design tips nov07

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DESIGN TIPS DESIGN TIPS

Power Systems Design Europe November 2007 16 17www.powersystemsdesign.com

By Dr. Ray Ridley, Ridley Engineering

W ithout a snubber, the leak -age inductance of the ybacktransformer rings with stray

capacitances in the circuit, producinglarge amplitude high-frequency wave -forms as shown in Figure 1.

Many application notes and designsignore the ringing waveforms and oper -ate the converter without addressing theissue. There are two problems with this:rstly, there is excessive voltage on thedrain of the FET which can lead to ava -lanche breakdown and eventually failureof the device. Secondly, the ringingwaveform will be radiated and conduct -ed throughout the power supply, load,and electronic system, creating noise issues and even logic errors. The ringing

frequency will also show up as a peak ofthe EMI spectrum in both radiated andconducted EMI.

In most designs, this is not accept -able, and it is necessary to add circuitelements to damp the ringing (usingan RC snubber), or to clamp voltages(with RCD clamps), or both. The designof these networks is a combination ofmeasurements and analysis to ensure arugged and dependable result.

Primary RCD Clamp for the FlybackConverter

Figure 2 shows an RCD clamp circuitused to limit the peak voltage on thedrain of the FET when an RC snubber isinsufcient to prevent switch overvolt -age. The RCD clamp works by absorb -ing the current in the leakage inductoronce the drain voltage exceeds theclamp capacitor voltage. The use of arelatively large capacitor keeps the volt -age constant over a switching cycle.

The resistor of the RCD clamp alwaysdissipates power. Even with very littleload on the converter, the capacitorwill always be charged up to the volt -age reected from the secondary of theconverter, vf. As the load is increased,more energy will ow into the capacitor,and the voltage will rise by an additionalamount, vx, above the ideal square waveyback voltage. The waveform deningthese voltages is shown in Figure 2.

Design Step 1 – Measure LeakageInductance

Flyback Conver terwith No Snubber s

All PWM converters have parasitic components that lead to ringing waveforms which must be properly suppressed. Without this, semiconductors can fail, and noise levels will be higher than necessary. This article describes the most commonly-used RCD clamp circuit used for the popular yback converter,

together with its design equations.

Figure 1: Flyback converter drain voltage.

crucial in determining the peak voltagevx, and it should be selected with thefollowing equation:

A larger value of resistor will slowthe discharge of the clamp capacitor,and allow the voltage to rise to a highervalue. A smaller value will result in alower clamp voltage, but the dissipationwill be increased.

Design Step 4 – Calculate Power

LossThe snubber design is now complete,

but we often need to know what the dis -sipation will be for currents other thanthe worst case current, Ip, in the equa -tions above. Use the following equationto calculate the voltage rise in a knownsnubber for a given peak current I, andleakage inductance L.

The value of the voltage rise, v x, abovethe yback voltage is given by:

The power dissipation is given by:

Design Step 5 – Experimental Veri -cation

Experimental verication of the designis essential since there will be effects

In other words, the higher we let theclamp voltage rise on the switch, thelower the overall dissipation. But of

course, we must balance this againstthe total voltage seen across the powerFET, so we cannot arbitrarily reduce dis -sipation.

A typical design is for the voltage v x

to be equal to � the yback voltage. Inthis case, the dissipation is equal to 3times the stored energy in the leakageinductance, which is not an immediatelyintuitive result. This is a conservativeestimate, however. It does not accountfor lossy discharge of the inductor, norfor stray capacitance. In reality, the de -sign will have less loss in the clamp thananticipated due to these effects.

For high-voltage ofine designs whichare often constrained to use a FET witha maximum voltage of 650 or 700 V,the voltage vx will have a hard limit setby the maximum input line, maximumcurrent, and FET breakdown voltage. Donot exceed the stated Vds of the FET,and be aware that the breakdown canvary with temperature. Some design -ers rely on the avalanche capability ofthe FET to let them regularly exceed thebreakdown voltage. This is not rec -ommended for rugged power supplydesign.

Design Step 3 – Select ClampResistor

The capacitor of the snubber needsto be large enough to keep a relativelyconstant voltage while absorbing theleakage energy. Apart from this consid -eration, its value is not critical, and willnot affect the peak voltage when thesnubber is working properly.

The resistor is the element that is

It is important to measure the leakageinductance of the yback transformerprior to designing the snubber. Details of

how to do this are given in [1]. Do not justguess at the value of inductance, andbe aware that worst-case specicationsfrom magnetics manufacturers are oftennot accurate enough to use for design.

Also, as explained in [2], the leakage in -ductance is a frequency-dependent, andmust be measured at the proper valueof frequency.

Design Step 2 – Determine PeakClamp Voltage

Now you must decide how muchvoltage can be tolerated on the powerMOSFET, and calculate the amountof power that will be dissipated in theclamp with this clamp level. The powerassociated with in the leakage induc -tance, L, with a current worst-case cur-rent Ip at turn-off is given by:

s pl f LI P 2

2

1=

Analysis of the RCD snubber has ap -peared in papers and numerous applica -tion notes. It is assumed that there areno stray capacitances to charge, andthat all the leakage energy is conductedinto the snubber capacitor from theleakage inductance. The capacitor is as -sumed to be large enough that its valuedoes not change signicantly during oneswitching cycle.

With these assumptions, the powerdissipated by the RCD clamp can beexpressed in terms of the energy storedin the inductor as follows:

Figure 3: Flyback converter drainvoltage with primary RCD clamp.

Figure 2: Flyback converter with primary RCD clamp .

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DESIGN TIPS

Power Systems Design Europe November 2007 18

direction in the diode, resulting in ring -ing. The type of diode chosen for theRCD snubber is crucial. It must be asfast as possible with the proper voltage

rating.

The severity of this ringing will dependon the reverse applied voltage acrossthe RCD diode. The higher you allow theclamp voltage to climb, the lower thedissipation, but higher voltage and dv/dtis applied to the diode, and the ringingincreases.

The ringing can subse -quently be damped out againby introducing the RC snub -ber, designed as describedin[1]. Figure 4 shows the drainwaveform with both an RCDclamp and RC snubber inplace. This provides the bestprotection for the FET, andthe lowest EMI signature, butresults in the highest powerdissipation.

Summary The RCD clamp circuit is

useful for all yback convert -ers to reduce the stress on thepower FET. Make sure that theclamp is designed to restrictthe voltage under worst-caseoperating conditions (high line,and maximum current limit) toless than the voltage rating ofthe part. The design equa -tions in this article remove theguesswork from the clampdesign.

Additional Reading [1] “Flyback ConverterSnubber Design” http://www.

switchingpowermagazine.com/downloads/12FlybackSnubber Design.pdf

[2] “High-Frequency PowerTransformer Measurement

and Modeling” http://www. powersystemsdesign.com/ design_tips_janfeb07.pdf

not accounted for in the equations, andyour circuit will have nonideal compo -nents. Figure 3 shows the effectivenessof the circuit in clamping the peak value

of the FET drain voltage.

This gure also shows a limitationof the RCD clamp. After the clampingperiod is nished, the circuit resumesringing. With ideal components, thiswould not happen. However, the diodeof the RCD clamp has a nite reverserecovery time which allows the leakageinductor current to ow in the opposite

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Figure 4: Flyback converter drainvoltage with primary RCD clamp andRC snubber.