Abstract:
Disclosure includes a control method for over voltage protection, suitable for a power converter. The power converter has a rectifier for rectifying an alternative-current voltage into a direct-current input voltage. The direct-current input voltage is sensed and determined whether it locates in a valley. A detection voltage is provided, capable of representing the direct-current input voltage. The detection voltage is compared with a predetermined reference voltage. When the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley, an OVP signal is provided to stop the power conversion of the power converter.

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to over voltage protection for power converters. 
         [0002]    Power converters, which usually supply power to appliances used in daily life, need to be equipped with protection mechanism to prevent abnormal satiations from damaging users or surroundings. For example, a power converter that powers light emitting diodes for lighting must have over-voltage protection (OVP) so as to avoid over voltage occurring in its outputs, which might cause electric shock to human beings if touched. 
         [0003]      FIG. 1  demonstrates a conventional power converter  10 . Bridge rectifier  12  provides full-wave rectification to alternative-current (AC) mains voltage V AC  to generate rectified direct-current (DC) input voltage V IN  and a ground line. Power converter  10  is a buck converter having LED module  14  as a load, which connects in series with a primary winding PRM in a transformer, between DC input voltage V IN  and the ground line. Power controller  17  has a power switch  18 , which, when turned on (as being in a conduction state), energizes primary winding PRM and conducts a driving current to illuminate LED module  14 . When power switch  18  is turned off (as being in a non-conduction state), primary winding PRM starts to release its stored energy to generate another driving current, which passes wheel diode  16  to keep LED module  14  illuminating. Current-sense resistor  20  provides to power controller  17  current-sense signal V CS , a representative of the current passing through power switch  18 . 
         [0004]    If a LED open event happens to LED module  14 , meaning that at least one LED in LED module  14  is open or cannot conduct current, driving voltage V LED  could rocket if there is no corresponding protection mechanism, or OVP, built in power controller  17 . The two end terminals of LED module  14 , which meanwhile has a drop voltage the same with the rocket-high driving voltage V LED , could cause severe electric shock to anyone whoever touches them, endangering human beings. 
         [0005]    Power controller  17  in  FIG. 1  detects driving voltage V LED , through the help from the combination of node VOP, voltage divider  22  and secondary winding SEC. When the transformer de-energizes to release its stored energy, the voltage across primary winding PRM is about the summation of driving voltage V LED  and the forward voltage of wheel diode  16 , and the voltage across secondary winding SEC is in proportion to that across primary winding PRM. Accordingly, in case that the voltage at node VOP exceeds a certain limit when the transforming de-energizes, it implies driving voltage V LED  is somehow over high, and, responsively, power controller  17  could continuously turn off power switch  18  to stop power conversion of power converter  10 , achieving OVP. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted. 
           [0007]    The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0008]      FIG. 1  demonstrates a conventional power converter; 
           [0009]      FIG. 2  demonstrates a power converter according to embodiments of the invention; 
           [0010]      FIG. 3  demonstrates a power controller and some peripheral devices; and 
           [0011]      FIG. 4  shows some signal waveforms of signals in  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Even power converter  10  in  FIG. 1  achieves OVP, it is a costly and bulky solution because of the necessity of the transformer composed of at least primary winding PRM and secondary winding SEC. A transformer, which has more than one windings coupled to each other, normally costs and occupies more than an inductor with only one winding does. 
         [0013]      FIG. 2  demonstrates power converter  60  according to embodiments of the invention, which uses an inductor L to replace the transformer in  FIG. 1 . Power converter  60  could achieve OVP without a costly and bulky transformer. This does not mean a power converter according to the invention must not have a transformer. Some embodiments of the invention might use one winding of a transformer to be an inductor. 
         [0014]    Resistors  64  and  66  for voltage dividing are connected in series between DC input voltage V IN  and a ground line GND, where the joint therebetween provides a detection voltage V PF  which is therefore a scaled version of DC input voltage V IN . 
         [0015]    In the embodiment of  FIG. 2 , power controller  62 , which could be in form of an integrated circuit, operates power converter  60  substantially in boundary mode. One operation mode is called discontinuous conduction mode (DCM), referring to that an inductor in a power converter is operated to empty completely the energy stored therein every time when a new switching cycle starts. Another operation mode is continuous conduction mode (CCM), referring to that a power converter is operated to start a new switching cycle while the inductor has not emptied the energy stored. Boundary mode operates a power converter in a way between DCM and CCM, generally referring to that a new switching cycle starts right after the inductor just empties the energy stored. 
         [0016]    Inductor L starts to increase its stored energy when the power switch in power controller  62  is turned on, and the voltage V L  and the current I L  of inductor L shall follow the relationship presented as the following equation (I). 
         [0000]        V   L   *T   ON   =L   L   *I   L , 
         [0000]      ( V   IN   −V   LED )* T   ON   =L   L   *I   CS-PEAK   (I),
 
         [0017]    where V L  and I L  denote the voltage drop across inductor L and the current through inductor L; L L  the inductance of inductor 
         [0018]    L; T ON  the duration or the ON time when the power switch in power controller  62  is turned on; and I CS-PEAK  the peak current flowing through current-sense resistor  20 . 
