Abstract:
An over temperature detector circuit for use in a switching converter including one or more power switches in accordance with an embodiment of the present application includes a silent sense generator connected to at least one power switch and operable to detect a noise level of the switch and to provide a generator output signal indicative of absence of switching noise and a comparator operable to compare a temperature sensor signal from a temperature sensor with a reference voltage to provide an alarm signal indicating an over temperature condition when the temperature sensor signal exceeds the reference voltage, wherein the alarm signal does not indicate an over temperature condition when the generator output signal does not indicate absence of switching noise

Description:
[0001]    The present application claims benefit of and priority to U.S. Provisional Patent Application Ser. No. 60/971,757 filed Sep. 12, 2007 entitled OVER TEMPERATURE DETECTION FOR UNIDIRECTIONAL NOISE, the entire contents of which are hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present application relates to a switched mode power supply with improved over temperature detection. 
         [0003]    In order to protect power switching devices in switching power converters from overheating, temperature sensors are provided as close as possible to the power switches. As power devices have been reduced in size, they are more commonly mounted directly on the printed circuit board (PCB) surface. Thus, the temperature sensors are typically provided on the electrodes of the switch, since the thermal resistance in the copper traces is better than that of the PCB bulk material. However, when such sensors are attached to the electrodes, a large common mode noise results in the terminals of the temperature sensor as a result of the switching of the switches. 
         [0004]    The block diagram of  FIG. 1  illustrates one conventional approach to providing over temperature sensing for a switched mode power converter.  FIG. 1  illustrates a temperature sensor  10 , a noise generator  12  and the prior art temperature detector circuit  14  which includes a low pass filter  14   a  and a comparator  14   b . The noise generator  12  triggers transient noise for each noise triggering event, typically based on switching activity of the power switches. The temperature sensor  10  produces a sensor signal which is disturbed by the switching noise and passed to the over temperature detector circuit  14 . An over temperature alarm signal is provided as the output of the comparator  14   b  when the sensor output indicates an abnormally high temperature. In  FIG. 1 , the temperature sensor signal includes noise generated by the switching of the switches which may lead to an unreliable alarm signal from the temperature detector  14 . 
         [0005]    Thus, it would be advantageous to provide a temperature detector that avoids these problems. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide an over temperature detector circuit that minimizes the effect of noise. 
         [0007]    An over temperature detector circuit for use in a switching converter including one or more power switches in accordance with an embodiment of the present application includes a silent sense generator connected to at least one power switch and operable to detect a noise level of the switch and to provide a generator output signal indicative of absence of switching noise and a comparator operable to compare a temperature sensor signal from a temperature sensor with a reference voltage to provide an alarm signal indicating an over temperature condition when the temperature sensor signal exceeds the reference voltage, wherein the alarm signal does not indicate an over temperature condition when the generator output signal does not indicate absence of switching noise. 
         [0008]    A switching power converter with over temperature detection in accordance with an embodiment of the present application includes a first power switch, a second power switch connected in series with the first power switch such that a desired voltage is provided at an output node between the first and second power switches, a first temperature sensor connected to the first power switch, a second temperature sensor connected to the second power switch and an over temperature detector circuit connected to the first temperature sensor, the second temperature sensor and a control terminal of at least one of the first and second power switches, the over temperature detector circuit operable to provide an over temperature alarm signal indicating an over temperature condition when a temperature of the first power switch or the second power switch exceeds predetermined threshold values, wherein the alarm signal does not indicate an over temperature condition when a noise level on one of the first and second power switches is high. 
         [0009]    An over temperature detector circuit for use in a switching converter including one or more power switches in accordance with an embodiment of the present application includes a silent signal generator connected to at least one power switch and operable to detect a noise level of the switch and to provide a generator output signal indicative of absence of switching noise and a comparator operable to compare a temperature sensor signal from a temperature sensor connected to the power switch with a reference voltage to provide an alarm signal indicating an over temperature condition when the temperature sensor signal exceeds the reference voltage, wherein the alarm signal does not indicate an over temperature condition when the generator output signal does not indicate absence of switching noise. 
         [0010]    Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         [0011]      FIG. 1  is a block diagram illustrating a convention temperature detector circuit for use with a switching power converter; 
           [0012]      FIG. 2  is a block diagram illustrating a temperature detector circuit in accordance with an embodiment of the present application; 
           [0013]      FIG. 3  illustrates a conventional temperature detector circuit connected to an exemplary power switching converter; 
           [0014]      FIG. 4  illustrates a more detailed diagram of the conventional detector circuit of  FIG. 3 ; 
           [0015]      FIG. 5A-FIG .  5 H are graphs illustrating typical waveforms of the circuit of  FIG. 4 . 
