AN2439

Board description

7/20

1.3

Input section

The schematic of the input section is given in Figure 4. We used two separated connectors

The system works correctly if we feed the converter with the Dc voltage in the AC input

voltage connector. We used a separated connector in order to gain efficiency.

Using the additional connector for the Dc input voltage we use two different NTCs to limit the

inrush current in the two cases. NTC1 has to limit the inrush current, considering as worst

case the maximum AC Peak input voltage. NTC2 has to limit the inrush current, considering

as worst case the maximum DC voltage (72 V) which means that NTC2 can be lower than

NTC1 and dissipates less energy.

The DC voltage is connected to the bulk capacitors through one diode (used for protection in

case we connect the Dc input with the wrong polarity) so we cut one half of power

dissipation compared to the power we would dissipate in the bridge.

Figure 4.

Input section

1.4

Buck DC-DC converter post regulator [3.]

The second output of 3.3 V, able to provide up to 5 A to its load, is obtained using a

switching post regulation. The post regulator is a synchronous buck converter based on the

L6725 controller and the schematic is given in Figure 5.

-+

BR1

BRIDGE

1

4

C1

1nF Y 1

C3

1nF Y 1

NTC1

8.2 2A

1

2

C5

33 uF

C4

33uF

J1

CON2

1

2

J2

CON2

1

2

F1

2A

C2

100nF

NTC2

.82 2A

1

2

C222

100nF

D8 600V 3A diode

FILTER1

Lin Filter

3

4

2

1

Primary +

Start up Circuit

Dc Input

Voltage

Ac Input

Voltage

+

-