MP4032-1—PRIMARY-SIDE-CONTROLLED, OFFLINE LED DRIVER WITH FULLY-INTEGRATED MOSFET
MP4032-1 Rev. 1.02 www.MonolithicPower.com 16
7/15/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
RIPPLE SUPPRESSOR
(Innovative Proprietary)
For dimming LED lighting application, a single
stage PFC converter needs large output
capacitor to reduce the ripple whose frequency is
double of the Grid. And in deep dimming situation,
the LED would have shimmer caused by the
dimming on duty which is not all the same in
every line cycle. What’s more, the Grid has noise
or inrush which would bring out shimmer even
flicker. Figure 9 shows a ripple suppressor, which
can shrink the LED current ripple obviously.
+
+
N
S
D
O
D
D
Z
C
O
C
R
R
O
M
Figure9: Ripple Suppressor
Principle
Shown in Figure 9, Resister R, capacitor C, and
MOSFET M compose the ripple suppressor.
Through the RC filter, C gets the mean value of
the output voltage V
Co
to drive the MOSFET M. M
works in variable resistance area. C’s voltage V
C
is steady makes the LEDs voltage is steady, so
the LEDs current will be smooth. MOSFET M
holds the ripple voltage v
Co
of the output.
Diode D and Zener diode D
Z
are used to restrain
the overshoot at start-up. In the start-up process,
through D and D
Z
, C is charged up quickly to turn
on M, so the LED current can be built quickly.
When V
C
rising up to about the steady value, D
and D
Z
turn off, and C combines R as the filter to
get the mean voltage drop of V
Co
.
The most important parameter of MOSFET M is
the threshold voltage V
th
which decides the
power loss of the ripple suppressor. Lower V
th
is
better if the MOSFET can work in variable
resistance area. The BV of the MOSFET can be
selected as double as V
Co
and the Continues
Drain current level can be selected as decuple as
the LEDs’ current at least.
About the RC filter, it can be selected by
RC LineCycle
50 / fτ≥ . Diode D can select 1N4148,
and the Zener voltage of D
Z
is as small as
possible when guarantee
O
DDZ C_PP
VV 0.5V+>⋅ .
Optional Protection Circuit
In large output voltage or large LEDs current
application, MOSFET M may be destroyed by
over-voltage or over-current when LED+ shorted
to LED- at working.
Gate-Source(GS) Over-voltage Protection
+
+
N
S
D
O
D
D
Z
C
O
C
D
G
R
R
O
M
R
G
Figure 10: Gate-Source OVP Circuit
Figure 10 shows GS over-voltage protection
circuit. Zener diode D
G
and resistor R
G
are used
to protect MOSFET M from GS over-voltage
damaged. When LED+ shorted to LED- at normal
operation, the voltage drop on capacitor C is high,
and the voltage drop on Gate-Source is the same
as capacitor C. The Zener diode D
G
limits the
voltage V
GS
and R
G
limits the charging current to
protect D
G
. R
G
also can limit the current of D
Z
at
the moment when LED+ shorted to LED-. V
DG
should bigger than V
th
.
Drain-Source Over-voltage and Over-current
Protection
As Figure 11 shows, NPN transistor T, resistor
R
C
and R
E
are set up to protect MOSFET M from
over-current damaged when output short occurs
at normal operation. When LED+ shorted to LED-,
the voltage v
DS
of MOSFET is equal to the v
Co
which has a high surge caused by the parasitic
parameter. Zener Dioder D
DS
protects MOSFET
from over-voltage damaged. Transistor T is used
to pull down the V
GS
of M. When M turns off, the
load is opened, then the OVP mode is triggered,
