Patent Abstract:
A circuit protects a power conversion system with a feedback control loop from a fault condition. The circuit has an oscillator having an input for generating a signal with a frequency and a timer connected to the oscillator input and to the feedback control loop. The timer disables the oscillator after a period following the opening of the feedback control loop to protect the power conversion system.

Full Description:
BACKGROUND  
       [0001]     The present invention relates to an off-line switched mode control system with fault condition protection.  
         [0002]     Quantum leaps in electronic technology have led to the development of “smart” electrical and electronic products. Each of these products requires a steady and clean source of power from a power supply. One power supply technology called switched mode power supply technology operates at a high frequency to achieve small size and high efficiency. In such a switching power supply, an integrated circuit (IC) regulator is connected in series with the primary winding of a transformer to a rectified and filtered alternating current (AC) power line. The energy is transferred from the primary winding through an output secondary winding to the power supply output in a manner controlled by the IC regulator so as to provide a clean and constant output voltage. Additionally, a third winding called a feedback or bias winding may be used to provide a feedback signal and power to the IC regulator.  
         [0003]     The voltage on the feedback winding tracks the output voltage present on the secondary winding. Thus, when a short occurs on the output of the secondary winding, the voltage on the feedback winding also goes low. Further, in the event of a short circuit condition, an overload condition on the output secondary winding or an open loop condition on the feedback winding, the regulator circuit responds to such conditions by delivering maximum power over a period of time. In such cases, the regulator circuit detects that the power supply is short circuited, overloaded at the output or has encountered an open loop condition. In any of these fault conditions, the regulator circuit goes into a mode called “auto-restart.” In the auto-restart mode, the regulator circuit tries to start the power supply periodically by delivering full power for a period of time (greater than needed for start up) and turns off the power supply for another period of time that is approximately four to ten times longer. As long as the fault condition is present, the regulator circuit remains in this auto-restart mode limiting the average output power to a safe, low value. When the fault is removed, auto-restart enables the power supply to start-up automatically.  
       SUMMARY  
       [0004]     The invention protects a power supply from fault conditions. The power supply has an output and a feedback control loop, the feedback control loop having a feedback signal which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. In a first aspect, the circuit includes a switching device for controlling power delivered to the output and a timer coupled to the switching device and to the feedback signal. The timer disables the switching device to prevent power delivery to the output in a first predetermined period after the fault condition exists.  
         [0005]     Implementations of the invention include one or more of the following. The timer may enable the switching device to deliver power to the output after a second predetermined period. The switching device may be alternately enabled for the first predetermined period and disabled for the second predetermined period when the fault condition exists. The switching device may be enabled upon removal of the fault condition. The switching device may be a power transistor. The timer may be a digital counter. An oscillator with a predetermined frequency may be coupled to the counter. The oscillator may have a control input for changing the predetermined frequency and a first current source coupled to the oscillator control input to generate a first frequency. A second current source may be coupled to the oscillator control input to generate a second frequency. The counter output may be coupled to the fist and second current sources. The timer may be a capacitor which is adapted to be charged at a first rate from a first threshold to a second threshold to generate a first predetermined period. The capacitor may be discharged from the second threshold to the first threshold at a second rate to generate the second predetermined period. The capacitor may also be reset to a voltage below the first threshold each time the feedback signal cycles. The fault condition includes one or more of an output overload fault condition, an output short circuit fault condition and an open feedback control loop fault condition.  
         [0006]     In a second aspect, a method for protecting a power supply having an output and a feedback control loop from fault conditions includes receiving a feedback signal from the feedback control loop, the feedback signal being adapted to cycle periodically when the power supply operates normally and to remain idle when the power supply is in a fault condition; timing the feedback signal to detect whether a fault condition exists in the power supply; and disabling the output after a first predetermined period after the fault condition is detected.  
         [0007]     Implementations of the invention include one or more of the following. A switching device may be enabled to deliver power to the output after a second predetermined period. The switching device may be alternatingly enabled for the first predetermined period and disabled for the second predetermined period. The switching device may be enabled upon removal of the fault condition. The enabling step may enable a power transistor. The timing step includes digitally countering periods of time. A signal may be generated with a predetermined frequency. The generating step includes oscillating at a first frequency and a second frequency. The second frequency may be used when the fault condition exists. The timing step includes charging a capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period; and discharging the capacitor from the second threshold to the first threshold at a second rate to generate a second predetermined period. The capacitor may be reset to a voltage below the first threshold each time the feedback signal cycles.  
