGM2 2.27V
PGM3 2.27V
CC 2.29V
BATT- .28V
DRV 3.55 V
V+ 5.22V
REF 2.28V

I hope that information gives you some insight into what is going on. My brain is fried for today, Ive got some mcu programming to do so Ill 'relax' doing that and come back to this in the morning

Regards,
Brendan



i Brendan,
Thanks for sending the information. I am having trouble identifying components on your layout picture. Please let me know where the inductor and MOSFET are located. Also, I need to see the actual scope shot showing the current through the inductor and the voltage at the node where the MOSFET and inductor connect. That way we'll know if the inductor is saturating and what value inductor to replace it with.

Regards,

Ben Wolde
 
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BrendanP

Senior Member
Ok, I've attached a jpeg of the pcb layout. There are four views.

top left tracks only

top right top and bototm layers

bottom left compent layer

bottom right solder side layer.

There are couple of misreprensentations in the print out. I use a MPSA06
in a TO-92 package in place of the CMPTA06.(Q1 on the schematic) The
print out shows the MPSA06 the wrong way around, I have the part the
correct way around on the board.

The 10uf electrolytic on the bottom left of the board (C3 on the
schematic) is shown polarity reversed. This is corrected on the pcb.
 

Attachments

BrendanP

Senior Member
Ive attached another view of the track only shot hopefull it will appear larger. No it didnt attach at all.

delete this please dippy.
 
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moxhamj

New Member
Thanks for the pics.

However, I can't work out which components go where. Where are the transisitors, and which way round are they?

I could reverse engineer from the schematic, but if it is unclear on the pcb there would still be a possibility a component is in the wrong way round.

It is really important that gate drive is swinging the full voltage range.

A photo will help a huge amount.

Failing that, could you load up a board with the mosfet, the two driver transistors, the resistor. Don't put the IC in, but try driving the drive pin high and low via a 1k resistor. Eg 1k to 0V and to the supply volts. Measure the volts on the mosfet gate and check it goes the full range. That will test both the transistors are working.
 

BrendanP

Senior Member
Thanks Dr. latest email form MAXIM is below. Im about to order some parts from Mouser, I'll get more inductors with higher I saturation rating and MOSFETS. I notice as the sat. rating increases the inductance rating decreases. Whats the minimum inductance I can use?
I've sent you private message Dr.

Ive tried taking pictures, the shots are a blurry mess. The camera wont take shots at such a close range.

I'll load a board as you suggest and get back to you.

OK, I think were making progress. I have done as you suggested.

With supply of 9V the following happens:

When I pull the drive pin low via the 1KR the gate V level drops 2.5V according to my scope.

When I pull it high nothing happens it sits still.

Therefore the gate is only swinging through half its voltage range?


Hi Brendan,
Given your time limit, at this point I would recommend that you try replacing your inductor with one that has at least twice as much saturation current rating as the one you are using currently. Make sure the PEAK current you measured through your inductor does not reach the specified saturation current. You can also confirm the maximum power dissipated through your MOSFET from the oscilloscope reading of the voltage across the MOSFET and current through the inductor. Once you determine the maximum power, you can replace the switch with one that can handle at least 1.5 to 2 times the maximum power measured.

Regards,

Ben Wolde

MTS Customer Applications Engineering

Maxim Integrated Products, Inc.

120 San Gabriel Dr. MS 165

Sunnyvale, CA 94086

New email: ben.wolde@maxim-ic.com
 
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wilf_nv

Senior Member
Why not order the turnkey MAX713 evaluation board. It would give you a deliverable solution, identical to the circuit you are using and by comparing the operation of the demo board with your own board may give you insight why your version does not work.

http://datasheets.maxim-ic.com/en/ds/MAX713SWEVKIT.pdf

One other comment, if inductor saturation is the problem, you can also test that by reducing peak current for example by increasing the frequency by reducing C2 or by reducing the programmed charging current.

Switch mode efficiency goes way down when the inductor saturates. After making adjustments, you will know if the circuit operates normally when the output power rises above 60%-70% of input power.
 

moxhamj

New Member
I think we are narrowing the problem down to the two drive transistors. Their job is to swing that gate from 0V to more than 10V.

Looking at the specs for the chip, pin 14 is a current sink. So it sinks current, not volts, so no base resistor is needed and it can directly connect to a transistor gate. 30mA I think from the spec charts. With a 10V supply we can simulate that with a 330 ohm resistor, so maybe swap the 1k resistor with a 330R.

The CMPT06 is a transistor and the datasheet I found is a surface mount device. The boards you have are hard to read but is there a surface mount device on there or some other package.

And with the other transistor there seems to be some confusion about the pinout.

If there is no photo, we need a way of working out what the components look like and which way they are inserted into the board.

I don't know how to do that best. Maybe a big sketch on an A4 piece of paper and scan it in?

I'm thinking of all the permutations on ways two transistors can be incorrectly inserted. At least 6 I think, and only one combination will work.

There must be a way of transmitting information on which way round a transistor is in a board...
 
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moxhamj

New Member
Back from seeing patients.

I have an idea to narrow this down further. See the attachment from the schematic.

