LYT5216-5228
reduction, this can be replaced by a fusible resistor (typically a flame
proof wire-wound type) which would need to be rated to withstand
the instantaneous dissipation induced when charging the input
capacitance when first connected to the input line.
It is important to note that for accurate output current regulation the
use of 1% tolerance for LINE SENSE pin resistors (R3 and R4) is
recommended. This recommendation also applies to OUTPUT
COMPENSATION pin resistor R5, FEEDBACK pin resistor R9 (capacitor
C6 at least X7R type), and DRIVER CURRENT SENSE pin resistor R7
and R10.
Selection of fuse (F1) type and rating is dependent on input surge
requirements. Typical minimum requirement for tube application is
500 V differential surges. This design meets a 3 kV surge specifica-
tion, so a 5-ampere slow blow fuse was used. A fast-blow fuse with
a high ampere energy (I2T) rating could also be used.
Diode D6 and C7 provides a bias supply for U1 from an auxiliary
winding on the transformer. Bias supply recommended voltage level
is 12 V. Filter capacitor C7 should be sized to ensure a low ripple
voltage. Capacitor C5 serves as local decoupling for the BYPASS pin
of U1 which is the supply pin for the internal controller. Current via
R6 is typically limited to 1 mA. During start-up, C5 is charged to
~5.25 V from an internal high-voltage current source internally fed
from the DRAIN pin. This allows U1 to start switching. After start-up
the operating supply current is provided from the bias supply via R6.
The recommended value for the BYPASS pin capacitor C5 is 1 mF.
The voltage rating for the capacitor should be greater than 7 V. The
capacitor can be a ceramic or electrolytic type, but tolerance should
be less than 50%. The capacitor must be physically located close to
BYPASS and SOURCE pins for effective noise decoupling.
An optional RCD surge clamp circuit (D5, R2 and C3) can be employed
for differential surge voltage requirement of 3 kV. Capacitor C3 can
also be increased to help meet a higher surge voltage requirement.
Typical Line Surge
Input Range
90 to 264 VAC
90 to 308 VAC
Protection
Requirement
500 V Differential Surge /
2.5 kV Ring Wave
275 VAC MOV
1 kV to 3 kV Differential
Surge / 2.5 kV Ring Wave
320 VAC MOV,
RCD Clamp
Output Rectification
During the switching off-state the output from the transformer main
winding is rectified by D7 and filtered by C8. An ultrafast 1 A, 600 V
with 35 ns reverse recovery time (trr) diode was selected for efficiency.
The value of the output capacitor C8 was selected to give peak-to-
peak LED ripple current equal to 30% of the mean value. For designs
where lower ripple is desirable the output capacitance value can be
increased unlike traditional power supplies, low ESR capacitors are
not required for the output stage of LED designs.
Table 3. Recommended Surge Protection.
LYTSwitch-5 Output Regulation
In order to maintain very accurate output current regulation – within
±3%, the FEEDBACK (FB) pin voltage (with an appropriately selected
low-pass filter comprising R9 and C6) is compared to a preset
average feedback voltage (VFB) of 300 mV. When the detected signal
is above or below the preset average VFB threshold voltage, the
onboard averaging-engine will adjust the frequency and/or on-time to
maintain regulation.
A small output pre-load resistor R11 discharges the output capacitor
when the driver is turned off, giving a relatively quick and smooth
decay of the LED light. Recommended pre-load power dissipation is
≤0.5% of the output power.
The bias winding voltage is proportional to the output voltage
(controlled by the turns-ratio between the bias supply and output-
main winding). This allows the output voltage to be monitored
without the need for output-side feedback components. Resistor R5
converts the bias voltage into a current which is fed into the OUTPUT
COMPENSATION (OC) pin of U1. The OUTPUT COMPENSATION pin
current is also used to detect output overvoltage which is set to 30%
above the nominal output voltage. Once the current exceeds the
ILOV+ threshold the IC will trigger a latch which disables switching
which prevents the output from rising further. An AC recycle is
needed to reset this protection mode once triggered.
Key Design Considerations
Device Selection
The data sheet power table (Table 1) represents the maximum
practical continuous output power that can be delivered in an open
frame design with adequate heat sinking.
Output Power Table
Output Power1
Product2
In order to provide line input voltage information to U1 the rectified
input AC voltage is fed into the LINE SENSE (L) pin of U1 as a current
via R3 and R4 (4 MW total resistance). This sensed current is also
used by U1 to detect input zero-crossing and set the input line
overvoltage protection threshold. In a line overvoltage condition
once this current exceeds the IOOV threshold, the IC will instanta-
neously disable switching to protect the MOSFET from further voltage
stress. The IC will start switching as soon the line voltage drops to
safe levels indicated by the L pin current dropping by 5 mA.
90-308 VAC
LYT5225D
9 W
16 W
25 W
LYT5216D, LYT5226D
LYT5218D, LYT5228D
Table 4. Output Power Table.
DER-515 is a 12 W driver. The LYT5226D IC was chosen for its higher
voltage MOSFET rating of 725 V because the topology chosen was a
buck-boost and the specification called for a maximum input voltage
of 308 VAC. In other applications where surge and line voltage
conditions allow, it may be possible to use the 650 V MOSFET option
to reduce design cost without impacting reliability.
The primary switched current is sensed via R10 and filtered with C4.
The signal is fed into the DRIVER CURRENT SENSE (DS) pin. A low ESR
ceramic capacitor of at least 10 mF is recommended for capacitor C4.
The internal frequency/on-time engine inside LYTSwitch-5 combines
the OUTPUT COMPENSATION pin current, the LINE SENSE pin current
and the DRIVER CURRENT SENSE pin current information to deduce
the FB signal. This is compared to an internal VFB threshold to
maintain accurate constant output current.
Magnetics Design
The core type selected was a low profile EFD15 with ferrite core
material and a wide winding window that allowed better convection
cooling for the winding.
5
Rev. C 03/16
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