What is step load current?

[Artlav]

I was looking through the math for the two switch-forward converter ( https://www.onsemi.com/pub/Collateral/AND8373-D.PDF or https://www.onsemi.com/pub/Collateral/TND378-D.PDF
 ), and they use something called "step load current" to calculate the output inductor value.
The number is just given in the example solution as if i'm supposed to know it, yet it is never mentioned or defined before or after.

What does it mean?

The transformer primary peak current is proportional to the size of the output inductor, and yet it is derived from the size instead of the size being dictated by the primary peak current. So this step load current seems to be more important.

[diyaudio]

I was looking through the math for the two switch-forward converter ( https://www.onsemi.com/pub/Collateral/AND8373-D.PDF or https://www.onsemi.com/pub/Collateral/TND378-D.PDF
 ), and they use something called "step load current" to calculate the output inductor value.
The number is just given in the example solution as if i'm supposed to know it, yet it is never mentioned or defined before or after.

What does it mean?

The transformer primary peak current is proportional to the size of the output inductor, and yet it is derived from the size instead of the size being dictated by the primary peak current. So this step load current seems to be more important.

Figure 27. Step Load Response from 0.5 A to 5.5 A should be obvious.

Its the load "steps" in increments as you can see 0.5 to 5.5A, in steps of 500mA per step.
 

[PSR B1257]

What does it mean?

As the name implies 
A current step from I₁ to I₂ (refere also to page 18)
Therefore it is nothing more than a differential current, hence the Delta in the formula.

It makes no difference whether you go from 0A to 5A or from 5A to 10A, the current difference is always the same.

[Artlav]

Ok then...
If i have a fixed load, does that mean that this is zero or near zero?
If so, all their math ceases to work, with a filter size approaching infinity.

What would be a good way to estimate the filter size in that case?

[Jay_Diddy_B]

Hi,

I described the circuit for doing these load current steps in this thread:

https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/msg288313/#msg288313

For the purpose of design you should choose to step the load current from 50% to 100%, even if your load current doesn't step. Then size the output capacitor for around 3% change in voltage.

The load step response is used to verify the stability of the control loop.

Regards,

Jay_Diddy_B

[coppice]

Ok then...
If i have a fixed load, does that mean that this is zero or near zero?
If so, all their math ceases to work, with a filter size approaching infinity.

What would be a good way to estimate the filter size in that case?

A fixed load? Really? It never switches on or off? It can never fail and suddenly go from full load to zero? You might want to consider the impulse response of your design under all its operating conditions before you use the notion of a fixed load in your design.

That said, good power supply design is all about achieving satisfactory impulse response, and a benign load makes the power supply designer's life very easy. Its often possible to treat sudden load disconnects as a special case, and arrange that the supply does an emergency shutdown, completely avoiding the need for continuous responsive control.