500Khz Opto Isolated Gate Drive

Discussion in 'The Projects Forum' started by nenadilic84, Dec 28, 2009.

  1. nenadilic84

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    Aug 7, 2009
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  2. Ron H

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    Apr 14, 2005
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    The recommended maximum high level input current is 15mA. Unless your 5V logic source is current limited, the 47 ohm resistor in your schematic will allow about 75mA of input current. 10mA should be adequate. I would make the input resistor 330 ohms.
     
  3. nenadilic84

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    I didn't notice that. Thank you for the suggestion.
     
  4. nenadilic84

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    I tested the circuit and it is working fine to about 10kHz but when I increase the frequency my frequency counter is measuring diffrent frequency than that was on the input. What may be the problem??

    Regards,
    Nenad
     
  5. Ron H

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    Is your counter designed to handle 15V p-p input?
     
  6. nenadilic84

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    Yes. It is a multimeter and can measure frequency to about 200kHz. I have measured the frequency to about 10kHz, and it was OK but when I increase it it showed me 190kHz but the input frequency was 20kHz.
     
  7. Ron H

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    Fast-rising edges and long leads can result in ringing that could cause frequency measurement errors.
     
  8. nenadilic84

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    How can I prevent "ringing"??
     
  9. Ron H

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    Try putting a 100 ohm resistor in series between the probe and the point you want to measure.
     
  10. nenadilic84

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    I have made some modifications to the original circuit. Dose anyone have simulation models of 6n137, 74ACT14 and MC34152 for LTSpice so I can test the circuit first before building a PCB. Also if you have any suggestions please point them out. Any help would be appreciated.
    Thanks

    Data sheets for the components used:
    http://www.datasheetcatalog.org/datasheet/fairchild/6N137.pdf
    http://www.datasheetcatalog.org/datasheet2/e/0lk8pzxo9lzr2tckseq0r4fsyify.pdf
    http://www.datasheetcatalog.org/datasheet/motorola/MC34152D.pdf
    http://www.datasheetcatalog.org/datasheet/irf/irf630n.pdf

    Regards,
    Nenad
     
  11. SgtWookie

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    Jul 17, 2007
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    You need a small resistor (perhaps 2.2 Ohms) between the MOSFET gate and the TC429; the resistor should be a non-inductive type (thick film SMT would work) and right next to the gate. Otherwise, you will get high-frequency oscillations on the gate due to the parasitic L of the wiring from the driver to the gate, and the C of the gate.

    It would be a good idea to use a 10k resistor between the gate and the source in case there is a failure of the drive circuit.

    The IRF630 is a bit "long in the tooth" technology-wise. It would help if you can describe the nature of your load; and what the expected current and voltages will be.
     
  12. SgtWookie

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  13. nenadilic84

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    I forgot to attach the modified circuit in the previous post, so I have attached it now. Also I have calculated the peak current for charging the parasitic capacitance of a Mosfet and it is 2.0 A. Because TC429 was not giving me good performance I have changed it with MC34152 that can output 1.5A by channel, and because I need 2A output I connected two channels to drive one gate. The time needed for charging a parasitic capacitance is 15 nS. I have designed this circuit to drive an inductive load at 5MHz at about 30 Watts. Also I was thinking in connecting a 6 ohm resistor between gate drive ic output and the gate of the mosfet to limit the current to 2A, and the Schottky diode (1N5819) between gate and ground but would that decrease the switching speed? Also because of spikes from the inductive load I need some protection, any suggestions on that???
    Thank you

    Regards,
    Nenad
     
  14. nenadilic84

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    Thank you for the information this group is going to be helpful. I didn't find a simulation models for MC34152 and 74ACT14. Do you now good tutorial for building a simulation models in LTSpice??
     
  15. SgtWookie

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  16. nenadilic84

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    I'm sorry for quoting my self, but questions asked here ware not answered. I think that is because I had another post after this one and this one was not noticed. Sorry, but I really need some answers to this questions if some one can help. Thanks

    Regards,
    Nenad
     
  17. SgtWookie

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    You may not have noticed, but we've been super-busy lately with a number of projects. Plenty of people need input, and those who are in a position to help have a limited amount of time available. If you don't get a reply after several days have passed, go ahead and "bump" your post up. But please be aware that the board is not interactive, and thorough replies take time.

    That might work - however, I see you are using an IRF630 MOSFET, which technology-wise is getting rather "long in the tooth". Are you certain that is the best match for your application?

    Very generally, the higher the Vdss rating, the higher your total gate charge will be for a given Id rating.

    If you can use a MOSFET that is rated for a lower Vdss, you will likely get much better specifications for gate charge and Rds(on). However, since the application is unknown at this point in time, it is impossible to suggest another more modern MOSFET that may result in a substantial improvement. The IRF630 has an Rds(on) of 0.4 Ohms when Id=5.9A, which is very high by contemporary standards.

    Why do you need to operate at such a high frequency? It will not be efficient, as you will expend much energy by charging/discharging the MOSFET gate.

    You WILL need a "snubbing" resistor between the driver and the gate, otherwise you will wind up with inductive "ringing" due to the parasitic L of the trace or wiring, and the C of the gate, making a series resonant LC circuit. A small value of R will snub this ringing. 6 Ohms may be a bit large. That will hurt your discharge time when it comes to the Miller charge portion.

    Diodes have capacitance, believe it or not. Don't forget to add that in to your charge/discharge requirements.

    A Schottky/ultrafast recovery diode rated for more than your load current, from the drain to V+. Also, a small cap across the diode; somewhere around 330pF.
     
  18. nenadilic84

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    Aug 7, 2009
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    The MOSFET needs to switch 150V at the frequency of 5Mhz max, somwhere about 60W so the current is not going to be high (400mA). The rise and fall time of IRF630N is 15ns and it uses 2A to charge the input capacitance, so I figure it out that might suit my application. If you have any suggestions please point them out. Thanks

    This circuit is suppose to drive my Solid State Tesla Coil, and the resonant frequency is about 5MHz. I want to build a small and portable version of it and that is why the resonant frequency is so high.

    Ok, I will use a 2 Ohm resistor, but I'm not sure about W ratings? How do I calculate that. I now that 3A at 12V is going across it but only for a 15ns and that is 1/13 of a one cycle at 5Mhz. If someone know how to calculate this please help.

    Yes, output from the gate driver IC is 1.5A max, but IC has two outputs, so connected gave the output of 3A, and the requirements for MOSFET input capacitance is 2A so I think 1A is enough for a diode.

    Thanks for the advices,
    Nenad
     
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