Basic working principle of undervoltage protection

Issue Time:2006-03-10
The figure shows a method of energy storage and transfer. When SW1 is turned on, the power adapter charges C1 and C2 through resistors R1 and R2, respectively, and their final voltage value will reach the power adapter voltage Vs.

If the circuit is removed from the power adapter and SW1 is closed, then C1 and C: will be connected in series and a voltage of 2Vs will appear across the circuit.

In the figure, the above circuit is applied to both ends of a linear regulator input regulating tube Q1, and the capacitor in this figure is in a state of being fully charged. Now if SW1 is closed under the undervoltage condition, capacitor C1 is connected in series with C2, providing a voltage of 2Vs at point A of the circuit.

Because the input voltage of the linear adjustment tube at point A exceeds the specified output voltage value V, Q1 can operate as a linear regulator, which provides a required transient current to keep the output voltage of the load terminal almost constant. The state will continue until C1 and C2 discharge to half of their initial voltage of 2V.

In the dynamic case, C1, C2, SW1, and Q1 form a series circuit. The position of each component in the series circuit does not affect the overall function of the circuit. In addition, both SW1 and Q1 can be used as one switch, one of which is redundant. In this example, SW1 is redundant.

The figure shows the actual application of the circuit; SW1 has been removed, and Q has moved to the original location of sW. Now Q1 completes the aforementioned SW1 switching function and the Q linear adjustment tube function. Although the performance of this replacement is not obvious, it can be verified that the circuit has the same characteristics as the circuit of the figure.

As mentioned earlier, as long as C1 and C2 are able to maintain a desired terminal voltage, the voltage regulation can be maintained. It is clear that the load current and the size of C1 and C2 determine the adjustment process. When the voltage on the capacitor reaches approximately half of the initial voltage value, the voltage at point A is too low and transistor Q will stop the adjustment behavior. Since the energy stored in the capacitor is proportional to V2 and 1/4 of this energy is consumed on the linear regulator, half of the energy storage is available.

Due to the efficient use of stored energy, a smaller capacitor can be selected than the usual bypass capacitor. Even if the capacitor voltage drops, the load voltage can be kept only within a few millivolts during the entire undervoltage, so better performance can be obtained by the dynamic transient suppression circuit.

It should be noted that the resistors R and R bring unwanted loads to the capacitors C and C when the circuit is in the SW and Q shutdown states, and the choice of R1 and R resistance should be compromised. When the large resistance value is taken, the load of the capacitor is small, but a long charging time is required.

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