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Date Submitted Wed. Jun. 12th, 2013 1:18 PM
Revision 1
Helper aussiegadget
Tags " laptop battery" | "power"
Comments 0 comments
A few days ago encountered a strange case, including the user's R & D engineers, project managers, old barefoot doctors and several experts brains, painstakingly. The results did not think the answer is so simple. But still allow users to research and development staff and project managers have a lingering fear, barefoot doctors are a bit long learning. Today show to share with you.
 
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One user, with our E3641A power supply for the DUT power supply. DUT is not complicated, includes a microcontroller, such as capacitors and resistors, power supply voltage is 25V, stable and normal operating current is 50-55mA. Power users through the GPIB interface for programming, the voltage is set at 25V, and set up a 100mA current limit to protect the DUT. This should be a very simple application. Only need to supply a very high purity, there is sufficient protection, and the need to control via computer interface.
 
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But is such a simple thing, or a bizarre story. Users found that when the power supply 100mA current limit setting, and turn the power supply output, the stable and sometimes work, but sometimes, the current display is very unstable, will appear from 20-50mA, and the DUT can not start properly. Users then very puzzled, but also particularly worried about the DUT burned out. He began to suspect that the power supply current measurement problem out. Thus, in the current loop in series with a 34401A digital meter to measure current, the results of measurements digital meter displays the same value as the power supply, the power work is normal. Even more incredible is that in order to solve this problem, the user changed a same level of other manufacturers programmable power supply, do the same settings, even working properly, and the current value and the user's expectations are the same!

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So very skeptical users of our power problem. Initially, we did make me baffled. The same voltage, the same limit, do not understand why there is a different result. This seems inconsistent with Ohm's law and common sense. Thus, painstakingly, repeatedly check wiring, check the grounding, did not find the crux. We have even changed a precision and speed, better performance of the system power supply, do the same settings, use up, and found the problem is even more serious than the E3641A, the DUT is basically not start. Toss several times, imagine a lot of programs but the problem is not solved.

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Later, I asked run for Agilent's a power supply expert. He based on past experiences, and made a bold inference: the user to supply a current limit set low. DUT is likely during the boot process, there are more than 100mA inrush current, at which point the power supply current is limited to a 100mA, before the system starts, the power started overcurrent protection, so that the DUT does not start properly. So, we try to gradually increase the current limit, up to 650mA, the user's system is actually normal! However, users in any case could not believe that in his design of the product, there are as ten times higher than the normal current inrush current, so that everyone is more incomprehensible that, in addition to a power supply at 100mA current limit setting, the system can normal startup. These two results are totally contradictory.
 
To verify this, we use the N6705B DC Power Analyzer, with high-precision power supply module, and cancel the current limit, open N6705 oscilloscope function captures the DUT power surge current waveform. This waveform, or allow users to surprise! As shown below:

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From N6705B-screen display can be clearly seen, the product starts instantly appeared a star inrush current peak value of 650mA, far beyond the current limit of 100mA, it is higher than the normal operating current of 50mA ten times ! Inrush current duration is about 800us, then the current value returns to 52mA normal. Users see this phenomenon is very surprised and acknowledged the existence of this design found no significant deficiencies. Because such a high inrush current may cause the user's entire system failure.
 
Further, we set on the N6705B limit of 100mA, to simulate a power failure earlier. We got the following results of this test:

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  As 100mA current limit function, inhibit the inrush current during startup. Once the product required current exceeds 100mA, it will automatically go into current limiting mode, the current stable at 100mA or less, while the voltage will fall, the DUT protection. However, in this case, the output will not be required to meet the DUT power supply conditions that lead to customer product into an unstable working state, consumption current value representing random values. Found the cause of the problem, but there is a greater wonder why another one of the programmable power supply 100mA current limit is also set up, but the system could work it?
     The expert explained that this is due to different performance limiting state power into the time required different. In order to protect the DUT, once the over-current situation, we will soon enter the power-limiting condition. We found, in varying degrees according to over-flow, E4641A enters current limit time is about 0.5ms - 1.5ms. The higher performance, for example, a high performance module N6705B in 50uS - 1mS. That manufacturers supply, this indicator is 10mS. Therefore had encountered such a large streams are, E3641A and N6705B in the tens to hundreds of microseconds entered the limiting state, limiting the need to start the inrush current. And that house limiting state power into the time needed to grow more than 10 times, in the absence of limiting access, the DUT can already started. But there is a huge potential danger for a long time overcurrent may burn DUT. It's like the car's braking system, if found dangerous able to stop within five meters, compared to 50 meters in order to stop the car, which is more secure do you feel? Faster startup time meant to protect the DUT is more secure, especially when you are measuring the price of gold and other heavy prototype, this indicator is particularly important, because we do not want to be too slow burn your power supply overcurrent response for several months of effort.
 
Agilent E3600 Series base power,