Abstract:
A switching power supply can include multiple power MOSFETs that receive an initial gate drive waveform comprising a fast slew rate region having a negative slope and a slow slew rate region also having a negative slope. The MOSFETs can turn off during the slow slew rate region of the initial gate drive waveform.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to power supplies and, more particularly, to low noise switching power supplies. 
       BACKGROUND 
       [0002]    Low noise power supplies generally require both the voltage and the current to be switched softly. Techniques such as zero voltage switching (ZVS) typically allow the voltage transitions to have controlled slew rates.  FIG. 1  illustrates an example of a push-pull DC-to-DC power supply  100  using ZVS. In the example, the power supply  100  includes two first metal-oxide-semiconductor field-effect transistor (MOSFETs) (here, MOSFET  1  and MOSFET  2 ) and an output voltage (V out ) at a load (Load). 
         [0003]    Softly switching the current off usually requires applying a series impedance that is large enough to force the current to ramp down but not so large that the current&#39;s slew rate results in an unacceptable level of noise. One generally accepted means for applying such a series impedance is to slowly drive the gate of a MOSFET through its threshold voltage. 
         [0004]    Most, if not all, power MOSFETs have threshold voltages that vary several volts from part to part and can vary with temperature as well. A slow gate drive transition will generally result in a large variation in the time at which the threshold is crossed. A gate drive that can swing though all possible threshold volts in a reasonable amount time will result in the MOSFET switching the current faster and generating more noise, however. 
         [0005]      FIG. 2  is a graphical representation of a gate drive waveform  200  illustrating the impact that a slow gate drive  202  and a fast gate drive  204  can have on the variation in time for the switching of a typical switching power supply. One can readily ascertain that a fast gate drive  204  results in a smaller variation in timing (e.g., between the maximum threshold  206  and the minimum threshold  208 ) than the slow gate drive  202 . 
         [0006]    Accordingly, there remains a need for improved low noise switching power supplies. 
       SUMMARY 
       [0007]    Embodiments of the disclosed technology are generally directed to a switching power supply where the primary power MOSFETs are driven on and/or off using a gate drive waveform that has both a fast slew rate region and a slow slew rate region for positive and/or negative slopes. After an initial turn-on of the power supply, the offset or shape of the gate drive waveforms may be adjusted independently for each MOSFET such that the threshold of each MOSFET is traversed by the slow slew rate region of its gate drive waveform at a controlled point in time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an example of a push-pull DC-to-DC power supply using zero voltage switching (ZVS). 
           [0009]      FIG. 2  is a graphical representation of an example of a gate drive waveform illustrating the impact that slow and fast gate drives can have on the variation in the timing of the switching for a typical switching power supply. 
           [0010]      FIG. 3  is a graphical representation of an example of gate drive waveforms illustrating the impact that initial and raised gate drives can have on switching times for certain implementations of the disclosed technology. 
           [0011]      FIG. 4  is a graphical representation of an example of initial gate drive voltages for certain implementations of the disclosed technology. 
           [0012]      FIG. 5  is a graphical representation of an example of raised gate drive voltages for certain implementations of the disclosed technology. 
           [0013]      FIG. 6  is a graphical representation of an example of common mode noise measured on an oscilloscope through a resistor before tuning in accordance with certain embodiments of the disclosed technology. 
           [0014]      FIG. 7  is a graphical representation of an example of common mode noise measured on an oscilloscope through a resistor after tuning in accordance with certain embodiments of the disclosed technology. 
           [0015]      FIG. 8  illustrates an example in which a MOSFET is turned off and subsequently turned back on in accordance with certain embodiments of the disclosed technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Embodiments of the disclosed technology may include at least one metal-oxide-semiconductor field-effect transistor (MOSFET), at least one waveform generator configured to drive a gate of each of the at least one MOSFET such that the off transition of each of the at least one MOSFET is shaped to have a faster slewing region followed by a slower slewing region, and a detection component configured to detect a time that each of the at least one MOSFET switches off. 
         [0017]    The at least one MOSFET may include an n-channel MOSFET in which the gate drive slews negative to turn the MOSFET(s) off. In alternative embodiments, the at least one MOSFET may include a p-channel MOSFET in which the gate drive would slew positive to turn the MOSFET(s) off. 
         [0018]    Unlike a signal fast gate drive of today&#39;s power supplies, implementations of the disclosed technology generally include a gate drive that is shaped to have a fast edge followed by a slow edge, e.g., such that the fast edge can cut through all possible thresholds in a relatively short amount of time.  FIG. 3  is a graphical representation of an example of gate drive waveforms  300  illustrating the impact that an initial gate drive  302  and a raised gate drive  304  can have on switching times for certain implementations of the disclosed technology. In the example, the starting shaped gate drive or initial gate drive  302  has a fast edge that crosses both the maximum threshold  306  and the minimum threshold  308  but is not followed by a slow edge until after it has completely crossed all possible threshold between the maximum threshold  306  and the minimum threshold  308 . 
