Switching power supply behavior

In article , Scott Miller wrote: [...]

You can make the circuit in LTSpice and study the effects of various modifications without smoking real parts.

The transformer is two inductors with a coupling K between them.

In article , Nico Coesel wrote: [...]

It has some "hacks" in it that makes it able to handle switching circuits more quickly than normal spices. This is doubly true if you use their feature of imbedding the resistance and capacitance right into inductors rather than adding parts to the model to handle them.

Didn't we tell you how to dump that asinine MOV crap in a previous thread you started about this p.o.s.? -PLONK

Yep, it's as close as possible...

Understood that part, that's why the cap is there.

Ok, I get it. Didn't the the inductance in a ferrite bead would be enough to make a difference. I use them for filtering out RFI (100 MHz and up) and never looked much at their characteristics beyond that. Some of this is starting to come back to me from my ham radio licensing long ago...

This sounds like a good setup to learn LTSpice with. I found something similar in the example files. I've also got some dual ferrite cores I can go throw in the circuit and see what that does to my duty cycle requirements.

Thanks again,

Scott

No, higher ESR increases the effect.

This is somethimes true and sometimes not.

Deep breath:

The current consumption of the transformer occurs in pulses (there is current when the fet is on and none when it is off). This pulsing load causes the 9 volt battery voltage to bounce because it does not have a zero internal resistance, and there is inductance in the wiring between the fet/transformer and the battery. Any circuits at the fet/transformer end of that wiring will see both the battery bounce and the additional inductive bounce caused by the pulses. The capacitor connected directly between fet source and transformer (you do have it very closely connected to those two points, I assume) acts as a small local battery that soaks up a lot of that pulsation, so that it does not all have to come from the battery through the wiring inductance.

But a capacitor can provide and absorb current only if it changes voltage. The formula that relates capacitive voltage to current is I=C*(dv/dt), with I in amperes, C in farads, dv/dt in volts per second. So if you want the battery to supply the average current while holding steady voltage and also let the cap bounce up and down with each pulse a bit, so it can supply the pulses and recharge between them, you need some elasticity between the battery and the cap.

That is what the resistor is doing. But it is also dropping voltage at least equal to the average current from the battery to this circuit.

An inductor instead, will provide the elasticity that allows the cap to soak up the pulses while the battery does not, but can waste very little voltage if its resistance is low. A ferrite bead (or a few in series) will only separate the harmonics of the pulse (the edges) from the battery, but that may be enough to make the wiring inductance fairly insignificant, so at least only the battery internal resistance is involved in any bounce that other circuits see. A small wound inductor will give better low pass filtering that separates the cap voltage changes from the battery. A bigger cap with lower equivalent series resistance and inductance will have to bounce less to soak up the transformer pulses, regardless of what size inductor you use.

That is enough to supply essentially an entire pulse with very little sag, but it will wiggle a bit because of its internal inductance. The small high frequency capable cap in parallel damps more of that.

Yes. LTSpice has provision to add specified resistance and inductance ot each capacitor so you can test the effect of various amounts of these parasitics.

The OP needs a good rule for "considerable". I think he is safe so long as N*Vin is less than about 50% of the desired. He should be sure to limit the duty cycle of the PWM to not exceed this same percentage.