Fan Control Advances: Consider

文章摘要：The MAX6650/MAX6651 free the designer from the complex issues associated with closed-loop amplifier design, leaving only selection and installation of the pass transistor. A resistive load, linear, control pass element will be subject to maximum dissipation when delivering half the total supply volt

Fan Control Advances: Consider,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

The MAX6650/MAX6651 free the designer from the complex issues associated with closed-loop amplifier design, leaving only selection and installation of the pass transistor.

A resistive load, linear, control pass element will be subject to maximum dissipation when delivering half the total supply voltage across the load. But does a resistive load represent a good model for fans? The graph in Figure 4 answers this question and shows that modeling the fan as a resistive load would be more demanding than the actual real-world fan behavior would indicate. This suggests that conservative design practice can treat the fan as a resistive load with the only consequence being a choice of a larger pass transistor than might be used with careful fan characterization.

Figure 4 shows that as supply voltage increases, the fan current is nonlinear and always below the current equivalent to what a resistor would draw (sized according to full fan voltage and current). For instance, the Papst fan drew approximately 240mA at 12V. An equivalent resistance would consume 120mA at the half-voltage output point and correspond to a dissipation of 0.72W:



where PD represents worst-case resistive load dissipation.

Note from the graph of Figure 4 that the real-world Papst motor showed a true dissipation of 0.65W on the pass element. Even more revealing is the current behavior of the Superred fan, which steps down to a very low level at around 6V. A conservative design that considers the fan a resistive load would have quite a bit of safety margin with the Superred fan.

Once a given pass transistor has been selected, if the following equation is satisfied then no additional heatsinking is necessary:



where:

TJMAX = Maximum allowable junction temperature from transistor manufacturer's data sheet.

TA = Maximum expected ambient temperature.

PD = Power dissipation (same as Equation 3 above).

θJ-A = Thermal resistance, junction to ambient, from transistor manufacturer's data sheet.

If the equation is not satisfied, a heatsink must be selected to satisfy the following:



where:

RθSA = Heatsink thermal resistance (from heatsink manufacturer's data sheet).

RθJC = Pass transistor junction-to-case thermal resistance (from pass transistor manufacturer's data sheet).

The possibility of a total short circuit across the fan has not been discussed. If this happens, then the full current available from the fan power supply will flow through the pass transistor. If this condition is a consideration, then that current and voltage value should be used in all the power dissipation and heatsinking calculations. Alternatively, current limiting circuitry could be included on the pass transistor, such as the circuit shown in Figure 5. Calculate the value of the current limit resistor according t

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