Re: DMOS handling precautions

From: Winfield Hill (Winfield_member_at_newsguy.com)
Date: 09/17/04 

Date: 17 Sep 2004 12:53:09 -0700

Ken Smith wrote...
>
> Paul Burridge wrote:
>>
>> ISTR reading somewhere that DMOS devices don't require as much care
>> with handling WRT ESD as do the more common CMOS types. Having just
>> bought a bunch of 2N7000s for use as MOSFET drivers, I was hoping
>> someone could confirm or deny this...
>
> I'll deny it.
>
> Almost all CMOS chips have ESD protection. Many DMOS parts have no
> ESD protection because they don't want to give up any performance.

 Right, 2n7000 are not protected, and are probably more fragile than
 a typical CMOS logic chip. However the gate's input capacitance is
 much higher than that of a logic gate, and provides some protection.
 For example a typical logic gate has 2 to 5pF of capacitance, which
 compares to 40 to 50pF for a 2n7000 at zero volts bias. Nonetheless,
 you can easily zap the 2n7000 straight to kingdom come, as they say.

 In the early 70s I always used a Supertex VN01 type that came with a
 gate zener. When they discontinued the zener version, I called and
 complained. An engineer there told me they had more failures of the
 protective zener than they had from the unprotected gate alone, so
 they discontinued it. There are still MOSFETs available with zeners,
 but in considering them I think back to the Supertex guy's claim. In
 using about 3000 pieces of 2n7000 in recent years, I have encountered
 only about 4 damaged ones. This is a bit on the high side compared
 to CMOS gates, but it's not too bad. If one uses good static-control
 procedures, a much lower failure rate should ensue. 

-- 
 Thanks,
    - Win
 (email: use hill_at_rowland-dotties-org for now)

Relevant Pages

  • Re: DMOS handling precautions
    ... > much higher than that of a logic gate, and provides some protection. ... > gate zener. ... Doesn't that screw-up the input capacitance to ...
    (sci.electronics.design)
  • Re: Determining the level of protection
    ... BNC or other physical connection logic/analog signal input from the external world on something that warrants being called an "instrument". ... This connection will be frequently connected and disconnected, and is likely to have bare wire adapters attached by the ordinary user while taking no ESD precautions. ... This should have the maximum protection. ... Inputs should have a resistor sized for acceptable balance between input bandwidth and current limiting during ESD events and overvoltage application, dual diodes to the rails for primary ESD shunting away from the device, and another resistor between that diode and the device. ...
    (sci.electronics.design)
  • Re: Determining the level of protection
    ... BNC or other physical connection logic/analog signal input from the external world on something that warrants being called an "instrument". ... This connection will be frequently connected and disconnected, and is likely to have bare wire adapters attached by the ordinary user while taking no ESD precautions. ... This should have the maximum protection. ... Inputs should have a resistor sized for acceptable balance between input bandwidth and current limiting during ESD events and overvoltage application, dual diodes to the rails for primary ESD shunting away from the device, and another resistor between that diode and the device. ...
    (sci.electronics.design)
  • Re: Determining the level of protection
    ... BNC or other physical connection logic/analog signal input from the external world on something that warrants being called an "instrument". ... This connection will be frequently connected and disconnected, and is likely to have bare wire adapters attached by the ordinary user while taking no ESD precautions. ... This should have the maximum protection. ... Inputs should have a resistor sized for acceptable balance between input bandwidth and current limiting during ESD events and overvoltage application, dual diodes to the rails for primary ESD shunting away from the device, and another resistor between that diode and the device. ...
    (sci.electronics.design)
  • Determining the level of protection
    ... This connection will be frequently connected and disconnected, and is likely to have bare wire adapters attached by the ordinary user while taking no ESD precautions. ... This should have the maximum protection. ... Inputs should have a resistor sized for acceptable balance between input bandwidth and current limiting during ESD events and overvoltage application, dual diodes to the rails for primary ESD shunting away from the device, and another resistor between that diode and the device. ... In my recollection from my recent SPICE experiments with ESD protection networks, RC networks just tended to shuffle charge from the external capacitor to the internal one, and causing the protection diodes in the device to be protected to bear an excessively large surge current. ...
    (sci.electronics.design)
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