source:Industry News Popular:adapter release time:2021-06-16 09:21:28 Article author:sznbone
Fold back (fold back) current limit, sometimes called restartable current limit. It is similar to constant current control, except that its voltage decreases as the load resistance becomes zero, and the current also drops. But it is this small characteristic change that has a great impact on the circuit performance. In order to explain the principle, we consider the use of a linear power adapter here.
In the linear power adapter, the purpose of the foldback current limit is to prevent the power adapter from being damaged when it fails. When using foldback current limit, the circuit reduces the current under overload conditions, thereby reducing the loss of the linear adjustment transistor. Because the linear adjustment transistor has high losses, the foldback current limit is a method often used in linear power adapters.
The principle of foldback current limit
The figure shows a typical restartable characteristic curve, which is the actual measurement result of the output of the foldback current-limiting power adapter.
A pure resistive load can be described as a straight load line, as shown in the figure as a 5Ω load line. The starting point of each resistive load line is at zero, and its current and voltage are proportional.
When there is no load, that is, the resistance is infinite, the load line is vertical. When the load resistance changes, the straight load line will rotate clockwise with the origin as the center point. When it is short-circuited, its resistance will be zero and the load line will be in a horizontal position. It should be noted that this straight load line can only have an intersection with the restartable characteristic curve of the power adapter, as shown in the figure P1. In the case of linear resistance load, even if the shutdown characteristic curve is restartable Yes, the "lock-in" situation is unlikely to occur.
In the example shown in the figure, when the load current increases from zero, the output voltage remains at 5V. But when the current value increases to Imax. When the current limit value of is merged to P2, if the load resistance is reduced (and the load is increased), it will cause a drop in voltage and current. Therefore, in the case of a short circuit, the output terminal only outputs a small current lsc.
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