Abstract:
An improved snubber is electrically switched to close a current path to a capacitor (C) in a series connected RC circuit at the onset of an abrupt voltage change otherwise producing ringing in a resonant circuit to which the snubber circuit is connected. The current path to the capacitor (C) is then interrupted before the capacitor (C) discharges and thereafter at each such voltage change in the resonant circuit the capacitor (C) is no longer charged from its totally discharged state but nevertheless damps the ringing by virtue of current flow to the nearly completely charged capacitor (C). By preventing complete charging and discharging of the capacitor (C) in the RC circuit every cycle, power dissipation in the resistance of the snubber circuit is greatly reduced.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to circuit provisions for damping resonant ringing in an electrical circuit, and more particularly to a switched RC circuit connected with a resonant circuit to damp ringing. 
   In power devices such as DC-DC converters, abrupt voltage changes can result in transient ringing in the primary and secondary circuits of a transformer. For example, for a regular DC-DC converter, the rectification in a power transformer secondary uses one or more ultra fast or Schottky rectifiers. The leakage inductance of the transformer may interact resonantly with the reverse recovery current and/or with the junction capacitance of the output rectifier diode(s). The leakage inductance and the junction capacitance together with all the parasitic inductances and capacitances of the layout determine the resonant frequency. This resonant circuit has low losses. As a result, many cycles of ringing will occur after the first spike. Also, the ringing generates an overvoltage that constitutes the maximum reverse voltage across the rectifier. This can exceed the diode maximum voltage rating, or it may require larger, more expensive components than would be needed in the absence of such overvoltage. The ringing is present in the current flowing in the secondary side as well. The transformer then reflects the ringing to the primary side of the circuit. This can affect a current sense signal used by the controller when it is located in primary side and the converter works in current mode control. To avoid these problems, the ringing must be damped. However, the damping should be optimized since an excessive damping will increase the switching time and switching losses will become more significant. 
     FIG. 1  illustrates, for purposes of explanation, what probably is the most common circuit  25  used to suppress the voltage transient spikes. This is a typical RC snubber. It is a serial RC circuit  26  connected in parallel with a switching element shown for purposes of explanation in  FIG. 1  as a simple switch SW. It can, however, be a discrete or integrated semiconductor switching device. In the absence of the snubber, when the switch abruptly opens the ringing occurs in the resonant circuit  28 . 
   The value of the snubber capacitor will help define the resonant frequency of the circuit. This is another advantage because this frequency will be less dependent on the parasitic capacitances of the switching element or circuit layout and will be dictated mainly by the value of the snubber capacitor. This helps with EMI filtering by limiting the high frequency harmonics. 
   The snubber  25  is dissipative. Transferring the energy stored in the leakage and parasitic inductances to the snubber capacitor, a fraction of it is dissipated across the snubber resistor. Thus the snubber absorbs some energy at every voltage transient across the switch SW. The effect is the damping of the parasitic ringing at the moment when the switch opens. To properly damp the ringing, the value of the resistor is important. It should be close to the characteristic impedance of the parasitic resonant circuit. 
   The main disadvantage of this usual snubber  25  is the power lost cycle by cycle to charge the capacitor of the snubber from zero to the maximum value of the overvoltage across the switch. This energy is proportional to the value of the capacitor, the switching frequency and the square value of the voltage swing on the capacitor. Depending on the value of the voltage swing, these losses may become significant for power devices such as high efficiency DC-DC converters. 
     FIG. 2  schematically illustrates the standard RC snubber in the context of the equivalent circuit of a DC-DC converter shown as Li is the leakage inductance of a power transformer primary binding. An ultrafast or Shottky diode D 2  serves as the output rectifier. The snubber circuit consists of a series-connected capacitor  30  and resistor  32  connected in parallel with the diode D 2 . An input approximating a square wave is applied at Vi. Power transformer inductance on the output side is represented at Lo and an output voltage Vo appears across the output capacitor Co. 
   In the single ended forward topology of  FIG. 2 , it is the free wheeling diode D 2  that is the switching element that causes ringing requiring a snubber. When the diode D 2  opens, ringing occurs as described above.  FIGS. 3 and 4  show the voltage wave form across the diode D 2  before and after application of the snubber. V c  the voltage across the capacitor  30  follows V D2 . Each time V D2  ramps up, capacitor C charges. Each time V D2  ramps down, capacitor C discharges completely. 
   For efficient power device operation, it would be beneficial, where a snubber is used to damp ringing, to reduce the losses that occur as a result of current flow in the RC snubber circuit during the charging and discharging of the capacitor. 
