Abstract:
In an integrated circuit (IC) for providing an enabling signal (EN) to a converter, the integrated circuit (IC) comprising:  
     a monitoring circuit (FET 1 , R 2 , D 1 , Io, M, S 1 ) for providing a control signal (CS) in response to a level of a line voltage (Vline) on a first connection terminal ( 8 ) of the integrated circuit (IC), and  
     a start-up circuit (FET 2 , Istrt_up, Vcc_strt-Lev, COMP, S 2 ) for providing the enabling signal (EN) to the converter in response to the control signal (CS) and a generated voltage level (Vcc), said generated voltage level (Vcc) being generated in response to the level of the line voltage (Vline) on the first connection terminal ( 8 ),  
     the monitoring circuit and the start-up circuit are sensing the level of the line voltage (Vline) only via the first connection terminal ( 8 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a switched mode power supply (SMPS) control circuit, and to a switched mode power supply (SMPS) comprising such a control circuit.  
         BACKGROUND OF THE INVENTION  
         [0002]    In an SMPS, the maximum output power is often dependent on the supply voltage. Below a certain input voltage level, the required output power cannot be supplied by the SMPS due to current limitations of the power switches in the SMPS, maximum allowed current in the fuses at the input of the SMPS or maximum allowed current through the transformer. In many cases these limitations will result in audible noise of the transformer.  
           [0003]    To prevent the SMPS from starting at a too low input voltage level (and thus preventing audible noise), a circuit has to disable the operation of the SMPS. In prior art systems operating at a rectified line (mains) voltage, external components are required in order to determine the level of the line voltage. These external components require that at least one extra pin is available on the circuit.  
           [0004]    In prior art SMPS systems that supply from a rectified line voltage (Vline), detection of the line voltage is performed using e.g. an external voltage divider. Alternatively, a voltage to current converter with an external resistor may be used.  
           [0005]    It is a disadvantage of the prior art systems that an extra pin on the integrated circuit (IC) is required in order to determine the level of the line voltage. In addition, the use of external components complicates the implementation of such systems.  
           [0006]    Integration of ohmic resistive dividers is very complicated for the following reasons. A high-ohmic resistive divider is not suitable for integration, because high ohmic resistive dividers occupy too much silicon area of the IC. Integration of lower ohmic resistive dividers would lead to too much dissipation—and thereby too much heat generation—and power consumption in the IC.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to provide an improved IC with integrated sensing/monitoring of the line voltage.  
           [0008]    The above-mentioned objects are complied with by providing an integrated circuit for providing an enabling signal to a converter, and an SPMS as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.  
           [0009]    According to a preferred embodiment, the use of external components for sensing/monitoring the line voltage can be avoided. Furthermore, no extra pin is needed in order to connect the control circuit to external components for sensing the line voltage.  
           [0010]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    In the drawings:  
         [0012]    [0012]FIG. 1 shows an embodiment of an SMPS control IC in accordance with the present invention with internal operation enabling function where the threshold level can be adjusted using an external resistor R 1 ,  
         [0013]    [0013]FIG. 2 shows an embodiment of an SMPS in accordance with the present invention with open elcap fault condition F, and  
         [0014]    [0014]FIG. 3 shows the signals of the SMPS controller during and after “missing elcap” fault condition F. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    The present invention relates to a control circuit for an SMPS, where the use of external components or an additional pin for voltage sensing is avoided. The control circuit according to the present invention offers internal determination of the level of the line voltage. Furthermore, in accordance with a preferred embodiment, the SMPS operation enabling level may be tuned/adjusted using a single external resistor R 1 .  
         [0016]    An additional advantage of the control circuit according to the present invention is the safety aspect provided by the present invention—e.g. when the line elcap (electrolytic capacitor C 2  in FIG. 2) is disconnected, as indicated at F in FIG. 2.  
         [0017]    As shown in FIG. 1, the line voltage sensing is done inside the IC. A (high voltage) field effect transistor (FET 1 ) limits the maximum voltage on the resistor. The pinch off voltage of the transistor FET 1  is above the desired enabling level (which can be as high as 100V), because the gate of the transistor FET 11  is connected to a relatively high (zener) voltage. Due to the high zener voltage of the internal zener diode D 1 , sensing can be done up to high Vline voltage levels, while keeping the current low. After the SMPS is enabled by the described function—see below, switch S 1  is closed. In this way, the pinch off voltage of the transistor FET 1  drops significantly wherefore low power consumption is guaranteed. Furthermore, a better resurf of the high voltage transistor FET 1  is obtained.  
         [0018]    Since switch S 1  is controlled by internal logic (not shown), the IC is able to operate down to very low line voltages, but is prevented from starting up at low line voltages.  
