Coil driver simulation and questions

On Tuesday, February 5, 2019 at 4:33:38 AM UTC-5, 69883925...@nospam.org wrote:
> dakupoto@gmail.com wrote
> 
> >On Monday, February 4, 2019 at 3:58:36 PM UTC-5, whit3rd wrote:
> >> On Monday, February 4, 2019 at 3:50:29 AM UTC-8, daku...@gmail.com wrote:
> >> 
> >> > The base of each 2N3055 is driven by a relaxation
> >> > oscillator at 150 Kilohertz. What is so fast about that ?
> >> 
> >> If those transistors go into saturation, the rise time when they turn off
> >> is very poor, by switchmode standards.  A bit of capacitance across
> >> the 10 ohm resistors would help.   So would a different transistor, with
> >> lower Rbb.
> >
> >Thanks, but I already tried out your suggestions 
> >before I made my initial post. For example, using a 0.01uF capacitor across the 10 Ohm and BC548(instead 
> >of 2N3055) boosts the current to 125 mA and the 
> >emitter voltage to 500 mV. The emitter output voltage consists of nice square wave
> >and the collector current is also a nuce square wave.  
> 
> The 2N3055 was not even up to audio frequencies with a beta of 20 to 70.. Ft of < 2.5 MHz
> The BC548 is low power with a beta of 200 (-B and -C version even higher), and an Ft of 100 MHz. 
> 
> For the current in an inductor i = t / L 
> where t is the time in seconds the transistor is 100% on (as a switch).
> A 3055 will likely not be 100% on any time soon in less than 1 / 150.000 seconds ;-)
> 
> A current limiting resistor in the emitter will create a current source,
> You do not want that if you use the transistor as a switch,
> unless it is a very low value and perhaps used to detect the current to dynamically trigger a cutoff.
> A small current transformer in the collector for feedback is better.
> 
> 
> Example of current sense feedback to a micro to dynamically current limit Ic every cycle:
>            +
>            |
>            L 
>            |
>       _____|_ 
> feedback |   }||
>         [ ]  }|( 10:1 current transformer
>       ___|__ }||
>            |  
>            c
>       -- b
>            e
>            |
>           ///
> 
> What are you trying to do, circuit diagram?
> 
> 
> I have used IRLZ34N MOSFETS as switch at that frequency.
> There are many suitable MOSFETS.

I agree that an emitter current limiting resistor acts as a current source, but connecting the emitter directly
to ground makes me queasy. So, I have a small resistance, 10 Ohm with a bypass capacitor. This is a side project, and I am trying to understand the workings of an air-core transformer.
  

dakupoto@gmail.com wrote
>I agree that an emitter current limiting resistor acts as a current source, but connecting the emitter directly
>to ground makes me queasy. So, I have a small resistance, 10 Ohm with a bypass capacitor. This is a side project, and I am
>trying to understand the workings of an air-core transformer.

The way I go about transformers is like this:

First the no-load situation, get the primary inductance.

If a switcher look at the maximum on-time of the switch.
 _______
|       | 
|+      )L
U       ) 
|-      |
|       / switch
|       |
--------

i
^   /
|  /
| /
--------->t

The current will then linear rise as U.t/L,
where t is the time the switch his on.
so in a 1 Henry coil after 1 second the current will be 1 Ampere.

L should be so big as to not create a huge no-load current (maybe 10% or less of full load).

So from L you can now calculate the number of primary turns,

Air-coils do not go into saturation, but if you keep the switch on too long the current will not rise linear
as it will be limited by the R of the coil.

And that is really, all there is too it, for a switcher that is.

The fun starts when the switch opens..... and a large flyback voltage appears across L if there is no load.