Abstract:
An electronic circuit that is normally used in a car cigarette lighter adapter is disclosed. It is designed to meet the high performance requirements of the Apple Computer specification for charging iPod and iPhone, while still retaining low cost. The circuit is composed of input protection components, surge regulator, FET switch, inductor-capacitor smoothing filter, gate drive components, voltage regulator, current regulator, current sense element, and voltage reference.

Description:
BACKGROUND-FIELD OF INVENTION 
       [0001]    The present invention relates generally to electrical chargers for electronic devices. More specifically, the present invention relates to vehicle electrical chargers for electronic devices with universal serial buses (USB). 
       BACKGROUND-DESCRIPTION OF RELATED ART 
       [0002]    Since Apple Computer has sold hundreds of millions of iPods, iPhones and iPads, a means to recharge their internal batteries while being used in the car is necessary. This is normally accomplished with a device that plugs into the vehicle&#39;s cigarette lighter socket that converts the vehicles loosely regulated voltage of 13.8 volts to the USB standard of 4.75 to 5.25 volts. 
         [0003]    Various cabling arrangements are being used from a standard USB cable to different length coil cords. The electronic circuits are typically step-down switching regulators. Most manufacturers of these adapters want to have them certified by Apple Computer as Works With iPhone and/or Made for iPod. To get this certification, the adapter must meet all the requirements of the Apple specification. When marketing a product in such a competitive environment, price is of prime concern. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    The efficient power supply/charger of the present invention is a car charger comprising an input protection component, a surge regulator, a voltage controller component, a current limiting component, and a cable compensation component. The voltage controller components further comprises of a power output component and a gate drive. 
         [0005]    An object of the car charger is to provide an efficient car charger capable of charging various electronic devices through their USB ports. A further object of the car charger is to provide an efficient car charger capable of regulating a surge versus attempting to clamp the surge. Another object of the car charger is to achieve low EMI without the necessity of using any ferrites. Yet another object of the car charger is to provide an efficient car charger that is capable of better response to transients and the capability to spread out EMI over a wider frequency range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows the complete schematic diagram of the preferred embodiment of the efficient power supply/charger. 
           [0007]      FIG. 2  shows the schematic diagram of the preferred embodiment of the input protection circuit. 
           [0008]      FIG. 3  shows the schematic diagram of the surge regulator circuit. 
           [0009]      FIG. 4  shows the schematic diagram of the power output circuit. 
           [0010]      FIG. 5  shows the schematic diagram of the gate drive circuit. 
           [0011]      FIG. 6  shows the schematic diagram of the voltage controller circuit. 
           [0012]      FIG. 7  shows the output and gate voltage under a variable load of 80 mA to 1000 mA. 
           [0013]      FIG. 8  shows the output voltage during dynamic load change of 80 mA to 1000 mA. 
           [0014]      FIG. 9  shows the schematic diagram of the current limiting and cable compensation circuit. 
           [0015]      FIG. 10  shows the output voltage loaded to 80 mA and subjected to a 40V input surge. 
           [0016]      FIG. 11  shows the output voltage loaded to 1 A and subjected to a 40V input surge. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The following description and figures are meant to be illustrative only and not limiting. Other embodiments of this invention will be apparent to those of ordinary skill in the art in view of this description. 
         [0018]    The preferred way to construct the invention is shown in  FIG. 1 . Vout+ and Vout− can be connected directly to a USB or other type of connector, or connected to a cable. Vin+ and Gnd are normally connected to a positive tip spring and ground clip. In the preferred embodiment, the efficient power supply/charger comprises an input protection component, a surge regulator, a voltage controller component, a current limiting component, and a cable compensation component. The voltage controller components further comprises of a power output component and a gate drive. 
         [0019]      FIG. 2  shows the input protection components comprised of F 1 , C 2 , and D 3 . F 1  provides protection to the vehicle&#39;s electrical system in case of a shorted part inside the adapter. A 2 amp PTC is shown, but any other type of fuse could be used, including a PCB trace fuse, which would be the lowest possible cost. C 2  is a 1 uF capacitor used to provide a small amount of filtering. D 3  is used to provide reverse input voltage protection. 
         [0020]      FIG. 3  shows the surge regulator comprised of Q 4 , Q 2 , Q 5 , Z 2 , R 7 , R 17 , R 18 , and R 19 . Normally, the adapter input current flows through Q 4 , which is biased on by R 17  and R 19 . If the input voltage surge rises above 27 volts, Z 2  starts to conduct current into the base of Q 2 . This causes Q 2  to pull base current out of Q 5 . As Q 5  starts to turn on, the gate to source voltage of Q 4  is reduced to its linear operating region, regulating the output to a little less than 28 volts. During the Apple specified 40 v, 16 ms surge, repeated 5 times, the power dissipated in Q 4  is the input current×the 12 volts across it. Worse case scenario testing has shown Q 4  can survive with more than 1 amp passing through it. 