         [0019]    Once the power supply is running, a measurement of when the MOSFET turns off with respect to the gate drive waveform can be made. Raising the gate drive waveform will cause the MOSFET to turn off later. Thus, the gate drive waveform can be raised  304  until the MOSFET turns off during the slow edge of the raised waveform  304 , as indicated by its crossing the actual threshold  310 . In this situation, the MOSFET is now turning the current off at a slower rate and, consequently, the noise generated will be advantageously reduced. 
         [0020]      FIG. 4  is a graphical representation of an example of initial gate drive voltages  400  for certain implementations of the disclosed technology. In the example, the initial gate drive voltages (V G1  and V G2 ) quickly pass through all possible thresholds (e.g., 1 to 3 volts). The drain currents (I D1  (here, ˜13 uS) and I D2  (here, ˜38 uS) turn off quickly. Once the drain current is off (i.e., the MOSFET is off), the drain voltages (V D1  and V D2 , respectively) of MOSFET  1  and MOSFET  2  flip voltages at ˜15 uS and at ˜40 uS, respectively. This allows the drain that is near zero volts to be turned on with approximately zero volts across it (i.e., ZVS). Here, MOSFET  2  is turned on at ˜24 uS and MOSFET  1  is turned on at ˜49 uS. 
         [0021]      FIG. 5  is a graphical representation of an example of raised gate drive voltages  500  for certain implementations of the disclosed technology. After a tuning (e.g., by raising the gate drive waveforms), the gate drives voltages (V G1  and V G2 ) pass through the MOSFET thresholds (here, ˜2V) slowly and the drain currents (I D1  and I D2 ) turn off slower. 
         [0022]      FIG. 6  is a graphical representation of an example of common mode noise measured on an oscilloscope through a resistor  600  before the gate drives have been tuned in accordance with certain embodiments of the disclosed technology. In the example, the resistor is a 2 kilo-Ohm resistor connected between the primary ground and the secondary ground of the power supply (here, 2 mV=&gt;1 uA). In the figure, Scope CH 2  is measuring one of the drain voltages. Before the gate drives have been tuned, the probe voltage (CH 1 )  602  has a measured range of ˜0.8*2m V, which translates to 0.8 uA peak-to-peak. 
         [0023]      FIG. 7  is a graphical representation of an example of common mode noise measured on an oscilloscope through a resistor  700  after tuning (e.g., after the measurement taken in  FIG. 6 ) in accordance with certain embodiments of the disclosed technology. In the example, after tuning has been completed, the probe voltage (CH 1 )  702  now has a measured range of ˜0.2*2 mV, which translates to 0.2 uA peak-to-peak—this represents an [approximate] fourfold improvement over the measurement taken in  FIG. 6 . 
         [0024]      FIG. 8  illustrates an example in which a MOSFET is turned off and subsequently turned back on  800  in accordance with certain embodiments of the disclosed technology. In the example, the starting shaped gate drive or initial gate drive  802  has a fast edge that crosses both the maximum possible threshold  806  and the minimum possible threshold  808  but is not followed by a slow edge until after it has completely crossed the the minimum possible threshold  808 . The gate drive waveform can be raised  804  until the MOSFET turns off during the slow edge of the raised waveform  804 , as indicated by its crossing the actual threshold  810 . 
         [0025]    Here, the initial gate drive  802  and raised waveform  804  both have upward slow and fast edges that roughly correspond to the initial, downward slow and fast edges, respectively, such that the MOSFET turns back on during the latter slow edge of the raised waveform  804 , as indicated by its crossing the actual threshold  810  a second time. 
         [0026]    The following discussion is intended to provide a brief, general description of a suitable machine in which embodiments of the disclosed technology can be implemented. As used herein, the term “machine” is intended to broadly encompass a single machine or a system of communicatively coupled machines or devices operating together. Exemplary machines may include computing devices such as personal computers, workstations, servers, portable computers, handheld devices, tablet devices, and the like. 
         [0027]    Typically, a machine includes a system bus to which processors, memory such as random access memory (RAM), read-only memory (ROM), and other state-preserving medium, storage devices, a video interface, and input/output interface ports can be attached. The machine may also include embedded controllers such as programmable or non-programmable logic devices or arrays, Application Specific Integrated Circuits (ASICs), embedded computers, smart cards, and the like. The machine may be controlled, at least in part, by input from conventional input devices such as keyboards and mice, as well as by directives received from another machine, interaction with a virtual reality (VR) environment, biometric feedback, or other pertinent input. 
         [0028]    The machine may utilize one or more connections to one or more remote machines, such as through a network interface, modem, or other communicative coupling. Machines can be interconnected by way of a physical and/or logical network, such as an intranet, the Internet, local area networks, wide area networks, etc. One having ordinary skill in the art will appreciate that network communication may utilize various wired and/or wireless short range or long range carriers and protocols, including radio frequency (RF), satellite, microwave, Institute of Electrical and Electronics Engineers (IEEE) 545.11, Bluetooth, optical, infrared, cable, laser, etc. 
         [0029]    Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments may be modified in arrangement and detail without departing from such principles, and may be combined in any desired manner. And although the foregoing discussion has focused on particular embodiments, other configurations are contemplated. 
         [0030]    In particular, even though expressions such as “according to an embodiment of the invention” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. 
         [0031]    Consequently, in view of the wide variety of permutations to the embodiments that are described herein, this detailed description and accompanying material is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.