Abstract:
Disclosed are methods and apparatus for isolating and driving a power supply or power amplifier circuit. The circuits and methods provide for using a modulated PWM input signal and its complement to drive one or more core-less transformers providing an isolated power supply circuit output signal reproducing the PWM input signal. Preferred methods of the invention are disclosed in which steps for receiving and modulating a PWM input signal and its complement are included. In further steps, the modulated PWM signal and its complement control an isolated output driver to provide a power supply circuit output signal reproducing the PWM input signal. Embodiments of the invention are disclosed in which a circuit is configured for receiving a PWM input signal, providing isolation, and outputting a PWM output signal. The circuits include modulators for modulating a PWM input signal and its complement. Pulse transformer stages deliver the modulated PWM signal and its complement to the respective gates of power transistors of gate driver output stages. At an output terminal, a PWM output signal responsive to the PWM input signal is produced.

Description:
TECHNICAL FIELD 
   The invention relates to electronics and electronic circuits. More particularly, the invention relates to electronic circuits using a transformer and transistor gate driver for the isolation and control of power supplies in electronic systems. 
   BACKGROUND OF THE INVENTION 
   In some electronic circuits, isolation of one portion of a circuit from others is required. Transformers are widely used for isolation in AC circuits. Conventional cored pulse transformers have a primary winding and a secondary winding and work on the principle that energy can be efficiently transferred by magnetic induction from one winding to another winding by a varying magnetic field produced by the alternating current. Pulse transformers are used extensively for isolation, for example in MOSFET gate driver circuits, but have several serious shortcomings. Conventional cored pulse transformers are expensive, bulky, and can vary significantly from unit to unit in terms of electrical characteristics. 
   Core-less PCB transformers use primary and secondary windings on opposing sides of a PC board. Such transformers lack a magnetic core and have a relatively small number of windings, with the result that they have a relatively low magnetizing inductance and higher leakage inductance. The use of core-less PCB transformers can save expense, ensure greater uniformity among units, and avoid saturation problems. However, the use of core-less PCB transformers for isolation in circuits offers technical challenges as well. To avoid high primary side drive current associated with the low magnetizing inductance, these transformers are typically operated at switching frequencies within a range of about 7–11 MHz. Since most power transistors cannot be switched at such high frequencies, a PWM waveform cannot practically be sent directly across these core-less transformers. In order to address this problem, it is known in the arts to differentiate a PWM input signal waveform by subtracting it from a delayed version of itself. This produces a positive pulse indicative of the rising edge of the PWM input and a negative pulse indicative of the falling edge. These pulses are fed into the primary side of the transformer. A latch is used on the secondary side of the transformer. The positive pulse sets the latch and the negative pulse resets it, thus reconstructing the original PWM input signal. Although ideally this approach would substantially reproduce the input PWM signal at the output, it is very difficult to build a latch that is able to operate reliably in a noisy environment. Noise events can act to set or reset the latch. Such accidental operation of the latch can result in damage to the circuit. An additional problem is that inductive flyback from the pulse transformer can cause the latch to reset immediately after being set, or vice versa, also potentially causing damage to the circuit. 
   Due to these and other problems, improved pulse transformer driver circuits would be useful and advantageous in the arts. 
   SUMMARY OF THE INVENTION 
   In carrying out the principles of the present invention, in accordance with preferred embodiments thereof, circuits and methods for driving and isolating a power supply circuit use a modulated PWM signal and its complement to drive one or more core-less transformers, providing an isolated power supply circuit output signal substantially identical to the PWM input signal. 
   According to one aspect of the invention, a method of driving and isolating a power supply circuit includes steps of receiving and modulating a PWM input signal and its complement. In further steps, the modulated PWM signal and its complement are used to control an isolated output driver to provide a power supply circuit output signal reproducing the characteristics of the PWM input signal. 
