Design of Snubbers for Power Circuits

Design Of Snubbers For Power Circuits-Free PDF

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current in the diode the switch voltage will remain at Eo Once all of the current has been transferred to. the switch the switch voltage can begin to fall At turn off the situation is reversed As the switch turns. off the voltage across it will rise The current in the switch will however not begin to fall until the. switch voltage reaches Eo because the diode will be reverse biased until that point Once the diode begins. to conduct the current in the switch can fall, This type of switching commonly referred to as hard switching exposes the switch to high. stress because the maximum voltage and maximum current must be supported simultaneously This also. leads to high switching loss, In practical circuits the switch stress will be even higher due to the unavoidable presence of. parasitic inductance Lp and capacitance Cs as shown in figure 3A Cp includes the junction capaci. tance of the switch and stray capacitance due to circuit layout and mounting Lp is due to the finite size. of the circuit layout and lead inductance Lp can be minimized with good layout practice but there may. be some residual inductance which may cause a ringing voltage spike at turn off as shown in figure 3B. The most common reasons for using a snubber are to limit the peak voltage across the switch. and to reduce the switching loss during turn off,RC snubber design. An RC snubber placed across the switch as shown in figure 4 can be used to reduce the peak. voltage at turn off and to damp the ringing In most cases a very simple design technique can be used to. determine suitable values for the snubber components Rs and Cs In those cases where a more opti. mum design is needed a somewhat more complex procedure is used. Quick snubber design To achieve significant damping Cs Cp A good first choice is to make Cs. equal to twice the sum of the output capacitance of the switch and the estimated mounting capaci. tance Rs is selected so that Rs Eo Io This means that the initial voltage step due to the current. flowing in Rs is no greater than the clamped output voltage The power dissipated in Rs can be esti. mated from peak energy stored in Cs, This is the amount of energy dissipated in Rs when Cs is charged and discharged so that the average. power dissipation at a given switching frequency fs is. Depending on the amount of ringing the actual power dissipation will be slightly higher than this. The following example shows how to use this procedure Suppose the switch is an IRF740 with. Io 5 A and Eo 160 V For this device Coss 170 pF and the mounting capacitance will be 40 pF. Doubling this capacitance Cs 420 pF A 500V Snubber Mike capacitor would be ideal for this appli. cation and the standard values available are 390 and 470 pF We will choose the closest standard value. and set Cs 390 pF Rs Eo Io 32W For fs 100 kHz Pdiss 1W A 2 Watt carbon composition. resistor would be ideal for Rs because it has very low self inductance Carbon film resistors can also be. used as long as those resistors which are trimmed with a spiral cut are avoided. If this very simple and practical procedure does not limit the peak voltage sufficiently then C s can. be increased or the optimizing procedure can be used. Optimized RC snubber In those cases where the peak voltage must be minimized and power dissipation. is critical a more optimum design approach should be used In a classic paper 1 Dr W McMurray. described the optimization of the RC snubber The following discussion presents the highlights of the. The following definitions will be used,Io Eo Zo initial current factor.
initial energy in Lp final energy in,Ps 2Zo damping factor. E1 peak switch voltage,E1 Eo normalized peak switch voltage. In the design process Io Eo and Lp will be given and it will be necessary to determine the values. for Rs and Cs which give an acceptable peak voltage E1 Figure 5 shows the relationship between E1. Eo and for different values of The key point which this graph makes is that for a given 1 Cs. there is an optimum value for Rs which gives the lowest peak voltage A second important point is. that the lowest value of peak voltage attainable is determined by the size of Cs If a lower peak voltage. is required then a larger Cs must be used This means that the power dissipation has to increase as the. peak voltage is reduced, The design of an optimized RC snubber is very easy using the graph given in figure 6 The design. proceeds in the following steps,1 Determine Io Eo and Lp. 2 Select the maximum peak voltage,3 Compute E1 Eo,4 From the graph determine the values for and.
5 Given and calculate the values for Rs and Cs, Here is an actual example If Io 5 A Eo 300 V Lp 1 H and E1 400V then E1 Eo 1 33. Following the dashed line and arrows on figure 6 o 0 65 and o 8 From this information Rs and. Cs can be determined, using a standard CDE Snubber Mike capacitor let Cs 680pF. using a standard resistor let Rs 62 Ohms, The graphs figures 5 and 6 do not take into account the effect of the switch shunt capacitance. or finite transition time In general the optimum value for Rs will be somewhat lower than calculated A. more precise optimum can be achieved by simulation of the switching with SPICE Starting with the. computed values Rs can then be easily varied to find the optimum In general the optimum will be quite. broad allowing the use of standard 5 resistor values. An example of optimizing Rs using an IRF840 for the switch is shown in figure 7 The optimum. value for Rs 51Watts and E1 363 V For Rs 39 and 62 Watts E1 is higher The final peak voltage. is less than 400 V because of the shunt capacitance of the switch If E1 is allowed to rise to 400 V then. a smaller value for Cs could be used saving some power dissipation. This example shows the importance of simulating and optimizing the snubber circuit using the. actual components The graphs get you into the ball park and the simulation