DIY - 1kW Push Pull Inverter - low efficiency - Page 1
Forum section: electronics / beginners

author: fodor95
author_rank: Newbie
timestamp: April 06, 2020, 09:18:49 am
content: Dear Colleagues,Recently I have tried to replicate as much as possible a commercial 1-2kW 12V->250V DC inverter. The main idea was that most of the hand power tools contain universal motors, which can be used directly from a DC supply, no AC power needed. I have looked up some commercial available dc to ac inverter schematics from elektrotanya for insiprational purposes. I have used the All in One software to calculate the needed turns for the SMPS ferrite transformer, which at 20-30kHz frequency, for a core of ETD59 came out to be, for a push pull topology to be 2x2 for primary and 45-50 turns secondary with full bridge rectification.I have attached the schematic below, and a few pictures. The problem is that the whole circuit works well(almost: i get an efficiency of around 50%-best), and I get on the secondary side a stable 200V output under a load of 0.75A. Above that the voltage starts to decrease, and the duty cycle on the primary circuit is up to 45% at around the maximum.I have tried to improve the efficiency by adding a 2+2 extra turns for the primary - but the efficiency this way has seriously decreased and the output voltage was unstable. I have tried to run at a higher frequency (at around 50kHz), again the efficency decreased.I need to mention the fact, that the transformer core did not got warm, only the transistors started to heat really seriously - so i propose that problem came from there. I have verified the gate drive signals - the transistors do not switch on each other. I have tried to measure the transistor drain-source voltages and there i have got some huge voltage spikes and i was unable to trigger the scope to get some usable measurements.Im in search for some ideas - or solutions from someone who have worked or designed such push pull converters. The interesting fact is that nor the commercial inverters have got any snubber circuits, only a 10R - 0.1uF capacitor connected in series - between the drains of the transistors.ftg_inverter.pdf(52.66 kB - downloaded 226 times.)

author: fodor95
author_rank: Newbie
timestamp: April 06, 2020, 09:19:29 am
content: PS: Thanks in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2020, 10:01:12 am
content: There's... no inductor? No current sense? Nothing, just wide open raw PWM? Yikes...Neither of the ICs have numbers shown.What is the winding diagram for your transformer?Tim

author: fodor95
author_rank: Newbie
timestamp: April 06, 2020, 12:58:43 pm
content: When mentioning inductor are u refering on the secondary side?I have added the ics, I have no current sensing - only secondary voltage - even commercial ones are lack of primary side current limit - they are only heavily fused (50-70A - many 15A fuses connected in parallel)Also i have made a figure in paint about how i have winded the transformer - primary coil is closer to the core - secondary is winded over the primary.Thanks.

author: langwadt
author_rank: Super Contributor
timestamp: April 06, 2020, 01:37:51 pm
content: what's the value of R13?

author: fodor95
author_rank: Newbie
timestamp: April 06, 2020, 01:39:49 pm
content: its just a higher impedance connection to pgnd, around 0.01R in order to protect the driver.

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2020, 05:29:04 pm
content: An inverter should operate with a fixed duty cycle close to 100% limited only by cross conduction and maybe reverse recovery. There is no regulation of the output voltage which tracks the input voltage.There needs to be provisions to either prevent the magnetic core from accumulating flux imbalance or for handling it; this can be caused by mismatched switching times or applied voltages. Typically the core is designed to handle some amount of DC flux with a small air gap but primary side current sensing could also be used to enforce balance.Flux imbalance in the magnetic core could explain the excessive transistor heating you are finding.While DC works for universal motors, phase control based speed controls will not function.

author: fodor95
author_rank: Newbie
timestamp: April 09, 2020, 09:53:59 am
content: Quote from: David Hess on April 06, 2020, 05:29:04 pmAnd how could be these ideas/laws be put in practice? I mean how i could measure a flux imbalance?Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: April 10, 2020, 11:31:01 am
content: Quote from: fodor95 on April 09, 2020, 09:53:59 amFor diagnostic purposes in a push-pull design, usually a current shunt resistor is placed in series with the transformer center tap which allows the current during each half cycle to be measured and compared. In theory the same thing could be done with a current mode controller but I have never seen anybody go to that much effort in an inverter.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2020, 06:39:16 am
content: Quote from: David Hess on April 10, 2020, 11:31:01 am* T3sl4co1l glances around nervously