           [0016]      FIG. 6  illustrates a temperature detector circuit in accordance with an embodiment of the present application; 
           [0017]      FIG. 7A-7H  are graphs illustrating exemplary waveforms of the detector circuit of  FIG. 6 ; and 
           [0018]      FIG. 8  illustrates the temperature detector circuit of  FIG. 6  being used with an exemplary power switching converter in accordance with an embodiment of the present application. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    A temperature detector circuit  114  in accordance with an embodiment of the present application is illustrated in  FIG. 2 , for example. In the temperature detector circuit  114 , a sensor signal from the temperature sensor  110  is gated when switching noise is present. This avoids the problem of producing a DC offset error based on the noise. 
         [0020]    The temperature sensor  110  provides an indication of the circuit temperature and may be connected to the electrode of a switch, as described above. When noise is generated by the noise generator, typically due to switching activity of the power switch, the output of the sensor  110  will include the generated noise. 
         [0021]    The temperature detector circuit  114  receives as an input, the output of the temperature sensor  110 . The detector circuit  114  includes a silent sense generator  114   a  which is used to detect a noise triggering event, such as switching of the power switches. The generator  114   a  monitors the switching activity or the noise triggering signals and activates a silent sense signal to pass the signal through the gates  114   b ,  114   d  when active switching noise is predicted to be low. That is, the output of the generator  114   a  is active, or high, when the noise level is low, while the output is inactive, or low, at other times. The noise gates  114   b ,  114   d  are connected to receive the sensor signal, and the comparator output, respectively, and pass the sensor signal and comparator output, respectively, when the silent sense signal is active, or high, that is, when the noise level is low. At all other times, the noise gates  114   b ,  114   d  block the sensor signal and/or comparator output such that no over temperature alarm can be triggered. While  FIG. 2  illustrates two noise gates  114   b  positioned between the sensor  110  and the comparator  114   c  and the gate  114   d  positioned at the output of the comparator, it is noted that it is not necessary to use both noise gates. A single noise gate may be used instead and may be positioned either between the sensor  110  and the comparator  114   c , or after the comparator, as desired. 
         [0022]    That is, in the detector circuit  114 , the sensor  110  output signal and/or the comparator  114   c  output signal are blocked when switching noise is present. This will improve the detector sensitivity and will reduce instances of false alarm where the noise in the sensor signal incorrectly triggers the alarm signal. While the noise gate is illustrated as positioned between the sensor  110  and the comparator  114   c , it is noted that these devices may switch position such that the output of the comparator, which serves and the alarm signal is gated based on the generator output. In either case, the alarm signal will not indicate an over temperature condition, that is go high, unless the noise level detected by the generator  114   a  is low, and thus has little affect on the output of the sensor  110 . 
         [0023]      FIG. 3  illustrates an example of a prior art temperature detector circuit  214  connected to a switched mode power converter  200  with dual temperature sensors B, S. The converter  200  includes switches Q 1 , Q 2  which are controlled at the control terminals HO and LO. An inductor Lout is connected to the common node positioned between the switches Q 1 , Q 2  along with an output capacitor Cout. The output terminal O is connected to a load represented by the resistance RL. Two separate power supplies +VB and −VB are connected to the positive and negative rails, respectively. The circuits to the left of the line A are preferably embodied as integrated circuits while those on the right side of line A are PCB level circuits. The temperature sensors B, S are connected between terminals VB and TB and terminals VS and TS, respectively, of the detector circuit  214 . 
         [0024]    The detector circuit  214  of  FIG. 3  is illustrated in further detail in  FIG. 4 . As noted above, this circuit is preferably embodied as an integrated circuit and includes power supply terminals VB and VS, sensor terminals TB and TS and output alarm terminal  10 . For each temperature sensor B, S a current source  220   a ,  220   b , a voltage reference  212   a ,  212   b  and an analog comparator CO 1 , CO 2  whose output is combined via an OR gate to the over temperature output terminal  10 . The current sources  220   a ,  220   b  drive the current from the node TB to VS and from the node VB to TS, respectively. The voltage reference circuits  212   a ,  212   b  generate the reference voltages VTHB and VTHS which are referenced to VB and VS respectively. Resistor Rp and capacitor Cp provide a pole frequency for the LC filters  216   a ,  216   b  between the sensor nodes and the comparator input nodes. 
         [0025]    The temperature sensors B, S are preferably thermistors whose resistance is a positive or negative function of temperature. The sensor voltage is provided by forcing a reference through the temperature sensor. The sensor voltage is low pass filtered and passed to the comparator(s) CO 1 , CO 2  and compared to the reference voltages VTHB and VTHS. An over temperature condition is reported when the filtered sensor voltage exceeds a reference voltage. The alarm signal at terminal  10  goes high at this point. 
