# Current-Sensing Topology with Multi Resistors in Parallel and Its Protection Circuit
## 2 Materials and Methods
### Parallel or Series Multi Resistor Current-Sensing Topology
#### 2.1.1 Current-Sensing Topology with Multi Resistors in Series  
Several current sense resistors were in series connection in the known power analyzer [15; 16]. A current-sensing circuit with two resistors is shown in Figure 2, while the overcurrent protection circuit is included.  
In Figure 2, \(R_{2}\) has a higher resistance value, covering lower ranges, while \(R_{1}\) has a lower value, covering higher ranges. \(CTR_{2}\) is the switch control signal from the module controller. When the user selects lower ranges, \(CTR_{2}\) is low level and switch \(S_{2}\) is open. The input current flows through \(R_{2}\). Voltage \(V_{O2}\) is selected and sent to the latter circuit. When switching to higher ranges, the switch \(S_{2}\) is closed. Most current is bypassed through \(S_{2}\) and voltage \(V_{O1}\) is selected. Whether the switch \(S_{2}\) is open or closed, all current flows through \(R_{1}\). Therefore \(V_{O1}\) can be monitored via the magnitude-detection circuit and  
Figure 1: Current channel of power analyzer.  
overcurrent protection can be performed by comparing the magnitude with threshold voltage \(V_{T2}\).  
Let the maximum allowed sensing current of resistor \(R_{2}\) be \(I_{T2}\). When the monitoring voltage \(V_{O1}\) meets (2), \(PT_{2}\) switches to a high level. The switch \(S_{2}\) is forced to close to protect \(R_{2}\) from overheating.  
\[V_{O1}>I_{T2}\times R_{1} \tag{2}\]  
\(D_{1}\) and \(D_{2}\) are bi-direction freewheeling diodes. Let the forward voltage of diodes be \(V_{F}\). When the input current exceeds the threshold current \(I_{T2}\) and the protection circuit has not yet activated, \(D_{1}\) and \(D_{2}\) limit the current \(I_{R2}\) flowing through \(R_{2}\) as follows:  
\[I_{R2}\leq V_{F}/R_{2} \tag{3}\]  
Through the above protection circuit, when the input current is high and the user chooses a lower range, bypassing of the current is forced. This prevents the current sense resistor \(R_{2}\) from being damaged by overheating.  
The series sensing circuit in Figure 2 can be extended to have more sensing resistors \(R_{3}\), \(R_{4}\),..., more bypass switches \(K_{3}\), \(K_{4}\),... and more freewheeling diodes \(D_{3}/D_{4}\), \(D_{5}/D_{6}\),... No matter how many sensing resistors there are, \(R_{1}\) can always detect the threshold current \(I_{T3}\), \(I_{T4}\),... The corresponding sensing resistors can be overcurrent protected by the combination of magnitude detection and comparison circuits similar to Figure 2.  
For the series sensing method, the protection circuit is easy to design. But there are two drawbacks: Each high side sensing resistor needs a bypass switch and two freewheeling diodes. On the other hand, all these switches and diodes need to conduct the maximum input current (usually tens of amperes) continuously or transitorily, respectively, when the highest range is selected. That is, these switches and diodes would have considerable size and need heat dissipation design. As the number of sensing resistors increases, the layout space and power dissipation of these switches and diodes will be far beyond the sensing resistors.  
Therefore, current sensing with multi resistors in series is generally only used in the case of 2\(\sim\)3 sensing resistors [15; 16]. For further expansion of the dynamic range of current  
Figure 2: Two resistors in series current-sensing topology with protection circuit.  
measurement, this article proposes a current-sensing prototype with multi resistors in parallel. When the number of sensing resistors increases, the layout space and power consumption of switches and diodes can be significantly reduced compared with the series counterpart.