         [0008]     In a third aspect, a circuit for protecting a power supply having an output and a feedback control loop from fault conditions includes means for receiving a feedback signal from the feedback control loop, the feedback signal being adapted to cycle periodically when the power supply operates normally and to remain idle when the power supply is in a fault condition; timing means coupled to the feedback signal to detect whether a fault condition exists in the power supply system; and means for disabling the output after a first predetermined period after the fault condition is detected.  
         [0009]     Implementations of the invention include one or more of the following. The circuit includes a means for enabling a switching device to deliver power to the output after a second predetermined period. A means for alternatingly enabling the switching device for the first predetermined period and disabling the switching device for the second predetermined period when the fault condition exists may be used. The circuit may have a means for enabling the switching device upon removal of the fault condition. The switching device may be a power transistor. The timing means includes a digital counter. The circuit includes means for generating a predetermined frequency. The generating means includes means for oscillating at a first frequency and a second frequency. The circuit may include a means for applying the second frequency when the fault condition exists. The timing means includes a means for charging a capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period; and a means for discharging the capacitor from the second threshold to the first threshold at a second rate to generate a second predetermined period. A means for resetting the capacitor to a voltage below the first threshold each time the feedback signal cycles may be used.  
         [0010]     In another aspect, a fault protected power supply includes a regulator coupled to a transformer having a primary winding. The transformer has a secondary winding coupled to a secondary output. The regulator receives a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The power supply includes a switching device coupled to the primary winding of the transformer for controlling power delivered to the secondary output; an oscillator for generating a signal with a predetermined frequency; and a timer coupled to the oscillator and to the feedback signal, the timer disabling the switching device after a predetermined period of existence of a fault condition.  
         [0011]     Implementations of the invention include one or more of the following. The power supply includes a means for changing the frequency of the oscillator. The timer alternatively enables and disables the switching means when the fault condition is present.  
         [0012]     In another aspect, a method protects a power supply having a regulator coupled to a transformer having primary winding, the transformer having a secondary winding coupled to a secondary output, the regulator receiving a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The method includes controlling power delivered to the secondary output using a switching device; generating an oscillating signal with a predetermined frequency; and timing the feedback signal with the oscillating signal and disabling the switching device after a predetermined period of existence of a fault condition.  
         [0013]     Implementations of the invention include one or more of the following. The method includes changing the frequency of the oscillating signal. The method also includes alternatingly enabling and disabling the switching device when the fault condition is present.  
         [0014]     In another aspect, a fault protected power supply has a regulator coupled to a transformer having a primary winding, the transformer having a secondary winding coupled to the secondary output. The regulator receives a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The power supply includes a switching device coupled to the primary winding of the transformer for controlling the power delivered to the secondary output; a capacitor; means for charging the capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period and discharging the capacitor from the second threshold to first threshold at a second rate to generate a second predetermined period; and means coupled to the switching device, the capacitor and the feedback signal for alternately enabling the switching device during first predetermined period and disabling the switching device during the second predetermined period in the presence of a fault condition.  
         [0015]     In yet another aspect, a method protects a power supply having a regulator coupled to a transformer having a primary winding. The transformer has a secondary winding coupled to a secondary output. The regulator receives a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The method includes controlling power delivered to the secondary output using a switching device; charging a capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period; discharging the capacitor from the second threshold to first threshold at a second rate to generate a second predetermined period; and alternatingly enabling the switching device during the first predetermined period and disabling the switching device during the second predetermined period in the presence of a fault condition.  
         [0016]     Advantages of the invention include one or more of the following. The invention protects the switched mode controller and associated components such as the diode and the transformer from various fault conditions. The feedback winding is not necessary. The protection is provided using a minimum number of components. Further, the power supply properly shuts down when it encounters a fault condition and automatically returns to an operating condition when the fault condition is removed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic illustration of a fault condition protection device of the invention.  
         [0018]      FIG. 2  is a plot illustrating the operation of the device of  FIG. 1 .  
         [0019]      FIG. 3  is a schematic illustration of a second embodiment of the fault condition protection device.  
         [0020]      FIG. 4  is a plot illustrating the operation of the device of  FIG. 2 .  
         [0021]      FIG. 5  is a schematic illustration of a switched mode power supply in accordance with the present invention. 
     
    
     DESCRIPTION  
       [0022]     Referring now to  FIG. 1 , a fault-protection circuit  200  is shown. The circuit  200  has a primary oscillator  111  which is connected to a counter  202 . The counter  202  can be reset by a feedback signal which clears registers Q 8 -Q 13  of counter  202 . The feedback signal is explained in more detail below.  