         [0026]      FIG. 5  illustrates typical waveforms for the circuit of  FIG. 4 . The graph of  FIG. 5A  is an exemplary waveform of the voltage at the control node HO for the high side switch Q 1 .  FIG. 5B  is the waveform of the supply voltage.  FIG. 5C  illustrates the sensor voltage of sensor B along with the reference voltage VTHB.  FIG. 5D  is an illustration of the reference voltage VTHB and the filtered sensor voltage of sensor B.  FIG. 5E  illustrates the reference voltage VTHS and the voltage of sensor S.  FIG. 5F  illustrates the over temperature alarm signal provided at terminal  10 .  FIG. 5G  illustrates a reference temperature THB for the sensor B and the temperature sensed by sensor B.  FIG. 5H  illustrates the temperature of sensor S. The sensor voltage is affected by unidirectional switching noise which causes a DC offset indicated by the solid line of  FIG. 5D . This offset may be added to the sensor voltage and results in errors in detecting over temperature. In  FIGS. 5A-H , the left dotted line indicates the beginning of the rise of temperature. The center dotted line indicates a false detector alarm signal and the right dotted line indicates the time when the alarm signal should have been triggered. Thus, the conventional circuit of  FIG. 4  clearly allows for false alarm signals. 
         [0027]      FIG. 6  illustrates a novel over temperature detector circuit  314  in accordance with an embodiment of the present application. The detector circuit  314  is connected to dual sensors B, S. That is, a single circuit  314  detects an over temperature condition for two switches Q 1 , Q 2 . Power supply terminals VB and VS are provide along with sensor terminals TB and TS. For each temperature sensor B, S, a current source  320   a ,  320   b , a voltage reference  312   a ,  312   b  and an analog comparator CO 1 , CO 2  are provided. The comparator outputs are gated with the silent gate signal from the generator  314   a  via AND gates  350   a ,  350   b . The generator  314   a  includes a delay element  360  and a pulse generator  362  with an input connected to the power switch, or the control terminal thereof HO. The current sources  320   a ,  320   b  drive current from node TB to node VS and node VB to node TS, respectively. The voltage reference circuits  312   a ,  312   b  generate threshold voltages VTHB and VTHS which are referenced to VB and VS respectively. 
         [0028]    The output of the generator  314   a  is active during a narrow time window and is triggered by the delayed positive flank of the power switch control signal HO. The delay is sufficiently long to allow switching noise to fade and avoid false alarms. The time window ends before the next switching action, by either switch such that the signal is high when a low noise condition is present. The generator  114   a  illustrated in  FIG. 2  and described above, may be implemented in the same manner as generator  314   a .  FIG. 7  illustrates exemplary timing diagrams for the circuit of  FIG. 6 . 
         [0029]      FIG. 7A  illustrates the waveform of the control signal HO.  FIG. 7B  is the supply voltage.  FIG. 7C  illustrates the time window signal of the generator.  FIG. 7D  illustrates the sensor output signal of sensor B and reference voltage VTHB and  FIG. 7E  illustrates sensor output signal of sensor S and reference voltage VTHS.  FIG. 7F  illustrates the output signal of the detector circuit, that is, the alarm signal provided at terminal  10 .  FIG. 7G  illustrates the temperature detected by sensor B and  FIG. 7H  illustrates the temperature detected by sensor S. 
         [0030]    In general, the switching activity represented in the waveform HO generates switching noise at the supply voltage and at the temperature sensor B. The temperature starts off low, but the temperature at sensor B increases gradually. When the sensor voltage approaches threshold VTHB, switching noise causes the sensor voltage to cross the threshold VTHB, in a manner similar to that described above. The output of comparator CO, however, is gated until the next generator  114   a  output window, so that it is prevented from triggering the alarm. The over temperature alarm is only triggered, that is, driven high, when the comparator output and the output of the generator  1114   a  are both high. That is, when the sensor temperature exceeds the threshold VTHS and the noise level is low. This eliminates false alarms that are triggered by noise. In  FIG. 7 , the left dotted line indicates the beginning of temperature rise. The second dotted line indicates a time when the noise signal drives the sensor signal over the threshold, but proper gating prevents a false alarm. The right line indicates a time when the generator window (output signal is high) and the over temperature alarm is triggered. 
         [0031]      FIG. 8  illustrates the over temperature detector circuit  314  connected to a switching power converter  400  including switches Q 1 , Q 2  and temperature sensors B and S monitoring the temperature of those switches. The terminals HO, LO control the switches Q 1 , Q 2 . An output inductor Lout and capacitor Cout are connected to the node between the switches Q 1 , Q 2 . Two power supplies provide the positive and negative rails +VB and −VB. The resistance is represented by the resistor RL. Components on the left side of the dotted line A are integrated circuits while those on the right side are preferably PCB level circuits. The sensors B, S are connected between the supply voltage and the sensor terminals of the circuit  314 . A power switch input HO is provided connected to the control terminal of the high side switch Q 1  to provide an indication of noise triggering events for the detector circuit. This information is used by the generator  314   a  to provide proper gating to avoid false alarms as is described above. 
         [0032]    Thus, the detector circuit of the present application provides accurate detection of an over temperature condition even when the sensors are connected directly to the electrodes of the switches and are distorted by switching noise. Properly gating the sensor input signals, and/or, the output alarm signal to prevent inadvertent triggering of the over temperature alarm signal during times of high noise improves reliability of the over temperature detecting circuit. 
         [0033]    Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.