homebrew inverter power - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: September 08, 2018, 12:20:47 am
content: hi all ive just cobbler together a square wave inverter,its a multivibrater running 50hz driving 4 n channel mosfets,ive rewound the secondary windings of a big old ex psu transformer,for 12v+12v center tapped,it works ok but wont put out more than about 100watts at most,the mosfets are well over rated the transfo is about 6kg,its running a 50/50 mark space ratio on the multivibrator,nothing gets warm and it works fine just guttless,any ideas?,cheers m3vuv

author: m3vuv
author_rank: Super Contributor
timestamp: September 09, 2018, 05:52:52 am
content: could it be low current passing thru the transfo due to the on resistance of the mosfets?,and if i parallel them will the on resistance be lower and increase current flow and give me more watts output?.

author: chemelec
author_rank: Contributor
timestamp: September 09, 2018, 01:40:41 pm
content: What MOSFETS are you using?What Value of Gate Resistors?Also For a Correct Output Voltage, Your Transformer should be wound to have 9-0-9 Volts out.(12.6 volts, times .707)

author: chemelec
author_rank: Contributor
timestamp: September 09, 2018, 01:45:48 pm
content: It should also be BIFILAR Wound for best efficiency.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 09, 2018, 02:06:01 pm
content: Hi, some remarks to your design:- If its a Royer converter, the turns ratio is the same as the (peak) voltage ration plus some percent to cover the losses.If you pick a line transformer, I should have 8-9V secondaries.- What Royers dont like at all is simulateous conduction on both legs caused by slow MOSFET turn-off. An appropriate dead-zoneshould eliminate this- MOSFETs should have a sufficiently low RdsON. If you want 200W, e.g., we have a current of close to 20A at 12V inputs.To keep losses low, the voltage across a MOSFET when ON should be less than 200mV, even that gives 2W per transistor.This means an RDsOn of 10mOhm. Not really much !- A soft-start circuit (PWM the duty cycle up within a few seconds) and an overcurrent protection (e.g., buy sensing drain voltagedurch conductive phase) are also important.- Toroids are not bad du to less stray field, but should overdimensioned by a factor of 1.5.Much success !Wolfgang

author: m3vuv
author_rank: Super Contributor
timestamp: September 11, 2018, 08:44:17 am
content: im using the attached schematic,except i have 4 mosfets,2 on each leg,i realise its a crappy design but think it will do what i want if i can get more power from it.cheers m3vuv

author: m3vuv
author_rank: Super Contributor
timestamp: September 11, 2018, 11:02:00 am
content: the mosfets are irf 3205 devices ive 12 of then now 6 paralell in each leg,not finised the heatsink yet tho so havent tried it yet.

author: chemelec
author_rank: Contributor
timestamp: September 11, 2018, 03:37:32 pm
content: Each MOSFET should have a Gate resistor of about 100 ohms.Here is a Better Inverter.If you use Just Two STP75NF75 MOSFETS (Rated for 75 V and 75A) in this circuit, it will work quite good for a power of over 250 Watts.Assuming a suitable transformer and bifilar wound.

author: m3vuv
author_rank: Super Contributor
timestamp: September 16, 2018, 01:30:49 am
content: well it woked,used 2 banks or fets with 100ohm rsustors on the gates,its putting out about 800 watts now.