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LT3798EMSE

Isolated No Opto-Coupler Flyback Controller with Active PFC

Linear Technology 

LT3798

OPERATION

to the input capacitor. This is a second-order network

composed of the parasitic capacitance on the switch node

and the magnetizing inductance of the primary winding

of the transformer. The minimum voltage of the switch

node during this discontinuous ring is VIN – VOUT • NPS.

The LT3798 turns the switch back on at this time, during

the discontinuous switch waveform, by sensing when

the slope of the switch waveform goes from negative to

positive using the dv/dt detector. This switching technique

may increase efficiency by 5%.

Sense Resistor Selection

The resistor, RSENSE, between the source of the external

N-channel MOSFET and GND should be selected to provide

an adequate switch current to drive the application without

exceeding the current limit threshold.

For applications without power factor correction, select a

resistor according to:

RSENSE

=

2(1– D)NPS

IOUT • 42

• 95%

where

D = VOUT • NPS

VOUT • NPS + VIN

For applications with power factor correction, select a

resistor according to:

RSENSE

=

2(1– D)NPS

IOUT • 42

• 47.5%

where

D = VOUT • NPS

VOUT • NPS + VIN

Minimum Current Limit

The LT3798 features a minimum current limit of approxi-

mately 18% of the peak current limit. This is necessary

when operating in critical conduction mode since low

current limits would increase the operating frequency to a

very high frequency. The output voltage sensing circuitry

needs a minimum amount of flyback waveform time to

sense the output voltage on the third winding. The time

needed is 350ns. The minimum current limit allows the

use of smaller transformers since the magnetizing primary

inductance does not need to be as high to allow proper

time to sample the output voltage information.

To help improve crossover distortion of the line input

current, a second minimum current limit of 6% becomes

active when the VIN_SENSE current is lower than 27µA. Since

the off-time becomes very short with this lower minimum

current limit, the sample-and-hold is deactivated.

Universal Input

The LT3798 operates over the universal input voltage

range of 90VAC to 265VAC. In the Typical Performance

Characteristics section, the Output Voltage vs VIN and the

Output Current vs VIN graphs, show the output voltage

and output current line regulation for the first application

picture in the Typical Applications section.

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

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.

3798fa

12

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