LYT3314-3328
LYTSwitch-3 Output Regulation
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.
In order to maintain very tight output current regulation – within
±3%, the FEEDBACK (FB) pin voltage (with an appropriately selected
low-pass filter comprising R15 and C8) 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.
Output Rectification
During the switching OFF-state the output from the transformer main
winding is rectified by D3 and filtered by C10. An ultrafast 1 A, 600 V
with 35 ns reverse recovery time (tRR) diode was selected for efficiency.
The value of the output capacitor C10 was selected to give peak-to-
peak LED ripple current equal to 30% of the mean value. However,
the output ripple current will also depend on the type and impedance
characteristic of the LED load, so it is recommended to, use the
actual LED load for sizing the capacitor value for the output ripple
current. 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.
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 R16
in Figure 7 converts the bias voltage into a current which is fed into
the OUTPUT COMPENSATION (OC) pin of U1. The OUTPUT COMPEN-
SATION pin current is also used to detect output overvoltage which is
set to 30% above the nominal output voltage. Once the current
exceeds the IOOV 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.
A small output pre-load resistor R19 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.
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 R20 and R18. This sensed current is also used by U1 to detect
the input zero crossing, type of dimmer (i.e. leading or trailing edge)
connected to the input and set the input line overvoltage protection
threshold. In a line overvoltage condition once this current exceeds
the ILOV+ threshold, the IC will instantaneously disable switching to
protect the power MOSFET from further voltage stress. The IC will
start switching as soon as the line voltage drops to safe levels
indicated by the LINE SENSE pin current dropping by 5 µA.
Phase-Cut Dimming
The biggest challenge in designing dimmable LED bulb is high
compatibility with a broad range of dimmer types and power rating.
As different type of dimmers have different minimum loading
requirements the dimmable LED bulb may manifest varying incompat-
ibility behavior depending on the dimming conditions from light
flickering or shimmering, ghosting, huge pop-on to low dim ratio.
There are two main types of phase-cut dimmers namely leading edge
(Figure 8) and trailing edge (Figure 9). Each type has its own
characteristic and nuances that particularly makes it challenging for
LED driver to achieve high compatibility and no one can ever know
what type of dimmer an LED bulb will be used with therefore it is
imperative that the designer must use a controller with bleeder with
the capability to satisfy the requirement depending the type of the
dimmer.
The primary switched current is sensed via R12 and filtered with C6.
The signal is fed into the DRIVER CURRENT SENSE (DS) pin. A low
ESR ceramic capacitor of at least 10 µF is recommended for capacitor
C6 for better regulation and reduced the AC RMS loss across R6. The
DRIVER CURRENT SENSE pin program resistor R13 is 6.04 kW 1% for
primary-side regulation for indirectly sensing of the output current.
The internal frequency/on-time engine inside the LYTSwitch-3 IC
combines the OUTPUT COMPENSATION pin current, the LINE SENSE
pin current and the DRIVER CURRENT SENSE pin current information
to deduce the FEEDBACK pin signal. This is compared to an internal
VFB threshold to maintain accurate constant output current.
The requirement to provide flicker-free output dimming with low-cost,
TRIAC-based, leading edge phase dimmers introduces a number of
trade-off in the design. Due to the much lower power consumed by
LED based lighting the current drawn by the overall lamp is below the
holding current of the TRIAC within the dimmer. This causes
undesirable behaviors such as limited dimming range and/or flickering.
The relatively large impedance the LED lamp presents to the line
allows significant ringing to occur due to the inrush current charging
the input capacitance when the TRIAC turns on. This too can cause
similar undesirable behavior as the ringing may cause the TRIAC
current to fall to zero and turn-off.
It is important to note that for accurate output current regulation the
use of 1% tolerance for LINE SENSE pin resistors (R20 and R18) is
recommended. This recommendation also applies to OUTPUT
COMPENSATION pin resistor R16, FEEDBACK pin resistor R15
(capacitor C8 at least X7R type), and DRIVER CURRENT SENSE pin
resistor R12 and R13.
Diode D2 and C11 provides a bias supply for U1 from an auxiliary
winding on the transformer. Bias supply recommended voltage level
is 20 V, when this voltage drops at low conduction angle during
dimming would be high enough to maintain supply for U1. Filter
capacitor C11 should be sized to ensure a low ripple voltage.
Capacitor C7 serves as local decoupling for the BYPASS pin of U1
which is the supply pin for the internal controller. Current via R14 is
typically limited to 2.5 mA. During start-up, C7 is charged to ~5.3 V
from an internal high-voltage current source internally fed from the
DRAIN pin. This allows U1 to start switching even at low conduction
angle when in dimming. After start-up the operating supply current is
provided from the bias supply via R14. The recommended value for
the BYPASS pin capacitor C7 is 22 µF. The voltage rating for the
Figure 9 shows the line voltage and current at the input of the power
supply with a trailing edge dimmer. In this example, the dimmer
conducts at 90 degrees. Many of these dimmers use back-to-back
connected power FETs rather than a TRIAC to control the load. This
eliminates the holding current issue of TRIACs and since the
conduction begins at the zero crossing, high current surges and line
ringing are minimized. Typically these types of dimmers do not
require damping circuits. However, would require a bleeder circuit to
provide a low impedance path for the internal supply to recharge and
reset its internal controller in order to operate normally and avoid
misfiring for the succeeding cycle of the incoming input.
6
Rev. D 04/16
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