   According to another aspect of the invention, a method of driving isolating a power supply circuit includes steps of receiving a PWM input signal, modulating the PWM signal to produce a modulated PWM signal and a modulated PWM signal complement, and providing a rising edge pulse to the modulated PWM signal and its complement. The signals thus produced are subsequently applied to respective pulse transformers in order to control the isolated power supply circuit output responsive to the PWM input signal. 
   According to yet another aspect of the invention, a transistor gate driver circuit is disclosed. According to preferred embodiments, the circuit is configured for receiving a PWM input signal, providing isolation, and outputting a PWM output signal. The circuits include modulators for modulating the PWM input signal and its complement. Pulse transformer stages deliver the modulated PWM signal and its complement to the respective gates of power transistors of gate driver output stages. At an output terminal, a PWM output signal responsive to the PWM input signal is produced. 
   According to further aspects of the invention, additional embodiments of the invention are disclosed in which methods and circuits are used to provide isolated drivers according to the invention alternatively using active-on/passive-off and passive-on/active-off approaches. 
   The invention provides technical advantages over the prior art including but not limited to improvements in reliability and a low susceptibility to noise. Advantages in cost are also achieved. These and other features, advantages, and benefits of the present invention can be understood upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from consideration of the following detailed description and drawings in which: 
       FIG. 1  is a flow diagram illustrating an example of a preferred embodiment of a method according to the invention; 
       FIG. 2A  through  FIG. 2E  are depictions of representative examples of waveforms for illustrating steps in preferred methods according to the invention; 
       FIG. 3  is a schematic circuit diagram illustrating an example of a preferred embodiment of the invention; 
       FIG. 4  is a schematic circuit diagram illustrating an example of an alternative embodiment of the invention; and 
       FIG. 5  is a schematic circuit diagram illustrating an example of an alternative embodiment of the invention. 
   

   References in the detailed description correspond to the references in the figures unless otherwise noted. Descriptive and directional terms used in the written description such as first, second, left, right, top, bottom, and so forth refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating the principles, features, and advantages of the invention. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In general, the preferred embodiments of the invention provide new, reliable, isolated gate driver circuits. The preferred embodiments of the invention use modulation techniques to pass signals across a core-less PCB transformer. 
   An illustration of steps in a method  10  of the invention is shown in  FIG. 1 . As indicated at box  12 , a PWM input signal denoted X is received. The PWM input signal necessarily has a complement, herein denominated X′. The PWM signal X is modulated  14  to produce a modulated PWM signal X mod . Additionally, the PWM signal complement X′ is modulated  16 , producing a modulated PWM signal complement X′ mod . Preferably, as shown at steps  18  and  20  respectively, the modulated PWM signal X mod  and its complement X′ mod  are each provided with a rising edge pulse P, P′. The rising edge pulses P, P′, ensure clearly defined rising edges in the modulated signals, X mod , X′ mod  to facilitate switching as further described herein. The rising-edge-enhanced modulated PWM signal, (X mod +P), is preferably fed into a first transformer  22 . Similarly, the rising-edge-enhanced modulated signal complement, (X′ mod +P′), is fed into a second transformer  24 . The first and second transformers  22 ,  24 , are preferably core-less PCB transformers, providing effective isolation of the output  30  from the input  12 . A first transistor  26  is configured for switching “on” responsive to the saturation of the gate by the signal (X mod +P) from the first transformer  22 . Also, a second transistor  28 , preferably of similar construction but opposite polarity, is configured for switching “off” responsive to the complementary signal (X′ mod +P′) from the second transformer  24 . Accordingly, the PWM output  30  is regulated in such a way as to provide a PWM output signal Y that is essentially a reproduction of the PWM input signal X. The PWM output  30  is connected to a power transistor, preferably a MOSFET or IGBT. 
   Those skilled in the arts should take note that the preferred method shown and described may be implemented with various modifications. For example, either the active “on” or active “off” provided by the respective transistors  26 ,  28 , may be omitted, relying instead on a passive “on” or “off”. In another alternative embodiment, one or both of the rising edge pulses P, P′ may be omitted. Such alternative embodiments may be used independently or in combination to implement a functional circuit in suitable applications without departure from the invention. 
