MIC24420 Datasheet, PDF (18/34 Pages) Micrel Semiconductor – 2.5A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
E.G. Where R_CS = 250Ω, C_CS = 82pF
Snubber
A snubber is used to damp out high frequency ringing
caused by parasitic inductance and capacitance in the
buck converter circuit. Figure 10 shows a simplified
schematic of one of the buck converter phases. Stray
capacitance consists mostly of the output capacitance
(C_OSS) of the two MOSFET’s. The stray inductance is
mostly package and etch inductance. The arrows show
the resonant current path when the high-side MOSFET
turns on. This ringing causes stress on the
semiconductors in the circuit as well as increased EMI.
Figure 10. Output Parasitics
One method of reducing the ringing is to use a resistor to
lower the Q of the resonant circuit. The circuit in Figure
11 shows an RC network connected between the switch
node and ground. Capacitor C_S is used to block DC and
minimize the power dissipation in the resistor. This
capacitor value should be between 5 and 10 times the
parasitic capacitance of the MOSFET C_OSS. A capacitor
that is too small will have high impedance and prevent
the resistor from damping the ringing. A capacitor that is
too large causes unnecessary power dissipation in the
resistor, which lowers efficiency.
The snubber components should be placed as close as
possible to the low-side MOSFET and/or external
Schottky diode since it contributes to most of the stray
capacitance. Placing the snubber too far from the
MOSFET or using traces that are too long or too thin
adds inductance to the snubber and diminishes its
effectiveness.
Proper snubber design requires the parasitic inductance
and capacitance be known. A method of determining
these values and calculating the damping resistor value
MIC24420/MIC24421
is outlined below.
1. Measure the ringing frequency at the switch node
which is determined by parasitic L_P and C_P. Define this
frequency as f₁.
2. Add a capacitor C_S (normally at least 3 times as big as
the C_OSS of the FET) from the switch node to ground and
measure the new ringing frequency. Define this new
(lower) frequency as f₂. L_P and C_P can now be solved
using the values of f₁, f₂ and C_S.
3. Add a resistor R_S in series with C_S to generate critical
damping.
Step 1: First measure the ringing frequency on the
switch node voltage when the high-side MOSFET turns
on. This ringing is characterized by the equation:
f₁ =
1
2π L_P ⋅ C_P
Where:
C_P and L_P are the parasitic capacitance and inductance
Step 2: Add a capacitor, C_S, in parallel with the
synchronous MOSFET, Q2. The capacitor value should
be approximately 3 times the C_OSS of Q2. Measure the
frequency of the switch node ringing, f_2.
f₂ = _2π
1
L_P ⋅ (C_S + C_P )
Define f’ as:
f' = ^f1
f₂
Combining the equations for f₁, f₂ and f’ to derive C_P, the
parasitic capacitance
C_P
=
C_S
2 ⋅ (f ^' )²
−1
L_P is solved by re-arranging the equation for f₁.
L_P
=
(2π)²
1
⋅ C_P
⋅ (f₁)²
Step 3: Calculate the damping resistor.
Critical damping occurs at Q=1
Q= ¹
R_S
Solving for R_S
L_{P = 1}
C_S + C_P
R_S =
L_P
C_S + C_P
Figure 11 shows the snubber in the circuit and the
June 2012
18
M9999-062012-C