You have some spare boards. Build another board, and this time put just two components on - the 5k1 resistor and Q1. Put 330R between pin 14 where the chip goes and ground. Measure the volts where the base of the mosfet is. You should get zero volts. Now disconnect the 330R resistor. You should get the same volts as the supply.

That will tell us if Q1 is working. If it is, then that narrows it down to Q2.
 

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BrendanP

Senior Member
Thanks for your input Wilf. I did consider getting the eval board and in future will do so with new parts. I've come this far this way and the soloution looks to be in sight ( I hope).

Dr. I will do the experiment you suggest now on a new board.
 

Dippy

Moderator
Brendan, scoping/checking the drive transistors is something I suggested doing days ago.
I noticed the Tech boys asked for scope shots, which you don't seem to have supplied.

Do you have a scope? You don't seem to have posted any scope shots here.
What's the reluctance? (Anyone with any knowledge about magnetic, inductory stuff without using Google* will realise that is a superb pun:) )

Testing transistors at low frequency and DC is one thing but at higher frequencies its a different kettle of fish.

Your artwork/layout. I can't really follow it too easily as if there is any ident layer its lost in the lousy, sorry, reduced resolution.
Where does the inductor go? Please don't tell me it's at the bottom end of the board (as viewed on screen) and your MOSFET is near the top....???

For switched mode stuff you should (generally) group the input caps/driver/mosfet/flyback/inductor/output caps as physically close together as possible with great fat tacks. And any feedback/sensing must be as far away from switching components/tracks as possible. Ground plane/screening should have been experimented with at the prototype/homebrew stage. You'e got to keep track impedances/inductances to a minimum and also minimise any parasitic effects.

Maybe it's a duff transistor? But didn't you say you tried a few boards with same result?

We can witter on about inductors saturating and this that and the other. But, assuming the Maxim boys prototyped their Data Sheet example and that are no typoes, then it shouldn't saturate.
Have you put the transistors into a transistor tester?
Have been around an unpopulated board with a multimeter to compare with schematic?
Have you checked there haven't been any pth errors?

(*Naturally people will now Google and pretend they knew all along....)
 

BrendanP

Senior Member
I have attached a edited view of my pcb layout that shows the way I have Q1, Q2 and M1 connected. This view is looking down at the board/components ie. birds eye view/


(I have corrected the orientation of Q1 so if you see discrepnacy between this view and the other layouts this is why.)

I am using using MPSA06 as Q1 in place of CMPTA06 because its though hole. It comes in a TO-92 package.

Q2 is 2N2907.

The MOSFET is IRFU9024PBF

I have used the MBRS3040T3 which are SMD as is the inductor.


I have connected up the complete what appears to be functioning but running hot board to the scope.

With 9V supply I am seeing a wave form at the gate of M1 that goes from from 0 to 8 V's. Ive got the scope set at 1 volt divisions and the wave form is going from the top to bottom of the screen.

I have replaced R1 (sense R which controls fast charge rate) which was R22 with a R47.

The inductor and mosfet feel markedly cooler to touch . Fast charge rate is now 346 trickle 26.

Have a look at the pcb layout Dr. and we will go from there. It will probably save any more experiments.
 

Attachments

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BrendanP

Senior Member
".....Where does the inductor go? Please don't tell me it's at the bottom end of the board (as viewed on screen) and your MOSFET is near the top....???"

Guilty as charged........

Have you put the transistors into a transistor tester? No. I have about 50 of each type and have tried changing them, same result

Have been around an unpopulated board with a multimeter to compare with schematic? Yes.

Have you checked there haven't been any pth errors? Yes.

The thing basically works except for running hot. It switches from fast to trickle charge. It actually does charge the battery pack to the correct level. It terminates fast charge at 4.7V.

Maybe it is working OK. I had a HP made notebook AC power supply that used to run so hot when I was watching DVD's on the notebook I couldnt touch it. It died a few weeks ago after three years use.

The circuit is designed to supply maximum of 1 amp which with a R sense of R25 Im doing with it.

My brain hurts.........



I have a glass scope can't get screen shot of it. When Ive got some cash I'll get a pc driven unit but until then I have to make do......

Its no big deal I can redo the pcb with what i have learnt from this experience.

I've got a box of dud/cock up pcbs, cost me a few thousand, thats what it cost me to learn to use the GSM module.
 
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Dippy

Moderator
Yes, but it may switch from main to trickle chargre even if you had mostly DC running through that coil.

So, what does the gate drive look like on your 'scope?

You've got two main areas of potential fault with switching regulators which you can diagnose with a scope and the end of your finger.
Preamble: Your complimentary BJTs are drivng that Gate high/low. Obviously with that little MOSFET the gate charge won't be very high so the transistors don't have to be very manly. But it is crucial (for good efficiency) that they switch high/low fully and with good timing.

1. You've scoped the Gate. And you're getting a square wave. Its driving high/low. Good. Turn up the speed to 1uS/Div. What are transition times 0-8V and 8-0V ??

