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P LOSS ( TRAN ) =
V IN × I LED × F SW
I DRV
V DRV
I DRV =
Micrel, Inc.
The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
× ( Q gs 2 + Q gd )
R GATE
where:
R GATE is total MOSFET resistance, Q gs2 and Q gd can be
found in a MOSFET manufacturer datasheet.
The total power loss is:
P LOSS ( TOT ) = P LOSS ( CON ) + P LOSS ( TRAN )
The MOSFET junction temperature is given by:
MAQ3203
Proper snubber design requires the parasitic inductance and
capacitance be known. A method of determining these
values and calculating the damping resistor value 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 f 1 .
2. Add a capacitor C S (normally at least 3 times as big as
the C OSS of the diode) across the diode and measure the
new ringing frequency. Define this new (lower) frequency
as f 2 . L P and C P can now be solved using the values of
f 1 , f 2 and C S .
3. Add a resistor R S in series with C S to generate critical
damping. If the snubber resistance is equal to the
characteristic impedance of the resonant circuit
(1/sqrt(L P C P )), the resonant circuit will be critically
damped and have no ringing.
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:
T J = P LOSS ( TOT ) × R θ JA + T A
The T J must not exceed maximum junction temperature
under any conditions.
f 1 =
1
2 π L P × C P
Snubber
A RC voltage snubber is used to damp out high
frequency ringing on the switch node caused by parasitic
inductance and capacitance. The capacitor is used to
slow down the switch node rise and fall time and the
resistor damps the ringing. Excessive ringing can cause
the MAQ3203 to operate erratically by prematurely
tripping its current limit comparator circuitry.
w here:
C P and L P are the parasitic capacitance and inductance.
Step 2: Add a capacitor, C S , in parallel with the Schottky
diode. The capacitor value should be approximately 3 times
the C OSS of D1. Measure the frequency of the switch node
ringing, f 2.
The snubber is connected across the Schottky diode as
shown in the evaluation board schematic. Capacitor C S
(C4) is used to block the DC voltage across the resistor,
minimizing the power dissipation in the resistor. This
f 2 =
1
2 π L P × (C S + C P )
capacitor value should be between two to five times the
parasitic capacitance of the MOSFET C OSS and the
Schottky diode junction capacitance C j . 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.
Define f’ as:
f' =
f 1
f 2
The snubber components should be placed as close as
possible to the Schottky diode. Placing the snubber too
far from the diode or using an etch that is too long or too
Combining the equations for f 1 , f 2 and f’ to derive C P , the
parasitic capacitance:
? C P =
?
C S
?
2 × (f ) ? 1 ? ?
thin adds inductance to the snubber and diminishes its
effectiveness.
March 2011
13
?
?
, 2
?
M9999-032411-A
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