Example: biology

Design of Snubbers for Power Circuits

1 Design OF Snubbers FOR Power CIRCUITSBy Rudy SevernsWhat s a snubber ? Power semiconductors are the heart of Power electronics equipment. Snubbers are Circuits whichare placed across semiconductor devices for protection and to improve performance. Snubbers can domany things: Reduce or eliminate voltage or current spikes Limit dI/dt or dV/dt Shape the load line to keep it within the safe operating area (SOA) Transfer Power dissipation from the switch to a resistor or a useful load Reduce total losses due to switching Reduce EMI by damping voltage and current ringingThere are many different kinds of Snubbers but the two most common ones are the resistor-capacitor (RC) damping network and the resistor-capacitor-diode (RCD) turn-off snubber . This appli-cation note will show you how to Design these two waveformsBefore getting into the Design of Snubbers it is important to understand the waveforms whichoccur naturally in Power Circuits .

The power dissipated in R s can be esti-mated from peak energy stored in C s: This is the amount of energy dissipated in R s when C s is charged and discharged so that the average power dissipation at a given switching frequency (f s) is: Depending on the amount of ringing the actual power dissipation will be slightly higher than this.

Tags:

  Design, Power, Circuit, Peak, Average, Snubber, Design of snubbers for power circuits, Average power

Information

Domain:

Source:

Link to this page:

Please notify us if you found a problem with this document:

Other abuse

Transcription of Design of Snubbers for Power Circuits

1 1 Design OF Snubbers FOR Power CIRCUITSBy Rudy SevernsWhat s a snubber ? Power semiconductors are the heart of Power electronics equipment. Snubbers are Circuits whichare placed across semiconductor devices for protection and to improve performance. Snubbers can domany things: Reduce or eliminate voltage or current spikes Limit dI/dt or dV/dt Shape the load line to keep it within the safe operating area (SOA) Transfer Power dissipation from the switch to a resistor or a useful load Reduce total losses due to switching Reduce EMI by damping voltage and current ringingThere are many different kinds of Snubbers but the two most common ones are the resistor-capacitor (RC) damping network and the resistor-capacitor-diode (RCD) turn-off snubber . This appli-cation note will show you how to Design these two waveformsBefore getting into the Design of Snubbers it is important to understand the waveforms whichoccur naturally in Power Circuits .

2 These provide both the motivation for using Snubbers and the infor-mation needed for their Design . There are many different types of Circuits used in Power converters,motor drives, lamp ballasts and other devices. Fortunately all of these different Circuits have a commonnetwork and waveforms associated with the switches. Figure 1 shows four widely used Circuits . All ofthese Circuits , and in fact most Power electronics Circuits , have within them the same switch-diode-inductor network shown within the dotted lines. The behavior of this network is the same in all thesecircuits which means that we only have to solve the snubber Design problem for one circuit to apply it toall of the others. This tremendously simplifies the problem and allows generalized snubber Design typical boost converter is shown in figure 2A.

3 For snubber Design we are concerned withcircuit behavior during the switch transition time which is much shorter than the switching period. Thisallows us to simplify the analysis. In normal operation the output voltage is DC with very little means that we can replace the load and filter capacitor with a battery since the output voltagechanges very little during switch transitions. The current in the inductor will also change very littleduring a transition and we can replace the inductor with a current source. The simplified circuit is givenin figure 2B. The voltage (E) and current (I) waveforms are given in figure the beginning of the switching cycle the switch is open and all of the current (Io) will beflowing through the diode into the battery. As the switch turns on, the current will gradually shift fromthe diode to the switch.

4 However, as long as there is2 Figure 13 Figure 24current in the diode, the switch voltage will remain at Eo. Once all of the current has been transferred tothe switch, the switch voltage can begin to fall. At turn-off the situation is reversed. As the switch turnsoff, the voltage across it will rise. The current in the switch will however, not begin to fall until theswitch voltage reaches Eo because the diode will be reverse biased until that point. Once the diode beginsto conduct the current in the switch can type of switching, commonly referred to as hard switching , exposes the switch to highstress because the maximum voltage and maximum current must be supported simultaneously. This alsoleads to high switching practical Circuits the switch stress will be even higher due to the unavoidable presence ofparasitic inductance (Lp) and capacitance (Cs) as shown in figure 3A.

