the negative voltage generated by the winding. As

long as the transformer is delivering some energy

on secondary side, the negated EOD signal

remains in the high state and the mosfet switch Q

is on. The duration of this state is noted tonsec

and corresponds to the time where the secondary

the demagnetisation function, refer to figure 6.

The average output current can be expressed as:

=

2

X

(1)

Where :

Taking into account the transformer ratio n

between primary and secondary side,

be expressed versus primary peak current

(2)

The value of the capacitor C is sufficiently high to

consider the voltage Uc as constant. This

capacitor is submitted to a charging current and

discharging current at the rhythm of the switching

frequency. As these currents are in the range of a

few mA (Iref is typically 1 mA), a 470 nF is a

suited value for a switching frequency of 60 kHz.

In steady state, it can be written that the charge is

equal to the discharge :

R

It comes :

(3)

As

can be also expressed as :

=

(4)

Combining (1), (2), (3) and (4) :

n

2

x

This last expression shows that the average

output current doesn’t depend any more neither

on the output voltage, nor on the duty cycle, nor

on the input voltage. The only parameters which

are setting its value are :

The transformer ratio n.

The sense resistor value

The product

This product corresponds to a voltage which is

noted Vreg in the specification tables. Figure 5

shows the test fixture for measuring it : The

DSENSE pin is held in the high state (In fact, it is

left open, as an internal pull up current source is

internally connected on this pin) and the mosfet

switch Q is always in the high state. In this case,

the voltage on the CREF pin establishes at

Note that the oscillator must be running for the

demagnetisation block to sample correctly the

DSENSE pin.

current can be finally written as :

0.175

A sense resistor of 1.3

Ω with a transformer ratio

of 6 gives a typical output current of about 800

mA.

The

schematics

of

figure

10

shows

a

compensation on the CSENSE pin with the two

resistances R5 and R7. These resistances are

connected on the Vin input voltage and are

providing an offset on the current sense pin. The

higher is the input voltage, and the higher is this

offset current. The purpose of this compensation

is to cancel the effect of the current control

propagation time td, which induces an extra

current on top of the theoretical peak current Ip

given by (4).

The

output

current

obtained

with

this

compensation can be seen on figure 11. The

typical ”flatness” is about +/-2.5 %, including the

input voltage variation from 100 VDC to 400 VDC.

If less accuracy is needed, these two resistances

can be omitted.

CONSTANT VOLTAGE OPERATION

An another part of the circuit is in charge of the

regulation of the output voltage, and generates

the vertical characteristic of figure 11. It consists

of

a

primary

feedback

regulation,

with

a

conventional

voltage

mode

control

:

An

operational amplifier with an internal voltage

reference of 2.6 V is configured in error amplifier

and defines the duty cycle of the power mosfet

switch by comparison with the oscillator sawtooth

(See block diagram on page 1).

As it is a primary feedback, the accuracy of the

output

voltage

depends

closely

on

the

transformer coupling quality. This is especially

VIPer31SP

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