Abstract:
A pulse detection circuit, a method of operation and a fan assembly test circuit employing the same. In one embodiment, the pulse detection circuit includes a charge pump that receives an input signal and varies a charge in a charge storage device based on the input signal. The pulse detection circuit further includes a level detector, coupled to the charge pump, that compares a voltage across the charge storage device with first and second reference voltages, and a signaling circuit, coupled to the level detector, that generates an output signal based on the comparison and indicating an existence of the pulse. The pulse detection circuit may be a part of a fan assembly test circuit adapted to receive an input signal from a cooling fan under test.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to power conversion and, more specifically, to a pulse detection circuit, a method of operation thereof and a fan assembly test circuit employing the same. 
     BACKGROUND OF THE INVENTION 
     Single or multiple brushless DC cooling fans are widely employed in AC-DC, DC-DC and DC-AC power conversion systems to remove the heat generated by semiconductor switching devices, various magnetic components and other circuit components that are part of the power conversion equipment. Typically, the cooling fan allows the power conversion equipment to be operated at higher temperatures. As a result, the proper operation of the cooling fan plays an important role in overall power system reliability and lifetime. 
     In most applications, a speed feedback signal from the fan is employed to govern a fan alarm or a system-wide safety interlock. Whenever the speed feedback signal is not detected, the alarm is tripped or the power conversion system is shut down to protect “on-board” circuit components of the power conversion system from excessive heating. In the normal course of assembly and operation, a fan alarm and possible shutdown may be caused by one or all of the following: a broken fan power cable(s), a shorted fan power cable, a loose fan cable connection, an incorrect wiring of the fan power cable, a malfunction of the fan&#39;s internal speed sensor, a problem with the fan&#39;s power supply and a malfunction of the fan&#39;s speed detection circuit. 
     Generally, after the fan has been assembled into the power conversion system, it becomes difficult (and sometimes impossible) to inspect the fan visually to determine whether it is operating properly or to identify the nature of a malfunction. Alternative, non-visual inspection methods typically require complex inspection/detection circuitry that increase the fan&#39;s material and manufacturing costs. The primary reason for the complexity of the inspection/detection circuitry is that the fan speed feedback signal can either be a variable frequency alternating signal, e.g., a 60 Hz to 120 Hz pulse train, or a constant voltage signal, e.g., a 0V or 5V signal. 
     The form of the fan feedback signal depends on the status of the fan. In the case of a brushless DC fan, the fan feedback signal assumes a constant voltage when the fan is stalled or not running. During normal operation, a pulse train with a peak amplitude of e.g., 5V, is provided at the fan&#39;s feedback signal terminal. 
     The different forms and signal levels that might be encountered results in existing on-board fan detection circuits that are quite cumbersome and complex. Accordingly, what is needed in the art is an improved fan operation detection circuit that overcomes the above-described limitations. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a pulse detection circuit, a method of operation and a fan assembly test circuit employing the same. In one embodiment, the pulse detection circuit includes a charge pump that receives an input signal and varies a charge in a charge storage device based on the input signal. A level detector, coupled to the charge pump, makes a comparison among a voltage across the charge storage device and first and second reference voltages. Finally, a signaling circuit, coupled to the level detector, generates an output signal based on the comparison and indicating an existence of the pulse. The pulse detection circuit may be part of an overall fan assembly test circuit that further includes a socket adapted to receive an input signal from a cooling fan under test. 
     In one embodiment of the present invention, the input signal is a fan speed feedback signal. Under normal operating conditions (fan operating normally), the feedback signal is a variable frequency alternating signal. It should also be noted that the present invention is not limited to determining the operational status of a fan and may also be advantageously employed where detection of a variable frequency alternating signal is required. 
     In one embodiment of the present invention, the signaling circuit further includes a current indicator that indicates a state of the level detector. In yet another embodiment, the current indicator is a light emitting diode (LED). Alternatively, audible devices, such as buzzers, may also be advantageously employed. Those skilled in the art should readily appreciate that any device capable of detecting current flow and generating a signal in response thereto is well within the broad scope of the present invention. 
     In one embodiment of the present invention, the level detector includes first and second operational amplifiers (op-amps) configured as open-collector comparators. In an embodiment to be illustrated and described, the first and second reference voltages, which are derived from taps of a voltage divider network, are coupled to the inverting nodes of the second comparator and the non-inverting node of the first comparator, respectively. 
