Abstract:
A circuit structured to drive an isolated high speed voltage metal-oxide-semiconductor field-effect transistor (MOSFET) switch, including a first MOSFET and a second MOSFET configured to operate as a switch, a capacitor, a charging component in parallel with the capacitor, a first switch in series with the charging component, and a second switch in parallel with the charging component and the capacitor. The stored voltage in the capacitor is sent to the gates of the first MOSFET and the second MOSFET when a second switch is open and a first switch is closed.

Description:
PRIORITY 
     This application claims priority to U.S. Provisional Application No. 61/936,568 filed Feb. 6, 2014, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an isolated high speed MOSFET switching circuit capable of switching on lower on resistance MOSFETs at an acceptable rate. 
     BACKGROUND 
     An isolated MOSFET switching circuit, as seen in  FIG. 1 , uses an opto-battery  100 , as described in more detail below, to provide isolation and power to the gates of the drive metal-oxide-semiconductor field-effect transistors (MOSFET)  102  and  104 , forcing the MOSFETs  102  and  104  to turn on. MOSFETs  102  and  104  are high voltage MOSFETs. These MOSFETs have a gate-source capacitance, shown as capacitors  106  and  108  for clarity in  FIG. 1 , and also referred to as capacitors Ca and Cb, respectively. To turn the isolated MOSFET switching circuit in  FIG. 1  on, switch  110  closes to provide 10-15 mA of current through the light emitting diode (LED)  112  of the opto-battery  100 . The light from the LED causes a small current (e.g., 10-30 μA) to flow in the current source of the opto-battery  100 . The current flows into capacitors  106  and  108 , increasing the voltage across them at a rate of dV/dt=i/(Ca+Cb). This rate continues until the zener-diode  114  zeners, which is around 6V, and begins stealing current from opto-battery  100 , limiting the voltage to 6V. As the voltages on the capacitors and gate-source of the MOSFETs  102  and  104  increase the MOSFETs  102  and  104  turn on. As the MOSFETs  102  and  104  turn on, the gate-drain voltage of each of the MOSFETs  102  and  104  begins to change, causing some gate current to flow into the gate-drain capacitance. The gate-drain capacitance of the MOSFETs  102  and  104  also affects the charging and turn-on speed of the MOSFETs  102  and  104 . The longer it takes to energize these collective gate-source capacitances to 6V, the longer it takes to turn on the isolated MOSFET switching circuit. Further, the voltage could be much higher for the gate-drain capacitances, also increasing the switching time. 
     Phototransistor  116  in  FIG. 1  is optional and used when the isolated MOSFET switching circuit is part of a protection circuit. When the switching circuit should protect, Phototransistor  116  will begin to reduce the gate-source voltages of MOSFETs  102  and  104  and begin to open the switch circuit. MOSFETs  102  and  104  are in the linear region when the circuit is in the protection mode. Phototransistor  116  must handle the current of de-energizing the MOSFETs  102  and  104  capacitances  106  and  108  from 6V to the voltage necessary for protection. 
     To turn off the switching circuit of  FIG. 1 , switch  110  opens, cutting off current to the LED  112 , and opto-battery  100 . A resistor in parallel with the current source (not shown for simplicity) discharges gate capacitances  106  and  108 , eventually lowering the gate-source voltage of MOSFETs  102  and  104  and turning off the switch circuit. 
     Newer MOSFETs, however, typically have a high gate capacitance, and the switching circuit shown in  FIG. 1  is slow to switch on these newer MOSFETs due to the higher gate capacitance. 
     One alternative to correct for the slow switching of the newer MOSFETs is to use a gate isolation transformer. The gate isolation transformer provides the required gate current to quickly switch on the MOSFET switching circuit. However, the gate isolation transformer requires complicated drive circuitry for the transformer, as well as a large and costly transformer itself. Another alternative is to include a floating power supply in the design using another winding on a supply transformer. The winding provides the high current at high signal voltages. Although this type of design is less complicated than adding a gate isolation transformer, it is still a major redesign and added cost. 
     The disclosed technology addresses these limitations of the prior art. 
     SUMMARY 
     Embodiments of the disclosed technology include a circuit structured to drive an isolated high speed metal-oxide-semiconductor field-effect transistor (MOSFET) switch, including a first MOSFET and a second MOSFET configured to operate as a switch, a capacitor, an opto-battery in parallel with the capacitor, a first switch in series with the opto-battery, and a second switch in parallel with the opto-battery and the capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conventional isolated MOSFET switching circuit. 
         FIG. 2  is an isolated high speed MOSFET switching circuit according to some embodiments of the disclosed technology. 
         FIG. 3  is another isolated high speed MOSFET switching circuit according to some embodiments of the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, which are not necessarily to scale, like or corresponding elements of the disclosed systems and methods are denoted by the same reference numerals. 
     The isolated MOSFET switching circuit of  FIG. 1  was fast enough for the original MOSFETs used in the circuit, however, these older MOSFETs have become obsolete. The trend for replacement parts is to have a lower on resistance for MOSFETs. Lower on resistance means less loss in the MOSFET when high currents are flowing, so higher efficiency circuits can be designed. To achieve the lower on resistance, MOSFET manufacturers increase the size of the silicon die. This size increase causes a significant increase in gate capacitance—an order of magnitude greater for the gate-source and gate-drain capacitances. This increase in capacitance causes the rate of voltage change to decrease. This decrease in rate increases the time it takes to turn on the switching circuit with the newer MOSFETs. 
