Rev. C 03/16

6

LYT5216-5228

www.power.com

Figure 6. DER-515 PCB Layout Example using SO-16B D Package. Shows the Location of Critical Components and Loop Areas on the PCB Layout.

To ensure that discontinuous conduction mode (DCM) operation of

LYTSwitch-5 is maintained over line input and inductance tolerance

variations, and to ensure for accurate output current regulation, it is

recommended that the LYTSwitch-5 PIXls spreadsheet located at

PI Expert web lab (http://piexpertweblab.power.com/site/login)

should be used for magnetics calculations.

EMI Considerations

Total input capacitance affects PF and ATHD – increasing the value

will degrade performance. With LYTSwitch-5, the combination of a

low-side switching configuration and frequency jitter reduces EMI and

enables the use of small and simple π (pi) filter. It also allows simple

magnetic construction where the main winding can be wound

continuously using the automated winding approach preferred for

low-cost manufacturing. The recommended location of the EMI filter

is after the bridge rectifier. This allows the use of regular film

capacitors as opposed to more expensive safety rated X capacitors

that would be required if the filter is placed before the bridge.

Surge Immunity Consideration

This design assumed a differential surge requirement of 3 kV which

can be met easily with LYTSwitch-5 line overvoltage protection and

using a RCD surge clamp circuit (D5, R2 and C3) and MOV (RV1). For

lower differential surge requirement such as 1 kV, capacitor C3 can

be reduced to 1 mF.

Thermal and Lifetime Considerations

Lighting applications present thermal challenges to the driver. In

many cases the LED load dissipation determines the working ambient

temperature experienced by the drive. Thermal evaluation should be

performed with the driver inside the final enclosure. Temperature

has a direct impact on driver and LED lifetime. For every 10 °C rise in

temperature, component life is reduced by a factor of 2. Therefore it

is important to verify and optimize the operating temperatures of all

components.

Quick Design Checklist

Maximum Drain Voltage

Verify that the peak drain voltage stress (VDS) does not exceed

maximum acceptable drain voltage under all operating conditions,

including start-up and fault conditions.

Maximum Drain Current

Measure the peak drain current under all operation conditions

(including start-up and fault conditions). Look for transformer

saturation (usually occurs at highest operating ambient tempera-

tures). Verify that the peak current is less than the stated Absolute

Maximum Rating in the data sheet.

Thermal Check

At maximum output power, for both minimum and maximum line

voltage and maximum ambient temperature; verify that temperature

specifications are not exceeded for the LYTSwitch-5, transformer,

output diodes, output capacitors and clamp components.

PCB Layout Considerations

The EMI filter components should be located close together to improve

filter effectiveness. Place the EMI filter components C1 and L1 as far

away as possible from any switching nodes on the circuit board

especially U1 drain node, output diode (D7) and the transformer (T1).

Care should be taken in placing the components on the layout that

are used for processing input signals for the feedback loop – any high

frequency noise coupled to the signal pins of U1 may affect proper

system operation. The critical components in DER-515 are R5, R9,

C6, R4, R7 and R8. It is highly recommended that these components

be placed very close to the pins of U1 (to minimize long traces which

could serve as antenna) and far away as much as possible from any

high voltage and high current nodes in the circuit board to avoid

noise coupling.

The bypass supply capacitor C5 should be placed directly across

BYPASS pin and SOURCE pin of U1 for effective noise decoupling.

As shown in Figure 6, minimize the loop areas of the following

switching circuit elements to lessen the creation of EMI.

•

Loop area formed by the transformer output winding (T1), output

rectifier diode (D7) and output capacitor (C8).

•

Loop area formed by transformer bias winding (T1), rectifier diode

(D6) and filter capacitor (C7).

•

Loop area formed by input capacitor (C2), transformer (T1) main

winding R10, C4 and internal MOSFET (U1).

Lastly, unlike discrete MOSFET designs where heat sinking is through

the drain tab and which generates significant EMI, the LYTSwitch-5

devices employ low-side switching and the ground potential SOURCE

pins are used for heat sinking. This allows the designer to maximize

the copper area for good thermal management but without having

the risk of increased EMI.

Design Tools

Up-to-date information on design tools can be found at the Power

Integrations web site: www.power.com

LYTSwitch-5 PIXls spreadsheet is located at PI Expert web lab:

http://piexpertweblab.power.com/site/login.

FEEDBACK Pin

Resistor R9 and C6

LINE SENSE Pin

Resistor R4

Drain Current

Sense R10 and C4

OUTPUT

COMPENSATION

Pin Resistor R5

BYPASS Pin

Capacitor C5

Bias Diode Rectifier D6

and Filter C7

Output

Capacitor

TRF

DRIVER CURRENT

SENSE Pin Resistor R7

DATA OUTPUT Pin

Resistor R8

Ouput Diode

Rectifier D7 and

Filter C8

PI-7795-120315