AN1484 - APPLICATION NOTE

4/11

The accuracy of this circuit is limited especially in

temperature but is unrivalled in terms of cost. The

addition of R5 base resistor is necessary in short

circuit

to

avoid

destroying

T1

Base-Emitter

junction. R3 is also necessary in short circuit

otherwise T1 collector current directly flows

through D6 and the optocoupler is not driven

anymore, leading to an increase of short circuit

power consumption. C8 limits the gain in frequency

of T1, stabilising the loop.

2.0 THE TRANSFORMER

An important part of a SMPS design lies in the

transformer. Its performances are a key to the

system performances.

The requirements for this application are: small

size and limited voltage on the drain.

Table 2: Transformer Target Specification

2.1 Primary inductance

A simple calculation gives the range of values of

primary inductance suitable for this application.

VIPer12A has a drain current limitation of 360mA

min. The energy transferred is E=1/2LpIp2*fsw in

discontinuous mode. Emin=5W, Ip=360mA and

fsw=50kHz giving Lp>1.54mH.

The transition mode is when Ton*Vin=Toff*Vr

(Ton*Vin=Lp*Ip). The expression of Lp is:

Lp=1/2*(Ton*Vin)2*fsw/E

With Vin=150V, Vr=50V, Ton=5us, E=5W give

Lp=2.8mH.

Vin was chosen to reach the continuous mode at

low input voltage level. Vr is low to limit the drain

peak voltage.

The transformer optimisation has led to a final

value of 2.5mH partly to reduce the primary turns

and their power dissipation with an E12.5 bobbin.

2.2 Transformer Structure

A standard transformer structure (so called with

windings order Primary/Auxiliary/Secondary) gives

the following results on VIPer12A supply:

Table

3:

VIPer12A

Vdd

with

a

Standard

Transformer

Two concerns can be seen from this table:

- the VIPer12A is not going in hiccup mode in Short

Circuit

- The supply voltage is too low in Standby with the

risk of a "Bad burst mode" with higher standby

consumption

and

poor

regulation

(VIPer12A

undervoltage is at 9V max with 8V Typical).

The solution, implemented in the demoboard, is

the optimized structure (so called with windings

order: Primary/Secondary/Auxiliary) shown on

figure 3.

Figure 3: Transformer Structure

Parameters

Value

Power

5W

Saturation Current

> 400mA

Primary Inductance

2.5mH

Reflected Voltage

50V

Leakage Inductance

< 100

µH or <3%

Primary Capacitance

> 20pF

Conditions

100VDC

380VDC

Stand-by

10V

8V

Load 6V/100mA

15V

15V

Load 6V/600mA

25V

26V

Short Circuit

9V

10V

CORE

Bobin

Primary

Secondary

Auxiliary

CORE

Bobin

Primary

Secondary

Auxiliary

Optimisation