Abstract:
The present invention discloses a capacitive high-side switch driver for a power converter. The capacitive high-side switch driver according to the present invention includes an inverter and two alternately conducting totem-pole buffers with complementary duty cycles. The duty cycles alternate in response to an input signal. The capacitive high-side switch driver further includes a low-side transistor and a high-side transistor. Once the low-side transistor is turned on, a bootstrap capacitor is charged to create a floating voltage via a charge-pump diode to supply power for the high-side switch driver. To supply additional power for the high-side switch driver, differential signals are produced to further charge the bootstrap capacitor via a bridge rectifier. The capacitive high-side switch driver utilizes a programmable load to provide variable impedance. Furthermore, an under-voltage protector supervises the supply voltage to ensure a reliable gate driving voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high-side switch driver, and more particularly relates to a driver circuit for driving the high-side switch of a power converter. 
     2. Description of the Prior Art 
     Many commonly used power converters utilize bridge circuits to regulate a voltage source in response to a load. Some of these power converters include power supplies and motor drivers. 
     The bridge circuits used by these power converters are normally built from a pair of switching devices connected in series across the voltage source. The switching devices include a high-side switch and a low-side switch. The high-side switch is connected to the voltage source while the low-side switch is connected to the ground reference. A common node between the high-side switch and low-side switch is coupled to the load. The switches are generally transistor devices (MOSFET, IGBT, etc). The switches are controlled to alternately conduct, so that the common node will periodically swing in between the voltage source and the ground reference. When the high-side transistor is turned on, the voltage of the common node will rapidly shift to the voltage level of the voltage source. When the high-side transistor is fully turned on, the bridge circuit will operate in low-impedance mode. To turn on the high-side transistor, the gate drive voltage must exceed the voltage of the voltage source. Thus, the gate-to-source voltage of the high-side transistor will be floated with respect to the ground reference. 
     FIG. 1 demonstrates a technique that employs a pulse transformer  5  to create a floating voltage for driving a high-side transistor  10 . However, the design of the pulse transformer  5  suffers from two disadvantages. First of all, its size is relatively large. Second, the pulse transformer  5  will require a substantially higher driving current due to magnetizing-current consumption. 
     FIG. 2 shows another prior-art bridge circuit using a bootstrap capacitor  30  and a charge-pump diode  40  to create a floating voltage for driving a gate of a high-side transistor  10 . Switching on a transistor  45  will pull down the voltage at the gate of the high-side transistor  10  to the ground reference. The gate voltage of the high-side transistor  10  will be pulled down via a diode  42 . This will turn off the high-side transistor  10 . Once the high-side transistor  10  is turned off and a low-side transistor  20  is turned on, the floating voltage of the bootstrap capacitor  30  is charged up to a bias voltage V B  via the charge-pump diode  40 . Switching off the transistor  45  will turn on the high-side transistor  10  by conducting the floating voltage to the gate of the high-side transistor  10  via a transistor  41 . The drawback of this circuit is that the gate voltage level might not be sufficient to ensure proper operation. Since the bootstrap capacitor  30  is charged via the low-side transistor  20 , its charge time and the bootstrap voltage may decrease to an unacceptable level due to the reduced duty cycle of the low side transistor  20 . Furthermore, the voltage drops across the diode  40  and the transistor  41  will cause the gate voltage level of the high-side transistor  10  to fall. 
     Recently, various techniques have been proposed for generating the required gate voltage for the high-side transistor in a more reliable manner. One such technique appears in U.S. Pat. No. 5,381,044 (Zisa, Belluso, Paparo), U.S. Pat. No. 5,638,025 (Johnson), and U.S. Pat. No. 5,672,992 (Nadd). 
     The drawback of these prior-arts is the need for a switch-off transistor such as the transistor  45 . The prior-art bridge circuits listed above use something like the transistor  45  of FIG. 2 to turn off the high-side transistor. This switch-off transistor must be manufactured according to a high-voltage process, in order to be safely used in high-voltage applications (200 volts or more). To be integrated into a silicon chip, this high-voltage transistor requires a relatively thick coat of oxide and silicon. Furthermore, the parasitic capacitance of this high-voltage transistor will slow down the slew rate of the switching signal, thus resulting in significant high-side transistor switching losses. Therefore, these prior-arts are not suitable for high-voltage applications, or for high-speed applications. 
