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1N4004

OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY

STMicroelectronics 

AN1262 APPLICATION NOTE

2 CONVERTER ELECTRICAL SPECIFICATION

The starting point of the design procedure is the properties of the converter as a black-box, that is the set of data

listed in the electrical specification table (table 1). Additional requirements, such as efficiency at zero load or

line/load regulation or maximum junction temperature, etc., can be added to that list and their impact will be con-

sidered where appropriate.

s Mains Voltage: Range and Frequency. There are basically the three possible options listed in table 2,

where a variation of ± 20% is assumed, according to common practice. There are exceptions like some

distribution lines rated at 277 VAC, where a ± 10% spread can be considered, or other special cases for

specific applications. Table 2 shows also the line frequency to be considered in the standard cases at

the minimum specified mains voltages. An additional specification may require the converter to be shut

down if the mains voltage falls below a "brownout level". This additional specification will be used for

setting up the brownout protection on the types where it is available.

Table 2. Mains voltage specifications

Input (VAC)

VACmin (VAC)

110

88

220

176

WRM (Wide Range Mains)

88

VACmax (VAC)

132

264

264

fL (Hz)

60

50

60

s Number of holdup cycles. The holdup requirement is the ability of the converter to keep the output volt-

age in regulation even in case of mains interruption (missing cycles). This is usually specified in terms

of number of mains cycles NH. This feature is not always demanded (in which case, NH = 0), otherwise

the typical requirement is 1 mains cycle, that is NH = 1. It impacts on the input bulk capacitor selection.

s Output voltage tolerance. It can be expressed either in absolute value or as a percentage of the nominal

voltage. This requirement, as well as the ones on line and load regulation, if specified, will affect the

choice of the feedback technique (primary or secondary).

s Output voltage ripple. The ripple superimposed on top of the DC output voltage is specified as the peak-

to-peak amplitude and includes both low frequency (at 2·fL) and high frequency (fsw) component.

Switching noise due to parasitics of the printed circuit board and random noise are beyond the scope

of this procedure. This requirement, if tight, may require the use of an additional filtering cell at the out-

put.

s Converter Efficiency. The efficiency is, by definition, the ratio of the output power to the input power.

This figure is strongly dependent on the output voltage, because of the losses on the secondary diode.

It should be set based on experience, using numbers of similar converters as a reference. As a rule of

thumb, 75% (η = 0.75) can be used for a low voltage output (3.3 V or 5 V) and 80% (η = 0.8) for higher

output voltages (12 V and above).

3 PRE-DESIGN CHOICES

Before starting the design calculations of the various parts of the converter, some parameters not defined at the

"black-box level" need to be fixed. There is some degree of freedom in the selection of these parameters, pro-

vided some constraints are taken into account.

s Reflected Voltage. In principle, the reflected voltage should be as high as possible. In fact this leads to

a greater duty cycle, which minimizes the RMS current through the IC's MOSFET for a given power

throughput. There are two possible limitations to the maximum reflected voltage. One is the maximum

duty cycle Dmax allowed by the devices (67% min.); some margin should be considered for load tran-

sients, thus the reflected voltage should be such that the maximum duty cycle (at minimum input voltage

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