     In one embodiment of the present invention, the charge storage device is a capacitor. However, it should also be readily apparent to those skilled in the art that the charge storage device is not only limited to capacitors, other energy storage devices may also be advantageously employed and are within the broad scope of the present invention. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the concepts and the specific embodiments disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a block diagram of a conventional fan operation detection scheme that provides a suitable environment for the practice of the present invention; 
     FIG. 2 illustrates a schematic diagram of an embodiment of a pulse detection circuit according to the principles of the present invention; and 
     FIG. 3 illustrates a schematic diagram of an embodiment of a fan assembly test circuit according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is a block diagram of a conventional fan operation detection scheme  100  that provides a suitable environment for the practice of the present invention. In the illustrated embodiment, a brushless DC cooling fan  110  is powered by a DC voltage source VDC and generates a feedback signal, e.g., pulse train, to a fan operation detection circuit  120 . The detection circuit  120  “interprets” the feedback signal and, in turn, generates a fan status signal to an equipment controller  130 . In a typical system, the equipment controller  130 , which generally functions as a main system controller, enables a power conversion equipment/unit  140  only after the fan status signal indicates that the fan  110  is functioning normally. 
     As discussed previously, the form of the fan feedback signal depends on the status of the fan  110 . In the case of the brushless DC fan  110 , the fan feedback signal is at a constant voltage, e.g., at 0V or 5V, when the fan is stalled or not running. During normal operation, a pulse train with a peak amplitude, e.g., of 5V, is provided at the fan  110  feedback signal terminal  150 . The feedback signal&#39;s different forms and signal levels, corresponding to the different operational status of the fan  110 , result in the fan operation detection circuit  120  being quite cumbersome and complex. 
     Turning now to FIG. 2, illustrated is a schematic diagram of an embodiment of a pulse detection circuit  200  according to the principles of the present invention. The pulse detection circuit  200 , which may be advantageously employed as the fan operation detection circuit  120  (illustrated in FIG.  1 ), includes a charge pump  210 , level detector  220  and a signaling circuit  230 . The charge pump  210  includes first and second resistors R 1 , R 2  coupled to a gate of a transistor Q 1 , a NPN field-effect-transistor (FET) is shown. The charge pump  210  also includes a charge storage device (capacitor C 1 ) that is coupled to third and fourth resistors R 3 , R 4 . Although a capacitor is shown in the illustrated embodiment, those skilled in the art should readily appreciate that other charge storage devices may also be advantageously employed. 
     The level detector  220  includes a voltage divider network comprising fifth, sixth and seventh resistors R 5 , R 6 , R 7 . First and second reference voltages Vref 1 , Vref 2  are derived from first and second taps  250 ,  260  located between the sixth and seventh resistors R 6 , R 7  and between the fifth and sixth resistors R 5 , R 6 , respectively. The first reference voltage Vref 1  is provided to the inverting node of a second comparator U 2  and the second reference voltage Vref 2  is provided to the non-inverting node of a first comparator U 1 . The level detector further includes diodes D 1 , D 2  that are coupled to the first and second op-amps U 1 , U 2  configured as open-collector comparators. 
     The signaling circuit  230  includes an eight resistor R 8  and an output terminal  270  that are coupled to the anode terminals of the first and second diodes D 1 , D 2 . The operation of the pulse detection circuit  200  will hereinafter be described in greater detail as part of the fan operation detection scheme  100  illustrated in FIG.  1 . It should be noted, however, that the practice of the present invention is not limited to determining the operational status of a fan and may also be advantageously employed where detection of a variable frequency alternating signal is required. In the following discussion, under normal operating conditions, i.e., fan operating properly, the fan speed feedback signal is assumed to be a pulse train with 5V peak amplitude. When a failure has occurred, the fan speed feedback signal is a constant voltage of 0V or 5V. Furthermore, the operation of the pulse detection circuit  200  will be described under three conditions: (1) normal fan operation, (2) fan inoperative with 0V output and (3) fan inoperative with 5V output. 
