Advanced electric current detection circuit and principle
Gao Duan / low-end examines the current circuit
Low-end examine to examine, flow the intersection of resistance and series connection get place (picture 1) current circuit ,And it is that the series connection to the high voltage terminal (Fig. 2) to examine examining and flowing the resistance of current circuit in high side . Two kinds of methods each have a characteristic: Low-end examine stream mode increase extra resistance in the intersection of ground and the intersection of line and loop, high side examine stream mode want, punish greater common mode signal.
Low-end that 1 Fig. show examine, flow operational amplifier regard ground level as reference level, examine, flow resistance answer the straight phase terminal. The common mode signal range in the input signal of operational amplifier is: (GNDRSENSE*ILOAD) . Though low-end examine current circuit to be simple, have several fault state whether low-end examine whom current circuit can not be found, this will make load in the dangerous situation, examine the current circuit and can solve these problems with high side.
High side examine current circuit connect the intersection of power and end to directly, can detect subsequent any trouble of loop and take the corresponding protection measure, especially suitable for the automatically controlled application, because usually adopt the ground for reference of chassis in the circuit of application.
Examine the current circuit in traditional high side
Traditional high side / low-end examine stream mode, have, realize scheme is most because of discrete or half discrete component circuit while being many kinds of. Examine the current circuit to usually need to use an accurate operational amplifier and electric capacity of some accurate resistance in high side, most frequently used high side examine current circuit adopt the intersection of differentiating and operational amplifier do gain amplification and signal level from shifting in high side to consult place (picture 3) :
VO=IRS*RS; R1 =R2 =R3 =R4
This scheme has already been applied to the real system extensively, but three main shortcomings that this circuit exists:
1)Input resistance is relatively lower, equal R1;
2)Generally there is greater magnitude of error in the input resistance of the input end;
3)Require match degree of the resistance to be high, in order to guarantee acceptable CMRR. Any resistance will make CMRR reduce to 46dB when producing 1% of the changes; 0.1% of the changes make CMRR reach 66dB, 0.01% of the changes make CMRR reach 86dB. Advanced electric current measure, need higher the intersection of measurement and skill, this promote high side examine, shed development of integrated circuit. The electric current detection technique of low-end seems that there is not corresponding progress.
Adopt the operational amplifier of the integrated differentiating to realize the advanced electric current is measured
It is more usable to adopt the differentiating and transport and put into the circuit that the advanced electric current of conduct measures, because introduced a variety of integrated circuit solutions recently. Include an accurate operational amplifier and match the resistance of one degree of a very good ones within the integrated circuit, CMRR is up to about 105dB. MAX4198/99 is such products, its CMRR is 110dB, the gain error is superior to 0.01%, and adopt 8 pins mMAX of the small volume to capsulate.
Examine and include finishing all functional units that the advanced electric current measures within the current circuit in specialized high side, can find advanced electric current, and the one that offer and is proportional to it, Tap taking ground level as point of reference under the common mode voltage up to 32V. Application needing to do precision measure and control to the electric current, such as power management and battery charge control, suitable for adopting this kind of scheme.
It examine high side of MAXIM by one that flow and use at operational amplifier if you can't examine, resistance last high side in power after flowing and between power input end of detection circuit, examine, flow resistance place high side on add extra impedance to ground wire loop, the technology raise whole characteristic of circuit and simplify the intersection of cloth and editions of requirement.
It is a series of and two-way for MAXIM to introduce and unidirectional current measure IC, some bidirectional current measure IC to be built-in to examine, shed resistance, can find, flow into or flow out, examine into magnitude of electric current of circuit and instruct the pin to reveal the current direction through a polarity. The gain to noise temperature ratio adjustable electric current measures IC, fixed gain (+20V/V, +50V/V, or +100V/V) The electric current finds the chip or electric current of fixed gain including single pair of comparator measures IC, adopt the small volume to capsulate, like SOT23, satisfiable application to that the size requirement is harsh. Advanced electric current detection circuit that Fig. 4 is formed with MAX4173.
The output voltage and relational expression of resistance of examining flowing are in the picture:
Vo =RGD*(Iload*Rsense) /RG1) *It is a image current coefficient that the b type hits b
Can it simplifies to be further while being aboving: Vo=Gain*Rsense*Iload; Gain=b*RGD/RG1 Gain is: 20(MAX4173T) ,50(MAX4173F) ,100(MAX4173H) .
Can be found out through the above-mentioned formulae, it determines (representative value & gt that CMRR is integrated the craft which examines the current circuit by inside; 90dB) ,No longer influenced by external resistance.
There are the following advantages to adopt and integrate and examine the current circuit:
1,The conformance of the device is good 2,Fairly good temp drift characteristic 3,Small 4,Low power consumption 5,Easy to use
Choose to examine flowing the attentive matters of resistance
Examining and flowing the resistance RSENSE should be chosen according to several following principles:
1,Voltage loss: Examine, flow electric resistance value pass conference, cause mains voltage reduce with number value of IR. In order to reduce the voltage loss, should choose examining and flowing the resistance of small resistance-value.
2,Precision: Greater survey accuracy that resistance can obtain more high trickle of examining flowing. This is because the larger the voltage examined and flowed on resistance is, the offset voltage of operational amplifier and influence of the input bias current are relative small.