     To remedy this shortcoming, a technique using a boost converter is proposed in U.S. Pat. No. 6,344,959 (Milazzo). The boost converter is essentially a voltage doubling circuit. While this technique generates a more reliable gate voltage to drive the high-side transistor, it requires an additional switching element and other circuitry. This increases the cost and complexity of the driving circuit. Moreover, severe noise will be generated by the voltage source and the ground reference, due to high frequency charging and discharging of the voltage doubling capacitor in the charge pump. 
     The objective of the present invention is to provide a high-side switch driver for high-voltage and high-speed applications that overcome the drawbacks of prior art high-side switch drivers. 
     SUMMARY OF THE INVENTION 
     The capacitive high-side switch driver according to the present invention includes an inverter and two totem-pole buffers. The switch driver controls the totem-pole buffers in response to an input signal, in such a manner that they alternately conduct with complementary duty cycles. The outputs of the two totem-pole buffers drive two high-voltage capacitors. These high-voltage capacitors are further coupled to the input of a high-side circuit. The high-side circuit comprises a comparator, a programmable load, an under-voltage protector, and a drive-buffer for driving a high-side transistor. The high-side circuit further consists of a charge-pump diode and a bootstrap capacitor. 
     When the low-side transistor is turned on, the bootstrap capacitor is charged to drive the high-side transistor. The two totem-pole buffers and the two high-voltage capacitors generate differential signals to drive the comparator, and further charge the bootstrap capacitor via a bridge-rectifier. The bootstrap capacitor is used to supply power for the high-side circuit. 
     One objective of the present invention is to provide protection against low gate-voltage levels. The under-voltage protector enables the drive-buffer whenever the floating voltage exceeds the start-threshold voltage, and disables the drive-buffer whenever the floating voltage drops below the stop-threshold voltage. The under-voltage protector further protects the high-side circuit from an insufficient supply voltage and ensures a sufficient gate-voltage level for the high-side transistor. 
     Another objective of the present invention is to provide a high-side switch driver with improved noise immunity. This is accomplished by connecting a programmable load in parallel to the input of the comparator. The programmable load provides a variable impedance to prevent noise interference. Furthermore, the two totem-pole buffers produce a differential voltage across the input of the comparator. This differential voltage further strengthens the noise immunity of the high-side circuit, so that it will be suitable for use in high-voltage applications. 
     To raise the floating voltage and improve the efficiency of the high-side switch driver, the bias voltage charges the bootstrap capacitor when the low-side transistor is turned on. The differential signals also provide additional power via the bridge-rectifier. 
     The capacitive high-side switch driver according to the present invention overcomes the drawbacks of prior-art high-side switch drivers. In particular, the present invention provides a capacitive high-side switch driver that is suitable for high-voltage and high-speed applications. Moreover, the capacitive high-side switch driver according to the present invention is substantially more efficient and has stronger noise immunity than prior-art switch drivers. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 shows a high-side switch driver using a pulse transformer. 
     FIG. 2 shows a conventional high-side switch driver. 
     FIG. 3 shows the schematic circuit of a capacitive high-side switch driver according to the present invention. 
     FIG. 4 shows a programmable load according to a preferred embodiment of the present invention. 
     FIG. 5 shows an under-voltage protector according to a preferred embodiment of the present invention. 
     FIG. 6 shows a schematic circuit of the buffer, for reference purposes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a capacitive high-side switch driver according to the present invention. The capacitive high-side switch driver comprises an inverter  53 , a totem-pole buffer  50 , and a totem-pole buffer  55 . The totem-pole buffers  50  and  55  are controlled to alternately conduct in response to an input signal S IN , so that their duty cycles are complementary. The totem-pole buffer  50  is driven by the input signal S IN . The input signal S IN  drives the totem-pole buffer  55  via the inverter  53 . An output of the totem-pole buffer  50  drives a capacitor  91 . An output of the totem-pole buffer  55  drives a capacitor  92 . A high-side circuit  60  has an output terminal for driving a high-side transistor  10 , wherein capacitors  91  and  92  are coupled to an input of the high-side circuit  60 . The totem-pole buffers  50  and  55  and the capacitors  91  and  92  generate differential signals and produce a differential voltage across the input of the high-side circuit  60 . 
