Abstract:
A boost DC/DC power converter is disclosed that has a low voltage source, an inductor and a switching device that forms a series loop, a diode in series with a capacitor coupled across the switching device, a voltage divider coupled across the capacitor and a pulse width modulator that is coupled to the voltage divider. The boost converter includes a first push controller coupled across the switching device to provide a first push voltage of sufficient magnitude to turn the switching device on where the low voltage source by itself is not capable of generating a voltage of sufficient magnitude to operate the switching device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a voltage converter and more specifically to switched low voltage converters that can operate when provided with an input voltage that is less than the threshold voltage of the switching devices used in the voltage converter. 
     2. Description of the Related Art 
     In today&#39;s electrical devices and equipment, voltage converters are widely used to convert an output voltage of a single power source, such as a battery, to a few voltage rails that, in turn, power different functional blocks within a system. 
     One example is a mobile handset powered by a single cell Li-Ion battery with a 3.6V nominal output voltage. Within the mobile handset, there are usually 3 to 5 voltage rails, such as 5V, 3.3V, 2.5V, 1.8V, and 1.5V, etc., that are needed to power different functional blocks within the handset. All these voltage rails are converted from the Li-Ion battery. Because some of the voltages, such as 5V, are higher than the nominal output voltage of the single-cell Li-Ion battery, voltage converters with a step-up or “boost” topology, are used. 
     A boost converter is a switching mode converter that typically contains a diode and an energy storage element. An example of a conventional boost converter is shown in  FIG. 1 . The boost converter  10  includes a high voltage power source (HV_BAT)  12 , an energy storage element in the form of an inductor (L 1 )  14  connected in series with the HV_BAT, a switching device (SW_ 1 )  16  connected between the inductor and ground (GND), a diode (D 1 )  17  connected in series with (L 1 ), a capacitor (C 1 )  18  connected between D 1  and GND, and a voltage divider, a first resistor (R 1 )  20  and a second resistor (R 2 )  22 , that is also connected between D 1  and GND. The voltage across the voltage divider is the output voltage (V OUT ) of the voltage converter that is provided to a load (RL)  24 . The voltage at the connection between R 1  and R 2  provides an input voltage to a Pulse Width Modulator (PWM)  26  that provides an input signal (PWM_SW) to the control terminal of switching device (SW_ 1 ). The PWM obtains power from terminals that are connected between D 1  and GND. 
     When the power source, HV_BAT is first attached to the converter, a voltage V D  appears between D 1  and the load (R L ). This “first” V D  serves two purposes: it provides an intermediate “first push” voltage that is required to activate the internal PWM block, and it provides the “initial” output voltage, V OUT , to the load (R L ). A constant V OUT  is achieved by using the PWM to switch the switching device SW_ 1  on and off to control the amount of energy stored in L 1  and C 1 . The PWM block uses the voltage at the junction between R 1  and R 2  (V FB ) to determine the duty cycle of the PWM output signal. When the desired voltage (determined by the values of R 1  and R 2 ) is obtained, the duty cycle of the PWM stabilizes and the output of the boost converter is a constant output voltage at the desired level. 
     For a boost converter to operate, the initial output of the PWM must be at a sufficient level (V TH ) to switch SW_ 1  ON. V TH  is defined by the nature of the switch. After the SW_ 1 &#39;s “first switch on,” the boost converter starts operation normally, and its output voltage V OUT  eventually reaches a constant voltage required by the load R L . If V TH  is insufficient, the SW_ 1  will not be switched on the first time, as a result, the boost converter illustrated in  FIG. 1  will not start. In a typical semiconductor based boost converter, V TH  is between 0.7V and 1.0V, depending upon the technology of the switching device (MOSFET, bipolar transistor, etc.) used in the voltage converter. This means the power source has to have an output of at least 0.7V to 1.0V. 
     Renewable power sources can be used to replace environmentally hazardous chemical and electrochemical based energy sources, such as a variety of Li-Ion, Li-Polymer, NiMH, and NiCd batteries, which are still being widely used in mobile handsets and other battery-powered consumer and commercial devices and equipment as the primary source of energy. A promising energy source is the solar cell. However, a typical single solar cell outputs a voltage that does not exceed 0.3V, which is significantly below the 0.7V to 1.0V V TH  of a typically switching element in conventional voltage converters. 
