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This is a Linear Regulator that outputs 9V and upto 0.8A. This circuit uses Foldback Current Limiting for short-circuit protection: http://everycircuit.com/circuit/6159356643770368
Why is Foldback Current Limiting used? Is it better than the traditional constant current type protection? Yes, this is sort of better than the traditional constant current type protection because the short circuit current in a foldback is a lot smaller than the maximum normal operation current.
Here's how that's helpful. The power dissipated across the pass transistor in a linear regulator is given by
P = V(drop) * I(load)
where V(drop) is the voltage dropped across the pass transistor, I.e., Vin-Vout. During normal operation, the maximum power dissipated in the pass transistor is (20-9)*(0.8) = 8.8W. Now, in the event of a fault, the short-circuit current is 8.33mA. Therefore, the power dissipated is (20-0)*(0.00833) = 0.16W. Now imagine that the regulator uses a traditional constant current type protection. The Short-Circuit current would be equal to the maximum normal operation current. So the power dissipated would be equal to (20-0)*(0.8)=16W, which is double the power dissipated under normal operation! A circuit using such a protection needs the pass transistor to be mounted on a huge heatsink and might even require an active cooling solution, which just increases the power consumption, size, cost and complexity of the regulator circuit.
With Foldback protection, the pass transistor only dissipates 0.18W in the event of a short circuit. Therefore, the maximum power dissipated across it is at maximum current output during normal operation (8.8W). The size of the heatsink required is a lot less for this kind of power dissipation.
The other advantage of this type of protection is that the current flowing through the faulty load is tiny, due to which it won't sustain a lot of damage.
So if foldback protection is so beneficial, then why isn't it used all the time? Well, this type of protection has some negatives. One, it doesn't have a constant current mode. A constant current mode is available in all kinds of power supplies and is a useful mode. With this, the power supply can be used as a constant current source and it gets it's overcurrent protection too. Two, Foldback protection is useless for regulators and PSUs that have a wide input and output voltage range. Let's assume a regulator can take in a voltage of 0-20V and output a voltage as low as 1V. Also, let's assume that the maximum current output is 1A. At 1V output, the power dissipated in the pass transistor is (20-1)*1=19W. In the event of a short, the power dissipated is (20-0)*1=20W, which isn't a lot more than the 19W dissipated during normal operation. Because of this, it just makes more sense to use constant current limiting and design a cooling system for 20W. And constant current, like I said before, is a useful feature.
And this circuit, in particular, has an inherent or parasitic feature: over-voltage lockout. This can be seen both as an advantage and disadvantage. The advantage is obvious: it protects the circuit from an over-voltage. A high input voltage to a linear regulator causes high power dissipation in the transistor, and protection against it is useful.
To discuss the disadvantage, the mechanism of this over-voltage lockout must be discussed first. The 330mohm, 1kohm, 33kohm and the 11ohm resistors form a voltage device. The voltage appearing at the base of the PNP transistor is equal to Vin*(33k+11R)/(0.33R+1k+33k+11R). If the difference between this voltage and the input voltage is ~0.5V or more, it turns on and starts injecting a current into the base of the NPN transistor, which pulls the pass transistor's gate to ground. Due to this, the pass transistor is forced shut. Here's how this can be a disadvantage: if care is not taken to ensure that the ratio of the resistors is a suitable value, over-voltage lockout can occur at very low input voltages, decreasing the input voltage range of the regulator. The other reason why this is a disadvantage is because the voltage at with the circuit locks out depends on the value of load resistance and also depends on the values of the resistors which set the knee current of the Foldback protection. Personally, I'm not comfortable having a resistor do two things, but it can be seen as a positive too.
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