Re: LF: How can you tell if a Class D amp output transformer is starting to saturate?

[N1BUG <paul@n1bug.com>]

Thanks Andy.

I've added a page on this in my handwritten notebook.

I do have some related questions which go beyond what Chris asked about.

What about stacked cores? If we stack three cores, each having a
cross sectional area of 1.58 cm^2, does it become 4.74 cm^2? Do the
cores have to be in physical contact with each other? What if each
is first wrapped with insulating tape, then they are stacked?

Finally, this dealt with ferrite cores. What about iron powder core
saturation?

Paul



On 05/03/2018 12:05 PM, Andy Talbot wrote:
> The peak of the fundamental sine component in a square wave is 4/pi
> times the square wave amplitude (yes, it is bigger).   The RMS of
> the sine is Peak / SQRT(2) .  Take the two together and the RMS of
> your sine is therefore 0.9 * Peak of the square wave
> 
> So if you had a Vdd of 50V you would have 45V rms across each half
> of the winding, or 90V rms across the two halves.  Which answers
> your other question
> 
> Actually, thinking about what I've just said, you are putting the
> square wave though the transformer, not a  sine.  So in that case
> you have a bit more leeway in that it can go to the same peak value
> as the sine would have given you.
> 
> Unless I'm building SMPSUs, I don't put square waves though
> transformers in transmitters - teh RF is always filtered
> beforehand.   There is an equivalent equation for square waves and
> used in SMPSU design,  it is   V.t = N.A.B    Now V is input
> voltage,  t is the on-time , and N.A.B as before
> 
> And yes, your area is correct.  1.58cm^2 = 0.000158m^2
> 
> Andy
> www.g4jnt.com <http://www.g4jnt.com>
> 
> 
> On 3 May 2018 at 16:36, N1BUG <paul@n1bug.com
> <mailto:paul@n1bug.com>> wrote:
> 
>     I am still not sure whether I am using that equation correctly.
> 
>     Chris, if you will forgive me for tagging along on your post, I'm
>     trying to get this equation sorted in my head. Put no faith in
>     anything I say with regard to this! ;-)
> 
>     Andy, could you please check me out on this?
> 
>     Let's use the case Chris asks about as an example.
> 
>     Suppose I want to calculate the maximum RMS voltage to stay out of
>     saturation.
> 
>     The manufacturer of the core states its Ae (equivalent cross
>     sectional area) is 1.58 cm^2. I believe that works out to
>     0.000158 m^2.
> 
>     F = 137000
> 
>     I do not know if I should be using 4 turns or 8 turns for the
>     primary. It's 4+4. I will assume 8.
> 
>     Vrms = 4.44 * 137000 * 8 * 0.000158 * 0.1
> 
>     Vrms = ~77V
> 
>     Does that look right?
> 
>     As for practical application I have no idea what this means. The
>     equation uses Vrms but we're talking about something closer to a
>     square wave than sine wave. But then it's not really a square wave
>     either...
> 
>     Paul
> 
> 
> 
>     On 05/03/2018 10:00 AM, Andy Talbot wrote:
>     > By measuring the voltage across the winding using a scope, then
>     > seeing what Bmax is using teh equation
>     > 
>     > Vrms = 4.44.F.N.A.B
>     > 
>     > F Hz     N turns   A core cross-sectional area in metres^2    B tesla
>     > 
>     > If B works out higher than about 0.1 for a ferrite core, then you
>     > are approaching saturation
>     > 
>     > Andy
>     > www.g4jnt.com <http://www.g4jnt.com> <http://www.g4jnt.com>
>     > 
>     > 
>     > On 3 May 2018 at 14:46, Chris Wilson <dead.fets@gmail.com 
> <mailto:dead.fets@gmail.com>
>     > <mailto:dead.fets@gmail.com <mailto:dead.fets@gmail.com>>> wrote:
>     > 
>     > 
>     > 
>     >     Hello LF'ers
>     > 
>     >     In  say a W1VD power amp, or in general, how can you test if the
>     >     output transformer is
>     >     starting to saturate please? Thanks.
>     > 
>     >     http://www.w1vd.com/137-500-KWTX.html
>     <http://www.w1vd.com/137-500-KWTX.html>
>     >     <http://www.w1vd.com/137-500-KWTX.html
>     <http://www.w1vd.com/137-500-KWTX.html>>
>     > 
>     > 
>     > 
>     >     -- 
>     >     Best regards,
>     >      Chris                          mailto:dead.fets@gmail.com 
> <mailto:dead.fets@gmail.com>
>     >     <mailto:dead.fets@gmail.com <mailto:dead.fets@gmail.com>>