RC Snubber for Class-D Audio Amplifiers
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AN_0454 RC Snubber for Class-D Audio Amplifiers INTRODUCTION High speed switching of power MOSFETs in the power stage of Class-D amplifiers results in output voltage over/undershoot and high frequency ringing on the output waveform, as shown in Figure 1. Figure 1 Class-D output over/undershoot and ringing The over/undershoot places additional stress on the power MOSFETs, potentially reducing MOSFET lifetime or, in extreme cases, causing avalanche breakdown. Copyright  Cirrus Logic, Inc., 2018 (All Rights Reserved)

AN_0454 The high frequency ringing can couple to nearby PCB planes and cables and is therefore a source of radiated emissions (see Reference 1). Radiated emissions can be particularly problematic in portable electronics where the Class-D amplifier may be some distance from the micro speaker and connected by a relatively long unshielded twisted pair or flex cable. An LC filter on the output of the Class-D amplifier can be used to attenuate the ringing, reducing radiated emissions. However, this does not reduce the voltage stress on the power MOSFETs. This application note describes the cause...

AN_0454 RC SNUBBER DESIGN Ringing can be almost eliminated (snubbed) by an RC network on the output of the Class-D amplifier as shown in Figure 3. RSN and CSN damp the voltage over/undershoot and ringing. Figure 3 Class-D power stage with RC snubber Adding an RC snubber damps out ringing, at the expense of increased switching losses. The snubber should be connected directly across the drain and source of the n-channel power MOSFET. The values for the power MOSFET output capacitance and IC package inductance are not always available from IC vendors. Similarly, capacitor package ESL is not...

Once COSS is known Lp can be calculated from Equation 1: Ұݐ߰ݑ = CALCULATING SNUBBER RESISTOR VALUE Once the parasitic capacitance and inductance are known, the snubber damping resistor value can be calculated. The damping factor of a parallel RLC circuit is given by: ưݜ= pݐ߰ݑ 1 ݢȚ 2ݑ Űݑưݑ pݐ ְݑ°ݑưݑ A damping factor of 1 (critically damped) is chosen for fastest possible rise time with no overshoot (see Reference 5). So, rearranging Equation 5 for RSN: ưݐ߰ݑ 1 ݰݑ Űݑưݑ = bȚ 2 ݐ ְݑ°ݑưݑ SNUBBER CAPACITOR VALUE A good choice of snubber capacitor, CSN, for effective damping and to minimize...

Figure 5 CS35L41 output voltage overshoot and ringing with 1 nF external capacitor The n-channel power MOSFET output capacitance is calculated as shown in Equation 2: ݐ ְݑ°ݑưݑ = Total parasitic inductance is then calculated using Equation 4: ٰݐ߰ݑ = Snubber resistor is calculated using Equation 6: 1 9.99ٰݑ۰ݐ ۰ݑ Űݑưݑ = bȚ = 3.45 Ω 2 205.45ݑݰݐ Preferred component value 3٢Ħ3 is chosen for snubber resistor and 560 pF for snubber capacitor (approximately 3 x COSS). For best performance, a C0G/NP0 ceramic capacitor is recommended. Average power consumption of the snubber resistor is calculated...

AN_0454 The common-mode spectrum of the PCB traces connecting Class-D amplifier CS35L41 to an 8 Ω, 33 µH load is measured in accordance with IEC 61967-4 (see Reference 7) using Rohde & Schwarz ESRP7 EMI test receiver. Figure 7 shows that with the RC snubber there is approximately 12 dBm attenuation at the ringing frequency. Figure 7 CS35L41 common-mode spectrum CONCLUSIONS Voltage over/undershoot and ringing on the outputs of Class-D amplifiers can cause additional stress to the power MOSFETs and lead to unacceptable radiated emissions. An LC filter on the outputs attenuates radiated...

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