TI中文支持网Other Parts Discussed in Thread:BQ76930, BQ76940
使用的是BQ76930芯片，采样电阻是用为1毫欧，，设置短路保护电流为155A，短路保护时间为70us。
1. 在开启MOS管之前先把负载短路，再开启MOS 。示波器紫色波形为探头负极接采样电阻靠近电池端，正极接放电MOS输出端D，短路电流峰值超过1000A。时间超过250MS都没有保护，直到母线烧断，MOS管没有损坏，能正常工作。黄色波形为MOS栅极的电平，一直为高电平。
2. 但是开启MOS后正常运行过程中，如果用铜柱短路负载，则可以保护，示波器采样到的最高电流为750多A。。从短路到MOS关断总时间约为2MS。感觉也应该不太对，但是短路保护能正常工作，，关断MOS。。。请问这种情况可能是哪里导致的呢？
3.以上第1点情况母线烧毁后 SYS_STAT寄存器读出来的值是0x00。 第2点情况短路后SYS_STAT寄存器读出来的值是0x82。 正常运行情况下SYS_STAT寄存器读出来的值是0x80。从寄存器值来看，检测部分没有检测到大电流的可能性比较大。
Star Xu：
您好，请参考下面的答复
1. The part responds to the voltage at SRN with respect to SRP.Check how much SRP is raising during the event.Also look at the rise time, the filter gives the pins an exponential characteristic.With about 1V rise as shown it should cross the threshold quickly.The recommended range of SRP is low, +/- 10 mV.If the inputs get too close to the power supply (REGOUT) they will saturate and not detect or detect more slowly.At 1V it does not seem that you would be close.2. The schematic shows some circuit between DSG from the BQ76930 and DSG_G the common feed to the FET gates.Be sure there is no unexpected delays to the gates.
3. During short circuit all the inputs pull on the power filter caps.See http://www.ti.com/lit/slua749 section 3.Input filter resistors and power filter caps are large to survive this type event.Rc = Rf = 1k and Cf = 10 uF are typical recommendations for the BQ76930 and BQ76940.. Be sure the VC5X is not pulling into the VSHUT region. If the voltage pulls into VSHUT the part loses power and can't time the event properly.Long delays are typical with a drop into VSHUT.
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Zhangzhang：
如您所说，我把Rc = Rf 改成1k和Cf 改成 10 uF，VC5X到GND电容由原来的10UF增加到30UF, VBAT到VC5X电容由原理的10UF增加到20UF，短路测试后VC5X电压还是由原理的12V拉低到2V，麻烦您帮忙看看是哪里导致的。
另外前面您的第二点回答，BQ76930的DSG与DSG_G之间的电路是一个MOS管快速关断电路，，前一天问题中的图片示波器通道1和通道2的波形分别就是BQ76930的DSG和MOS管G极的波形
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Star Xu：
您好，请参考下面的内容
I think you have found the reason the part can't respond, it can not operate at 2V.The mystery is why VC5X can drop so far before the part responds.An example short circuit with 6 cells is shown in this video at about 7:10.training.ti.com/getting-started-our-monitoring-and-protection-ic-high-cell-count-batteries It is difficult to compare the capture to the waveforms above since the time scales are so different.As a rough approximation if the design has 2 power filter resistors Rf and 4 input resistors Rc pulling on the top group and 3 gate resistors, if we estimate this as 8 1k resistors in parallel or 125 ohms acting on the 30 uF, an exponential time constant would be 3.75 ms to reach 67% of its final value or here 33% of its initial value so 12V x 0.33 = 3.96 V expecting it to fall to 0V.So the part should still be operational here, and the maximum SCD delay is 400 us. This is a rough approximation but it seems something is not as expected.
Check that the SRP and SRN pins are connected and the DSG response.You might see if the part responds to a similar (approx 1V) voltage step applied at C21 and switches DSG without the power swing.If the part responds look more carefully at the event edge to try to determine why the power is lost.If the part does not respond look for unconnected pins or perhaps it is damaged.
Please let us know what you find.
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Zhangzhang：
您好，我用示波器查看采样线电压，发现电池电压确实被拉到3V以下，增加VBAT和VC5X引脚电容，让GND到VC5X引脚电压和VC5X引脚到VBAT引脚电压大于3.6V确实能够关闭MOS，，但是时间不太正常，约为5MS才能关闭， 我的电流采样线没有走差分，这个影响会有那么大吗？
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Star Xu：
您好，您参考下面的内容
5 ms is longer than any SCD_Delay and shorter than the shortest default OCD_Delay for the part. Again typically those long delays for an SCD indicate a power loss, but it seems you have found and corrected that situation.The SRP and SRN lines do not necessarily need to be differential, but must be connected to represent the current.Typically if they are routed separately they may have pick up locations away from the sense resistor which adds extra trace resistance to the sense resistor for an apparently larger sense resistor and the part will trip at a lower than expected current threshold.However check the connection and effect in your design.I expect you have already checked, but be sure the SRP and SRN pins are soldered well, also the traces and resistors to the current path.Check the capacitors are not shorted and if necessary the value.Since capacitor values are not normally marked you may need to replace them or check the rise time.
One technique which may be helpful is to apply a 1 V step to an approximate 900 ohm test resistor to SRNWith 100 ohm filter resistor to GND (Battery-) neglecting the sense resistor you get about 10% of your supply voltage.See if the rise time is appropriate, if you have a long rise time there is unexpected capacitance present.If you are using a lower SCD threshold you should be able to see the timing of the SCD response to the DSG output.If you have a higher threshold you will need a higher voltage or smaller test resistor.If the SCD response works as expected, look for some coupling from the current which is shifting GND or interfering with SRP or SRN when current is flowing in the actual SCD event.
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Zhangzhang：
好的，谢谢您，我尝试一下
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user4745244：
你好这个问题怎么解决的
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Zhangzhang：
上面回复有的，加大电容可以解决
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user4745244：
哪个地方加到电容烦请告诉下谢谢
