LYT2002-2005
A pre-load resistor R10 was employed to discharge the output
capacitor and extinguish the LED light immediately after turn-off. The
resistor will also keep the output from rising higher than the permitted
maximum output voltage (usually determined by the output capacitor
voltage rating) when the load is disconnected.
LYTSwitch-2 Layout Considerations
Circuit Board Layout
The LYTSwitch-2 family of ICs present a highly integrated power
supply solution that integrates, both, the controller and the high-
voltage power MOSFET onto a single die. The presence of high
switching currents and voltages together with analog signals makes
it especially important to follow good PCB design practice to ensure
stable and trouble free operation of the power supply. See Figures 5
and 6 for a recommended circuit board layout for LYTSwitch-2.
Output Regulation
The LYTSwitch-2 family regulates the output using ON/OFF control in
the constant voltage (CV) regulation region of the output characteris-
tic and frequency control for the constant current (CC) region. The
feedback resistors (R7 and R8) were selected using standard 1%
resistors to center both the nominal output voltage and constant
current regulation thresholds. Resistor R6 acts as filter to limit the
voltage spike (caused by the coupling of the bias winding to the
primary winding), improving regulation.
When designing a printed circuit board layout for the LYTSwitch-2 based
power supply, it is important to follow these guidelines:
Single Point Grounding
Use a single point (Kelvin) connection at the negative terminal of the
input filter capacitor for the LYTSwitch-2 SOURCE pin and bias
winding return. This improves surge capabilities by returning surge
currents from the bias winding directly to the input filter capacitor.
Key Application Considerations
Output Power Table
The data sheet maximum output power table (Table 1) represents the
maximum practical continuous output power that can be obtained
under the following assumed conditions:
Bypass Capacitor
The BYPASS pin capacitor should be located as close as possible to
the SOURCE and BYPASS pins for effective noise decoupling.
1. The minimum DC bus voltage is 100 V at 90 VAC input. The value
of the input capacitance should be made large enough to meet
this requirement for AC input designs – typically 2-3 mF/W for
low-line or universal input designs and 1-2 mF/W for high-line input
designs.
2. The secondary output rectifier diode should withstand peak
inverse voltage (PIV) for 55 V output voltage for open load
condition.
3. Assume efficiency of >80%.
4. Discontinuous mode operation (KP >1.3).
5. The LYTSwitch-2 part is either board mounted with SOURCE pins
soldered to a sufficient area of copper to keep the SOURCE pin
temperature at or below 100 °C, or (in the case of the E package)
attached to a sufficiently sized heat sink to limit device tempera-
ture to below 110 °C.
Feedback Resistors
Place the feedback resistors (R7 and R8) very close to the FEEDBACK
pin of the LYTSwitch-2 device. This minimizes noise coupling.
Thermal Considerations (D and K Package)
The copper area connected to the SOURCE pins provides heat sinking.
A good estimate of expected power dissipation is to assume is that
the LYTSwitch-2 will dissipate 5% of the output power. Provide
enough copper area to keep the SOURCE pin temperature below
100 °C. Higher temperatures are allowable but output current (CC)
tolerance will increase. In this case a maximum SOURCE pin tempera-
ture below 100 °C is recommended to provide margin for part-to-part
RDS(ON) variation.
Secondary Loop Area
To minimize leakage inductance and EMI the area of the loop
contained within the connections between the secondary winding
(T1), the output diode (D3) and the output filter capacitor (C6) should
be minimized. In addition, sufficient copper area should be to the
rectifier diode for heat sinking preferably connected to the quiet
cathode terminal. A large anode area can increase high frequency
radiated EMI.
6. Ambient temperature of less than 50 °C for open frame designs
and an internal enclosure temperature of 60 °C for enclosed
ballast-type designs.
Note: Higher output powers are achievable if an output CC tolerance
> ±10% is acceptable, and allowing the device to be operated at a
higher SOURCE pin temperature.
Output Tolerance
Electrostatic Discharge Spark Gap
LYTSwitch-2 K and E package parts provides an overall CC mode
output current tolerance of ±5% including line voltage, normal
board-to-board component variation and across a temperature range
of 0 °C to 110 °C. For the D package (SO-8) additional CC variance
may occur due to stress caused by manufacturing (i.e. solder-wave
immersion or IR reflow).
A trace is placed at one of the AC line inputs to form one electrode of
a spark gap. The other electrode on the secondary is formed by the
output return node. The spark gap directs most ESD energy from the
secondary back to the AC input during a surge event. The trace from
the AC input to the spark gap electrode should be spaced away from
other traces to prevent unwanted arcing occurring and possible circuit
damage. If R1 and R2 are removed additional spark gaps across the
EMI filter inductors (L1 and L2) to prevent excessive build-up of
voltage across them during surge.
A sample power supply build is recommended to verify production
tolerances for each design.
BYPASS Pin Capacitor Selection
A 1 mF BYPASS pin capacitor is recommended. The capacitor voltage
rating should be greater than 7 V. The capacitor can be ceramic or
electrolytic but tolerance of capacitor should be ≤ ±50%. The
capacitor must be physically located close to the LYTSwitch-2 BYPASS
pin for effective noise decoupling.
5
Rev. B 09/15
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