dc to ac converter - Page 1
Forum section: electronics / projects

author: franticEEV
author_rank: Newbie
timestamp: December 08, 2014, 07:27:26 pm
content: Hi,i'm new in this forum.I'm working on the dc/ac convertor shown below in the attached file.a digital piloting of mos is generated by fpga with pwm technique.Dead times for mosfet are generated in digital way.The aim is to generate a 50hz sine wave, 150Vac @100W.The frequency of pwm is 50K.The bridge doesn't work very well. With a 70Vdc supply the output sine wave starts to distort on positive and negative peaks where the consumption is maximum.The waveforms on the gates of the mosfets are bed, seems to ringing.The 10V drivers supply drops in that moments (positive and negative peaks)Have you have some advice to modify the circuit in order to obtain the best performance? Is the circuit right and components i'm using suitabale for this application?Thanks

author: bobcat
author_rank: Regular Contributor
timestamp: December 08, 2014, 08:03:34 pm
content: In the schematic, the h bridge is connected to the load incorrectly. Both sides of the load should be driven by the h bridge. The schematic shows a dead short.

author: splin
author_rank: Frequent Contributor
timestamp: December 08, 2014, 08:07:08 pm
content: Firstly, you don't have a bridge circuit - you have two push-pull drivers in parallel. Is that a 'typo' in your diagram? For a full bridge, circuit which is what I expect is what you intended it to be, you need to have the load between the two pairs of FET drivers so that you drive one side of the load to +Vcc and the other to ground, and then reverse it. That gives you an output of +/- Vcc peak to peak across the load which in your case would be 500V peak to peak, 250V AC peak (less voltage drop in FETs) or 176V RMS.With the load returned to ground as shown, you'll get a DC offset AC signal, with some rather odd waveforms depending on the values of L C and R compared to the switching frequency.

author: Yansi
author_rank: Super Contributor
timestamp: December 08, 2014, 08:16:26 pm
content: The circuit is completely wrong, this is a no no.On the schematic, there is NO bridge. There are two halfbridges with outputs parallel. Doesn't make sense, if needed more current, use parallel mosfets/igbts directly.Also, basic error here is that this circuit simply cannot produce AC. It is only DC supply of around 120V with a modulation on top. Cannot understand, what I am talking about now? Well... on the load resistor (200ohm) will not be present any alternaticng current. The circuit cannot reverse current flow through the load. Only positive current is being output.If you need AC on the output, the load must be half-the-supply-voltage-referenced. So use two beefy caps to make a divider, or use symetrical supply or make an actual full bridge with two lowpass filters.Bad gate signals are a sign of wery poor design practice, eg bad earthing style, long wires flapping in the breeze,...Power electronics is not an arduino with LED diodes... this is serious stuff, which can't be done right without proper knowledge, at least power basics.Note: I can't understad, why some people do stuff such as FPGA, while they have only little (or no) knowlede of the basics?

author: franticEEV
author_rank: Newbie
timestamp: December 08, 2014, 08:25:36 pm
content: I've made a mistake in drawing a real schematic...This is the right oneSorry

author: Yansi
author_rank: Super Contributor
timestamp: December 08, 2014, 09:10:11 pm
content: That was a bad mistake.That is also not good, because one output is not being filtered. Sure, the load will see (might see) sinus now, but this would never comply to any basic EMC requirements. You must filter both ouputs and the load connect between them.Can we see the actual circuit you have made? To guess the problems with bad gate waveforms...

author: franticEEV
author_rank: Newbie
timestamp: December 08, 2014, 09:59:59 pm
content: At the moment i can't make a pitcure of the circuit but trying to simulate it i see the same problems...and others...What could be the issue? Wrong MOSFET? Wrong bootstrap cap?Is the voltage the mosfets work too high? Is there another way to realize that kind of inverter?

author: Yansi
author_rank: Super Contributor
timestamp: December 08, 2014, 10:31:31 pm
content: Without any further info, we can only guess problems. And there is so little info, we cannot even guess that.We know only the PWM frequency, output voltage (and current) and that you will control that by FPGA (god why? there a lot of MCUs around designed specificaly for power supply management/control with a lot better peripherals, than you can make inside an fpga).We don't know a type of gatedriver, your whole setup schematic and/or how did you measure that, what kind of experience you have with power electronics (if any),...Ale least, please give us moooooooore detail of your whole experiment. Full schematic, photo, oscilograms of waveforms (gate-source, output of the fpga, ...) ... and link all neccessary datasheets (mosfets, drivers, diodes,...) - it is a little bit annoyng everyone must find them again and again.Full bridge with output filtering is quite good way how to do that. No neeed to think out innovations, it will work well, if designed properly!