     The disclosed technology includes an isolated high speed MOSFET switching circuit that is capable of switching the newer lower-on-resistance MOSFETs at a much quicker rate. The switching circuit of  FIG. 2  includes MOSFETs  202  and  204 , with gate capacitances  206  and  208 . When MOSFETs  202  and  204  are on, the output of the switching circuit is equal to the input and the switching circuit is on. When MOSFETs  202  and  204  are off, the output is not equal to the input and the switching circuit can stand off as much voltage as MOSFET  202  and  204  are rated for individually. 
     In  FIG. 1 , since capacitances  106  and  108  are too high and cannot be reduced, the current output to the MOSFETs  102  and  104  must be increased. In the disclosed technology, as shown in  FIG. 2 , capacitor  220  is used as an energy storage tank to hold energy from a charging component  200  so the energizing time of capacitances of  206  and  208  can be reduced. The charging component  200 , may be, for example, an opto-battery. An opto-battery is an isolation device that pushes a current through an LED, which causes the LED to shine on a set of diodes, which in turn produces a small current, which is isolated from the current through the LED. Opto-batteries are also known as photocouplers. However, the charging component  200  can be any component capable of charging capacitor  220 . 
     Switch  110 , shown in  FIG. 1 , has been removed from the circuit in  FIG. 2 , so whenever power is applied to the system, charging component  200  is providing current. That is, charging component  200  constantly provides current to the switching circuit. When the switching circuit is off, that is, the output does not equal the input, switch  222  is open so all of the current from charging component  200  goes into energizing capacitor  220 . Capacitor  220  is much larger than capacitances  206  and  208 . That is, typical values of capacitances  206  and  208  are on the order of 1 nF, while capacitor  220  is on the order of 10 μF. Capacitor  220  may be a couple orders of magnitude greater than capacitors  206  and  208 . For example, the capacitor  220  may be anywhere from 0.1 μF to 100 μF. Preferably, capacitor  220  is 10 μF. 
     When the system requires the switching circuit to be on, switch  224  opens and switch  222  closes. This connects capacitor  220  in parallel with capacitances  206  and  208 . Since capacitor  220  is energized and capacitances  206  and  208  are not, current flows out of capacitor  220  and into the MOSFET capacitances  206  and  208 . Capacitor  220  therefore acts as a low impedance source, since there is only a small resistance in switch  222  and the printed circuit board traces. This limited resistance causes current on the order of amps to flow, so capacitors  206  and  208  charge quickly. Because capacitor  220  is much larger than capacitors  206  and  208 , the voltage on capacitor  220  decreases only slightly. The result is that the MOSFETs  202  and  204  turn on very quickly. 
     The high turn on current is provided solely by capacitor  220 . After the switching circuit is on, charging component  200  replaces the energy lost in the transfer to capacitors  206  and  208 , and this increases the voltage on capacitor  220  back to what it was before the switching circuit turned on. 
       FIG. 3  illustrates the circuit of  FIG. 2  with the added protection of phototransistor  216 . In this circuit, however, phototransistor  216  must drain the energy from capacitor  220  to turn off the MOSFETs  202  and  204 . This may slow down the protection circuit&#39;s reaction time. To improve the reaction time, a transistor  300  is added to the switching circuit, as shown in  FIG. 3 . The transistor  300  quickly drains MOSFET  202  when protection is needed. When protection is no longer needed, charging component  200  must re-energize capacitor  220 . This re-energizing of capacitor  220 , however, may take a significant amount of time, meaning that it may take longer for the switching circuit to resume normal operation after the protection circuit has been engaged due to an overload of the switching circuit. 
     The circuits of  FIGS. 2 and 3  are turned off by opening switch  222  and closing switch  224 . Switch  224  is required because in the original circuit of  FIG. 1 , the resistance in opto-battery  100  is used to drain capacitances  106  and  108  to turn off the MOSFETs  102  and  104 , and with switch  222  open, there is no path to drain capacitances  206  and  208 . Switch  222  must be opened first, however, or switch  224  will short out capacitor  220 , and the stored energy needed for the speed improvement will be lost. Switch  224  has the advantage of quickly turning off the switching circuit because it offers much less resistance than the resistor used in  FIG. 1 . 
     The zener-diode  114  shown in  FIG. 1  has been removed from the circuits of  FIGS. 2 and 3 . If the zener-diode  114  was still in the circuit, capacitor  220  would provide current to the zener-diode  114  until the voltage is lowered to 6V. This voltage drop would reduce the switching frequency by draining energy stored for future switching actuations. The original purpose of the zener-diode  114  was to protect MOSFETs  102  and  104  from electrostatic discharge, and to create a more consistent switch turn off since the gate source was limited to the zener voltage. 
     However, with the disclosed technology, this protection is no longer required. MOSFETs  202  and  204  have built in 20V zener protection diodes. Since the opto-battery is limited to 10 v, the built-in zener protection diodes are only useful for protection. 
     Switches  222  and  224  in  FIGS. 2 and 3  are optically isolated transistors, driven by LEDs (not shown). A digital signal provides current to the LED, which shines on the transistor to turn on the switch. Switches  222  and  224  require 10 mA of continuous current through the LEDs to operate, but the switch turn-on time is relatively slow at 10 mA. Accordingly, a circuit (not shown) can be added to the LED driver to create a brief 100 mA pulse to the LED before reducing the drive to 10 mA. The 100 mA pulse quickly turns on switch  222  or switch  224 , and then 10 mA continuous current keeps the switch on, further reducing overall switching time. 
     The disclosed technology allows the switch circuit to use newer MOSFETs with lower on resistance to switch the isolated high speed MOSFET switching circuit on at an acceptable rate. 
     Having described and illustrated the principles of the disclosed technology in a preferred embodiment thereof, it should be apparent that the disclosed technology can be modified in arrangement and detail without departing from such principles. We claim all modifications and variations coming within the spirit and scope of the following claims.