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AN1620 APPLICATION NOTE

The realisation of the three different functional modes requires dedicated controls.

The voltage and current controls are located on the secondary side of the transformer, and the error signal is

transferred to the primary controller via an optocoupler in order to have an isolated feedback.

The TSM101A a dual op/amp, with an internal 1.24V reference is used. One op/amp is dedicated to the voltage

mode control and the second one to the constant current control.

The two output are OR-ed and the common point drives the optocoupler.

The current signal is taken across the 40mOhm current sense resistor, R16, while the R18 potentiometer ad-

justs the output voltage.

The constant power characteristic is easy to achieve with discontinuous mode operation, since constant primary

peak current means constant output power. The value is programmed by adding a proper offset voltage, defined

by R3 value (fig 4), to the current sense ramp at pin 13.

Figure 4. Schematic diagram for output constant power.

The value of this resistor is related to the desired output power Pout by the below relationship:

where L1 is the primary inductance,

η is the expected efficiency, f is the switching frequency and Rs is the cur-

rent sense resistance.

The L5991A introduces a delay, td @ 100ns, at turn-off, on the current loop, and due to this, the primary peak

current value increases according to its slope.

The consequence is that the regulated output power is a function of the input voltage. R28 is introduced in order

to compensate this error.

Its value can be calculated with the following formula:

A primary auxiliary winding is required to supply the IC after turn-on, and it has to be designed to generate a dc

voltage within the limits of the IC supply voltage range. It should be loosely coupled with the secondary winding

in order to minimise the reflected secondary-to-primary effects when the output voltage is going down towards

short circuit.

A second auxiliary winding, on the secondary side transformer, forward coupled with the principal primary wind-

ing, has been added. When the system is in constant current mode, and the output voltage is reduced, this wind-

ing provides the supply voltage for the TSM101A.

The Table A summarises the efficiency performance of the complete system, and Table B shows the electrical

system performance.

L5991A

2.7M

10K

1K

0.5

Ω

4

10

13

IN438_mod

10

100pF

R

3

R

30

4

1

2

L

1

f

η

⋅⋅

--------------------



 R

S

P

out

⋅⋅

–

------------------------------------------------------------------------

⋅

=

R

28

R

() L

1

⋅

t

d

R

S

⋅

--------------------------------------

=