         [0023]     An inverter  204  receives the 13-th bit output of counter  202 . The output of inverter  204  is provided to an AND-gate  206  whose other input is connected to a switching signal. The switching signal is derived from the oscillator  111  output and the feedback signal. This switching signal cycles periodically when the power supply operates normally. The switching signal is idled when the power supply encounters a fault condition. The output of AND-gate  206  in turn is provided to the gate of a switching transistor  208 . Counter  202  eventually causes an AND-gate  206  to shut-off switching transistor  208  and to perform auto-restart.  
         [0024]     Turning now to oscillator  111 , a current source  122  generates a current I from a supply voltage  120 . The output of current source  122  is connected to the source of a p-channel MOSFET transistor  125 , whose drain is connected to a node  123 . Also connected to the node  123  through a p-channel MOSFET  182  is a second current source  184 . Current source  184  can supply current which is ¼ of the current I. The drain of transistor  182  is also connected to node  123 . The gate of transistor  182  is driven by an inverter  180 , whose input is connected to the gate of transistor  125  and to the counter output Q 13 .  
         [0025]     The node  123  is connected to the sources of p-channel MOSFET transistors  126  and  132 . The drain of MOSFET transistor  126  is connected to the drain of an n-channel MOSFET transistor  128 . The source of transistor  128  is grounded, while the gate of transistor  128  is connected to its drain. The gate of transistor  128  is also connected to the gate of an n-channel MOSFET transistor  130 . The source of transistor  130  is grounded, while the drain of transistor  130  is connected to the drain of transistor  132  at a node  131 . Transistors  126 ,  128 ,  130  and  132  form a differential switch. The input of inverter  124  and the gate of transistor  132  are driven by a hysteresis comparator  136 . Output of inverter  124  drives the gate of MOSFET transistor  126 . Comparator  136  has an input which is connected to node  131  and to a capacitor  134 . The other node of the capacitor is connected to ground. In combination, transistors  126 ,  128 ,  130  and  132 , capacitor  134 , inverter  124  and hysteresis comparator  136  and current source  122  form an oscillator. The output of hysteresis comparator  136  is provided as an oscillator output and is also used to drive the clock input of counter  202 .  
         [0026]     During operation, the feedback signal periodically pulses between a low state and a high state depending on the amount of power required on a secondary winding  922  ( FIG. 5 ). Every time the feedback signal is low, the feedback signal resets a counter whose states are reflected by outputs Q 8 -Q 13  of counter  202 . The resetting of the counter associated with outputs Q 8 -Q 13  thus occurs regularly when no fault is present in the power supply. The cycling of the feedback signal constantly clears the output bit Q 13  such that the power transistor  208  is controlled by the switching signal when no fault is present. However, in the event of a fault condition, the feedback signal remains high for a sufficiently long time such that the counter associated with output bits Q 8 -Q 13  has enough time to increment output bit Q 13 . The setting of the output bit Q 13  causes inverter  204  output to go low and thus causes the output of AND-gate  206  to be deasserted. The deassertion of AND-gate  206  in turn disables switching transistor  208 . Also, when the counter output Q 13  goes high transistor  125  turns off to isolate primary current source  122  from node  123 . This turns on the transistor  182  via inverter  180 , thus allowing the ¼ I current to flow from the secondary current source  184  to node  123 . The state change of the counter output Q 13  causes the oscillator to switch at one-fourth of its normal frequency to achieve about 20% on time and 80% off time. This operation reduces the power delivered by the power supply under a fault condition as well as avoids the possibility of damage to the regulator device and other power supply components such as the output diode or the transformer (not shown).  
         [0027]      FIG. 2  shows a timing diagram for the device of  FIG. 1 . The timing diagram of  FIG. 2  shows three periods:  211 ,  213  and  215 . Period  211  is normal operation with the feedback signal going “low” more often than a predetermined count such as approximately 4096 clock cycles, thereby resetting the Auto Restart Counter before it counts up to 4096.  
         [0028]     In Period  213 , the feedback signal has been “high” for 4096 continuous clock cycles due to a fault condition such as an output overload or short, so the circuit of  FIG. 1  goes into the auto-restart mode. The oscillator frequency is divided by four and switching transistor  208  has been inhibited from switching, remaining in its off state. After 4096 clock cycles, switching transistor  208  is activated and the oscillator frequency switches back to normal frequency. This sequence will repeat itself as long as the feedback signal stays “high.” 
         [0029]     In Period  215 , the overload condition or the short condition on the output of the power supply is removed and the feedback signal goes low, indicating the power supply output is in regulation. The circuit is now in normal operation with the feedback signal going “low” at least once every 4096 clock cycles. It is to be noted that the auto-restart capability as been described may not be used in all applications. Particularly, certain applications may disable the power regulator after detecting a fault condition and the disabling of the power regulator may continue until a user resets the power regulator, or until AC power is cycled OFF and then ON to the power regulator.  
