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TC654 Scheda tecnica(PDF) 10 Page - Microchip Technology |
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TC654 Scheda tecnica(HTML) 10 Page - Microchip Technology |
10 / 38 page TC654/TC655 DS20001734C-page 10 2002-2014 Microchip Technology Inc. 4.1 Fan Speed Control Methods The speed of a DC brushless fan is proportional to the voltage across it. For example, if a fan’s rating is 5000 RPM at 12V, it’s speed would be 2500 RPM at 6V. This, of course, will not be exact, but should be close. There are two main methods for fan speed control. The first is pulse width modulation (PWM) and the second is linear. Using either method the total system power requirement to run the fan is equal. The difference between the two methods is where the power is consumed. The following example compares the two methods for a 12V, 120 mA fan running at 50% speed. With 6V applied across the fan, the fan draws an average cur- rent of 68 mA. Using a linear control method, there are 6V across the fan and 6V across the drive element. With 6V and 68 mA, the drive element is dissipating 410 mW of power. Using the PWM approach, the fan is modulated at a 50% duty cycle, with most of the 12V being dropped across the fan. With 50% duty cycle, the fan draws an RMS current of 110 mA and an average current of 72 mA. Using a MOSFET with a 1 RDS (on) (a fairly typical value for this low current) the power dis- sipation in the drive element would be: 12 mW (Irms2 * RDS(on)). Using a standard 2N2222A NPN transistor (assuming a Vce-sat of 0.8V), the power dissipation would be 58 mW (Iavg* Vce-sat). The PWM approach to fan speed control causes much less power dissipation in the drive element. This allows smaller devices to be used and will not require any spe- cial heatsinking to get rid of the power being dissipated in the package. The other advantage to the PWM approach is that the voltage being applied to the fan is always near 12V. This eliminates any concern about not supplying a high enough voltage to run the internal fan components, which is very relevant in linear fan speed control. 4.2 PWM Fan Speed Control The TC654 and TC655 devices implement PWM fan speed control by varying the duty cycle of a fixed fre- quency pulse train. The duty cycle of a waveform is the on time divided by the total period of the pulse. For example, given a 100 Hz waveform (10 msec.) with an on time of 5.0 msec, the duty cycle of this waveform is 50% (5.0 msec/10.0 msec). An example of this is illustrated in Figure 4-2. FIGURE 4-2: Duty Cycle Of A PWM Waveform. The TC654 and TC655 generate a pulse train with a typical frequency of 30 Hz (CF = 1 µF). The duty cycle can be varied from 30% to 100%. The pulse train gen- erated by the TC654/TC655 devices drives the gate of an external N-channel MOSFET or the base of an NPN transistor (Figure 4-3). See Section 7.5 “Output Drive Device Selection” for more information on output drive device selection. FIGURE 4-3: PWM Fan Drive. By modulating the voltage applied to the gate of the MOSFET Qdrive, the voltage applied to the fan is also modulated. When the VOUT pulse is high, the gate of the MOSFET is turned on, pulling the voltage at the drain of Qdrive to 0V. This places the full 12V across the fan for the Ton period of the pulse. When the duty cycle of the drive pulse is 100% (full on, Ton = T), the fan will run at full speed. As the duty cycle is decreased (pulse on time “Ton” is lowered), the fan will slow down proportionally. With the TC654 and TC655 devices, the duty cycle can be controlled through the analog input pin (VIN) or through the SMBus interface by using the Duty-Cycle Register. See Section 4.5 “Duty Cycle Control (VIN and Duty-Cycle Register)” for more details on duty cycle control. T Ton Toff T = Period T = 1/F F = Frequency D = Duty Cycle D = Ton / T TC654/ TC655 FAN 12V Qdrive VDD GND VOUT G D S |
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