     Research is underway to develop semiconductor processing technologies on which ultra low threshold switching devices (i.e. switching devices with thresholds below 0.3V) can be made. However, even if such new semiconductor processing technologies can indeed be developed, the lower the threshold voltage becomes, the more complicated, and therefore more costly the processing technology is likely to become. In addition, no matter how low the threshold voltage of the switching devices becomes, conventional topologies cannot achieve a voltage converter that has a zero voltage threshold. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the invention the low voltage converter is a boost converter that include a “first push” stage. The “first push” stage provides an initial push voltage enabling the switching device within the boost converter to commence switching irrespective of the voltage level of the power source. The “first push” stage includes a zero-voltage-input switching device to generate the initial “first push”. 
     In an embodiment of the invention the low voltage power source can be a single solar cell. 
     In an embodiment of the invention the low voltage converter uses an isolated topology such as a flyback topology or a forward topology. 
     In an embodiment of the invention the low voltage power converter uses conventional semiconductor processes. 
     The foregoing has outlined, rather broadly, the preferred feature of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features 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 can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention and that such other structures do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claim, and the accompanying drawings wherein similar reference numerals in the various FIGS. Relate to similar parts. 
         FIG. 1  is a circuit diagram of a conventional boost converter; 
         FIG. 2  is a circuit diagram of a boost converter powered by an ultra low power source with a “first push” block in accordance with an embodiment of the invention; 
         FIG. 3  is a circuit diagram of a low voltage converter that includes a zero-threshold-voltage switch as a “first push” block in accordance with an embodiment of the invention; 
         FIG. 4  is a chart showing a simulation of the response of a low voltage converter in accordance with an embodiment of the invention; 
         FIG. 5  is a circuit diagram of a low voltage converter that utilizes a photo-coupler as a “first push” block in accordance with an embodiment of the invention; 
         FIG. 6  is a circuit diagram of a low voltage converter that uses a zero-threshold-voltage depletion mode MOSFET and a control circuit as a “first push” block in accordance with an embodiment of the invention; 
         FIG. 7  is a circuit diagram of a low voltage converter that uses a flyback topology in accordance with an embodiment of the invention; and 
         FIG. 8  is a circuit diagram of a low voltage converter that uses a forward topology in accordance with an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, embodiments of low voltage converters in accordance with the principles of the invention are shown. The low voltage converters are boost converters that include a first push block, which provides an initial voltage capable of ensuring that regardless of how low the power source voltage is, the voltage of the very first PWM pulse applied at the control terminal of the switching device, can overcome the threshold voltage of the switching device to turn it on the very first time. 
     Basic Architecture 
     A low voltage converter in accordance with the principles of the invention is shown in  FIG. 2 . The low voltage converter  50  is similar to the prior art voltage boost converter shown in  FIG. 1  with the exception that a “first push” block  52  is connected between the inductor L 1  and GND and the high voltage power source (HV_BAT) is replaced by a low voltage power source (LV_BAT)  54 . The low voltage power source is incapable of generating a voltage sufficiently large to cause the switching device (SW_ 1 )  16 ′ to switch. However, the “first push” block that is now included in the circuit is able to provide a “first push” voltage that is of sufficient magnitude to enable SW_ 1  to switch ON. The ability of the “first push” block to provide the initial voltage allows a low voltage power source to be used. In an embodiment of the invention the LV_BAT can be a single-cell solar panel that produces an output voltage (V LV     —     BAT ) of approximately 0.3V. In another embodiment of the invention a low voltage power source can be used that has an output voltage (V LV     —     BAT ) that is even lower than 0.3V, for example between 0.2V and 0.3V. In another embodiment of the invention, V LV     —     BAT  can be only slightly above GND, such as above 0.1V and less than 0.2V. In another embodiment of the invention the V LV     —     BAT  can be between 0.2V and 0.9V. 
     Operation of Low Voltage Converter 
     A “first push” block in accordance with the principles of the invention includes a current path through the “first push” block that is controlled by a control input (FP). Referring to  FIG. 2 , when a first push signal is applied to the input (FP)  56  of the “first push” block  52 , the current path created within the “first push” block connects inductor (L 1 )  14 ′ to GND. When the first push signal is removed from FP, L 1  is disconnected from GND. At this moment, because of L 1 &#39;s inductance, the voltage at the junction between L 1  and D 1  raises to a level that is higher than LV_BAT. The voltage across L 1  can be calculated as:
 