         [0019]    It can be derived from equation (I) that I CS-PEAK  is about 0 when DC input voltage V IN  is the same with driving voltage V LED , and inductor L cannot be energized. Bridge rectifier  12  causes DC input voltage V IN  to follow the absolute value of AC mains voltage V AC  if DC input voltage V IN  is about less than that absolute value. That absolute value has no influence to DC input voltage V IN  nevertheless if DC input voltage V IN  exceeds that absolute value. Accordingly, when that absolute value is less than driving voltage V LED , DC input voltage V IN  will have the same voltage as that of driving voltage V LED  because inductor L stops energizing at the same condition. When that absolute value exceeds driving voltage V LED , DC input voltage V IN  is about the same as that absolute value. It can be concluded that the local minimum of DC input voltage V IN  should be about the same as driving voltage V LED . A local minimum of DC input voltage V IN  happens in a valley of the waveform of DC input voltage V IN . 
         [0020]    One embodiment of the invention detects a local minimum of DC input voltage V IN  to decide whether to trigger OVP. 
         [0021]    Power controller  62  in  FIG. 2  determines the occurrence of a local minimum of DC input voltage V IN  by detecting current-sense signal V CS . For example, DC input voltage V IN  seems to be in a valley and have a local minimum if current-sense signal V CS  continues to be about 0 (or less than a predetermined value V CS-REF ) for a predetermined period of time. When DC input voltage V IN  is in a valley, it could be used to represent driving voltage V LED . 
         [0022]    Power controller  62  compares detection voltage V PF  with a reference voltage for OVP (V OVP-REF ). If DC input voltage V IN  is having a local minimum and detection voltage V PF  exceeds reference voltage V OVP-REF  driving voltage V LED  is deemed to be over high and, in response, power controller  62  provides an OVP signal S Protection  to stop the power conversion of power converter  60 . 
         [0023]      FIG. 3  demonstrates power controller  62  and some peripheral devices. Power controller  62  has, but is not limited to have, valley detector  79 , OVP comparator  82 , ramp-signal generator  84 , logics  83  and  88 , etc. 
         [0024]    Valley detector  79  includes valley comparator  80  and delay time generator  81 . Valley comparator  80  compares current-sense signal V CS  with a predetermined reference V CS-REF  which is 50 mV in one embodiment. If the input of delay time generator  81  indicates that current-sense signal V CS  has been less than predetermined reference V CS-REF  for a predetermined period of time T OVP-DELAY , delay time generator  81  makes its output 1 in logic, meaning the occurrence of a local minimum of DC input voltage V IN . 
         [0025]    If a local minimum of DC input voltage V IN  occurs and OVP comparator  82  deems detection voltage V PF  exceeding reference voltage V OVP-REF , logic  83  sends out OVP signal S Protection  with logic 1 to stop the power conversion of power converter  60 , thereby driving voltage V LED  being prevented from going higher. 
         [0026]    Ramp-signal generator  84  generates ramp signal V RAMP , whose slope is determined by a peak value of detection voltage V PF . For example, the peak value of detection voltage V PF  can be sensed or recorded by power controller  62 , and it represents a swing magnitude of AC mains voltage V AC .In one embodiment, the higher the peak value of detection voltage V PF , the higher the slope of ramp signal V RAMP . Both ramp signal V RAMP  and a compensation signal V COMP  are forwarded to two inputs of comparator  86 . For instance, ramp signal V RAMP  starts to ramp up at the same time when power switch  18  is turned on. Once ramp signal V RAMP  exceeds compensation signal V COMP , comparator  86  makes logic  86  to turn off power switch  18 . Ramp-signal generator  84  and comparator  86  together can determine the ON time T ON  of power switch  18  during which it is turned on. 
         [0027]      FIG. 4  shows some signal waveforms of signals in  FIGS. 2 and 3 . AC mains voltage V AC  has for example a sinusoidal waveform with a swing magnitude of 110V and a frequency of 60 Hz. Shown in  FIG. 4  is also DC input voltage V IN , whose local minimums occur in valleys and always even with driving voltage V LED . Detection voltage V PF  is in proportion to DC input voltage V IN . 
         [0028]    In  FIG. 4 , it is supposed that LED module  14  mistakenly becomes open since time t LED-OPEN . Accordingly, as an open LED module  14  does not consume electric power and the switching of power switch  18  continues the power conversion, driving voltage V LED  ramps up after time t LED-OPEN . 
         [0029]    A first valley VL 1  occurs in the waveform of DC input voltage V IN , shown in  FIG. 4 . In the meantime, OVP is not triggered though because detection voltage V PF  has not exceeded reference voltage V OVP-REF . 
         [0030]    Following the first valley VL 1 , a second valley VL 2  occurs in the waveform of DC input voltage V IN . Meanwhile, detection voltage V PF  has exceeded reference voltage V OVP-REF . At time t OVP  which is the moment when current-sense signal V CS  has continued to be about 0, or less than reference V CS-REF  for a predetermined period of time T OVP-DELAY  OVP is triggered. Current-sense signal V CS  becomes a constant 0V after time t OVP  because power switch  18  is constantly turned off. 
         [0031]    Different from  FIG. 1 , which needs a bulky and costly transformer,  FIG. 2  shows power converter  60 , which needs only an inductor and is capable of achieving OVP. Power converter  60  could render a product with more market competitiveness in view of its compact size and low cost. 
         [0032]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.