AN628 APPLICATION NOTE

26/35

Some design peculiarities

A magnetic snubber solution has been adopted in order to limit/control the ramp rate of the current at the turn-

on edge of the Power Mos, referring to the schematic, the parts involved in this function are:

L2, D5, D6, D7, C14 and R18.

The function of the auxiliary inductor L2 is to reduce the ramp rate of the current reducing in this way the peak

current, due to the recovery of the boost diode (D4), and the associate noise emission.

The additional benefit of this magnetic snubber is also the reduction of the switching power dissipated in the

Mos. D5 and D6 needs to provide a path that allows the demagnetization of the inductor L2, clipping the neg-

ative spike thus avoiding the Breakdown activation of the booster diode D4.

D7, C14 and R18 will clamp the energy of L2 at the turn off edge of the Mos.

The inrush current limiter NTC is placed between the cathode of the boost diode and the bulk capacitor C15.

In this way, the current flowing in to the NTC limiter is the same of the boost diode well below the mains current

especially at minimum mains value.

The demo is provided with "self-supply" circuit. The proposed supply circuit, using a Graetz bridge is much

efficient anyway; it can be inadequate when the output power is reduced down to less than 5W.

On board, it has been recovered place for the Raux1 and Raux2. The two resistors will be connected in case of

L4981B version evaluation.

Here follows some comment concerning the design and the selection of the power parts of the demo.

88 to 264 Vac

BRIDGE

B1 8A

L1=0.5mE42*21*15

gap=1.9 58/6 turns

20*.2mm

C2

330n

C3

330n

R2

33k

R5

220k

R4

1.2M

R6

500k

74

C1

330nF

400V

L4981A/ B**

15

16

R7

500k

19

C8

100n

13

R1

460

R3

2.2k

R8

17k

C6

3.3n

R13

2.2k

C9

1n

R15

24k

1

17

10

18

9

5

8

2

R9 (RS)

50m

// 3*0.15)

R10

5k

R11

56k

R12

56k

R16

220k

14

3

C14

100n

R17

15

# // Q1&Q2

TO220*2

STM12NM50

/ 7C/W

D7-STTA406

20

C12

1u

C13

1u

R21

19.6k

R24

16.9k

R19

750k

R22

750k

D4-STTH8R06

to220 (/40CW)

6

11

12

C15

220uF

450V

-

D3

L2

3u

R18

6.8

2W

R23

750k

C11

220n

R20

750k

R14

68

B2= D1+D2

+D8+D9

VCC

F1

T15A250V

NTC

2.5

V+ BUS=400V

D5-

STTA106

D6

DZW06-48

RAux1

RAux2

Cf

.22uF

600V

TP1

VCC

C10

150uF

Dz1

18V

Q4

4007

R25

1k

2W

Cs

330pF

Q1+Q2#

**