 V   L1   =LΔi   MAX   /Δt   (5)
         where V L1  is the voltage across L 1 ,
           L is the inductance of L 1 ,   Δt is the time duration that the current path through the “first push” block exists while the first push signal is applied to FP, and   Δi is the current flowing from LV_BAT to GND through L 1  while the first push signal is applied to FP.   
           In which, the Δi is limited as:
 
Δ i   MAX   =V   LV     —     BAT   /R   ON   (6)
           where V LV     —     BAT  is the output voltage of the ultra low voltage power source LV_BAT, and R ON  is the resistance of the current path through the “first push” block.   
               

     As such, after the first push signal is removed, the total voltage that is seen on the right side of the L 1 , or the anode of the D 1 , is given as:
 
 V   HV   =V   LV     —     BAT   +V   L1   (7)
 
     where V HV  is the voltage on the anode of D 1 , V LV     —     BAT  is the output voltage of the low voltage power source LV_BAT, and V L1  is the voltage across L 1 . 
     And the voltage on the cathode of the D 1 , is given as:
 
 V   D   =V   HV   −V   D1   =V   LV     —     BAT   +V   L1   −V   D1   (8)
 
     where V HV  is the voltage on the anode of D 1 ,
         V LV     —     BAT  is the output voltage of the low voltage power source LV_BAT,   V L1  is the voltage across L 1 , and   V D1  is the forward voltage drop of the diode D 1 .       

     Although V LV     —     BAT  can be ultra low, such as 0.3V from a single-cell solar panel, the V L1  can be high depending upon the value of L 1 , how long (Δt) FP signal stays on, and the magnitude of the current that flows through L 1  while the FP signal is ON. When the signal is removed from FP, V D  can be high enough to activate the PWM block and the voltage of the output PWM signal&#39;s ON state (V SW ) will be at a value that can switch SW_ 1  ON. The actual value of V D  that will enable the output of the PWM to switch SW_ 1  ON typically depends upon the nature of SW_ 1 . 
     Low voltage converters that include different types of switching devices in accordance with the principles of the invention are considered below. 
     N-Channel Enhancement Mode MOSFETs and NPN Transistors 
     If SW_ 1  is an N-channel enhancement mode MOSFET or a NPN transistor, when the first PWM pulse (PWM_SW 1 ) arrives at the control terminal of SW_ 1 , its voltage during the ON state (PWM logic HIGH), V SW , must be higher than GND by V TH  to switch on the SW_ 1 . Since the voltage of PWM_SW 1 &#39;s ON state cannot be higher than V D  (i.e. the voltage used to power the PWM block), V D  must be higher than GND by V TH . The following formula gives the condition that must be satisfied in order to switch on SW_ 1  for the first time:
 
 V   D   &gt;V   TH   (9a)
 
→ V   HV   −V   D1   &gt;V   TH   (9b)
 
→ V   LV     —     BAT   +V   L1   −V   D1   &gt;V   TH   (9c)
 
→ V   LV     —     BAT   +LΔi   MAX   /Δt−V   D1   &gt;V   TH   (9d)
 
→ LΔi   MAX   /Δt&gt;V   TH   +V   D1   −V   LV     —     BAT   (9e)
 
     According to Equation 9e, appropriate selection of the inductance value of L 1 , the duration of the first push signal applied to FP, and the current flowing through the “first push” block while the FP signal is applied to its control terminal, can produce a voltage sufficient to turn on SW_ 1 . 
     For example, if V TH  is 0.7V, V D1  is 0.7V, and V LV     —     BAT  is 0.3V, Equation 9e becomes:
 
→ LΔi   MAX   /Δt&gt;V   TH   +V   D1   −V   LV     —     BAT =0.7V+0.7V−0.3V=1.1V  (9f)
 
→ L ( V   LV     —     BAT   /R   ON )/Δ t&gt; 1.1V  (9g)
 
     From Equation 9f, if L 1  is a 100 μH inductor, and Δt is 100 μs, then a Δi MAX  that is higher than 1.1 A can turn SW_ 1  ON. To achieve a Δi MAX  that is higher than 1.1 A, Equation 9g indicates that R ON  (i.e. the resistance of the current path through the “first push” block) should be less than roughly 270 mΩ. Implementations of various “first push” blocks that possess resistances of this order of magnitude are discussed below. 
     P-Channel Enhancement Mode MOSFETs and PNP Transistors 
     If the SW_ 1  is a P-channel enhancement mode MOSFET or a PNP transistor, when the first PWM pulse (PWM_SW) arrives at the control terminal of SW_ 1 , its voltage (V SW ) during the ON state (PWM logic LOW) must be lower than the V HV  by V TH  to switch SW_ 1  ON. Since V SW  cannot be lower than GND, the V HV  must be at least V TH  higher than GND. The following formula gives the condition that must be satisfied in order to switch on SW_ 1  for the first time:
 
