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Snubber Circuit Design - Practical Tips - TI.com
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Snubber Circuit Design - Practical Tips
Snubbers are used to limit switching transients and help lower
EMI.
1. Measure the ring frequency at the top of the MOSFET switch
node (SW) turn-off waveform. Solder a good quality COG or f ilm
type capacitor from the MOSFET SW node to GND, causing the
ring frequency to be half of the original value. The effective
capacitance at this node is 1/3 of the external
capacitor value. The stray inductance is calculated
using:
where CT is the total (effective plus external)
capacitance.
2. In order to critically damp the ringing, double the size of
the external cap. Use a series R calculated from:
where d is the damping factor, set to a value of 1. CS is the
calculated effective capacitance at the SW node, not the external
snubber capacitance.
3. Install the snubber resistor in the ground side of the RC
circuit. If the ringing is not critically damped, the external
capacitor value is too low. However, increasing this value may
not be practical. Measure the positive and negative voltage spikes
across the snubber resistor, which will be highest at the
maximum input voltage. The power dissipated in the
resistor is calculated by:
where VP and VN represent the positive and negative voltage
spikes across the snubber resistor, and FSW is the switching
frequency. This could be
an unacceptably high amount of power dissipation. If so, a
compromise between efficiency and damping may be required.
For parallel damping, t he relationship is:
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8/21/2019 Snubber Circuit Design - Practical Tips - TI.com
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Snubber Circuit Design - Practical Tips - TI.com
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Buck Converter Snubber Circuit Design Example
Measure the switching signal at the MOSFET/inductor junction.
You will see a voltage spike and ringing at the top of this signal.
Let's say you measure a ring
period of 7 ns, which represents 143 MHz. Solder a
capacitor directly across the low side FET from the switch node to
ground. Let's say 330 pF causes the
period to double to 14 ns.
8/21/2019 Snubber Circuit Design - Practical Tips - TI.com
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Snubber Circuit Design - Practical Tips - TI.com
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Install RSNUB and CSNUB. Lets say at 40V input you measure Vpos
= 40V and Vneg = 20V across the resistor. For 150 kHz
operation:
For DC/DC converter ICs, visit the Texas Instruments Power
Management website.
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