Re: Snubbers (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Fri, 1 Apr 2005 17:07:51 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Snubbers (fwd)

Hi Matthew,
               Some quick tips on designing a snubber:

On 28 Mar 2005, at 8:06, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Sun, 27 Mar 2005 18:09:07 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Snubbers
> 
> Hi All
> 
> I know it's not the first time I've asked questions on snubbers, but
> they are still something of a black art to me...
> 
> I found the following article:
> <http://www.ridleyengineering.com/snubber.htm>
> 
> My interpretation is that I can size the resistor simply by giving it
> the same impedance value of the leakage inductance of the transformer.
> If I put an inductance meter (or my DMM) on the primary of a
> transformer, how much of the inductance measured is leakage
> inductance?
> 
> I'm still working on building a PWM flyback driver where none of the
> bits get hot...
> 
> Cheers
> 
> M
> 
> -- 
> Matthew Smith
> Kadina Business Consultancy
> South Australia
> http://www.kbc.net.au

The energy remaining in the core (call it E) has to be got rid of 
before the switching transistor next switches on. The resistive part 
of the snubber deals with this bit. The capacitor has to absorb E 
when the switch switches off. The voltage rise across the capacitor 
is determined by E and the capacitance according to: V = SQRT(2E/C).
C must therefore be sized to limit the voltage rise to an acceptable 
value (if for instance you have a switching transistor with a Vceo of 
500V then a safe (conservative) allowable voltage rise would be 450V, 
maybe a bit more. The resistor must be sized so that it dissipates 
this stored energy during the switch "off" time - ideally 5 CR time 
constants would be enough to do the job completely so the resistor is 
sized according to the value of C and the time from switch-off to 
switch-on. The power rating of the resistor is related to the average 
power dissipation: W = E/Tcycle and the major consideration when 
choosing a power rating for the resistor is the allowable temperature 
rise and the removal of heat i.e. the resistor may have to dissipate 
1W on average but a 1W resistor is operating at its ratings with no 
temperature rise so a 2W resistor (where, say, a fan is removing 
heat) or a 5W resistor where there is no fan may be required.
      The value of E is the trick - it depends on how much core 
energy is passed to the load and how much remains when the transistor 
switches off so a ready answer is not available out of the blue. 
Where the power drain may vary from little to a lot, an energy 
recovery winding is often employed together with a steering diode to 
channel unused power back to the DC power supply. There are too many 
unknowns to give a quantitative answer so experimentation for your 
particular supply would be warranted. The final part of a snubber is 
a steering diode so the capacitor is not switched to ground through 
the transistor when it switches on - i.e. it steers the excess energy 
into the capacitor.

Malcolm