     (1) Normal fan operation. When the fan is running and its internal speed sensor (not shown) is functioning properly, the charge pump  210  receives a series of 50% duty cycle 0V to +5V pulse train at an input terminal  240 . The values of the first and second resistors R 1 , R 2  are chosen so that the transistor Q 1  is turned off when the fan feedback signal received at the input terminal  240  is at 0V. This allows the capacitor C 1  to be charged up via the third resistor R 3 . The transistor Q 1  is turned on when the input signal at the input terminal  240  is at +5V to discharge the capacitor C 1  via the fourth resistor R 4 . The values of the third and fourth resistors R 3 , R 4  are chosen so that the voltage across the capacitor C 1  is built up to a value, Vc 1 . 
     The voltage divider circuit consisting of the fifth, sixth and seventh resistors R 5 , R 6 , R 7  sets two reference levels Vref 1  and Vref 2 , where Vref 2 &gt;Vref 1 . The voltage across the capacitor C 1 , i.e., Vc 1 , is set to satisfy the following condition (at normal fan operating condition), 0V&lt;Vref 1 &lt;Vc 1 &lt;Vref 2 &lt;Vcc (+15V). Consequently, the outputs of the first and second comparators U 1 , U 2  are at a high logic state. In the case where the pulse detection circuit  200  is used as an on-board detection circuit, a high logic level “OK” signal is sent via the output terminal  270  to a main controller, e.g., equipment controller  130  illustrated in FIG. 1, which allows a system, e.g., power conversion unit  140  illustrated in FIG. 1, to be operated as required. 
     (2) Fan inoperative with 0V output. When the fan is not running and the speed feedback signal is at 0V, the transistor Q 1  within the charge pump  210  is turned off. As a result, the capacitor C 1  is fully charged to Vcc, i.e., Vc 1 =Vcc&gt;Vref 2 . Therefore, the output of the first comparator U 1  is at a low state, i.e., logic low. Consequently, the first “Oring” diode D 1  is turned on (forward biased) and a fan alarm signal is sent via the output terminal  270  to the main controller to shutdown the system if necessary. 
     (3) Fan inoperative with 5V output. When the fan is not running and the speed feedback signal received at the input terminal  240  is at +5V, the transistor Q 1  within the charge pump  210  is turned on. As a result, the capacitor C 1  is fully discharged to 0V, i.e., Vc 1 =0V&lt;Vref 1 . Consequently, the output of the second comparator U 2  is at a low state. Therefore, the second “Oring” diode D 2  is turned on (forward biased) and a fan alarm signal is also sent to the main controller to shutdown the system if necessary. 
     Turning now to FIG. 3, illustrated is a schematic diagram of an embodiment of a fan assembly test circuit  300  according to the principles of the present invention. The fan assembly test circuit  300  is analogous to the pulse detection circuit  200  illustrated in FIG. 2, except for a plurality of sockets (generally designated  310 ) , which have been adapted to power a fan under test and to receive a fan speed feedback signal from a fan assembly  320 , and a current indicating device (a light emitting diode is shown)  330  series-coupled to a current-limiting ninth resistor R 9 . The type of device used for sockets  310  is dependent on the fan assembly  320  fan feedback signal and power connections and may include conventional test leads or a mating connector if the terminals that provide the feedback signal and power connections are embodied in a connector. In another advantageous embodiment, the current indicating device  330  is an audible warning device such as a buzzer. 
     The operation of the fan assembly test circuit  300  is similar to that of the pulse detection circuit  200  (illustrated in FIG. 2) discussed previously. In the case where the fan assembly  320  is functioning normally, i.e., generating a pulse train at the socket  310 , the resulting logic high signal at an output terminal  340  reverse biases first and second “Oring” diodes D 1 , D 2 . Concurrently, the LED  330  is forward biased and indicates a current flow. The resulting LED  330  signal indicates that the fan assembly  320  including the speed sensor, cables and connectors are functioning properly. 
     On the other hand, if the fan assembly  320  is inoperative and a constant signal of 0V or 5V is received at the socket  310 , the first diode D 1  (for 0V) or the second diode D 2  (for 5V) becomes forward biased. In either case, the LED  330  is turned off due to low terminal voltage, indicating a problem or failure in the fan assembly  320 . 
     From the foregoing, it is apparent that the present invention provides a novel pulse detection circuit that can be used for fan alarm generation in a power conversion system and for a system safety interlock that prevents the power system from being operated when heat is excessive. Furthermore, with an addition of a visual warning (LED) and/or audible warning (buzzer) device, the disclosed circuit may be used in the manufacturing/assembly process to provide a test circuit to verify that the fan assembly of the power conversion unit is functioning properly. 
     Although the present invention and its advantages have been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.