SO16N datasheet(27/51 Pages) STMICROELECTRONICS

27 / 51 page

L6566BH

Application information

Doc ID 16610 Rev 2

27/51

the theoretical one. It is therefore more practical to provide a first cut value, simple to

calculate, and then to fine tune experimentally.

Assuming that the system operates exactly at the boundary between DCM and CCM, and

neglecting propagation delays, the following expression for kopt can be found:

Equation 6

Experience shows that this value is typically lower than the real one. Once the maximum

peak primary current, IPKpmax, occurring at minimum input voltage Vinmin has been found,

the value of Rs can be determined from (5):

Equation 7

The converter is then tested on the bench to find the output power level Poutlim where

regulation is lost (because overcurrent is being tripped) both at Vin = Vinmin and

Vin = Vinmax.

If Poutlim @ Vinmax > Poutlim @ Vinmin the system is still undercompensated and k needs to

increase; if Poutlim @ Vinmax < Poutlim @ Vinmin the system is overcompensated and k

needs to decrease. This goes on until the difference between the two values is acceptably

low. Once the true kopt is found in this way, it is possible that Poutlim can turn out slightly

different from the target; to correct this, the sense resistor Rs needs to be adjusted and the

above tuning process is repeated with the new Rs value. Typically, a satisfactory setting is

achieved in no more than a couple of iterations.

()

max

min

max

min

opt

⋅

max

PKp

min

opt

−

Figure 17.

Left: overcurrent setpoint vs. VFF voltage; right: line feedforward function block

AM11493v1

Vcsx [V]

0.5

1.5

2.5

33.5

0.2

0.4

0.6

0.8

1.2

VVFF [V]

VCOMP= Upper clamp

Rectified Line Voltage

DRIVER

COMP

VFF

L6566BH

1.5 V

Hiccup

DISABLE

VOLTAGE

FEED

FORWARD

Optional for

OVP settings

OCP

PWM

Vcsx

Clock/ZCD

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