LT3575 Просмотр технического описания (PDF) - Linear Technology

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LT3575

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3575

APPLICATIONS INFORMATION

ERROR AMPLIFIER—PSEUDO DC THEORY

In the Block Diagram, the RREF (R4) and RFB (R3) resistors

can be found. They are external resistors used to program

the output voltage. The LT3575 operates much the same way

as traditional current mode switchers, the major difference

being a different type of error amplifier which derives its

feedback information from the flyback pulse.

Operation is as follows: when the output switch, Q1,

turns off, its collector voltage rises above the VIN rail. The

amplitude of this flyback pulse, i.e., the difference between

it and VIN, is given as:

VFLBK = (VOUT + VF + ISEC • ESR) • NPS

VF = D1 forward voltage

ISEC = Transformer secondary current

ESR = Total impedance of secondary circuit

NPS = Transformer effective primary-to-secondary

turns ratio

The flyback voltage is then converted to a current by

the action of RFB and Q2. Nearly all of this current flows

through resistor RREF to form a ground-referred voltage.

This voltage is fed into the flyback error amplifier. The

flyback error amplifier samples this output voltage

information when the secondary side winding current is

zero. The error amplifier uses a bandgap voltage, 1.23V,

as the reference voltage.

The relatively high gain in the overall loop will then cause

the voltage at the RREF resistor to be nearly equal to the

bandgap reference voltage VBG. The relationship between

VFLBK and VBG may then be expressed as:

⎛

α ⎝⎜

VFLBK

RFB

⎞

⎠⎟

=

VBG

RREF

or,

VFLBK

=

VBG

⎛

⎝⎜

RFB

RREF

⎞

⎠⎟

⎛

⎝⎜

1⎞

α ⎠⎟

α = Ratio of Q1 IC to IE, typically ≈ 0.986

VBG = Internal bandgap reference

In combination with the previous VFLBK expression yields

an expression for VOUT, in terms of the internal reference,

programming resistors, transformer turns ratio and diode

forward voltage drop:

VOUT

=

VBG

⎛

⎝⎜

RFB

RREF

⎞

⎠⎟

⎛

⎝⎜

α

1

NPS

⎞

⎠⎟

−

VF

−

ISEC

(ESR)

Additionally, it includes the effect of nonzero secondary

output impedance (ESR). This term can be assumed to

be zero in boundary control mode. More details will be

discussed in the next section.

Temperature Compensation

The first term in the VOUT equation does not have a tem-

perature dependence, but the diode forward drop has a

significant negative temperature coefficient. To compen-

sate for this, a positive temperature coefficient current

source is connected to the RREF pin. The current is set by

a resistor to ground connected to the TC pin. To cancel the

temperature coefficient, the following equation is used:

δVF = − RFB • 1 • δVTC or,

δT RTC NPS δT

RTC

=

−RFB

NPS

•

1

δVF / δT

•

δVTC

δT

≈

RFB

NPS

(δVF/δT) = Diode’s forward voltage temperature

coefficient

(δVTC/δT) = 2mV

VTC = 0.55V

The resistor value given by this equation should also be

verified experimentally, and adjusted if necessary to achieve

optimal regulation overtemperature.

The revised output voltage is as follows:

VOUT

=

VBG

⎛

⎝⎜

RFB

RREF

⎞

⎠⎟

⎛

⎝⎜

1

NPS

⎞

α ⎠⎟

−

VF

−

⎛

⎝⎜

VTC

RTC

⎞

⎠⎟

•

RFB

NPS α

–

ISEC

(ESR)

3575f

8

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