2. Touch the BJTs. Are they getting hot? If they are getting excessively warm, you are suffering from 'shoot-through' (I didn't make that term up so feel free to use it). This is where one BJT switches ON before the other went OFF and you get a brief short cicuit due to the 'overlap' where the current shoots through the driver. Idealy this overlap is zero, but that is impossible. A little bit of underlap would be better.

3. Scope the COLLECTOR of the MPSA06 i.e. the positive supply. If possible, on your second channel scope Gate drive. Trigger on the leading edge of Gate drive. There may be a brief little dip on the collector. If it's big then you may be getting the shoot-through.
(If this is the case, and it is unlikely, then oh dear.)

I would assume the Maxim boys selected those specific transistors to minimise this.

With proper driving and supplying 1 Amp max at 100% duty that MOSFET shouldn't get much more than tepid.
If the MOSFET is tepid and your diodes are the right way round then I'd expriment with a lower value inductor with a lower DC value.


Brendan, whats the point of redoing the PCB if you haven't diagnosed the fault? And it sounds like you've got money to burn :)
 

moxhamj

New Member
Darn, now my brain hurts. Conclusion is that the PCB and transistors look fine. I went off on a bit of a tangent with transistors that are TO94 cases instead of TO92 and have different pinouts, but these seem really rare and I'm sure TO92 is the standard sort of case.

I still don't understand the readings you are getting. Board with 5k1 and the two transistors and a 330R to pull the bases down to ground. Mosfet gate should swing rail to rail.

If it didn't, I'd be dropping in BC547/557s just to prove I wasn't going crazy. Or 337/328s (they probably would work ok in the final circuit). Then swap one out at a time. I'd be doing that on a breadboard (I'd do it myself but the shed is really cold tonight).

Sounds like you have a lot of dead boards lying around. What price are you paying for fabrication?

There is no reason this circuit can't be breadboarded. Then you can swap components in and out easily.

Digital photo of a cro trace should be fine. But you can describe it anyway. We need to get the DC voltages right on that gate, then go to testing AC waveforms to look for ringing etc.
 

BrendanP

Senior Member
I have attached a sketch I made of the scope screen. This is the gate pin of M1.

scope settings volt/div 1 time/div 1 uS

scope screen is composed of 10 increments horizontally 8 vertically.

BJT's feel faintly warm no more.

N.B. I have replaced the R.22 current sense with a R.5 which lowers the fast charge rate from around 870 to 345. But even at the higher charge rate I didn't notice the BJT getting hot.
(I'm familiar with shoot through from work with H bridges. tracks going up in smoke and so on.)

I have attached a sketch of what the screen looks like with a probe attached to the collector of Q1.

The charger has now gone into trickle mode. The wave for the collector of Q1 has gone virtually flat. The M1 gate wave looks preety much the same except it now has a sort of small nipple on its rising edge before it begins to plateau and then drop back down.

I won't get another board made until I sort out what the problem is here. One thing doing this sort of thing has taught me is that with peristence problesm will be solved.

I haven't got money to burn but there are circmustances when time is more valuable than cash. Thats where I'm at now.
 

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Dippy

Moderator
Well that looks fairly sharp, prob good enough for low power. Curvy top is odd, never seen that before. Come to think of it that shows that your MOSFET is in the resistive zone for a fair proportion of the time. To put it in perspective, the driver I'm doing has 0-20V rise/fall times of 200nS.

"I won't get another board made until I sort out what the problem is here." - hurrah!

"One thing doing this sort of thing has taught me is that with peristence problesm will be solved." - and maybe (or maybe not) with the suggestions supplied free of charge by others?? :)

I don't know Brendan, I've run out of brain on this one. I honestly would be tempted to put this board down, dig out a breadboard, have a cuppa coffee and start from scratch.
Or, better, make up a home-brew pcb with plenty of Fat tracks and close component positioning. (Take a note of the track layout and size on the Eval board that Wilf linked).

Would the Maxim eval board fit in your enclosure?

If you desparately need to use that chip etc. and have the time then I would buy the Maxim eval board and compare it side by side with yours with meters and scope.
Best of luck.
 

BrendanP

Senior Member
No desperate need to use the max chip. If you or anyone else has any suggestions I'll act on them re. other approaches.

The eval board won't fit in the enclosure. I will take note of its layout if I go on with the max part.
 

BrendanP

Senior Member
indulge me some more

I've been onto Maxim re. a eval kit for the MAX713 and the part is now obsolete ergo kit no longer availiable.

You thought the MAX713 had a big BOM, you should see the replacement part, at least its data sheet has suggested pcb track layouts.

I'm thinking to ditch the whole MAX/ switch mode thing and go with a picaxe based soloution.

Heres what Im thinking as a basis for begining experimenting/breadboarding, a picaxe controlling a transistor via pwm to control current and a DS1820B in the battery pack to allow temp rise,V slope and time terminated charge controll, it would also allow trickle charge/no charge if pack too cold or too hot.

A potential divider of high value R's across battery to allow V of battery to checked from time to time during charging to alow V slope terminated charging as well. High value to minimise drain on battery when not charging

I can easily bread board the idea and I have used all these parts before.

Can anyone give me some input if this concept has merit?
 
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