 V   HV   &gt;V   TH   (10a)
 
→ V   LV     —     BAT   +V   L1   &gt;V   TH   (10b)
 
→ V   LV     —     BAT   +LΔi   MAX   /Δt&gt;V   TH   (10c)
 
→ LΔi   MAX   /Δt&gt;V   TH   −V   LV     —     BAT   (10d)
 
     Equation 10d indicates that appropriate selection of the inductance of L 1 , the duration of the first push signal applied to FP, and the current flowing through the “first push” block while the signal is applied to FP can result in SW_ 1  being turned ON. 
     For example, if V TH  is 0.7V and V LV     —     BAT  is 0.3V, Equation 10d becomes:
 
→ LΔi   MAX   /Δt&gt;V   TH   −V   LV     —     BAT =0.7V−0.3V=0.4V  (10e)
 
→ L ( V   LV     —     BAT   /R   ON )/Δ t&gt; 0.4V  (10f)
 
     If L 1  is a 100 μH inductor, Δt is 100 μs, then according to Equation 10e, a Δi MAX  higher than 400 mA can result in SW_ 1  turning ON. A Δi MAX  higher than 400 mA can be achieved provided R ON  is less than roughly 750 mΩ. 
     As such, the output voltage of the LV_BAT (V LV     —     BAT ) does not have a decisive impact on the SW_ 1 &#39;s first switch ON. Selection of L 1 , Δt, and R ON  in accordance with the principles of the invention can enable SW_ 1  to switch ON for very low values of V LV     —     BAT . Hence, V LV     —     BAT  can be ultra low and to the level of only slightly higher than the GND, and SW_ 1  does not have to be a low threshold voltage switch manufactured using a low threshold voltage semiconductor process. 
     After the first switch on, SW_ 1  starts operating like a normal switch that is used in a typical boost converter, and the boost converter in  FIG. 2  starts operating like a typical boost converter, except that, the power source (LV_BAT) is allowed to have a ultra low output voltage. 
     Implementation of “First Push” Block 
     In  FIG. 2 , the “first push” block includes a zero-threshold-voltage switching device that can be turned on with a zero voltage applied to FP. Turning the zero-threshold-voltage switch on once provides the “first push,” which subsequently jump starts the rest of the boost converter. Various implementations of “first push” blocks that include zero-threshold-voltage switching devices in accordance with the principles of the invention are discussed below. Although specific embodiments are discussed, “first push” blocks can be implemented in other ways that achieve a device that can create a sufficiently low resistance current path in response to the application of a first push signal to a control input. 
     A “First Push” Block Including a Zero-Threshold-Voltage Mechanical Switch 
     A low voltage converter that includes a “first push” block implemented using a zero-threshold-voltage switch in accordance with an embodiment of the invention is shown in  FIG. 3 . The low voltage converter  60  includes a “first push” block that is implemented using a zero-threshold-voltage switch (ZE_TH_SW)  62  that is connected between L 1   14 ′ and GND. In the illustrated embodiment, ZE_TH_SW is a mechanical switch that, when a physical pressure is applied to the input FP, is turned ON, and after the physical pressure is removed, is turned OFF. 
     A chart of a simulation of the low voltage converter  60  illustrated in  FIG. 3  is shown in  FIG. 4 . The chart shows current through L 1  building as ZE_TH_SW is turned ON at time  0 . When ZE_TH_SW is released, the voltage across L 1  is sufficient for the PWM to generate a signal that causes SW_ 1  to commence switching. At which point, the low voltage converter settles into steady state operation. The output voltage can include “ripples” associated with the voltage regulator regulating the output voltage. The ripples can be “smoother” if a larger output capacitor is used. 
     A “First Push” Block Including a Zero-Threshold-Voltage Photo-Coupling Switch 
     A low voltage converter that includes a “first push” block implemented using a photo-coupling switch is shown in  FIG. 5 . The low voltage converter  80  includes a “first push” block implemented using a a zero-threshold-voltage photo-coupler (ZE_TH_PC)  82 . In the illustrated embodiment, ZE_TH_PC is an optoelectrical switch that, when it is activated by an optical signal, is turned ON and, after the optical signal is removed, is turned OFF. A sufficiently long application of an optical signal to the optoelectrical switch enables L 1  to store sufficient energy to enable the PWM to switch SW_ 1  ON. 
     The internal resistance of a photo coupler can be comparable to that of a non-zero resistance switch. As long as the intensity of the light is sufficient, the optoelectrical switch will be turned on, and as long as it can be turned on, its threshold voltage is no longer important. In embodiments where an optoelectrical switch is used, an important design parameter is, how high the intensity of the light source has to be to turn it on the very first time. 