         [0021]    The present invention has the benefit of regulating the surge, versus trying to clamp it to less than 30 volts using traditional parts and methods. Traditional parts would have to be very large and expensive, severely limiting the size of the enclosure and marketability. Using higher voltage parts, like most products do, is considerably more expensive. 
         [0022]      FIG. 4  shows the power output stage made from Q 3 , C 1 , C 3 , L 1 , D 2 , C 5 , and R 2 . When Q 3  is turned on, the input voltage causes a current to start ramping up through L 1 , which charges C 3 . As Q 3  is turned off, the reverse voltage caused by the collapsing magnetic field of L 1 , is clamped by D 2 . An R-C snubbing network is comprised of C 5  and R 2  to reduce EMI. 
         [0023]      FIG. 5  shows the gate drive circuit made from R 12 , R 13 , D 1 , and Q 1 . When U 1 -B is off (open collector) the FET&#39;s gate to source voltage is held to less than 0.6 volts by R 13  and Q 1 . As U 1 -B turns on, Q 1  is turned off by D 1  and R 12  slowly pulls the gate voltage down, turning on the FET. 
         [0024]    Careful control of the gate drive to the FET is the key to achieving low EMI without having to use any ferrites. Traditional methods switch the FET on and off as quickly as possible for high efficiency. This introduces large amounts of EMI. By turning the FET on slowly, when the current is at a minimum, EMI reduction is achieved. The FET must be turned off quickly, since the current is at a maximum, but this can be easily dealt with by the R-C snubber. The other unique feature of the gate drive circuit is that operation in both the digital and linear regions is possible, depending on the load. 
         [0025]      FIG. 6  shows how the voltage controller works. U 1 -B&#39;s inverting input is normally biased at 1.24 volts. The output voltage is controlled by the ratio of R 11  to R 6 . R 1  and C 6  provide a “speed up” function by coupling more of the output ripple into the comparator input. As can be seen, this is a very simple on-off controller. 
         [0026]    Extremely good line and load transient response is difficult to achieve with traditional constant frequency PWM controllers. The on-off controller with conditioned gate drive can respond to transients much better due to its cycle-by-cycle nature. Another advantage is the inherent frequency jitter. This tends to spread out the EMI, instead of concentrating it at a single frequency. 
         [0027]      FIG. 7  shows the output and gate voltages under a variable load of 80 ma to 1000 ma. As can be seen, the gate voltage drive changes in amplitude as well as pulse width. 
         [0028]      FIG. 8  shows a closer inspection of the output voltage. As can be seen, the output easily stays in spec during a dynamic load change of 80 ma to 1000 ma at a slew rate of 100 ma/us. This is at the end of a standard 0.25Ω USB cable. 
         [0029]      FIG. 9  shows how the current is limited and cable compensation is performed. R 15  and R 16  are the current sensing elements with U 2  “riding on top” of the current sense voltage. U 1 -A&#39;s non-inverting input is biased by the 1.24 volt reference as well as the output voltage. This provides a current foldback as well as a soft start function. If the returned load current exceeds the threshold, U 1 -A&#39;s output pulls the reference on U 1 -B&#39;s inverting input low, turning off the FET. As the output current decreases below the threshold, U 1 -A&#39;s output turns off, returning control of the FET to U 1 -B. As the load current increases from minimum to maximum, the reference voltage seen at U 1 -B&#39;s inverting input increases by 50 mv/amp. This increase causes the output voltage between Vout+ and Vout− to increase, compensating for the I×R loss of the cable. This effect can be seen in  FIG. 8 . 
         [0030]      FIG. 10  shows the output loaded to 80 ma and subjected to a 40 volt input surge. As can be seen, the surge is “transparent”, having little effect on the output. 
         [0031]      FIG. 11  shows the output loaded to 1 amp while subjected to a 40 volt input surge. The output stays within the spec of 4.75 to 5.25 volts. 
         [0032]    An alternative construction of the current invention replaces F 1  with a PCB trace fuse to lower cost. Yet another embodiment of the current invention replaces U 2  with a 2.5 volt reference to reduce the amount of cable compensation. Another alternative embodiment of the efficient power supply/charger removes all the surge regulator components and replaces Q 3  with a more expensive 40 volt FET to save space. Additional embodiment of the efficient power supply/charger use R 15  and R 16  with different values to achieve different current limiting and cable compensation characteristics. 
         [0033]    Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.