         [0019]    The operational characteristics of the control circuit will now be described in detail. Normal start-up of an SMPS application applies that a line switch (not shown) is activated, and subsequently the line voltage capacitor C 2  in FIG. 2 will be charged whereby the line voltage—Vline—applied to the IC starts to increase.  
         [0020]    When Vline is high enough to enable transistor FET 2  in the start-up circuit to operate, an internal current source Istrt-up will start trying to charge the external capacitor C 1 . This is however prevented by the internal switch S 2  as long as this switch is closed. In its closed position, switch S 2  shorts the external capacitor C 1  to ground.  
         [0021]    Switch S 2  is controlled by the operation enabling sense function, which is connected to the same HV terminal. Transistor FET 1  is constantly monitoring the line voltage Vline. Since the detection function is done with very low current, the source voltage of transistor FET 1  (source=connected to the resistor R 2 ) is almost the same as the drain voltage (=Vline).  
         [0022]    The zener diode D 1  has a high breakdown voltage whereby Vline may be monitored even at relatively high voltages keeping the current at very low levels. Preferably the break-down voltage of the zener diode D 1  is approximately 80V. Preferably, the break-down voltage is externally adjustable to e.g. 40V.  
         [0023]    When Vline exceeds the enabling level, the zener diode D 1  goes into breakdown causing a current to flow through transistor FET 1 , resistor R 2  and the zener diode D 1 . This current is being mirrored by current mirror M and compared with a reference current Io, and via some (latched) logic switch S 2  is opened and the internal current source Istart-up will start charging the external capacitor C 1 .  
         [0024]    The voltage Vcc on the external capacitor C 1  is constantly compared to a predetermined starting level—Vcc_strt_lev. When Vcc exceeds Vcc_strt_lev, an enabling signal EN is provided and the SMPS starts operating.  
         [0025]    Once the start level Vcc_strt_lev is reached, the operation enabling function is disabled and therefore no longer dependent on Vline. Also, and at the same time, the Istrt_up circuit is disabled. The operation enabling function is disabled by closing switch S 1  whereby the gate of transistor FET 1  is connected to ground. By connecting the gate to zero, the pinch off voltage of transistor FET 1  is lowered dramatically, with the result that the zener diode D 1  is no longer in breakdown and only a small leakage current will flow through transistor FET 1  and the resistor R 2 . The small leakage current reduces the power consumption of the IC whereby green function is achieved. Preferably, the power consumption is less than 50 mW.  
         [0026]    The moment the control circuit stops operating for whatever reason—e.g. over temperature protection, fault detections over voltage protection etc.—the supply voltage to the control circuit drops to a so-called “Under Voltage Lock Out” (take over supply is no longer present because the converter does not switch anymore), and the IC re-activates Istrt_up whereby the capacitor C 1  is recharged. Provided that Vline is above the enabling level, the function is enabled again when Vcc exceeds Vcc_strt_lev. Therefore, the IC will re-start only when Vline is above the predetermined level, but is able to operate down to very low line voltages.  
         [0027]    The resistor R 1  provides the opportunity to tune or adjust the line voltage enabling level. The current drawn from the HV pin  8  is always the same during start-up (for example 1 mA). Thus, it is easy to adjust the operation enabling level by a level 1 mA*R 1 , whereby the HV pin&#39;s voltage is lowered by 1 mA*R 1 .  
         [0028]    It is an advantage of the present invention that it protects control circuit operating in an open elcap situation as illustrated at F in FIG. 2. The rectified, but not buffered, line voltage charges the external capacitor C 1  (via the start-up current source) only to a low value. When the line voltage is below the low-line trip level (twice every period of the line frequency), the external capacitor C 1  is discharged. The voltage Vcc across the external capacitor C 1  will never reach the start-up voltage level Vcc_strt_lev as indicated in FIG. 3. Only as soon as the elcap C 2  is re-connected, indicated by -F in FIG. 3, the voltage Vcc will reach the start-up voltage level Vcc_strt_lev and the control circuit will start operating.  
         [0029]    An external resistor R 1  may be connected between the first connection terminal  8  and the mains voltage Vmains so as to generate a voltage drop over the said external resistor R 1  and thereby adjust the voltage on the first connection terminal  8 . The resistance of the external resistor R 1  may be within the range 1-1000 kΩ, such as within the range 10-100 kΩ. The breakdown voltage of the voltage limiting means—e.g. a zener diode D 1 —may be within the range 25-150V (including preferred typical values at 40V and 80V), such as within the range 50-100V, such as within the range 60-90V, such as approximately 80V. The current limited means may comprise a transistor, such as a MOST transistor.  
         [0030]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.