   Now referring primarily to  FIG. 2A  through  FIG. 2E , an additional view of the operation of the invention is provided.  FIG. 2A  provides a representation of an exemplary PWM input signal X. For the purposes of example, arbitrary pulses  34  are shown.  FIG. 2B  illustrates the modulated PWM signal X mod , exhibiting modulated pulses  36  with a rising edge pulse P added, (X mod +P).  FIG. 2C  depicts the compliment X′ of the PWM input signal X of  FIG. 2A , showing pulses  38  complementary to those of the PWM input signal X.  FIG. 2D  shows the modulated PWM signal complement X′ mod , having modulated pulses  40  to which a rising edge pulse P′ has also been added, (X′ mod +P′). In  FIG. 2E , the PWM output signal Y is shown to be a reproduction of the PWM input signal X of  FIG. 2A . 
   A schematic diagram of a preferred embodiment of the invention is shown in  FIG. 3 . The modulated gate driver circuit  44  shown includes an input terminal  46  for receiving the PWM input signal from an external source. A modulation signal is also accepted at a modulation input terminal  48 . The modulation signal is preferably used to modulate both the PWM input signal and its complement. The PWM input signal is modulated at a first modulator  50 , and the modulated PWM signal is used as the set signal at a first pulse transformer driver stage  52 . The first pulse transformer driver stage  52  includes a core-less PCB pulse transformer  54 . A complement of the PWM input signal is derived at the input terminal  46  and is modulated at a second modulator  56 . The modulated PWM signal compliment is used as a reset signal at a second pulse transformer driver stage  58 . The second pulse transformer driver stage  58  also includes a core-less PCB pulse transformer  60 . 
   Further referring primarily to  FIG. 3 , preferably, a first rising edge pulse generator  62  provides a pulse which is “or-ed” with a free running oscillator. The resulting signal is then gated by the PWM input signal. The use of the rising edge pulse in this manner avoids a random delay between the rising edge of the PWM signal and the first rising edge the modulated PWM signal. A second rising edge generator  64  is preferably applied to the modulated PWM signal compliment as well. Artisans will appreciate that the gates shown in the circuit  44  may be substituted with their logical equivalents without departing from the principles of invention. Of course, functional alternative versions of the invention may also be implemented omitting one or both of the rising edge pulse generators. 
   As described above, both the modulated PWM signal and the modulated PWM signal complement are fed into respective drive stages  52 ,  58 . There are many equivalent circuit components which may be used by those skilled in the arts to implement the modulators  50 ,  56  and drive stages  52 ,  58  within the scope of the invention, so long as the modulated PWM signals are each applied to the primary sides  66 ,  68  of their respective core-less PCB transformers  54 ,  60 . Of course, many alternative core-less PCB transformers may also be used. 
   On the secondary side  70  of the first PCB transformer  54 , the modulated PWM signal is preferably tied to a first gated transistor  72 . Preferably, an NPN type BJT  72  is used, with the secondary side  70  of the first transformer  54  coupled between the base and emitter. In this arrangement, the rising edge of the modulated PWM signal saturates the NPN  72 , causing charge to be dumped on the gate of a power transistor  75 , preferably as MOSFET, bringing the MOSFET  75  into conduction. Successive high frequency pulses of the modulated PWM signal maintain the charge on the gate of the power transistor  75 . 
   In the same manner, at the second PCB transformer  60 , the modulated PWM signal complement emerges from the secondary side  74 . The secondary winding  74  is tied to a second transistor  76 , preferably, a PNP type BJT  76 . With the secondary side  74  of the second PCB transformer  60  coupled between the base and emitter, the rising edge of the modulated PWM signal complement saturates the PNP  76 , removing the charge between the gate and the source of the power transistor  75 . Successive pulses of the modulated PWM signal complement prevent any charge injected by noise pulses from accumulating at the gate of the power transistor  75 . 