allows for optimization. Determination of Lp, Eo and Io come directly from the circuit The value for E1 is a judgment call and will depend on the. voltage rating of the switch and the voltage derating factor The designer must choose the peak accept. able voltage All of these quantities are fairly obvious Lp however is a characteristic of the particular. circuit layout and is not usually easy to calculate Lp can be determined from the circuit by measuring the. period of one ringing cycle T1 then adding a known capacitor Ctest in parallel with the switch and. finally re measuring the period T2 Lp can be computed from. Usually Ctest is approximately equal to twice the switch capacitance. An alternate method for determining Lp in higher power circuits is to take advantage of the. voltage step Vstep which appears in Vce or Vds due to the dI dt of the current flowing in Lp at turn on. At switch turn on Cs will be charged This means that there will be a current spike in the switch due to. the discharge of Cs through Rs in addition to the normal current flow This is a short transient pulse. which rapidly decays but it can add substantially to the turn on current and should be taken into account. It should be noted that the RCD snubber discussed in the next section will also have this turn on current. spike but that spike can be controlled more easily because the value for Rs does not have to be optimized. for maximum damping, RC snubbers are very useful for low and medium power applications but when the power level is.
more than a few hundred watts the loss in the snubber can be excessive and other types of snubbers need. to be considered The RC snubber does have a place in high power applications as a secondary damping. network to suppress high frequency ringing which does not have a lot of energy associated with it. RCD snubber design, The RCD snubber as shown in figure 8 has several advantages over the RC snubber. In addition to peak voltage limiting the circuit can reduce the total circuit loss including. both switching and snubber losses, Much better load lines can be achieved allowing the load line to pass well within the SOA. For a given value of Cs the total losses will be less. The shunt capacitance across the switch Cp is a useful part of the snubber. There is one disadvantage however Because of the diode across Rs the effective value for Rs. during the charging of Cs is essentially zero This is not the optimum value and for a given Cs E1 will. be higher than it would be in an optimized RC snubber. Typical turn off waveforms for this snubber are given in figure 9 These waveforms assume that. Lp 0 The effect of Lp will be considered shortly The key feature of these waveforms is that the switch. voltage rises slowly as the switch current falls This means that the high peak power associated with. simultaneous maximum voltage and current is eliminated The net result is much lower peak stress and. switching loss Voltage waveforms for two different values of Cs are shown In this example Io 10 A. and Eo 300 V As Cs is made larger the peak power and the switching loss will be lower However. larger Cs means greater loss in Rs when the switch turns on and Cs is discharged through Rs and the. switch Again we see the tradeoff between snubber efficacy and loss. Depending on the size of Cs the switch voltage may reach Eo before at the same time or after the. switch current reaches zero The case where E Eo at the instant that I 0 is defined as a normal. snubber and Cs Cn where 2, Where ts is the fall time of the switch current see figure 9 For the example given in figure 9 Cn. The relationships between Cn switching loss peak switch stress snubber loss and total loss are. shown in figure 10 Snubber size is shown relative to Cn When even a small snubber is used Cs Cn the. switching loss drops quickly As Cs is made larger however the improvement in switch loss decreases. For example for Cs Cn the switch loss is down to 16 Making Cs larger will reduce the switching. loss only a small amount but will increase the snubber loss substantially There is in fact a broad mini. mum loss around Cs 0 45 Cn where the total loss is reduced to 53 of what it would have been. without the snubber It is important to remember that Cp is part of Cs and that the actual value for Cs. 45 Cn Cp For Cs Cn 2 the total loss is equal to what it would have been if no snubber were used. however the switching load line will have very low stress. In those cases where the primary concern is to reduce the total switching loss the value for Cs. is usually set to 5 Cn In this case Rs is selected to allow the voltage on Cs to decay to a small value. during the minimum switch on time ton min The capacitor voltage decay is a simple RC exponential. and in two time constants RsCs will be down to 0 14 Eo This is usually sufficient The value of. Rs is then, When Lp is significant there will be voltage overshoot during turn off If E1 must be controlled then. different values for Cs may be necessary and a compromise made Figure 11 gives a comparison be. tween the RC snubber developed earlier and an RCD snubber using the same component values Rs. 51W and Cs 680 pF Note that the loss will be lower but the peak voltage is higher for the RCD. snubber This is typical For similar total loss Cs can be larger in the RCD snubber which will reduce E1. Increasing Cs to 1 2 nF reduces E1 to 424 V Cs could be increased further but for the same total loss E1. will still be higher in the RCD snubber, Combined snubber When it is important to minimize both the loss in the switch and E1.
12 Usually C test is approximately equal to twice the switch capacitance An alternate method for determining Lp in higher power circuits is to take advantage of the voltage step Vstep which appears in V ce or Vds due to the dI dt of the current flowing in L p at turn on At switch turn on C s will be charged This means that there will be a current spike in the switch due to

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