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Power Adapter Forced Current Sharing and Remote Sampling

Issue Time:2006-12-07
Forced current sharing
A voltage-controlled power adapter is essentially a device with low output impedance. Because the output voltage and performance characteristics of any two or more devices are not exactly the same, when these devices are operated in parallel, the load current is not evenly shared.

Many methods can be used to force current sharing. However, in most cases these techniques force the current sharing by reducing the output impedance of the power adapter (and thus the load regulation), so in parallel forced current sharing applications, the load The regulation performance is lower than the performance of a single device application.

One possible exception is the master-slave setup technique for voltage-controlled current sources. However, master-slave setup technology is now unpopular because it does not provide redundant parallel operation, and failure of the main module system often results in failure of the entire system.

If forced current sharing is not provided, one or more of the power adapters will operate in maximum current limit mode, and the remaining power adapters will operate almost at no load, however, as long as the power adapter is capable of continuous operation in current limit mode, this limit The flow allows the power supply to have a reasonable service life, so a simple direct parallel connection can also be used. This method should not be ignored.

Remote sampling
If the load is far from the position of the power adapter, the voltage on the power line drops significantly, and the performance can be improved by using remote voltage sampling in the power adapter. In principle, the reference voltage and the input of the comparison amplifier are connected to the remote load through a separate voltage sampling line to counteract the connection voltage drop effect. The current on this sampled wire is small and the voltage drop is negligible. This connection arrangement compensates for the lead drop caused by the output lead of the main circuit by increasing the required supply voltage to ensure that the voltage supplied to the load is normal. This is especially useful in low voltage, high current applications. But users need to know at least three limitations of this technology.

(1) The maximum allowable external line voltage drop on the power line is usually limited to 250mV per line, so that the voltage drop between the two leads is 500mV. In a 100A, 5V application, this will increase the additional power consumption of the power adapter 50w, which is lost on the power line.

(2) When the power adapter is to be connected in parallel redundancy mode, it is common practice to connect a diode in series to isolate each power adapter. The principle is that if a power supply is shorted, the diode will isolate the power adapter from the rest of the device. If this connection is used, ignoring all lead losses, the voltage at the power supply terminal is at least 0.77V higher than the load terminal unless the power adapter is specifically Designed for this mode of operation, otherwise the required power adapter terminal voltage may exceed the maximum specified by the design. It must also be noted that in the event of a power failure in parallel redundancy mode, the amplifier detects that the sample leads will still be connected to the load and detect the voltage of the load. The remote sampling circuit should maintain this condition without causing more damage.

The general practice is to connect the remote sampling terminal to the power adapter output with a resistor inside the power adapter to avoid runaway and voltage overshoot when the sample line is disconnected. When this resistor is used for parallel redundant connections, it must be able to consume a certain amount of power v/R, and there will be no failure when the output voltage drop of the power adapter is zero.

(3) The remote sampling is connected to the high gain portion of the power amplifier loop, so any noise received on the remote sampling line is transmitted to the power supply terminal as output voltage noise, thereby reducing the power supply performance and is caused by the lead inductance and resistance. The additional phase shift may have an unstable effect. Therefore, it is recommended to use a two-wire as a remote sampling line to minimize inductance and noise interference.

Transient characteristics are degraded due to distributed capacitance, and coaxial cable connections are not recommended unless the coaxial cable is properly matched.

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