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DF04M

OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY

STMicroelectronics 

AN1262 APPLICATION NOTE

Output Capacitor

Capacitor's ripple current rating should exceed 3 A. The minimum capacitance value should be 373 µF (eqn.

18) and the maximum ESR should be less than 5 mΩ. For long-time reliability the capacitor(s) should also be

able to withstand at least 3.08 A current ripple. Three Rubycon's ZL series 470 µF/16V paralleled capacitors

were selected, for a total ripple capability of nearly 3 A and a total ESR of about 20 mΩ. To meet the requirement

on the output voltage ripple an LC post filter is needed that attenuates ripple at least four times. Choosing a

standard value of L = 4.7 µH, the maximum ESR of the additional capacitor should not exceed 300mΩ.

An additional 220 µF/10V ZL capacitor has been added.

Self-supply circuit

The self supply circuit will include an 1N4148 diode and a 22 µF supply capacitor. A 10 Ω resistor will be added

in series to the diode to reduce Vcc voltage variations with the load current. This value is likely to be adjusted

after bench verification.

Control loop design

The crossover frequency will be selected as high as 10 kHz, worst case. The objective will be to get 70° phase

margin.

The plant transfer function is:

G2(jω) = G20 ⋅ 1--1---+--+----ω---ω---------Ejj----o-ωω-----S---u---------Rt----

with G2o = 11.5, fESR = 5464 Hz, fout = 90.3 Hz (@ max. load and max. Vin). A type 2 amplifier will be used for

G1(jω).

Going step-by-step:

a) The gain and phase of G2 at f=10 kHz are 0.281 and -29° respectively;

b) In order for the overall open-loop gain to cross the 0 dB axis at f=10 kHz with 70° phase margin, the gain and

phase of G1(jω) will be 3.56 and -81° respectively;

c) the compensating zero will be placed at 360 Hz (α = 4, to maximize 100Hz gain);

d) the compensating pole will be placed at 2270 Hz;

e) the unity gain factor is 35.4·104 s/rad.

Since a tight tolerance on the output voltage is required, an optoisolated feedback will be used and G1(jω) will

be realized with the schematic of figure 13. The TL431 and an optocoupler PC817A from Sharp will be used.

The CTR is specified between 0.8 and 1.6.

Using a 6.8kΩ resistor as RC, the resulting part values are:

RL = RH = 2.43 kΩ; RB = 560 Ω; RF = 2kΩ; CF = 100 nF; CCOMP = 22 nF.

Electrical Schematic, BOM and evaluation results

In fig. 22 the electrical schematic of the test board is illustrated and table 24 lists the relevant BOM. The dia-

grams of figure 23 show the evaluation results of the board, figure 24 shows some typical waveforms and figure

25 the effect of the frequency change on the output voltage transient.

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