01-08-2012, 04:01 PM
Implementing Temperature-Based Variable Fan Speed Control in NLX Power Supplies
500Implementing Temperature.pdf (Size: 193.8 KB / Downloads: 59)
INTRODUCTION
The NLX Power Supply Specification released by Intel®
Corporation 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. 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.
2. 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
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.
APPLICATION CIRCUIT
The application circuit using the Microchip TC646 is
shown in Figure 2. Please refer to the TC646 datasheet
(DS21446) for details on pin descriptions. The overall
circuit can be broken down into five basic functional
blocks, plus the TC646 itself:
1. FanC “Y-Network” Input From Motherboard:
This section accepts the 0V–12 VDC signal
from an NLX-compliant motherboard. The Ynetwork
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 +1 VDC. 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.
2. 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.
3. Output Stage: This section consists of the fan
and its drive components: R5, Q2. The signal
output from the VOUT pin is a 30 Hz nominal
5 Vp-p PWM waveform. Q2 is a 2N2222A smallsignal
BJT. The fan is driven to a full +12V
(minus Q2 saturation voltage).
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 its supporting
components, make it the ideal choice for highvolume
applications.