   SUMMARY OF THE INVENTION 
   In accordance with this invention, there is provided a resonant circuit that includes a snubber circuit that damps resonant ringing and a switch to, first, complete a current path to a capacitive element at the onset of an abrupt voltage change and to, second, interrupt the current path to the capacitive element to prevent a complete transition in the charge state of the capacitive element thereafter. More particularly, the switch is preferably an electrically controlled switching device. In a preferred embodiment, the switch is electrically closed during the period of abrupt voltage change in a resonant circuit and is electrically opened once damping is completed. 
   In a preferred embodiment of the invention, the switch is a semiconductor device in series with the capacitive element of the snubber. The snubber can be an RC circuit path in parallel with a circuit&#39;s primary switching device. Where the circuit is a power device, that switching device may be an ultrafast or Shottky diode across the secondary of a power transformer. 
   Preferably, in accordance with a preferred embodiment of the invention, the resistance of the RC snubber circuit is close to the characteristic impedance of the parasitic resonant circuit that the snubber serves. In one preferred embodiment of the invention, the electrically controlled switch is a MOSFET device in series with the RC circuit that is the snubber. The electrically controlled switch can be, however, any of a number of fast switching devices including NPN or PNP discrete transistors, a CMOS transistor, either discrete or integrated, an SCR or any other precisely electrically controlled fast switch or switching device. Further, in accordance with a preferred embodiment of the invention, the electrically controlled switch completes a current path during each abrupt voltage increase in the resonant circuit to cause charging of the capacitor of the snubber to begin before the first spike or resonant overshoot occurs across the primary switch. The electrically controlled switch then interrupts the current path to the charged snubber capacitor to prevent its complete discharge. Subsequently, the capacitor is not charged from a totally discharged state to its fully charged state and is, in use, never completely discharged. Rather, the capacitor has held a large part of its charge and little current is required to recharge it during the snubber&#39;s damping. Consequently, there is considerably less power dissipation in the resistor of the snubber circuit. 
   The above and further objects and advantages of the invention will be better understood from the following detailed description of at least one preferred embodiment of the invention, taken in consideration with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration, partially in block diagram, of a prior art snubber connected across a switching device, preventing ringing in a resonant circuit; 
       FIG. 2  is a schematic illustration of an equivalent circuit of a prior art DC-DC converter with a snubber circuit like that of  FIG. 1 ; 
       FIG. 3  is a plot against time of voltage across an output rectifier diode in a DC-DC converter like that illustrated in  FIG. 2 , but without a snubber; 
       FIG. 4  is a plot against time of a voltage across the output rectifier diode or snubber capacitor of  FIG. 2 ; 
       FIG. 5  is a generalized schematic illustration of a resonant circuit having an output rectifying diode and no damping; 
       FIG. 6  is a series of plots versus time of input voltage, inductor current, and voltage across the diode in the circuit of  FIG. 5 ; 
       FIG. 7  is a diagrammatic block diagram of a generalized power device having a snubber circuit including an electrically controlled switch in accordance with the present invention; 
       FIG. 8  is a schematic illustration of a circuit like that of  FIG. 5  and including an active snubber in accordance with the present invention; 
       FIG. 9  is a series of plots like those of  FIG. 6  showing voltages and currents in the circuit of  FIG. 8 ; 
       FIG. 10  is a schematic illustration of an equivalent circuit of a DC-DC converter having a MOSFET switch connected with an RC damping circuit in accordance with the invention; 
       FIG. 11  is a series of plots against time of voltages in the circuit like that of  FIG. 10 , and shows correctly timed snubber switch control signals and the resultant voltages across the output rectifier diode and the capacitor of the snubber; 
       FIG. 12  is a series of plots against time like those of  FIG. 11 , but shows the effect of too early switching “on” of the snubber switch; 
       FIG. 13  is a series of plots like those of  FIGS. 11 and 12  and shows the effect of too late switching “on” of the snubber switch; 
       FIG. 14  is a schematic illustration of a boost converter employing an active snubber circuit, in accordance with the invention; and 
       FIG. 15  is a series of plots of voltages of a circuit like that of  FIG. 14  absent a snubber. 
   

   DETAILED DESCRIPTION 
   Turning now to  FIG. 5 , a generalized resonant circuit  40  is shown there in accordance with the prior art. An input voltage Vin is applied at  42 . The input voltage has abrupt rise and fall times and is of the nature of a square wave as shown. In  FIG. 6 , the input voltage is depicted with an abrupt rise time at time T. The circuit  40  has an inductance Lp as would a power device with a transformer primary  44  to be driven by the input voltage Vin. A capacitor  46  has a capacitance C p  which could be, for example, parasitic capacitance in the circuit  40 . The inductance Lp and capacitance Cp, then, form a resonant tank circuit. An output rectifier diode  48  switches from its non-conducting to its conducting state responsive to Vin. Current in the diode  48  is plotted at  50  in  FIG. 6 . Ringing is apparent at turn-on in the region  52 . Ringing is likewise apparent in the plot  54  of Vp versus time in the region  56 . Spiking at the outset of turn-on causes stress on the diode  48  to be almost twice that where just Vin is present. 
