LT3799IMSE-1(RevA) Просмотр технического описания (PDF) - Linear Technology

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LT3799IMSE-1
(Rev.:RevA)

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

Operation

current (description to follow). The dv/dt detector waits

for the ringing waveform to reach its minimum value and

then the switch turns back on. This switching behavior is

similar to zero volt switching and minimizes the amount of

energy lost when the switch is turned back on, improving

efficiency as much as 5%. Since this part operates on the

edge of continuous conduction mode and discontinuous

conduction mode, this operating mode is called critical

conduction mode (or boundary conduction mode).

Primary-Side Current Control Loop

The CTRL1/CTRL2/CTRL3 pins control the output current

of the flyback controller. To simplify the loop, 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 an 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. 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 LT3799-1 calculates it using the informa-

tion 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 N is the primary

to secondary winding ratio. Instead of taking the area of

the triangle, 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 dur-

ing the rest of the cycle. The equation for expressing the

output current is:

IOUT = 0.5 • IPK • N • D´

where D´ is equal to the percentage of the cycle represented

by the flyback time.

SECONDARY

DIODE CURRENT

IPK(sec)

SWITCH

WAVEFORM

TFLYBACK

TPERIOD

37991 F01

Figure 1. Secondary Diode Current and Switch Waveforms

The LT3799-1 has access to both the primary winding

current, the input to the current comparator, and when

the flyback time starts and ends. Now the output current

can be calculated by averaging a PWM waveform with the

height of the current limit and the duty cycle of the flyback

time over the entire cycle. In the feedback loop previously

described, the input to the integrator is such a waveform.

The integrator adjusts the peak current until the 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.

10

For more information www.linear.com/LT3799-1

37991fa

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