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LT3799EMSE-1-PBF

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

OPERATION

Universal Input

The LT3799-1 easily operates over the universal input

range of 90VAC to 265VAC, but is not limited to this range.

Applications with input voltages above 500VAC can be

implemented with the LT3799-1. Output current regulation

error may be minimized by using two application circuits

for the wide input range: one optimized for 120VAC and

another optimized for 220VAC. The first application pictured

in the Typical Applications section shows three options:

universal input, 120VAC, and 220VAC. The circuit varies by

three resistors. In the Typical Performance Characteristics

section, the LED Current vs VIN graphs show the output

current line regulation for all three circuits.

Selecting Winding Turns Ratio

Boundary mode operation gives a lot of freedom in selecting

the turns ratio of the transformer. We suggest to keep the

duty cycle low, lower NPS, at the maximum input voltage

since the duty cycle will increase when the AC waveform is

decreases to zero volts. A higher NPS increases the output

current while keeping the primary current limit constant.

Although this seems to be a good idea, it comes at the

expense of a higher RMS current for the secondary-side

diode which might not be desirable because of the primary

side MOSFET’s superior performance as a switch. A higher

NPS does reduce the voltage stress on the secondary-side

diode while increasing the voltage stress on the primary-

side MOSFET. If switching frequency at full output load is

kept constant, the amount of energy delivered per cycle by

the transformer also stays constant regardless of the NPS.

Therefore, the size of the transformer remains the same at

practical NPS’s. Adjusting the turns ratio is a good way to

find an optimal MOSFET and diode for a given application.

Switch Voltage Clamp Requirement

Leakage inductance of an offline transformer is high due

to the extra isolation requirement. The leakage inductance

energy is not coupled to the secondary and goes into

the drain node of the MOSFET. This is problematic since

400V and higher rated MOSFETs cannot always handle

this energy by avalanching. Therefore the MOSFET needs

protection. A transient voltage suppressor (TVS) and

diode are recommended for all offline application and

connected, as shown in Figure 3. The TVS device needs

a reverse breakdown voltage greater than (VOUT + Vf)*N

where VOUT is the output voltage of the flyback converter,

Vf is the secondary diode forward voltage, and N is the

turns ratio.

VSUPPLY

GATE

37991 F03

Figure 3. Clamp

Transformer Design Considerations

Transformer specification and design is a critical part of

successfully applying the LT3799-1. In addition to the

usual list of caveats dealing with high frequency isolated

power supply transformer design, the following informa-

tion should be carefully considered. Since the current on

the secondary side of the transformer is inferred by the

current sampled on the primary, the transformer turns

ratio must be tightly controlled to ensure a consistent

output current.

A tolerance of ±5% in turns ratio from transformer to

transformer could result in a variation of more than ±5% in

output regulation. Fortunately, most magnetic component

manufacturers are capable of guaranteeing a turns ratio

tolerance of 1% or better. Linear Technology has worked

with several leading magnetic component manufacturers

to produce predesigned flyback transformers for use with

the LT3799-1. Table 1 shows the details of several of these

transformers.

37991f

14

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