R:[SI-LIST] : Zener used to clamp Vcc?

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From: Vigliarolo Roberto (Vigliarolo@asamrt.interbusiness.it)
Date: Tue Feb 29 2000 - 01:24:22 PST


If the zener is a TVS (e.g. SMC5.0) it can dissipate 1500W peak power for an
exponential pulse of 1msec and much higher power for shorter pulses (e.g. 15
kW for 10 usec) while clamping the voltage very effectively (Vclamp/Vz =
about 1.3)

Regards,

Roberto Vigliarolo

 Ferrovie dello Stato SpA
 Tecnica e Ricerca
 Unificazione Tecnica
 Viale Lavagnini, 58
 50129 Firenze
 Italy
 
 Tel: +39-055-235-3848
 Fax: +39-055-235-3581
 Email: Vigliarolo@asamrt.interbusiness.it

> ----------
> Da: Tom Dagostino[SMTP:tom_dagostino@mentorg.com]
> Risposta a: si-list@silab.eng.sun.com
> Inviato: luned́ 28 febbraio 2000 23.57
> A: si-list@silab.eng.sun.com
> Oggetto: RE: [SI-LIST] : Zener used to clamp Vcc?
>
> I'm curious what the actual function of the Zener diode will be. A simple
> model of a Zener diode is a reference supply of 5.6 volts in this case,
> with
> an ideal diode with Vforward of zero volts and a resistor in series with
> it.
> Parallel with these 3 components is a capacitor. In devices like the one
> you are proposing to use the R value is on the order of 2 to 5 Ohms and C
> is
> 100 to 200 pF. If you are trying to clamp narrow transients generated
> on
> the board with a single Zener diode, where are you going to place it? To
> be
> effective the Zener has to be near the source of the spikes. Otherwise
> the
> spikes may see the entire power supply bypass system before it sees the
> Zener. If the spikes are narrow they should be taken care of by the
> supply
> bypass system.
>
> If you are trying to protect the board from spikes coming from the power
> supply this seems like too small of a device to protect the board. To get
> any action from the Zener you will have the output impedance of the Zener
> to
> contend with, in this case 2 to 5 Ohms. Your 500 watt supply can source
> 100
> amps. For any reasonable spike from the supply the 2 Ohms series
> resistance
> will not effectively protect the board from over voltage. I suspect the
> Cdv/dt of the bypassing system will do a better job.
>
> True, the diode may be able to dissipate 30 watts for a few microseconds
> but
> to get 30 watts you will have almost 3 Amps flowing in the Zener. The
> terminal voltage of the Zener will be over 10 volts (5.6 volts plus (3
> amps
> * 2 Ohms), likely to blow any devices on the board. To clamp you need low
> impedance compared to the impedance of the source power supply in this
> case.
>
> Tom Dagostino
> ICX Modeling Group
> tom_dagostino@mentor.com
> 503-685-1613
>
>
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of WAUGH,RAY
> (HP-SanJose,ex1)
> Sent: Friday, February 25, 2000 2:46 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: RE: [SI-LIST] : Zener used to clamp Vcc?
>
>
> Chris...
>
> The Zener will do fine if the spikes you are clamping are not too fast
> (sub
> nano-second). Fast spikes must be clamped with a Schottky diode
> (picosecond
> fast).
>
> 400mW refers to the continuous power your diode can dissipate if you
> maintain the leads at 25 degrees C. That rating has no meaning when you
> are
> considering short duration current spikes. You can safely dissipate far
> more than 400mW in that diode if you only do it for a couple of
> microseconds. It really is a fairly complicated thing to figure out
> current
> carrying capacity (which is what you are really interested in).
>
> Ray
> ------------------------------------
> ray_waugh@agilent.com
> ------------------------------------
>
> -----Original Message-----
> From: Chris Bobek [mailto:cbobek@cadence.com]
> Sent: Friday, February 25, 2000 2:20 PM
> To: Si-list
> Subject: [SI-LIST] : Zener used to clamp Vcc?
>
>
> Hi,
>
> I was instructed to use a 5.6V Zener diode on the 5V Vcc plane of our
> board to clamp the voltage in the case of voltage spikes on the power
> supply. The Zener diode they chose is a 400mW diode. In your
> experience, is this the best way to handle power supply spikes? Is
> 400mW enough to accomplish anything? Our power supply is a 500W supply,
> I believe. The board typically draws 2A.
>
> I'm not sure what power supply spikes they are referring to. When I
> look at Vcc on a scope, the rising edge is a nice, quick, R-C charging
> curve. There's a lot of capacitance on our board and supply that
> dampens this response.
>
> Thanks for your help,
>
> Chris
>
>
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