     A “First Push” Block Including an Active Component 
     A low voltage converter  91  that includes an active component in accordance with an embodiment of the invention is shown in  FIG. 6 . The low voltage converter  90  includes a depletion mode MOSFET switch (ZE_TH_DE)  92 , and a control circuit that includes a transformer with L 1 ,  14 ′ as a primary coil and L 2 ,  94  as a secondary coil, a capacitor (C 2 )  96  connected between the grounded terminal of L 2  and the control input of ZE_TH_DE, a once-pressed-always-on RELAY  98  connected between L 1  and D 1 , and a switch (SW)  100  that is on if a physical pressure is on and off when the physical pressure is removed that is connected between the other terminal of L 2  and the control input of ZE_TH_DE. Both the RELAY and SW are controlled by the control input FP,  56 ′. When the voltage (V G ) applied to the control terminal (Gate) of ZE_TH_DE is higher than its negative threshold voltage (−V TH ), then ZE_TH_DE is ON. As such, it can only be turned off when a negative voltage, of which the absolute value is higher than |V TH | (more negative than V TH ), is applied to its control terminal. 
     When the RELAY is turned ON (and stays ON) L 1  is connected to the GND through ZE_TH_DE, which was ON prior to the RELAY being turned ON. SW is also turned on, which connects L 2  to C 2 . The current in L 1  and, because L 2  has the same polarity as L 1 , the current induced in L 2  flows in the opposite direction charging C 2 . Since the left side of C 2  is connected to GND, the voltage (V C ) at the side connected to the control input of ZE_TH_DE becomes negative. When the pressure on SW is removed, SW is turned OFF, and L 2  is disconnected from the rest of the circuit. Since there is no path for C 2  to discharge, V C  stays the same. Depending on the values of the L 2  and C 2 , and the duration of the FP applied to SW, V C  can be “negative” enough to turn ZE_TH_DE OFF, and make it stay OFF. After ZE_TH_DE is OFF, L 1  is disconnected from GND. If at that time V HV  satisfies either Equation 9b or 10a, SW_ 1  is switched on by the PWM  26 ′ for the first time and a sustainable boost conversion is created. 
     A “First Push” Block Used in an Isolated Topology 
     Voltage converters that provide galvanic isolation between an input power source and the output of the voltage converter are useful in a variety of applications. A low voltage converter that includes galvanic isolation between the low voltage power source and the output of the voltage converter in accordance with an embodiment of the invention is illustrated in  FIG. 7 . The low voltage converter  110  uses a flyback topology. In the illustrated embodiment, LV_BAT,  54 ′, L 1 ,  14 ′, and ZE_TH_SW  112  are connected in series. A photo-coupling switch (SW_ 1 )  16 ′, such as an optocoupler, is connected between the terminal of L 1  that is connected to ZE_TH_SW, and GND. L 1  and a second inductor (L 2 )  113  are windings of a transformer. L 2  is connected between D 1   17 ′ and GND. The PWM  26 ′ is not physically connected to the control input of SW_ 1 . Instead, the PWM  26 ′ is connected to a light emitting device  114 , such as an LED. The optical coupling between the LED,  114  and SW_ 1 ,  16 ′ enables the PWM to turn SW_ 1  ON. 
     Application of a first push input to the control input (FP) of ZE_TH_SW  112  causes energy to collect on L 1 . When FP is released, a current is induced in L 2  and the magnitude of the induced current determines the potential at the terminal of L 2  connected to D 1 . When the voltage at the terminal of L 2  connected to D 1  is sufficiently large, the PWM is capable of activating the LED and switching ON SW_ 1 . 
     Another low voltage converter that includes galvanic isolation between the low voltage power source and the output of the voltage converter in accordance with an embodiment of the invention is illustrated in  FIG. 8 . The voltage converter  130  uses a forward topology, which uses similar galvanic isolation techniques to those illustrated in  FIG. 7 . However, D 1  is connected to the load via a third inductor (L 3 )  132  and ground via a second diode (D 2 )  134 . 
     While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, it will be understood that various omissions and substitutions and changes of the form and details of the apparatus illustrated and in the operation may be done by those skilled in the art, without departing from the spirit of the invention.