     A charge-pump diode  40  and a bootstrap capacitor  30  are connected in series. An anode of the charge-pump diode  40  is connected to a bias voltage terminal V B . A negative terminal of the bootstrap capacitor  30  is connected to a source of the high-side transistor  10 . A cathode of the charge-pump diode  40  and a positive terminal of the bootstrap capacitor  30  are tied together. When a low-side transistor  20  is turned on, the bias voltage V B  will charge the bootstrap capacitor  30  and create a floating voltage to supply power for the high-side circuit  60 . A rectifier  81 , a rectifier  82 , a rectifier  83 , and a rectifier  84  form a bridge-rectifier having an input terminal and an output terminal. The input terminal of the bridge-rectifier is connected to the input of the high-side circuit  60 . The output terminal of the bridge-rectifier is connected in parallel to the bootstrap capacitor  30 . The differential signals further charge the bootstrap capacitor  30  via the bridge-rectifier to supply power for the high-side circuit  60 . 
     The high-side circuit  60  further comprises a comparator  70  and a drive-buffer  75 . An input of the comparator  70  is connected to the input of the high-side circuit  60 . An output of the drive-buffer  75  drives a gate of the high-side transistor  10 . An input of the drive-buffer  75  is connected to an output of the comparator  70 . The comparator  70  has a turn-on threshold. When the differential voltage across the inputs of the comparator  70  exceeds the turn-on threshold, the comparator  70  will output a logic-high signal. A programmable load  100  is connected in parallel with the inputs of the comparator  70 . The programmable load  100  provides variable impedance in response to the magnitude of the voltage at the output of the comparator  70 . An input of the programmable load  100  is connected to the output of the comparator  70 . When the voltage at the output of the comparator  70  is logic-low, the programmable load  100  will generate low impedance. When the voltage at the output of the comparator  70  is logic-high, the impedance of the programmable load  100  will increase in proportion to the length of the logic-high output period of the comparator  70 . 
     The capacitive high-side switch driver according to present invention further includes an under-voltage protector (UVP)  200  connected in parallel with the bootstrap capacitor  30 . The under-voltage protector  200  is used to detect the floating voltage of the bootstrap capacitor  30 . The output of the under-voltage protector  200  is connected to an enable terminal of the drive-buffer  75 . The under-voltage protector  200  will enable the drive-buffer  75  whenever the floating voltage exceeds a start threshold voltage. Furthermore, the under-voltage protector  200  will turn off and/or disable the drive-buffer  75  whenever the floating voltage goes below a stop-threshold voltage. A resistor  65  is connected from the output of the drive-buffer  75  to a negative terminal of the bootstrap capacitor  30 . The resistor  65  is used to shut off the high-side transistor  10  when the drive-buffer  75  is disabled. 
     FIG. 4 shows a programmable load  100  according to a preferred embodiment of the present invention. The programmable load  100  has a pl-ground terminal connected to the negative terminal of the bootstrap capacitor  30 . The programmable load  100  further includes a voltage terminal connected to the positive terminal of the bootstrap capacitor  30 . The programmable load  100  further comprises an inverter  110 , a constant current source  120 , a transistor  123 , a capacitor  125 , three buffers  151 ,  152  and  153 , three transistors  171 ,  172  and  173 , and three resistors  191 ,  192  and  193 . 
     An input of the inverter  110  is connected to the output of the comparator  70 . The transistor  123  has a gate connected to an output of the inverter  110 . A source of the transistor  123  is connected to the pl-ground terminal of the programmable load  100 . The constant current source  120  is connected to a drain of the transistor  123 . The capacitor  125  is connected from the drain of the transistor  123  to the pl-ground terminal. The buffer  151 , the buffer  152 , and the buffer  153  have a first input-threshold, a second input-threshold, and a third input-threshold respectively. An input of the buffer  151 , an input of the buffer  152 , and an input of the buffer  153  are tied to the drain of the transistor  123 . The transistor  171  and the resistor  191  are connected in series, and further connected in parallel with the inputs of the comparator  70 . The transistor  172  and the resistor  192  are connected in series, and further connected in parallel with the inputs of the comparator  70 . The transistor  173  and the resistor  193  are connected in series, and further connected in parallel with the inputs of the comparator  70 . A gate of the transistor  171  is connected to an output of the buffer  151 . A gate of the transistor  172  is connected to an output of the buffer  152 . A gate of the transistor  173  is connected to an output of the buffer  153 . 
