Mosfet's gate driver easily burnt by my osscillscope

Discussion in 'General Electronics Chat' started by raychar, Dec 5, 2011.

  1. raychar

    Thread Starter Member

    Nov 8, 2011
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    When using my osscillscope's probes to touch input and output of gate driver of mosfet for measurement. I sometimes observe small sparking when the ground clip touchs the pin. Then, the driver became dead. Same thing happens when connecting them to micro-controller, but contoller won't burnt off.
    Can anyone help me what's wrong with it?

    Thanks,
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Your scope´s ground is connected to mains protection earth, which is in turn connected to your power supplies ground, especially on SMPS supplies. By connecting your scopes ground to anything else but the circuit ground you are shorting something.

    Use two probes in a differential manner, one channel normal and the other one inverted and set scope to sum, and keep the ground connected to ground or leave it open.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you are using a typical mains powered oscilloscope, its input grounds will usually be effectively connected to the mains earth.

    If the device you are trying to monitor has any connection to the AC mains, or to earth, then you should only connect the oscilloscope probe ground clip to those points on the device under test which are at ground potential.

    Connecting the probe ground to any point on the circuit which is at some voltage away from earth potential can cause a damaging short-circuit.

    Edit: To answer your original question, unless your oscilloscope is provided with a "floating" input, which most are not, there need not be anything wrong with it. It is most common for scopes to be made this way.
     
    Last edited: Dec 5, 2011
  4. raychar

    Thread Starter Member

    Nov 8, 2011
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    I have limited knowledge on it so I don't understand fully.

    In my project of high-side mosfet driver, the VS is floating. That means I should not connect the ground clip to VS and positive probe to HO for measurement or it will burn the driver. Please help me for clarifying.. What method can I use for such measurement.

    Thanks,
     
  5. kubeek

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    Sep 20, 2005
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    Please post your circuit, but like I said, best is to use two probes like with a volt meter.
    The grounds is basically for shielding, so if you don´t exceed your scope´s max voltage wrt. ground, you will be able to measure what you need.

    Another option is lifting the ground ( disconnect protection earth) either on the circuit or your scope. Lifting ground on scope is very dangerous and shouldn´t be done by inexperienced people, because you then may connect your scope´s ground to a mains line which will put line voltage straight to your scope´s case.

    Another possibility are mains isolation transformers.
     
  6. colinb

    Active Member

    Jun 15, 2011
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    How do you use a mains isolation transformer? If the scope runs off the transformer, what do you do with protective ground? If it's left floating, you still have the same safety issue where the scope's grounds might be at high voltages w.r.t. earth ground.

    If you run the device under test (DUT) on the isolation transformer, instead of the oscilloscope, then you have a safer solution, I think. But I'll let those more experienced comment on that.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    That's what I meant, but if you use it without thinking you can easily end up with lethal voltage on the case of the DUT as well.

    If you use isolation transformer or disconnect the safety ground from anything, please use a GFI with small trip current like 3mA just to be sure.
     
  8. thatoneguy

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  9. MrChips

    Moderator

    Oct 2, 2009
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    Your scope is already grounded. You do not touch the grounding click of the scope probe to any active part of your circuit. The only place you should connect the probe grounding clip is the part of the circuit that you know for certain is at ground potential.
     
  10. sheldons

    Active Member

    Oct 26, 2011
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    you need to transfer your ground lead to the primary side of the smps(if you are dealing with an smps)...dont directley connect your probe to the gate of the mosfet driver as loading it in this way can upset the drive conditions and destroy the device causing a lot of unnecessary damage...always take it from the smps ic op pin directly....another trick you can use is connect an external supply to the control ic (with the mains switched off)and then check the drive to your fet.(most are 10-12v dc,with some early tv smps ics a 9v battery was ok-but do not have the mains on to do this).....when checking on the secondary side of your supply transfer your ground lead to the secondary side ground point.if you dont ,expect a big bang and smoke.......
     
  11. jimkeith

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    Oct 26, 2011
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  12. ashokcp

    Active Member

    Mar 8, 2007
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    Connecting the Ground clip to Vs is connecting Mains Earth to Vs.
    For measuring the voltage on the gate,
    You could use 2 channels of scope. One channel (A) probe is to be connected to Gate, and, the other channel (B) is to be connected to Vs. Ground Clips of both the channel probes are to be connected to ground of your power supply (which should be at mains earth). Set the scope in A-B mode, or A+B Invert mode, basically setting it up for A-B so that A's potential is measured w.r.t. B's potential. Both the channels should be at the same Y-Scale setting. Hope this is helpful.
     
  13. raychar

    Thread Starter Member

    Nov 8, 2011
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    Attached is the circuit to test the IR2110 component. With a channel of +ve probe to HIN and ground clip to circuit ground. Another channel of +ve probe to HO and ground clip to VS. The floating voltage VS is 0V or maximum +90V. Is there any problem and why the driver sometimes burnt off?

    Thanks
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    that is nice, but you need to post the rest of the circuti, i.e. how is Vb and Vs generated and how are the outputs and output mosfets connected together at least.
     
  15. raychar

    Thread Starter Member

    Nov 8, 2011
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    Here is the application circuit. I want to check the VG-VS waveform. The seperated 15V power supply to VB-VS is a transform type.

    It's difficult for me to use two channels in differential mode in scope to take this measurement, may be my scope has't this function or I need to study the user manual more...

    Thanks
     
  16. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Youre missing connection between the Vs pin and the mosfet source.
    Also you don´t show how the +80V is created, but if it´s ground is connected to protection earth and you stick your scope´s ground to source, then when the mosfet is on youre shorting the +80 through the protection earth back to the supplys ground.
     
  17. raychar

    Thread Starter Member

    Nov 8, 2011
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    Yes, I missed to draw it. The +80V is from transform type power supply, it's earth doesn't connect ground, only the case of power supply does.

    Ah, I know the (X-Y) function on scope, thank.

    By the way, the circuit can't function (even VS connect to th source of mosfet). Could anyone help to advise what's wrong?

    Thanks,
     
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    the part at bottom right is a relay switchnig at 1khz?
    Other than that, the circuit should work fine if the Vb and Vs are connected properly and with the right voltage.
    Maybe the 240 ohms gate resistor is too high to turn the mosfe on properly.
     
  19. raychar

    Thread Starter Member

    Nov 8, 2011
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    With gate resistor changed to 39 ohm and omitted the 1KHz relay, that is, to short its ends. The light bulbs still don't lit up. Maybe I need to check the waveform of each points...

    Thanks,
     
  20. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Make it simple. Get rid of the 1khz wave at the input and put there either 5 or 0V, then the bulb should be either on or off. This way you can measure DC voltages with normal multimeter to verify each point is where it should be.

    After you make it work like this, switching at 1khz should be a breeze.

    I don´t know what relay you plan to use, but I am pretty sure none will work at 1kHz switching rate.
     
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