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LT3799EMSE

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

OPERATION

not solve the problem completely for other supply voltages.

Another systematic error is that the current comparator

cannot instantaneously turn off the main power device.

This delay time leads to primary current overshoot. This

overshoot is less of a problem when the output current is

close to its maximum, since the overshoot is only related

to the slope of the primary current and not the current

level. The overshoot is proportional to the supply voltage,

so again this affects the line regulation.

Universal Input

The LT3799 operates over the universal input range of

90VAC to 265VAC. 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 4. 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

3799 F04

Figure 4. Clamp

3799p

14

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