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Online monitoring of resistive power load (Part 2)

Posted: 29 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:MOSFET? SMD resistors? decoupling capacitors? software-automation? PWM?

In Part 1, we covered the models and topologies. In Part 2, we will discuss interesting cases and examples.

Interesting particular cases
The generic model has a considerable number of parameters, allowing many different implementations. However, there are some particular parameter combinations and restrictions that give some interesting particular cases, on aspects such as easiness of adjustment and low component count. These particular cases are described next. They are ultimately defined by imposing a specific relationship between Ut1 and Ut2 at gains-calculation phase. All the discussion carried out in generic model concerning model adjustment and model performance applies to them. The effects of U and Rp variations that can occur during the monitoring will be discussed for each particular case.

Ut1 = Ut2 = Ut

This is maybe the most obvious particular case. Just choose an appropriate Ut value and insert it in gains formulae ((2), (3), (4), (5)), replacing Ut1 and Ut2 by Ut. The variation of U doesn´t affects the equality Ut1 = Ut2. However, the variation of Rp destroys the equality Ut1 = Ut2. Nevertheless, this particular case can be faced as a simple starting-point for model adjustment.

Gu1 = Gu2 = Gu.

The great advantage of this particular case is the reduction of the number of components necessary for model implementation, due to the elimination of one gain stage. Under this particular case, the voltages Ut1 and Ut2 must be chosen so that their ratio respects (6). Note that Ut1 Ut2 and, since Gu1 = Gu2 = Gu, Uu1 = Uu2 = Uu. In (6), the Rp value should be the same that will be used for model-gains calculation.

Being Ut1 and Ut2 chosen, the model-gains can be calculated as described in the general model discussion. Note that Gu can be calculated either from (2) or (3), because these formulae give the same result. Regarding U and Rp variations, there are no special considerations beyond those already addressed in general model discussion.

Gi1 = Gi2 = Gi

The previous approach can be applied in a similar way to gain stages Gi1 and Gi2, yielding the same advantages. Under this particular case, the voltages Ut1 and Ut2 must verify the ratio (7). Note that Ut1 > Ut2 and, since Gi1 = Gi2 = Gi, Ui1 = Ui2 = Ui.

The considerations for model adjustment and performance of the previous particular case apply completely to this one. Regarding Gi calculation, note that it can be calculated either from (4) or (5), because these formulae give the same result.

Ud1 and Ud2 with equal slope moduli on δi and δs.

In the particular cases presented so far, the modulus of the slope of Ud1 on δi is different from that of Ud2 on δs. As a consequence, the threshold-points are affected differently for a given comparator IOV. If desired, the model can be adjusted to avoid this situation, by equating the moduli of the slopes of Ud1 and Ud2 on the respective threshold-points. It can be done by imposing the ratio (8) between Ut1 and Ut2. Note that Ut1 Ut2.

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