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Online monitoring of resistive power load (Part 1)

Posted: 23 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:power-control? online monitoring? current monitoring? PWM? MOSFET?

In some power-control applications, it is ideal to do the continuous assessment of the working condition (health) of a resistive power-load for reliability or safety reasons. The heating resistors used in devices for medical purposes (heating pads, towels and blankets) are examples of such applications. To be effective, the assessment should be done by monitoring the resistance of the power-load continuously, without disturbing system's normal operation (online monitoring). The monitoring system should provide at least a digital alarm-signal, which is active when the resistance is outside a predefined range.

A typical power-control application with simple resistive load-current monitoring can be modelled as depicted in figure 1, discarding any reactive phenomena. In this lumped model, U is the supply-voltage; I is the current in the circuit; R is the power-load (purely resistive); Rp1, Rp2 and Rp3 represents all parasitic resistances, modelling the resistance of interconnecting wires, connectors and any eventual mechanical or electronic switches (when closed); and Rs is the current-sensing resistor. Let Rp be the total parasitic resistance, defined as Rp = Rp1 + Rp2 + Rp3. If U and Rp are constant, I can only change if R changes, since Rs is constant. Consequently, only current monitoring is required for assessing R deviations. However, in most cases, U and Rp are actually not constant. In fact, even in the usual constant-voltage PWM power-control, U can drift from the expected value due to power-supply excessive internal impedance (poor regulation) and/or voltage tolerance. The parasitic resistance Rp comprises the resistance of wires, connectors and switches, which usually changes with temperature, usage and ageing. If the switch is a power MOSFET, for example, its Rds(ON) would increase with temperature, due to its positive temperature coefficient.

Figure 1: A typical power-control application with simple resistive load-current monitoring.

Clearly, the variations of U and Rp compromise the accuracy of the simple current-based resistance-monitoring method. To overcome such situation, the resistance monitoring could be based on the calculation of the actual load resistance (R), by measuring both load-current and load-voltage, and performing their division (Ohm's Law). Nowadays, the typical approach is making such division in the digital domain, which requires at least one Analogue-to-Digital Converter (ADC) with two multiplexed input channels and some processing unit, that is, a microcontroller. This approach is attractive, mainly if there is already a microcontroller in the system. However, it may not be the case, or it may be undesirable at all having such task performed by software due to reliability or safety reasons.

In medical-grade equipment, for example, standard IEC 60601-1 (clause 14) states that if essential safety is ensured by programmable systems, then the development cycle must follow a specific process, further complicating the development and posterior certification of the final system. Alternatively, the division could be done in the analogue domain, by using a precision analogue-divider IC (Integrated Circuit). However, such ICs are usually expensive and quite uncommon nowadays. Yet in the analogue domain, we could explore the possibilities of the classic Wheatstone bridge, a well-known circuit in the context of low-power resistance measuring. It will be our starting point.

Prior to the discussion, it is convenient to define R as R = Rn(1+), where Rn is the nominal value of R and is the relative error of R, defined as = R/Rn C 1. Furthermore, let's define the threshold-points i and s as the values beyond which the monitoring system actuates (inferior and superior, respectively), signalling a fault condition. In figure 2 a), a Wheatstone bridge and a comparator are used to generate a logic signal that indicates if R is above or below a certain threshold. It is easy to demonstrate that the resistance threshold is independent of U, which is a characteristic aspect of this bridge topology. In figure 2 b), the topology is expanded to implement a resistance window comparator, by using an extra resistor (R3) in the reference branch and two comparators. The threshold-points i and s are set up by the ratios between R1, R2 and R3, as they define the threshold-voltages of the comparators (Ut1 and Ut2).

Figure 2: Wheatstone bridge topologies.

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