Patent Publication Number: US-6707101-B2

Title: Integrated series schottky and FET to allow negative drain voltage

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/045,451, filed Nov. 7, 2001 in the name of Niraj Ranjan and entitled “INTEGRATED SERIES SCHOTTKY AND FET TO ALLOW NEGATIVE DRAIN VOLTAGE” now U.S. Pat. No. 6,529,034B1. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to integrated circuit gate drivers and more particularly to such drivers for driving high side power MOSFETs or IGBTs, and to a novel planar MOSFET and integrated series connected Schottky diode. 
     BACKGROUND OF THE INVENTION 
     Integrated circuit MOSFET drivers are well known, for driving the low side and/or high side MOSgated device of a power control circuit. Thus high side drivers are known for controlling the turn on and turn off of a power MOSFET which then permits the connection of electrical power to a load. High side drivers of this kind are known for example, as the IR2015 chip sold by International Rectifier Corporation of El Segundo, Calif. 
     Such chips will typically consist of a single silicon chip which has a first plurality of control devices integrated in its main body, which is at ground potential, and will also have a second plurality of control devices contained within a high side floating well which is at a high potential relative to ground. The chip will have a number of input pins, including V cc  (control voltage), an input control pin, a comm (or ground) pin, all connected to components in the low voltage portion of the chip and all referenced to ground. 
     The output to the gate of high side switch (MOSFET or IGBT) can be at a high voltage, so that the input signal to the input pin must be level shifted up. This is commonly done by circuitry in the floating high side well in the integrated circuit chip. The high side circuit “floats” at the potential of the Vs pin, which is normally connected to the source of the high side switch (MOSFET or IGBT). The output pin HO is connected to the gate of the high side switch to be driven and it provides the drive signal. The voltage difference between the voltages on the Vb and Vs pin provides the supply for the floating high side circuit within the integrated circuit. There are many ways in which the Vbs floating supply can be generated; the bootstrap technique being the simplest and least expensive. In this technique the supply is formed by a high voltage diode and capacitor as shown in FIG. 1 to be later described in detail. This invention is primarily aimed at applications in which the bootstrap technique is used. 
     When Vs in FIG. 1 is at ground potential the bootstrap capacitor  36  is charged through the bootstrap diode  35  from the 15V Vcc supply. Once this capacitor is fully charged, it retains its charge even when the Vs pin floats to a high voltage, because the bootstrap diode  35  becomes reversed biased. The bootstrap capacitor  36  provides supply current for the high side circuit as well as the gate charge necessary to turn ON the external MOSFET to be driven. However, the bootstrap capacitor  36  must be refreshed by some means before it is discharged significantly. 
     If the high side switch drives a resistive or inductive load, the bootstrap capacitor  36  is easily refreshed by simply turning the switch off periodically and waiting for the Vs potential to drop to ground (Comm) potential through the load. Once the Vb potential reduces to 0.7V below Vcc the bootstrap diode  35  conducts and re-charges the bootstrap capacitor. 
     Additionally, in a half bridge circuit the bootstrap capacitor  36  is charged by turning the high side switch (MOSFET or IGBT) off and turning the low side switch (MOSFET or IGBT) on, thus connecting Vs to ground. If the Vb potential is significantly below Vcc the bootstrap diode conducts and refreshes the capacitor. 
     In absence of resistive (or inductive) loads or a synchronized low side switch, the Vs potential may not automatically drop to ground potential when the high side switch is turned off. In this situation it is desirable to add an internal high voltage MOSFET to the gate driver IC which will connect Vs to ground in order to refresh bootstrap capacitor  36 . It was found, however, that such an added transistor could not meet the (−)Vs condition which is often experienced in many applications where Vs goes a few volts below ground potential. During such (−)Vs excursions the inherent drain to body diode of the refresh transistor conducts in its forward conduction direction, generating minority carriers. These minority carriers are injected into the control circuit, and some are collected in the high side floating well and by nearby level shift FET drain regions. This results in small amount of drain current, resulting in malfunction of R-S latch used in level shift circuits [see U.S. Pat. No. 5,545,955 (Wood) for such level shift circuits]. Therefore, the output state of the HO pin can change from low to high (or vice versa) without any input signal. 
     It would be desirable to provide a means to refresh a bootstrap capacitor in the absence of resistive/inductive loads without danger of producing false control signals. It is also desirable in many application of MOSFETs in general, to prevent conduction of its parasitic diode under forward bias and to prevent injection of minority carriers into nearby control circuits. 
     In accordance with this invention, a Schottky diode is placed in series with the internal high voltage MOSFET which is used to connect Vs pin to ground in order to refresh the bootstrap capacitor. The refresh transistor and the Schottky can be integrated into the chip and the Schottky device can be formed in series with the drain of refresh transistor. 
     BRIEF SUMMARY OF THE INVENTION 
     The novel Schottky operates to add an approximately 0.5 volt drop to the V DS(ON)  of the refresh transistor during its on state. However, in the reverse direction, the blocking voltage is increased from (−)0.5 volts to up to about (−)8 volts. Thus, the device body diode does not conduct when V s  goes to (−)v e  when the body to drain diode would have otherwise started to conduct and inject minority carriers into the high side well. 
     A novel high voltage FET and Schottky diode is also formed by a novel process in which the vertical conduction FET is a lateral device, and the drain (or source) is connected to N −  silicon to define the Schottky. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block and circuit diagram of a high side integrated circuit chip and a load circuit therefore in which the novel refresh transistor and Schottky are integrated into the chip. 