     When the output of the comparator  70  becomes logic-high, the constant current source  120  will start to charge the capacitor  125 . When the voltage of the capacitor  125  exceeds the first input-threshold voltage, the transistor  171  will be shut off. When the voltage of the capacitor  125  exceeds the second input-threshold voltage, the transistor  172  will be shut off. When the voltage of the capacitor  125  exceeds the third input-threshold voltage, the transistor  173  will be shut off. Thus, the impedance of the programmable load  100  is increased in proportion to the voltage of the capacitor  125 . In this manner, the impedance of programmable load  100  is also increased in proportion to the length of the logic-high output period of the comparator  70 . 
     FIG. 5 shows an under-voltage protector  200  according to a preferred embodiment of the present invention. A voltage V+ is supplied from the positive terminal of the bootstrap capacitor  30 . The negative terminal of the bootstrap capacitor  30  is connected to a uv-ground reference. Two resistors  231  and  232 , and two zener diodes  210  and  220  are connected in series. The voltage V+ is supplied to the zener diode  210 . The resistor  232  is connected to the uv-ground reference. 
     An n-transistor  250  has a gate connected to the junction of the zener diode  220  and the resistor  231 . A source of the n-transistor  250  is connected to the uv-ground reference. The drain of the n-transistor  250  is connected to a gate of a p-transistor  271 . A source of the p-transistor  271  is connected to a cathode of the zener diode  210 . A drain of the p-transistor  271  is connected to the junction of the zener diode  210  and the zener diode  220 . A resistor  240  is connected in parallel between the gate and the source of the p-transistor  271 . The drain of the n-transistor  250  is further connected to an input of a buffer  290 . An output of the buffer  290  is further connected to the enable terminal of the drive-buffer  75 . The output of the buffer  290  drives a gate of an n-transistor  272 . A drain of the n-transistor  272  is connected to the junction of the resistor  231  and the resistor  232 . A source of the n-transistor  272  is connected to the uv-ground reference. The voltages of the zener diodes  210  and  220  determine the start-threshold voltage for the under-voltage protector  200 . The voltage of the zener diode  220  determines the stop threshold voltage for the under-voltage protector  200 . 
     Referring to FIG. 3, the totem-pole buffers  50  and  55  drive the high-side circuit  60  via the capacitors  91  and  92 . The capacitors  91  and  92  generate differential signals to enable high-speed switching. Two totem-pole buffers  50  and  55  produce a differential voltage across the inputs of the comparator  70 . The differential voltage is produced via the two capacitors  91  and  92 . The programmable load  100  uses the differential voltage to strengthen the noise immunity of the high-side circuit  60 . This feature is designed specifically for high-voltage applications. The bias voltage V B  is supplied to charge the bootstrap capacitor  30  when the low-side transistor  20  is turned on. 
     To further raise the floating voltage and improve the efficiency of the high-side switch driver, the differential signals are also used to charge the bootstrap capacitor  30 . The differential signals charge the bootstrap capacitor  30  via the bridge-rectifier. The under-voltage protector  200  further protects the high-side circuit  60  from an insufficient supply voltage and ensures a sufficient gate voltage level. 
     FIG. 6 shows a schematic circuit of a buffer, for reference purposes. This demonstrates how to build the totem-pole buffers  50  and  55 , the buffers  151 ,  152 ,  153  and  290 . The buffer includes two n-transistors  310  and  360 , a p-transistor  350 , and a current source  320 . A gate of the n-transistor  310  is connected to an input of the buffer. A drain of the n-transistor  310 , a gate of the n-transistor  360 , and a gate of the p-transistor  350  are tied together. The current source  320  is coupled to the drain of the n-transistor  310 . A drain of the p-transistor  350  and a drain of the n-transistor  360  are connected to an output of the buffer. The operation of this circuit will be well known to those skilled in the art and does not need to be discussed in further detail here. 
     The capacitive high-side switch driver according to the present invention can overcome many of the shortcomings of prior-art high-side switch drivers. In particular, the capacitive high-side switch driver according to the present invention is particularly well suited for use in high-voltage and high-speed applications. Some of the advantages of the capacitive high-side switch driver according to the present invention include significantly higher efficiency and strengthened noise immunity. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims or their equivalents.