Limiting current to 20A with 48V in
Unfortunately, there's no simple way to do this without adding your own DC-DC converter on the output.

The Problem

The problem is, the power supply you have will ALWAYS produce 48V.  The only way to limit the current, while keeping the output voltage constant, is to put something (a resistor or transistor, for example - this would be Q2 in your first image and T2/R4 in your second image) in series with your load.  The effect is you drop some voltage across that something so that the load sees a voltage at which it will draw less than 20A.

The problem?  Whatever the something is is dissipating enormous power - if the load draws 20A at 43V, you're dissipating 20A * 5V = 100W.  That's not impossible... but it's not practical.  

The Solution

The correct answer is to introduce a feedback loop which reduces the voltage set-point on the power supply to keep the output voltage wherever it needs to be for the load to draw 20A.  Unfortunately, this requires detailed knowledge of (and physical access to) the power supply circuit, and may or may not be possible with a given power supply.

So, if you don't have access to modify the 48V supply, and can't switch to a different one with a built-in current limit, here's what I propose:  

Place a DC-DC converter (48V to 48V) between your 48V supply and your load.  Connect it with a feedback loop in such a way that it operates in constant voltage mode as long as the current is less than 20A, then reduces the voltage to operate in constant current mode once the current limit is hit.

The Vicor app note AN:211 Constant Current Control for DC-DC Converters gives an example of how do this.  Rshunt (which will probably be in the milli-ohms to 10's of milli-ohms to avoid high power dissipation) acts as a current sense resistor, so that the voltage Vsense at the negative terminal of the op amp is proportional to load current.  

When Vsense &lt; Vref, the op amp rails high - the diode doesn't conduct, and the DC-DC operates in constant voltage mode voltage with a setpoint determined by Ru and Rd.

When Vsense > Vref, the op amp pulls low, causing the diode to conduct and reducing the voltage at the Trim pin.  This reduces the voltage set point, and therefore the load current, closing a negative feedback loop that keeps Vsense from going above Vref - that is, keeps your current from going above your current limit.



One final note:  You're actually interested in limiting the output current from your power supply, not your current to the load.  You could operate the effectively the same circuit, but sense the supply current instead of the load current, to get more consistent operation.