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Old 2nd June 2008, 11:32 AM   #1
Tahmid is offline Tahmid  Bangladesh
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Unhappy SMPS Inverter

I am making a 12v to 310v step up section for a SMPS IPS. I have read in many places that in SMPS, MOSFETs are not to get very hot. In my circuit I used SG3525 as the 30kHz oscillator and 2 IRFZ44N MOSFETs. But with just 20W (whereas IRFZ44N is supposed to take 110W load), my MOSFETs are getting very hot, even with a heatsink and fan. Can anyone tell me the reason? I'm following the ESP circuit, but with a few modifications.

http://sound.westhost.com/project89.htm

I changed soft start capacitor to 22uF, changed resistor between pin 15 and 13 to 4.7 ohm, omitted the external totem pole transistor drivers, and omitted the snubbers.

Help would be specially appreciated.
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Old 2nd June 2008, 02:25 PM   #2
areza is offline areza  Bangladesh
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you should use totem pole, 4.7 ohm is not necessary at all - if you follow standard layout methods for smps, be aware about fake mosfet sell allover stadium market place and other places,

my guess is if u dont make your own pcb then its a grounding problem , as well as cross conduction between mosfet,

at this power lavel pcb is must and gate driver totem pole should be very close to mosfet without and ground loop, i m running my “IPS” upto 300 watt (all load are inductive “ FAN + TUBE” ) with 2 IRF3205 with out significant heat,

IF ITS YOUR FIRST DIYING THEN YOU HAVE LONG WAY TO GO,

NICE TO SE SOME ONE FROM DHAKA DOING SUCH A ADVANCED INVERTER,
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Old 2nd June 2008, 04:09 PM   #3
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Dear Tahmid,
Carefully I read your posting. First, you should be aware regarding square wave characteristics for applying proper voltage. For square wave inverter, dc 12v to ac conversion should be 220volts, not 310v as for square wave, rms and peak value is same and hence there is no necessity for multiplying 220v with 1.4 and it is required only if you make dc to sine ac. So, you have applied over voltage and you have to change the secondary turn accordingly. You don't require external totem pole transistor drivers if you use 2/4 Mosfets as sg3525 is having internal totem pole drivers and it can supply 500 mA current to the gate of Mosfets. Dc supply from Battery to ic/mosfet should be clean dc and hence filter properly. High frequency can create ringing and parasitic capacitance and hence snubber circuit is required in most cases. You may use transzorb diode sometimes for clipping the spikes sometimes. Moreover, use of proper inductors/lc filter is also required for onrush of current as you can not use resistors as it dissipate heat.Using soft start capacitor to 22uf is alright I think and you can use 12/15 uf also as I use that.
Stadium market and Patuatuli is flooded with fake irf z44. You can use irf3205 as areza suggested which is having low rds on value-8mohm only.
Selection of proper ferrite core with proper air gap is also important for transformer for high frequency purpose.
I have made a prototype of 400w high frequency inverter with discreet ic/parts and working one with Pic Microcontroller controlled sine wave inverter. Hope to finish by October.
Success on Power Electronics require lot of studies, patience, hard work and Practical application. Work hard and you will be successful.
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Old 2nd June 2008, 04:09 PM   #4
Tahmid is offline Tahmid  Bangladesh
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Areza,
From where do you collect the MOSFETs? What is the shop's name?
Is your 300 Watt IPS switched mode or linear? If it is switched mode, please let me know some technicalities if you do not have any objection. If it is switched mode, from where have you collected ferrite core and what is the type of core? Please let me know. My email address is inferno-rage@hotmail.com . You can contact me in that email. It is very encouraging that you from Bangladesh are also interested in SMPS.
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Old 2nd June 2008, 05:59 PM   #5
areza is offline areza  Bangladesh
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IRF3205 can be found in many shops , carefully pick the best one , i have seen IRF3205 from FAIRCHILD possibly fake , not sure though, RDS-ON test show high ohm then IOR part, so try to pick IOR part , if you look the part under sunlight then you might se some tampering on the package , look carefully,

its a switch mode one , core can be snatched from beaten up computer supply, or even local tv repair shop, though the core from tv or monitor is gaped – you can always grind with sandpaper and make them un-gap, i use both atx core and flyback core (grind down ) with out much problem , you have to use several transformer if you need more watts or want to use ATX core, core material is unknown , dont really matter unless its a KW smps,

IRFZ44N is better then IRF3205 if you dont use discrete totem pole stage , cos 3205 has a gate charge 2.3 times higher then 44n, also use some dead time,

post a schematics and a picture if possible , its almost impossible to find out a problem with out looking at the real thing,

i like this forum and we should communicate through this thread ,
YOU WILL GET MORE HELP OVER HERE FROM OTHER MEMBERS THEN EMAILING ME,
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Old 3rd June 2008, 01:53 PM   #6
Tahmid is offline Tahmid  Bangladesh
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Areza,
When you say gaped, do you mean air gap or something else?
If it is air gap, then why remove it?
Where do YOU buy the IRF3205 from, could you tell me, if you have no objection?
And, do I need external discrete totem pole? Doesn't SG3525 have internal totem pole stages for driving MOSFETs directly? That's what they showed in the datasheet.
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Old 3rd June 2008, 01:53 PM   #7
Tahmid is offline Tahmid  Bangladesh
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1 more thing.
If you don't object, could you tell me the turns you gave in your transformer. And also the #SWG of wire.
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Old 3rd June 2008, 04:28 PM   #8
areza is offline areza  Bangladesh
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air gaped core is for flyback which store energy, you dont want to do it in push pull, cos it will increase magnetising current and loss some energy for nothing, for 3205 try swimming pool, if its a low power converter say 100-300 watts and/or you dont mind loosing some energy as heat through mosfet then you dont need a external totem poll stage, make sure you use some dead time,

you can just flow esp project for now, only thing you need to consider is core and construction of the transformer as well as pcb, which core you want to use,

primary turn is completely depends on cross section of the core and bmax you want to use for that particular core

the one i m using rite now has, if i remember correctly 2+2 turn for primary , and ~50+50 for secondary, primary is made with copper foil and secondary with 2 x 24awg,
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Old 3rd June 2008, 04:46 PM   #9
Tahmid is offline Tahmid  Bangladesh
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Areza,
Thank you for the turn details. I have collected IRF3205 of IR from Patuatoly.
If you are using 2+2 turns for primary, what frequency are you using for your transformer?
What do you mean by gap? And ungapping with sandpaper? - I am not understanding. Please make me understand, if you don't mind. I am learning a lot from you. Thank you very much.
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Old 3rd June 2008, 04:56 PM   #10
areza is offline areza  Bangladesh
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you can learn more from this books, look for them in net

BROWN, M. (1990). Practical Switching Power Supply Design
BROWN, M. (2001). Power Supply Cookbook (2nd ed.)

you must calculate turn according the core cross section you have, dont guess a value for turn, sandpaper= seris kagoz, got it,
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