     FIG. 2 is a cross-section of a portion of the novel refresh transistor and integrated Schottky. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows relevant portions of a high side driver chip  20  such as the IR 2015  chip which has a low voltage section  21  and a high side floating well  22 . The low voltage section has pins  23 ,  24  and  25  which are V cc  (15 volts), an input signal pin, and a comm (ground) respectively. The high side floating well has pins  30 ,  31  and  32 , which are the V B  pin which is at (+)v e  of a floating power supply, the HO output pin and the V s  pin which is at (−)v e  of the floating power supply and, for example, swings between 0 and 200 volts relative to ground. The voltage V B  at pin  30  is for example, set at (V B =V s +15 volts). The potential of COMM pin  25  is typically the same as the return terminal for the load. 
     A boot strap diode  35  is connected between V cc  pin  23  and V B  pin  30  and boot strap capacitor  36  is connected between V B  pin  30  and V s  pin  30 . A bipass capacitor  37  is connected between V cc  pin  23  and COMM  25 . 
     The main Mosgated device is shown as a power MOSFET  40  connected to a high voltage power pin  41  and to a load  42 . Load  42  may be any type of load which way be controlled by pulse frequency modulation of the MOSFET  40  under control of chip  20  end the input signal at pin  24 . 
     If load  42  is not resistive or inductive, it would be desirable to provide a refresh transistor to re-charge the bootstrap capacitor  36  by connecting pin  32  to pin  25  (COMM). However, in the circuit described Vs can go a few volts below Comm momentarily. When this occurs the body to drain diode of the refresh transistor becomes forward biased and injects minority carriers into the control circuit, causing malfunction or even destructive latch-up failure. 
     The boot strap capacitor charge must be refreshed through diode  35  from V cc . Thus, V B  must be below V cc  for capacitor  36  to be charged. If V B  goes higher than V cc  capacitor  36  will not discharge due to the blocking action of diode  35 . However, capacitor  36  will tend to discharge and must be charged or refreshed. Note that the circuit to charge capacitor  36  includes the series circuit of bipass capacitor  37  (15 volts); V cc  pin  23 ; diode  35 ; V B  capacitor  36 ; V s  pin  32 ; and back to COMM pin  25 . 
     If the load  42  is resistive or inductive, the refresh transistor is not needed at all because the bootstrap capacitor  36  can be refreshed by simply turning the MOSFET  40  off. The load itself will then connect the Vs pin to the ground potential, thereby causing the Vb potential to reach almost the Vcc potential through the conduction of bootstrap diode  35 . However, if the load  42  is capacitive, for example, or is otherwise not resistive or inductive, the node Vs will not go immediately to ground potential when MOSFET  40  is turned off. Therefore the bootstrap capacitor will not be refreshed as needed. 
     If load  42  is resistive or inductive the novel structure of the invention is not needed because V s  pin  32  will not go negative when the main MOSFET  40  turns off and bootstrap capacitor  36  will be refreshed. That is, since V s  is at zero if V B  goes lower than V cc , diode  35  will conduct after the diode forward drop is exceeded. However, if load  42  is, for example, capacitive, the node at V s  will not go immediately to zero volts when the MOSFET  40  turns off. Therefore, the bootstrap capacitor will not be refreshed as needed. 
     To solve this problem and to ensure the continuous refreshing of boot strap capacitor  36  a vertical conduction refresh MOSFET  60  is added to the circuit of FIG. 1, either as a discrete part, or integrated into silicon  21 , and is connected from V s  to comm. The purpose of MOSFET  60  is to bring V s  close to V COMM . When the main MOSgated device  40  (a Power MOSFET or IGBT) now turns off, the potential at pin  32  can be suitably connected to ground by turning MOSFET  60  on. However, MOSFET  60  has a parasitic diode  61  end this diode will turn on as soon as V s  goes below about (−) 0.5 volt and minority carriers will then be injected into the control circuits. 
     In accordance with the invention, a Schottky diode  62  is connected in series with MOSFET  60  in a direction to block forward conduction of its parasitic diode  61 . The addition of Schottky  62  slightly increases the on resistance of the MOSFET  60  circuit, but, when the MOSFET  60  and Schottky  62  are integrated into the chip  20 , minority carriers are not injected into the control circuit when the Vs node (pin  32 ) goes a few volts below COMM (pin  25 ). 
     FIG. 2 shows one embodiment of the refresh MOSFET  60  and Schottky  62 . More specifically, the device of FIG. 2 is the same as the lateral MOSFET transistor for a high side switch shown in U.S. Pat. No. 4,866,495, except that the N+ contacts for the drain connections are removed so that a Schottky contact is made to N −  silicon. Thus, FIG. 2 shows the chip area  21  as a P −  region with an N −  epitaxial layer  100  thereon. The region  21  is separated from the high side floating well and/or other components by P +  sinkers  101  and  102 . P −  resurf diffusions  105 ,  106 ,  107  and  108  are formed in the chip upper surface and a field oxide  109 . Spaced P −  channel diffusions with deepened P +  regions  110  and  111  contain respective N +  source regions  112  and  113  and are covered by a gate oxide and a polysilicon gate  114 . The conductive gate  114  is insulated by an interlayer oxide  115  from the source electrode  116 . The drain contacts  120  and  121  are connected directly to N −  silicon  100 , rather than to N +  contact regions as in U.S. Pat. No. 4,866,495. The drain contacts, like source contact  116  are of aluminum, and will form a novel Schottky connection to the N −  silicon  100  to define Schottky device  62 . 
     In operation, when the MOSFET  60  is turned on by a signal on gate  114 , currents I will flow as shown, through N −  epi  100  and under the channel areas, to Schottky drain contacts  120  and  121 . 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein.