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EDN Access--12.18.97 Boost converter controls 12V fan from 5V supply

-December 18, 1997



December 18, 1997


Boost converter controls 12V fan from 5V supply

John McNeill, Worcester Polytechnic Institute, Worcester, MA

The temperature-controlled PWM boost converter in Figure 1 allows operation of a 12V brushless dc fan from a 5V supply. The circuit is based on IC1, a single-chip, BiCMOS, PWM controller that contains all the necessary circuitry (voltage reference, error amplifier, comparator, MOSFET gate drive, and oscillator) for closed-loop PWM power-supply control. At maximum boost, the circuit provides a voltage of 10V to the fan, which operates at about 80% of its rated (12V) speed. The circuit controls fan speed by sensing the ambient temperature in the system enclosure and reducing the fan's supply voltage when maximum cooling is unnecessary. Reducing the operating speed of the fan saves power, extends operating life, and reduces acoustic noise.

IC1, which you can configure for either voltage- or current-mode feedback, operates in a voltage-mode configuration to control the boost converter comprising Q1, D1, L1, and C1. Q1's switch element is a logic-level MOSFET that IC1 drives from the 5V-swing gate-drive signal OUT at Pin 6. As the timing diagram shows, the on-chip comparison of the COMP control voltage (Pin 1) with the CS sawtooth waveform (Pin 3) determines the duty cycle of the switch's drive signal. The R3/R4 network attenuates the charge-discharge waveform on the oscillator timing capacitor, C2, to develop a sawtooth waveform at CS of the proper amplitude for comparison with the control voltage COMP. R2 and C2 set the frequency of the on-chip PWM oscillator to 120 kHz. The oscillator uses the on-chip 4V reference to keep the frequency independent of variations in the 5V supply voltage.

For this boost converter with a 5V input, the approximate relationship (assuming the voltage drop across Schottky diode D1 to be negligible) between the output voltage and duty cycle D is

 26q21291


Thus, as the duty cycle increases, VOUT increases. At the minimum duty cycle of zero, VOUT reaches its minimum value of 5V. This minimum value prevents the fan from stalling, which occurs at a fan supply voltage of approximately 4V. An on-chip control block limits the minimum duty cycle to 50%, corresponding to a maximum fan voltage of 10V. This limit is necessary because the output of the boost converter deviates significantly from Equation 1 for duty cycles near unity, causing the control loop to become unstable.

When VOUT is 5 to 10V, the system acts as a closed-loop controller, increasing fan voltage and thus fan speed when the ambient temperature increases. The loop uses proportional control because the long thermal time constant associated with sensing ambient temperature makes it difficult to compensate an integral control loop.

RT, a negative-temperature-coefficient thermistor located in the fan's airflow path, acts as the sensor that incorporates temperature information into the control voltage COMP at the input of the comparator. The RT/R5/R1 temperature-sensing network uses the on-chip reference so that the temperature-control characteristic is unaffected by supply variations.

The negative feedback loop that controls fan speed based on temperature consists of the control signal's path around the loop from RT to the fan supply voltage. When the ambient temperature increases, the value of RT decreases and the current through RT increases. The final result is an increase in the value of the COMP control voltage at the output of the error amplifier. This increase changes the duty cycle of the comparator output and MOSFET gate drive such that the amount of Q1's on-time increases. An increase in Q1's on-time increases the output voltage of the boost converter supplying the fan, which ultimately increases the fan's operating speed.

Two temperature points characterize the fan-speed-  vs-temperature profile: T1, below, which the fan runs at a supply voltage of 5V, and T2, above, which the fan runs at the maximum of 10V. The desired temperatures for T1 and T2 determine the values of R5 and R1, which scale and shift the control voltage at the output of the error amplifier. You determine the resistances from the values of the thermistor at temperatures T1 and T2 or at RT(T1) and RT(T2), respectively:

 26q21292


and

 26q21293


The circuit in Figure 1 is for an enclosure that contains circuitry that dissipates 5 to 30W of power. The values chosen for the temperature profile are T1=77ºF and T2=86ºF, at which the thermistor has values of RT(T1)=200 kiloohms and RT(T2)=154 kiloohms, respectively. Measurements of the circuit show VOUT=9.83V with corresponding dc fan current IFAN=117 mA and 5V-dc supply current ICC=258 mA. At the minimum measured fan voltage of VOUT=4.91V, IFAN=60.4 mA, and ICC=63.5 mA.

If you need a different temperature profile to suit your enclosure or power-dissipation range, you can determine the new values of R5 and R1 by substituting the thermistor values at the new T1 and T2 temperatures into equations 2 and 3. To use this circuit with a load that requires a higher operating current, change the values of C1 and L1. You need to increase the value of C1 in proportion to the increase in current to keep the same ripple amplitude. You must decrease the value of L1, which needs to change in inverse proportion to the increased current, because the maximum output current is inversely proportional to the inductance. For large increases in current, you may also need to change Q1 and D1.  (DI #2129)


Acknowledgments

I would like to acknowledge Luis Menezes, Jeff Kulesza, and Nina Tjoa, who completed the detailed design of the circuit and built and tested the prototype.

Figure 1
26DI21291
A single-chip PWM controller and negative-temperature-coefficient thermistor implement a boost converter that operates from a 5V supply and that varies fan voltage from 5 and 10V, depending on the sensed ambient temperature.


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Copyright c 1997 EDN Magazine, EDN Access. EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company, a unit of Reed Elsevier Inc.

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