video of waveform

Hi, A 555 timer at 10 kHz is driving a IR2117 gate driver.
IR2117 datasheet here

enter image description here

The load is a capacitor in parallel with a light bulb.
The IR2117's Vcc is supplied by a bench power supply.
A permanent magnet alternator (=PMA) output is connected to a bridge rectifier and the DC output of the rectifier is connected to the Drain pin of the MosFet. I am manually spinning the PMA. The varying magnitude of the load wave form during the off cycle seems to suggest the MOSFET is not turning off properly.
I do not understand why the MOSFET is not turning off when the IR2117 input is low.
This is a first attempt and I do not have any real experience with this type of circuit. Thank you for any help received.

Frame from video.
Lower trace = IR2117 input.
Upper trace = load waveform.
The load is on for some of the time when the IR2117 input is low.
Watching the video will allow the timing of this relative to the overall cycle of events to be seen much more clearly.

enter image description here

  • 2
    \$\begingroup\$ Huh? What "load battery"? What "permanent magnet alternator"? What is VDR? "The PMA"? It should be obvious you are not defining your terms and assuming a lot of information we can't possibly have. You didn't even include a diagram. It is difficult to tell what is being asked here. I am voting to close because until you clear this up I cannot answer the question. \$\endgroup\$ – Olin Lathrop Mar 4 '12 at 14:16
  • 2
    \$\begingroup\$ Please edit your question to include a schematic or SPICE netlist so it can be properly analyzed. \$\endgroup\$ – tyblu Mar 4 '12 at 14:40
  • \$\begingroup\$ Lauri - you need to urgently tidy up yu question and improve on the diagram provided. If the description in the question is not quite correct please improve it. If yu do not do this then people who have difficulty understanding what you are saying will close thequestioj and you will need to re-ask it. The question is a good one but some people have difficulty following your description. A proper diagram would help a lot. \$\endgroup\$ – Russell McMahon Mar 4 '12 at 21:52
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    \$\begingroup\$ @Y'All - there is enough information here to allow he (or she)that has an ear to address a very unusual and interesting technical question. How about rising to the challenge of English as a second language (probably) and seeing if you can stretch your brains far enough to follow what is above plus my answer. \$\endgroup\$ – Russell McMahon Mar 5 '12 at 2:09
  • \$\begingroup\$ Lauri - did you try my suggestions? What happened? \$\endgroup\$ – Russell McMahon Mar 7 '12 at 4:50

I think that you may be running into problems with the way you are using the driver IC. This is not very intuitive. The driver is driving an N Channel FET so the gate needs to be driven somewhat higher than its source to turn it on. Usually V+ applied to the load (here your PMA) is greater or much greater VCC or at least equal to VCC and the "bootstrap" capacitor pump (cap between Vb and Vs and diode) provide adeaute gate drive voltage.

However, when the PMA is stationary Vhigh = 0, then it rises as you spin the PMA from below to equal to above VCC. The gate bootstrap follows this up but then the PMA stops spinning and voltage declines again. BUT the MOSFET is now reverse biased (or may be) because there is voltage on the output cap and this flows back though the MOSFET body diode and potentially back into the bootstrap circuit via the source.

The end result is unusual and possibly undefined and ...

TEST: Power the bridge rectifier input from Vcc - what result? Now from > Vcc but steady DC. What result?


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