Editing Current sense
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Latest revision | Your text | ||
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for measuring current: | for measuring current: | ||
* resistive shunt | * resistive shunt | ||
− | + | * low-side current shunt | |
** high-side current shunt | ** high-side current shunt | ||
* magnetic field sense | * magnetic field sense | ||
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== resistive shunt == | == resistive shunt == | ||
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− | [ | + | [http://electronicdesign.com/article/articles/what-s-all-this-shunt-stuff-anyhow-2144.aspx "What's All This Shunt Stuff, Anyhow?"] |
by Bob Pease 2002 | by Bob Pease 2002 | ||
describes one way to custom-build a high-current shunt. | describes one way to custom-build a high-current shunt. | ||
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Even though a copper shunt has a pretty bad resistance variation with temperature (tempco), other parts in this circuit compensate for it. | Even though a copper shunt has a pretty bad resistance variation with temperature (tempco), other parts in this circuit compensate for it. | ||
− | + | == low-side current shunt == | |
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Low-side is (electrically) the simplest. | Low-side is (electrically) the simplest. | ||
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=== high-side current shunt === | === high-side current shunt === | ||
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In situations where low-side sensing is difficult ( automobile electronics bonded to the "GND" car frame; other systems where it is inconvenient to put a resistor on the "lo" power wire), we turn to high-side sensing. | In situations where low-side sensing is difficult ( automobile electronics bonded to the "GND" car frame; other systems where it is inconvenient to put a resistor on the "lo" power wire), we turn to high-side sensing. | ||
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== MOSFET voltage == | == MOSFET voltage == | ||
− | [http://www.4qdtec.com/mircl.html pseudo 'mirror' current sensing a MOSFET] -- sampling the voltage across a MOSFET while it is turned on. That voltage is linear with current but varies with temperature. If the purpose of measuring current is to turn off the MOSFET before it overheats, the variation with temperature doesn't matter. (''A true [ | + | [http://www.4qdtec.com/mircl.html pseudo 'mirror' current sensing a MOSFET] -- sampling the voltage across a MOSFET while it is turned on. That voltage is linear with current but varies with temperature. If the purpose of measuring current is to turn off the MOSFET before it overheats, the variation with temperature doesn't matter. (''A true [http://en.wikipedia.org/wiki/Current_mirror current mirror] isn't useful for motors, right?'') |
This uses the turned-on resistance of the MOSFET as if it were a shunt resistor. | This uses the turned-on resistance of the MOSFET as if it were a shunt resistor. | ||
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==== lossless average inductor current sensing ==== | ==== lossless average inductor current sensing ==== | ||
The "lossless average inductor current sensing" technique: | The "lossless average inductor current sensing" technique: | ||
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The current through an inductor with some known internal parasitic resistance DCR can be sensed with a series resistor (Rs) and capacitor network connected in parallel with the inductor-resistor network. | The current through an inductor with some known internal parasitic resistance DCR can be sensed with a series resistor (Rs) and capacitor network connected in parallel with the inductor-resistor network. | ||
− | The designer picks the sense components Rs and C such that Rs*C -- the time constant of the RC network -- is close enough to L/DCR -- the time constant of the inductor resistor network | + | The designer picks the sense components Rs and C such that Rs*C -- the time constant of the RC network -- is close enough to L/DCR -- the time constant of the inductor resistor network -- then the voltage across C is proportional to the current through L. |
(IR3508Z data sheet | (IR3508Z data sheet | ||
[http://www.irf.com/product-info/datasheets/data/ir3508zmpbf.pdf]) | [http://www.irf.com/product-info/datasheets/data/ir3508zmpbf.pdf]) | ||
current through L == voltage across C * (1/DCR). | current through L == voltage across C * (1/DCR). | ||
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---- | ---- |