LT3798IMSE-PBF 查看數據表（PDF） - Linear Technology

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LT3798IMSE-PBF

Isolated No Opto-Coupler Flyback Controller with Active PFC

Linear Technology 

LT3798

OPERATION

Primary Side Current Control Loop

The CTRL1/CTRL2/CTRL3 pins control the output current

of the flyback controller. To simplify the loop, let’s assume

the VIN_SENSE pin is held at a constant voltage above 1V

eliminating the multiplier from the control loop. The error

amplifier, A5, is configured as integrator with the external

capacitor C6. The COMP+ node voltage is converted to a

current into the multiplier with the V/I converter, A6. Since

A7’s output is constant, the output of the multiplier is

proportional to A6 and can be ignored. The output of the

multiplier controls the peak current with its connection to

the current comparator, A1. The output of the multiplier is

also connected to the transmission gate, SW1, and to a

1M resistor. The transmission gate, SW1, turns on when

the secondary current flows to the output capacitor. This

is called the flyback period when the output diode D1 is

on. The current through the 1M resistor gets integrated by

A5. The lowest CTRL input is equal to the negative input

of A5 in steady state.

A current output regulator normally uses a sense resistor

in series with the output current and uses a feedback loop

to control the peak current of the switching converter. In

this isolated case, the output current information is not

available so instead the LT3798 calculates it using the in-

formation available on the primary side of the transformer.

The output current may be calculated by taking the average

of the output diode current. As shown in Figure 1, the diode

current is a triangle waveform with a base of the flyback

time and a height of the peak secondary winding current.

In a flyback topology, the secondary winding current is N

times the primary winding current, where NPS is the primary

to secondary winding ratio. Instead of taking the area of

the triangle, let’s think of it as a pulse width modulation

(PWM) waveform. During the flyback time, the average

current is half the peak secondary winding current and

zero during the rest of the cycle. The equation to express

the output current is:

IOUT = 0.5 • IPK • NPS • D

where D is equal to the percentage of the cycle that the

flyback time represents. The LT3798 has access to the

primary winding current, the input to the current com-

parator, and when the flyback time starts and ends. Now

the output current can be calculated by averaging a PWM

SECONDARY

DIODE CURRENT

IPK(sec)

SWITCH

WAVEFORM

TFLYBACK

TPERIOD

3798 F01

Figure 1. Secondary Diode Current and Switch Waveforms

waveform with a height of the current limit and a duty cycle

of the flyback time over the entire cycle. In the feedback

loop described above, the input to the integrator is such

a waveform. The integrator adjusts the peak current until

calculated output current equals the control voltage. If the

calculated output current is low compared to the control

pin, the error amplifier increases the voltage on the COMP+

node thus increasing the current comparator input.

Primary Side Voltage Control

The output voltage is available through the third winding on

the primary side. A resistor divider attenuates the output

voltage for the voltage error amplifier. A sample-and-hold

circuit samples the attenuated output voltage and feeds

it to the error amplifier. The output of the error amplifier

is the V pin. This node needs a capacitor to compensate

the output voltage control loop.

Power Factor Correction

When the VIN_SENSE voltage is connected to a resistor divider

of the supply voltage, the current limit is proportional to

the supply voltage. The minimum of the two error ampli-

fier outputs is multiplied with the VIN_SENSE pin voltage. If

the LT3798 is configured with a fast control loop, slower

changes from the VIN_SENSE pin would not interfere with

the current limit or the output current. The COMP+ pin

would adjust to the changes of the VIN_SENSE. The only

way for the multiplier to function is to set the control loop

to be an order of magnitude slower than the fundamental

frequency of the VIN_SENSE signal. In an offline case, the

3798fa

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