Re: gEDA-user: OT: help needed; asymmetric load after rectifier seems to disrupt its working.

[Andy Fierman <andyfierman@xxxxxxxxxxxxxxx>]

The most recent circuit you posted is not the same as your original
and as Gene pointed out, you have now made a series resonant circuit
between the 220nF cap and the 200uH primary inductance.

In the simulation, the source resistance is zero, the ESR of the cap
is zero and there is only 0.25R series R and a bit over 500R parallel
R (admittedly highly nonlinear) to damp the resonance. This is why the
output voltage is so high. The value of output voltages you see in
simulation may depend somewhat on exactly how the simulator is set up
(and which simulator you use). Gnucap, LTspice and QUCS show rectified
output voltages of around 180V for the 500k side output and 153V for
the 500R side. I haven't run this in ngspice.

A real transformer would have a K < 1 but for this level of simulation
setting K to 1 instead of 0.999 will significantly speed up the
simulation with negligible effect on the output. Setting K to <1
introduces some leakage inductance but at 0.999 this is very small so
it has a high resonant frequency with the 220nF input cap. The
nonlinearity of the load switching between 500R and 500k through the
diodes kicks this into ringing so the simulator spends ages
calculating each ring.

As Wojciech suggested, if you use an inductor with a much higher value
then the resonance drops to well below your band of interest and the
output voltages are about where you'd expect. The primary currents
also fall dramatically. In your circuit, with an ideal inductor they
are about 4.1A rms. Your little pulse transformer probably saturates
some way below that current. With a 2mH primary inductance this falls
to about 108mA rms. Of course, you then lose the bandpass filter
effect of the resonance at 23kHz.

The same general discussion applies to if you use a single inductor
instead of a 1:1 transformer except of course there is no leakage
inductance to worry about.

What were the scales on the scope traces you sent?

Cheers,

         Andy.

signality.co.uk




On 27 June 2011 01:27, Wojciech Kazubski <wk0@xxxxx> wrote:
> Dnia piątek 24 czerwca 2011 o 13:10:35 myken napisał(a):
>> This is strange in my simulation the attached circuit works fine. In
>> real life it kinda works but the signals are distorted like you can see.
>> I think that has something to do with the fact we used a pulse
>> transformer to try the circuit. If we disconnect Vx the signals stay the
>> same, so the distortion is in the transformer. If you say it doesn't
>> work then why doesn't it work?
>>
> Probably the transformer has too low inductance for that frequency, it should
> be in mH range. Magnetizing current    I=Uin/(2*pi*f*L)   is high and
> saturates the core so waveforms are not sinusoidal.
>
> Wojciech Kazubski
>
>
>
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