5 Cp includes the junction capaci-tance of the switch and stray capacitance due to circuit layout and mounting. Lp is due to the finite sizeof the circuit layout and lead inductance. Lp can be minimized with good layout practice but there maybe some residual inductance which may cause a ringing voltage spike at turn-off as shown in figure most common reasons for using a snubber are to limit the peak voltage across the switchand to reduce the switching loss during turn-off. RC snubber designAn RC snubber , placed across the switch as shown in figure 4, can be used to reduce the peakvoltage at turn-off and to damp the ringing. In most cases a very simple Design technique can be used todetermine 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 snubber Design : To achieve significant damping Cs > Cp.

6 A good first choice is to make Csequal 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 currentflowing 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 averagepower dissipation at a given switching frequency (fs) is:Depending on the amount of ringing the actual Power dissipation will be slightly higher than 36 Figure 47 The following example shows how to use this procedure. Suppose the switch is an IRF740 withIo = 5 A and Eo = 160 V. For this device Coss = 170 pF and the mounting capacitance will be 40 this capacitance, Cs = 420 pF.

7 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 valueand set Cs = 390 pF. Rs = Eo/Io = 32W. For fs = 100 kHz, Pdiss = 1W. A 2 Watt carbon compositionresistor would be ideal for Rs because it has very low self inductance. Carbon film resistors can also beused as long as those resistors which are trimmed with a spiral cut are this very simple and practical procedure does not limit the peak voltage sufficiently then Cs canbe increased or the optimizing procedure can be RC snubber : In those cases where the peak voltage must be minimized and Power dissipationis critical, a more optimum Design approach should be used. In a classic paper [1] Dr. W. McMurraydescribed the optimization of the RC snubber .

8 The following discussion presents the highlights of following definitions will be used:1/2Zo = (Lp/Cs) = (Io/Eo)Zoinitial current factor 222 = LpIo/CsEoinitial energy in Lp/final energy inCs = Ps/2 Zodamping factorE1 = peak switch voltageE1/Eo=normalized peak switch voltageIn the Design process Io, Eo and Lp will be given and it will be necessary to determine the valuesfor 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/.5Cs)there is an optimum value for ( Rs) which gives the lowest peak voltage. A second important point isthat the lowest value of peak voltage attainable is determined by the size of Cs. If a lower peak voltageis required, then a larger Cs must be used.

9 This means that the Power dissipation has to increase as thepeak voltage is Design of an optimized RC snubber is very easy using the graph given in figure 6. The designproceeds in the following steps:1. Determine Io, Eo and Lp2. Select the maximum peak voltage8 Figure 5 9 Figure 6 103. Compute E1/Eo4. 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 = the dashed line and arrows on figure 6, o = and o= .8. From this information Rs andC s can be determined:using a standard CDE snubber Mike capacitor let Cs = 680pFusing a standard resistor let Rs = 62 OhmsThe graphs (figures 5 and 6) do not take into account the effect of the switch shunt capacitanceor finite transition time.

10 In general the optimum value for Rs will be somewhat lower than calculated. Amore precise optimum can be achieved by simulation of the switching with SPICE. Starting with thecomputed values, Rs can then be easily varied to find the optimum. In general the optimum will be quitebroad allowing the use of standard 5% resistor example of optimizing Rs, using an IRF840 for the switch, is shown in figure 7. The optimumvalue for Rs = 51 Watts and E1 = 363 V. For Rs = 39 and 62 Watts, E1 is higher. The final peak voltageis less than 400 V because of the shunt capacitance of the switch. If E1 is allowed to rise to 400 V thena smaller value for Cs could be used, saving some Power example shows the importance of simulating and optimizing the snubber circuit using theactual components. The graphs get you into the ball park and the simulation allows for of Lp:Eo and Io come directly from the circuit .


Related search queries