   In  FIG. 7 , the component parts of the active snubber in accordance with the invention are shown in functional block diagram form. A voltage Vin, again approximating a square wave is applied at  42  as in the prior art circuit of  FIG. 5 . Two way resonant circuit  40 , a damping circuit  50 , however, is connected with the resonant circuit  40 . An electrically controlled switch  59  is connected in series with the damping circuit  50  to complete or interrupt current flow to the damping circuit as controlled by a control signal source  60 . The timing of the opening and closing of the switch  59  by the control signal source  60  is described in greater detail below in connection with specific exemplary embodiments. The signal applied to the switch  59  by the source  60  may be derived from the input voltage  42 , from a separate auxiliary coil on a power transformer (not shown in  FIG. 7 ), from a controller (not shown) controlling a main switching element as represented by the switch SW of  FIG. 1 , or in any convenient fashion capable of precisely tiring the switch activation signals applied to open and close the switch  59 . 
   In  FIG. 8 , a generalized schematic like that of  FIG. 5  is shown wherein like elements are given like reference numerals. In the circuit of  FIG. 8  an active snubber circuit of the present invention is employed. A snubber capacitor  62  and snubber resistor  64  from a series RC damping circuit  50  are connected in parallel with the capacitor  46  and diode  48 . Unlike the prior art snubber circuits used to damp resonant ringing, the serial snubber circuit that includes the capacitor  62  and resistor  64  also includes a switch  65  connected in series. 
   In  FIG. 9 , the plot  49  is again the input voltage Vin illustrated in  FIG. 4 . The plot  66  illustrates at  67  the highly damped current I p  as compared to I p  illustrated in the region  52  of  FIG. 3 . Likewise, the voltage V p  across the diode  48  is plotted at  68  and shows in the region  70  the effect of damping by the RC circuit made up of the resistor  64  and the capacitor  62 . 
   In the circuit of  FIG. 8 , switching by the switch  65  is timed. At the time the switch  62  is turned on (i.e. closed), the snubber capacitor  62  charges parasitic capacitor  46  through the resistor  64  quickly, before an additional amount of energy accumulates in the inductor  44 . Thus the current I p  in the inductor  44  remains close to Io. The snubber capacitor  62  is then allowed to charge through the snubber resistor  64 . This restores any energy lost during the charging of the parasitic capacitor  46 . Any residual ringing is damped by the resistance  64 . At a time T 2  after the capacitor  62  is charged, the switch  65  is opened, conserving the charge on the capacitor before the input voltage drops from Vin to 0. The switched RC snubber circuit has the advantage that it prevents energy going into the tank circuit of inductor  44  and capacitor  46  and only dissipates the energy needed to dampen ringing. Energy is conserved by keeping capacitor  62  charged. 
   In  FIG. 10 , the equivalent circuit schematic is shown for a DC-DC converter like that of  FIG. 2 , but employing the active snubber circuit of the invention as described with respect to  FIGS. 8 and 9 . As stated, the conventional RC snubber was designed to damp any ringing generated by the transitions of the voltage across the main switch (Sw in  FIG. 1 , D 2  in  FIG. 2 ). During damping, the current flows through the RC snubber only for two or three cycles. After that, the ringing disappears. The snubber capacitor is charged. No current flows through the snubber resistor. At this moment there is no reason to allow the snubber capacitor to be discharged by the free-wheeling diode D 2  through the snubber resistor. For this purpose, then, the additional switch in series in the RC snubber circuit is provided in accordance with the invention. In the specific exemplary embodiment depicted in  FIG. 10 , that switch is a MOSFET transistor  80 . Illustrated in  FIG. 11 , the driving signal for the MOSFET switch  80  is gate voltage V G . As shown, the MOSFET switch  80  is kept off until the next positive transition appears across the diode D 2 . Synchronously with this transition, the MOSFET switch is switched on and the snubber becomes active, damping the ringing. In  FIG. 11 , the correctly timed MOSFET driving signal V G  appears at  82 . The resultant, damped voltage V D2  across the output rectifying diode D 2 , appears at  84 . As shown at  86  in  FIG. 11 , the only voltage swing across the snubber capacitor is from the overshoot of the voltage across the free-wheeling diode D 2  at  88 . Assuming that the MOSFET switch is ideal, and that the RC snubber is trimmed to limit the overshoot of the voltage to 10 percent, power losses in the ordinary, prior art RC snubber are 121 times higher than in the active snubber of this invention. In the non-ideal, real-life embodiment, the ratio is a little lower because of the additional power losses in the MOSFET switch  80 . 
