AN2252

Application circuit description

11/21

2.2.2

Transformer design

The initial power transformer specification is as follows:

●

Minimum input voltage 320VAC = 450VDC

●

Minimum switching frequency at full load and minimum input voltage 50kHz

●

Reflected flyback voltage 500V

●

Converter efficiency at full load and minimum input voltage 90%

The initial power transformer design was further optimized by VOGT Electronic Components

GmbH to cope with such a high voltage. Special attention was paid to guarantee VDE

distances by padding the winding ends.

To improve magnetic coupling, not only the primary winding but also the secondary was

split. The two primary windings so obtained have half the total number of turns each and are

connected in series while the two secondary windings have the nominal number of turns

and are connected in parallel. The windings are laid out as follows starting from the winding

nearest the core: W1 (Primary 1), W2 (Secondary 1), W3 (Primary 2), W4 (Secondary 2)

and W5 (Auxiliary). The transformer is designed so as to comply with the EN60950 safety

standard for CE certification. If the transformer must be compliant with the UL standard – on

the flammability of isolation material – whose compliance is mandatory for applications

implemented in the USA, then some modifications are required. The transformer’s physical

appearance, dimensions and winding arrangement are shown in Figure 6.

Figure 6.

Power transformer dimensions and winding arrangement

R13, R14

2

82k

Ω

Resistor, size 0414, metal film, 500V, 2W, 5%

R17, R20

2

4,7k

Ω

Resistor, size 0204, metal film, 250V, 0.185W, 1%

R18

1

39k

Ω

Resistor, size 0204, metal film, 250V, 0.185W, 1%

R19

1

120k

Ω

Resistor, size 0204, metal film, 250V, 0.185W, 1%

T1

1

SL 040 923 11 03 VOGT-electronic, Power transformer, ETD39, N67

T2

1

SL 040 903 21 02 VOGT-electronic, Current transformer, RM13*7*4.5, Fi 340

U1

1

L6565

STMicroelectronics, quasi-resonant SMPS controller, DIP-8

U2

1

TL431AI

STMicroelectronics, shunt Reference, 2.5V, 1 to 100mA, 2%, TO-91, –40 to

105°C

U3

1

PC817

optocoupler, SHARP, Viso = 5kV, CTR = 50..600% at IF = 5mA, DIP-4

Table 2.

Bill of materials

Referenc

e

Qty

Value

Description