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3575FE

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3575

APPLICATIONS INFORMATION

Zener clamps. Among them, RCD is widely used. Figure 9

shows the RCD snubber in a flyback converter.

A typical switch node waveform is shown in Figure 10.

During switch turn-off, the energy stored in the leakage

inductance is transferred to the snubber capacitor, and

eventually dissipated in the snubber resistor.

1

2

LS I2PK

fSW

=

VC

(VC

−N

R

•

VOUT )

The snubber resistor affects the spike amplitude VC and

duration tSP, the snubber resistor is adjusted such that

tSP is about 150ns. Prolonged tSP may cause distortion

to the output voltage sensing.

The previous steps finish the flyback power stage design.

5. Select the feedback resistor for proper output voltage.

Using the resistor Tables 1-4, select the feedback resistor

RFB, and program the output voltage to 5V. Adjust the

LS

RTC resistor for temperature compensation of the output

voltage. RREF is selected as 6.04k.

A small capacitor in parallel with RREF filters out the

noise during the voltage spike, however, the capacitor

should limit to 10pF. A large capacitor causes distortion

on voltage sensing.

6. Optimize the compensation network to improve the

transient performance.

The transient performance is optimized by adjusting the

compensation network. For best ripple performance, select

a compensation capacitor not less than 1.5nF, and select

a compensation resistor not greater than 50k.

7. Current limit resistor, soft-start capacitor and UVLO

resistor divider

Use the current limit resistor RLIM to lower the current

limit if a compact transformer design is required. Soft-start

capacitor helps during the start-up of the flyback converter .

Select the UVLO resistor divider for intended input opera-

tion range. These equations are aforementioned.

C

R

D

VC

VIN

NVOUT

3575 F09

Figure 9. RCD Snubber in a Flyback Converter

tSP

3575 F10

Figure 10. Typical Switch Node Waveform

3575f

17

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