Download Implementing Temperature-Based Variable Fan Speed Control in

AN38
Implementing Temperature-Based Variable Fan Speed Control in NLX Power Supplies
Author:
1.
Abid Hussain and Danny Allred,
Microchip Technology, Inc.
INTRODUCTION
The NLX Power Supply Specification released by Intel Corp.
defines the requirements for next-generation PC system power
supplies. There are several enhancements outlined in this specification as compared to the “old” PS/2 power supply form factor.
One of these features is control and monitoring of the cooling fan(s)
inside the NLX power supply. The NLX specification designates
two interface fan control signals:
1.
2.
FanM signal is an output from the NLX-compliant power
supply. This signal allows the host (typically a system
management ASIC) to monitor fan RPM. FanM is an
open-collector signal consisting of two pulses per fan
rotation.
FanC is an input to the NLX-compliant power supply used
by the motherboard to regulate fan speed and to shut the
fan down. FanC is a 0V to 12V analog signal.
BRUSHLESS DC FAN BASICS
2.
APPLICATION CIRCUIT
The application circuit using the Microchip TC646 is shown in
Figure 2. Please refer to the TC646 datasheet for details on pin
descriptions. The overall circuit can be broken down into five basic
functional blocks plus the TC646 itself:
1.
Brushless DC (BDC) fans are popular for cooling electronics and
come in many voltage, current, and CFM ratings. The most
common versions in PCs are +5V and +12V. The nominal voltage
rating is typically the input voltage at which the fan runs at
approximately 100% RPM. Some fans have a third terminal that
outputs pulses as a tachometer signal.
2.
100
RPM %
3.
50
PWM Drive
4.
Linear Drive
0
0
6
VFAN (Avg.) Volts
12
FIGURE 1: Linear voltage control vs. pulse-width modulation.
© 2001 Microchip Technology, Inc.
Linear Voltage Control: This method varies the DC
voltage at the VDD terminal of the fan in a linear manner.
For example, a +12V nominal fan modulated linearly to
+8V would theoretically run at 66.7% RPM. Though this
seems relatively simple, there are drawbacks. All BDC
fans have a “stall voltage” that can vary from 35% to 60%
of nominal voltage, making wide range speed control
impossible. Additionally, there is considerable start-up
hysteresis after a stall, complicating matters further.
Lastly, linear speed control is inefficient, generating
excessive heat.
Pulse Width Modulation: This method applies the full
DC nominal voltage to the fan, but only for a duty cycle
that corresponds to the desired speed. The average
energy delivered to the fan is, therefore, variable over a
wide range, and stall voltage is not an issue.
5.
FanC “Y-Network” Input From Motherboard: This section accepts the 0V–12VDC signal from an NLX-compliant
motherboard. The Y-network formed by R12, R1, and
R2 performs level shifting and range compression to
match the 1.25V–2.65V analog input range of the TC646.
The FanC signal indicates a shutdown request at or below
+1V DC. Above that, the signal is used for proportional fan
speed control. If FanC becomes disconnected or is not
present, the fan will be driven at full speed via the R13
pull-up resistor.
Thermistor-Controlled Fan Override: T1 (thermistor
10 KΩ at 25°C), R6 and Q1 form the “thermal override
function.” This circuit provides a local override in the event
of high ambient temperature. Under normal operation, Q1
is off.
Output Stage: This section consists of the fan and its
drive components: R5, Q2. The signal output from the
VOUT pin is a 30Hz nominal 5Vp-p PWM waveform. Q2 is
a 2N2222A small-signal BJT. The fan is driven to a full
+12V (minus Q2 saturation voltage).
FanM Signal: To fully comply with NLX specification,
the power supply must provide a signal back to the
motherboard (FanM). The NLX specification requires this
signal to be an open collector output from the tachometer
of a 3-wire fan.
Minimum Speed and Auto-Shutdown: R3 and R4 form
a divider network which defines the shutdown threshold
of the circuit. This corresponds to the +1V NLXspecification for fan shutdown.
DS00764A-page 1
AN38
System operation is straight-forward: fan speed ranges from
approximately 30% to 90% for a FanC voltage range of 1V to 12V.
The fan will be held in shutdown when FanC is less than 1V. FanM
is returned to the motherboard as prescribed in the NLX specification. FanM also is monitored by the TC646. FAULT is asserted if the
fan fails to operate (see TC646 datasheet for details). Additionally,
C4 sets the 30Hz PWM frequency, and R7 is the pull-up resistor for
the FAULT output.
SUMMARY
The Microchip TC646 Fan Speed Controller allows a computer
designer to implement a robust system cooling design and be
fully NLX-compliant. Additionally, the PWM control mode is superior to traditional linear control methods. The low cost of the TC646
and supporting components make it the ideal choice for highvolume applications.
+12V
R8
10kΩ
Q3
2N3906
+5V
FANM
(To Motherboard)
R10
Q4
2N2222A
R9
2.7kΩ
(TACH)
4.7kΩ
(+)
(23CFM)
F1
(∆AFB0912M-FOO)
R11
36kΩ
C9
0.47µF
(-)
Override
+5V
+5V
T1
NTC
10kΩ
C1
10µF
Q1
2N2222A
R6
4.7kΩ
C6
0.1µF
SENSE
+12V
R13
10kΩ
FANC 0V-12.0V
(From Motherboard)
VDD
30kΩ
+5V
R12
400kΩ
R2
51KΩ
VIN
+
C2
0.01µF
–
TC646
+5V
1µF
R7
C4
FAULT
R3
33kΩ
R4
16kΩ
R5
C7
0.47-1µF
(Optional)
CF
+
+5V
Q2
2N2222A
VOUT
D1
1N4148
R1
100kΩ
C8
0.01µF
VAS
10kΩ
System
Shutdown
Gnd
C3
.01µF
FIGURE 2: BDC fan control for NLX power supply (3-wire fan).
DS00764A-page 2
© 2001 Microchip Technology, Inc.
AN38
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with
express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property
rights.
Trademarks
The Microchip name and logo, the Microchip logo, PIC, PICmicro,
PICMASTER, PICSTART, PRO MATE, KEELOQ, SEEVAL,
MPLAB and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
Total Endurance, ICSP, In-Circuit Serial Programming, FilterLab, MXDEV, microID, FlexROM, fuzzyLAB, MPASM,
MPLINK, MPLIB, PICC, PICDEM, PICDEM.net, ICEPIC,
Migratable Memory, FanSense, ECONOMONITOR, Select
Mode and microPort are trademarks of Microchip Technology
Incorporated in the U.S.A.
Serialized Quick Term Programming (SQTP) is a service mark
of Microchip Technology Incorporated in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2001, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received QS-9000 quality system
certification for its worldwide headquarters,
design and wafer fabrication facilities in
Chandler and Tempe, Arizona in July 1999. The
Company’s quality system processes and
procedures are QS-9000 compliant for its
PICmicro® 8-bit MCUs, KEELOQ® code hopping
devices, Serial EEPROMs and microperipheral
products. In addition, Microchip’s quality
system for the design and manufacture of
development systems is ISO 9001 certified.
 2001 Microchip Technology Inc.
DS00764A-page 3
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DS00764A - page 4
Microchip Technology Inc.