   Thus, the driver transistors  72 ,  76  coupled to their respective PCB transformers  54 ,  60  are preferably used to control a driver output stage  78  to provide a PWM output signal at the output terminal  79 . Although the invention is shown and described using examples implemented with MOSFETs, other transistors with suitable operating characteristics, for example IGBTs, may be substituted. It should also be appreciated from an understanding of description and figures, that the invention may also be used to advantage in alternative implementations using passive rather than active turn-on or turn-off in suitable applications. 
     FIG. 4  depicts an example of an alternative embodiment of a modulated gate driver circuit  80  according to the invention using an active turn-on, passive turn-off topology. An input terminal  82  accepts a PWM signal, and a modulation input terminal  84  accepts a modulation signal. The PWM input signal is modulated at a modulator  86 , and the modulated PWM signal is used as the set signal at the primary side  88  of a core-less PCB pulse transformer  90  in a pulse transformer driver stage  92 . Preferably, a rising edge pulse generator  94  provides a pulse at the rising edge of the modulated PWM signal. The use of a rising edge pulse is preferred in order to prevent a random delay between the rising edge of the PWM signal and the first rising edge of the modulated PWM signal. Where appropriate for the application, the rising edge pulse generator  94  may be omitted. The secondary side  96  of the PCB transformer  90  is coupled to a transistor  98 , preferably an NPN BIT  98 . The secondary side  96  of the transformer  90  is coupled between the BIT  98  base and emitter. In this configuration, the rising edge of the modulated PWM signal saturates the BIT  98 , causing charge to build up on the gate of a power transistor  104 , bringing the power transistor  104 , preferably a MOSFET, into conduction. The successive high frequency pulses of the modulated PWM signal maintain the charge on the gate of the power transistor  104 , causing the output stage  100  to output a PWM signal at the output node  102 . Following the final pulse of the modulated PWM signal, the charge on the MOSFET  104  dissipates through the pull-down resistor, and the gate switches off. 
     FIG. 5  illustrates an example of an alternative embodiment of a modulated gate driver circuit  106  according to the invention using a passive turn-on, active turn-off topology. A complement of the PWM input signal is derived at the input terminal  108  and modulating signal from a modulation input terminal  110  is applied at a modulator  112 . The modulated PWM signal compliment is used as a reset signal at a pulse transformer driver stage  114 . The pulse transformer driver stage  114  has a core-less PCB pulse transformer  116 . The circuit  106  also preferably includes a rising edge pulse generator  118  for applying an extra pulse to the modulated PWM signal compliment. The rising edge pulse is used to prevent a random delay between the rising edge of the PWM signal complement and the first rising edge of the modulated PWM signal complement. The invention may also be implemented without the rising edge pulse generator  118 . At the pulse transformer  116 , the modulated PWM signal complement enters at the primary side  120  and emerges from the secondary side  122 , which is coupled to a transistor  124  in an output stage  126 . Preferably, the transistor  124  is a PNP type BJT. With the secondary side  122  of the second PCB transformer  116  coupled between the base and emitter, the rising edge of the modulated PWM signal complement cuts off the power transistor  130 , removing the charge between the gate and the source and causing the signal to turn off at the output  128 . Successive pulses of the modulated PWM signal complement prevent any charge injected by noise pulses from accumulating at the gate of the power transistor  130 , thus preventing an erroneous signal to appear at the output terminal  128 . Turn-on in this circuit  106  is passive, using a pull-up resistor R 4 . That is, in the absence of a reset signal at the power transistor  130 , the output stage  126  is allowed to remain in the “on” state. 
   Thus, the invention includes methods and apparatus for providing modulated transistor gate driver circuits using planar pulse transformers for isolation. While the invention has been described with reference to certain illustrative embodiments, the methods and apparatus described are not intended to be construed in a limiting sense. It should be appreciated that the invention may be used with power supply circuitry of various configurations or power amplifiers in a variety of applications. Artisans will appreciate that the circuits shown and described are examples only and that many components may be substituted with their logical equivalents without departing from the principles of invention. Various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the arts upon reference to the description and claims.