lowing equation. The MOSFET must be mounted on a
board as per manufacturer specifications to dissipate
the heat.
The RMS current rating of the switching MOSFET Q1 is
calculated as follows for boost and buck-boost configu-
rations:
where ID
RMS
is the MOSFET Q1’s drain RMS current in
amperes.
The MOSFET Q1 will dissipate power due to both
switching losses as well as conduction losses. The con-
duction losses in the MOSFET is calculated as follows:
where R
DSON
is the on-resistance of Q1 in ohms with
an assumed junction temperature of +100°C, P
COND
is
in watts, and IL
AVG
is in amperes.
Use the following equations to calculate the switching
losses in the MOSFET:
Boost configuration:
Buck-boost configuration:
where IG
ON
and IG
OFF
are the gate currents of the
MOSFET Q1 in amperes when it is turned on and
turned off, respectively, V
LED
and V
INMAX
are in volts,
IL
AVG
is in amperes, f
SW
is in hertz, and C
GD
is the
gate-to-drain MOSFET capacitance in farads.
Choose a MOSFET that has a higher power rating than
that calculated by the following equation when the
MOSFET case temperature is at +70°C:
Rectifier Diode
Use a Schottky diode as the rectifier (D1) for fast
switching and to reduce power dissipation. The select-
ed Schottky diode must have a voltage rating 20%
above the maximum converter output voltage. The max-
imum converter output voltage is V
LED
in boost configu-
ration and V
LED
+ V
INMAX
in buck-boost configuration.
The current rating of the diode should be greater than
I
D
in the following equation:
Dimming MOSFET
Select a dimming MOSFET (Q2) with continuous current
rating at +70°C, higher than the LED current by 30%.
The drain-to-source voltage rating of the dimming
MOSFET must be higher than V
LED
by 20%.
Feedback Compensation
The LED current control loop comprising of the switch-
ing converter, the LED current amplifier, and the error
amplifier should be compensated for stable control of
the LED current. The switching converter small-signal
transfer function has a right half-plane (RHP) zero for
both boost and buck-boost configurations as the induc-
tor current is in continuous conduction mode. The RHP
zero adds a 20dB/decade gain together with a 90°
phase lag, which is difficult to compensate. The easiest
way to avoid this zero is to roll off the loop gain to 0dB
at a frequency less than one-fifth of the RHP zero fre-
quency with a -20dB/decade slope.
The worst-case RHP zero frequency (f
ZRHP
) is calculat-
ed as follows:
Boost configuration:
Buck-boost configuration:
where f
ZRHP
is in hertz, V
LED
is in volts, L is the induc-
tance value of L1 in henries (H), and I
LED
is in amperes.
The switching converter small-signal transfer function
also has an output pole for both boost and buck-boost
configurations. The effective output impedance that
determines the output pole frequency together with the
output filter capacitance is calculated as:
(27 Seiten)
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