Getting Odd Current Waveform from H-Bridge

[Scruffles]

Hi there,

I am trying to solve an issue with an H-Bridge circuit with respect to the current it is drawing. Using a current probe on a scope, I am see a 200KHz waveform of the current. It rises slowly up to about 2A, and then right before the FETs switch, the current jumps to 4A and down to 0A, then ramps slowly back to 2A and so on... Looks something like this:

~4A .
..
..
. .
. .
. .
~2A .. .
...... .
....... . .....
..... . ....
.. . ..
0 .... ....

Dunno if that helps. I understand that maybe some back EMF would cause a bit of a jump at the end of the circuit, but really, the waveform should be sort of a wavy sine wave (ideally DC) if the switching of the H-Bridge was done properly as I understand it.

Any thoughts on how to get rid of this excess current? Could the input capacitance be too low?

Thanks

 

[ScottyUK]

What is the horizontal timescale? Is this effect at the end of each switching cycle? If it is, this may be the current caused by reverse recovery charge flowing in to the diode junction. You must be using a good probe to see it.

Reverse recovery is a well-documented phenomenon - a Google search or a good power electronics text will give far more detail than I can reasonable fit in here, especially without diagrams.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[baltic]

For H-bridge inductive load, load current should be piecewise linear. Assuming that current probe is good, 4A current peak may be caused by inductance drop due to magnetic saturation for current levels 2A and higher.

Reverse recovery current is seen by FET (not by load) at turn-on (not at turn-off). It is caused by an opposite diode of the same leg that was conducting on off portion of PWM interval. Reverse recovery current amplitude may be controled by FET turn-on rate.