   To optimize the performance of the active snubber of this invention, timing is important. This is illustrated in  FIGS. 12 and 13 .  FIG. 12  illustrates premature switching “on” of the active snubber by the MOSFET switch  80 . The driving signal  82  is applied to the MOSFET switch too early in the onset of V D2 . Here, the snubber capacitor C is discharged more than necessary at  90  by virtue of being switched on before V D2 , the voltage across the output free-wheeling diode D 2  has risen to the voltage V c  across the snubber capacitor C. As illustrated in  FIG. 13 , turning “on” the MOSFET switch  80  too late results in the RC snubber circuit not being able to receive the energy stored in the parasitic inductances for the beginning of the cycle. The snubber will damp the ringing only after the MOSFET is switched on and the overvoltage shown at  92  with respect to the free-wheeling diode and at  94  with respect to the capacitor, will be much larger. In both of the situations, graphically illustrated in  FIGS. 12 and 13 , the power losses in the snubber circuit are higher than where the timing is correct. This is due to the increased voltage swing across the snubber capacitor. In addition, in the second situation,  FIG. 13 , the voltage stress on the free-wheeling diode D 2  is much higher than when the timing of the MOSFET switching element  80  is correct. 
   When the timing is correct, the active snubber has the following advantages: 
   1. Reduction in a very efficient manner of the power losses on the snubber circuitry increasing the efficiency of, in the case of  FIG. 10 , the DC-DC converter. 
   2. The damping does not affect the switching time of the main switch because the snubber becomes active only at the end of the switching time, i.e. well along in the onset of the applied voltage V D2 . 
   3. Reduction in the size, tolerances and cost of the circuit components by virtue of reduced current and voltage swings with reduced stress. 
   Turning to  FIG. 14 , a boost converter  100  is shown having a transformer  102 . The transformer has a primary inductor  104  with a leakage inductance L 1 , a diode  103  and capacitor  106  all in the primary circuit. The capacitor  106  may be parasitic capacitances and it forms a tank circuit with the inductance L 1  of the primary winding  104  of the transformer  102 . A primary switch is, in this case, a MOSFET switch  108 . This switch  108  functions in the manner of the switch SW of  FIG. 1 . In the secondary  109  of the transformer  102 , L 2  is the leakage inductance of the secondary winding  109 , resistor  110  is the snubber resistor, and the capacitor  112  is the snubber capacitor. The switch  114  is the timed snubber switch and may be a MOSFET switch like that of  FIG. 10 . Here, the active snubber made up of the snubber resistor  110 , snubber capacitor  112  and switch  114  are connected in parallel with the secondary winding of the transformer  102 . In  FIG. 15  at  116 , the drive voltage Vc applied to the gate of MOSFET switch  108  is plotted. At  118  the voltage Vd across the MOSFET. switch  108  in the absence of damping is plotted and ringing is observed in the region  120 . At  122  the voltage across the secondary winding or choke is plotted and the ringing is reflected as shown in the region  124 . The drive voltage Vc snubber is plotted at  130  as shown. This closes the switch  114  at the onset of the ringing depicted at  120  and  124  and opens the switch  114  upon completion of damping and before the snubber capacitor  112  discharges. At  116  in  FIG. 15 , the active snubber  110 ,  112  and  114  of the invention is in use. The drive voltage Vc of the primary MOSFET switch  108  plotted at  116  is the same. At  126 , the voltage across the secondary V(L 2 ) is plotted and good damping of the ringing observed at  124  is evident in the region  128 . 
   Although preferred embodiments of the invention have been described in detail, it will be readily appreciated by those skilled in the art that further modifications, alterations and additions to the invention embodiments disclosed may be made without departure from the spirit and scope of the invention as set forth in the appended claims. For example, although the change in capacitor charge state that occurs to damp ringing at the onset of an abrupt voltage change across a main switch or free-wheeling diode in embodiments of the invention described above is associated with a charging of the snubber capacitor, the current flow through a snubber resistor when damping occurs could, in particular circuits, be associated with a discharge of the snubber capacitor. What is important is the correctly timed change of the charged state of the capacitor and the prevention of the capacitor&#39;s reversion fully to its one charged state subsequent to damping.