         [0030]      FIG. 3  shows an analog auto restart circuit. A current source  525  produces a fixed magnitude current  530 . Fixed magnitude current  530  is fed into first transistor  535  and mirrored to transistors  540  and  545 . Third transistor  545  is connected to a capacitor  550  via transistor  595 . Transistor  600  is also connected to the capacitor  550 . Transistor  600  is controlled by the feedback signal provided to inverter  605  whose output drives the gate of the transistor  600 . Node  400  is generated by the charging and discharging of capacitor  550 . Capacitor  550  has a relatively low capacitance which allows for integration on a monolithic chip in one embodiment of the IC regulator of the invention. Node  400  is provided to a hysteresis comparator  560  which compares its input with a lower limit of about 1.5 volts and an upper limit of about 4.5 volts. The output of comparator  560  is provided to the gates of transistors  585  and  595 . AND-gate  570  receives at one input the output of comparator  560 . AND-gate  570  enables switching transistor  572  to turn on and off. AND-gate  570  receives at a second input a switching signal which modulates the regulator output.  
         [0031]     In operation, after the feedback signal goes high, capacitor  550  begins to charge from a level below 1.5 volts to an upper threshold of about 4.5 volts. Upon reaching 4.5 volts, the output of comparator  560  switches and discharges the capacitor  550  through transistors  545  and  595 . Node  400  then switches between the upper threshold of about 4.5 volts and the lower threshold of about 1.5 volts.  
         [0032]     Signal  401  output of comparator  560  will be high until node  400  exceeds the upper threshold limit. When signal  400  is high, p-channel transistors  585  and  595  are turned off. By turning off transistors  585  and  595 , current can flow into and steadily charge capacitor  550  and increase the magnitude of node  400 . The current that flows into capacitor  550  is derived from current source  525  because the current through transistor  590  is mirrored from transistor  580 , which current is derived from transistor  540 .  
         [0033]     Referring to  FIGS. 3 and 4 , in period  600  feedback signal  402  is switching and the system is in normal operation with switching transistor  572  controlled by the switching signal. At the end of period  600  a fault condition has been detected and the feedback signal stays high for an extended period of time (period  601 ). In period  601 , transistor  600  turns off, allowing capacitor  550  to be charged by current source  590 . When the voltage on node  400  has reached the second threshold, the output  401  of comparator  560  goes low, disabling the switching transistor  572 . Capacitor  550  will be discharged to the first threshold by current source  545  with switching transistor  572  disabled. This mode of oscillation continues until the feedback signal goes low again, indicating that the fault condition no longer exists. When the feedback signal  402  at the end of period  601  goes low, transistor  600  turns on and discharges capacitor  550  to a voltage below the first threshold. Comparator  560  output will go high and enable the switching signal to control the switching transistor  572 . In period  602 , the system has returned to normal operation with the feedback signal  402  going low at least once during a defined time period indicating that the regulator circuit is in regulation.  
         [0034]     Referring now to  FIG. 5 , a switched mode power supply is shown. Direct current (DC) input voltage is provided to a Zener diode  912  which is connected to a diode  914 . The diodes  912 - 914  together are connected in series across a primary winding of a transformer  920 . A secondary winding  922  is magnetically coupled to the primary winding of transformer  920 . One terminal of the secondary winding  922  is connected to a diode  930 , whose output is provided to a capacitor  932 . The junction between diode  930  and capacitor  932  is the positive terminal of the regulated output. The other terminal of capacitor  932  is connected to a second terminal of the secondary winding and is the negative terminal of the regulated output. A Zener diode  934  is connected to the positive terminal of the regulated output. The other end of Zener diode  934  is connected to a first end of a light emitting diode in an opto-isolator  944 . A second end of the light-emitting diode is connected to the negative terminal of the regulated output. A resistor  936  is connected between the negative terminal of the regulated output and the first end of the light-emitting diode of opto-isolator  944 . The collector of the opto-isolator  944  is connected to current source  172 . The output of current source  172  is provided to the switching regulator logic  800 .  
         [0035]     Connected to the second primary winding terminal is the power transistor  208 . Power transistor  208  is driven by AND gate  206  which is connected to inverter  204  and switching regulator logic  800 . Switching regulator logic  800  receives a clock signal  101  from an oscillator  111 . A counter  202  also receives the clock signal  101  from the primary oscillator  111 . The output of counter  202 , Q 13 , is used to switch in the current source  184  to supply current in lieu of the current source  122  when Q 13  is high.  
         [0036]     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape, materials, components, circuit elements, wiring connections and contacts, as well as in the details of the illustrated circuitry and construction and method of operation may be made without departing from the spirit of the invention.

Technology Classification (CPC): 5