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H-bridge voltage output not AC

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GTBorderlines

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Hey guys, new to this forum. Looking forward to going through it and learning.


I've got a quick question for you- it's bothered be for a couple of weeks now.

I'm working on an h-bridge and am experiencing issues with none other than the output voltage waveform. In short, it's just not AC. Looking at the current;however, is much more appealing as it is nearly perfectly AC. So my question is- where have I gone wrong? My guess is the coil that I'm using (as the load). Maybe the possible combination of series inductance and capacitance produces some kind of overmodulation at my switching frequency?

This output is being driven by 4 mosfet fired by 2 lm2726 drivers.





I think my image is attached (I hope). I'm posting this on an iPad so I will have to check when I get home.


Btw, this is a from the ground up design. All components have been selected an individually bought. This is not an item you could buy on digikey.

Thanks in advance for your help


 

What's the load, how did you measure voltage and current? Most likely you didn't measure the voltage correctly, e.g. not the voltage difference between both H-bridge legs.
 

What's the load, how did you measure voltage and current? Most likely you didn't measure the voltage correctly, e.g. not the voltage difference between both H-bridge legs.

The load is just a coil. We've got a guy who's been studying how to build them over the last year. Its gotta have some combination of inductance and capacitance built into the design of it, as well as some small external caps. To measure the voltage, we used two proves- one on either side of the output. Then we used the math function to do Ch1-Ch2. The current is measured using a differential probe.
 

You are showing Ch1 & the math function, and not stating how you generate the math function on the scope, full circuit with values and showing how you measure and load details needed to fault find your 500kHz creation....!
 

May be it's just normal operation. You didn't tell how the H-bridge is driven.

Let's assume the output voltage is measured correctly. A square wave distorted by the load current related voltage drop. Of course it is AC but no sine waveform.

It looks like the load is a resonant circuit tuned to the fundamental. Looks familiar, e.g. like a resonant converter with series LC circuit.
 

**broken link removed**

attached is the basic circuit layout of the system. I didnt bother to put the gate drive signals for the mosfets. The math function is performing the operation CH1-CH2 where CH1 is one leg of the RC output and CH2 is the other leg of the RC output. The trouble is, I don't know those RLC values as i have trusted the guy who made the coil knew what he was doing. And im thinking therein lies the problem. The circuit is pretty basic - so i feel as if im missing something fairly obvious.
 

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