AN2872

Description

Doc ID 15305 Rev 1

9/23

capacitors are the same, no problem is presented. But in cases where there is a difference

in leakage currents (which is the case in practical applications) this difference will cause the

capacitor with lower leakage to be charged by this difference and the voltage over this

capacitor will rise, risking exceeding the voltage defined in the datasheet. To avoid this

correct value of the balancing resistors can be calculated from the difference in the leakage

currents and applied voltage. Unfortunately, the maximum difference in leakage currents is

not defined. It is possible, however, that the calculations for the balancing resistors are

available the technical documentation of some aluminum capacitor manufactures. An

examination of these documents reveals that there is not a single method and each

manufacturer uses different calculation methods, each of which unfortunately can give very

different values. Technical notes describing the calculations of balancing resistors can be

found in Section 6: References: 4, 5, 6, 7. Typically, the value of balancing resistors is in the

range of hundreds of k

Ω to several MΩ.

The drawback of using balancing resistors is the constant input load. The additional

constant load caused by balancing resistors is mainly visible in the standby consumption

and reduction of efficiency at low power SMPS. It is recommended to set the value of the

balancing resistors to the maximum level, respecting the technical recommendations of the

manufacturer of the aluminum capacitors selected.

3.2

Converter schematic

The schematic of the converter is shown in Figure 4. It consists of three sections: an input,

a high voltage DC-DC converter and a linear regulator.

DC converter when the input voltage is low. Because there is 500 VAC applied, which

means a DC level over 700 V, the foil and electrolytic capacitors are connected in series.

This is due to the fact that the standard foil safety capacitors are produced for 440 VAC, and

the maximum DC voltage for standard electrolytic capacitors is 450 V. As the electrolytic

applied to guarantee that the voltage of each capacitor does not exceed the maximum rating

even at maximum input voltage.

Figure 4.

Schematic of the step-down converter based on the VIPer16L

The high voltage DC-DC section is a buck converter based on the VIPer16. It converts the

R

9 1 kΩ

C

7 1.8 nF

R

2 750 kΩ

R

5 750 k

Ω

R

6

750 k

Ω

R

8

750 k

Ω

C

1

150 nF / X2

D

3

1N4007

L

1

2.2 mH

D

5

STTH108

D

6

15 V

D

2

STTH108

C

3

10 µF / 450 V

C

9

10 µF

/ 35V

C

5

220 µF

/ 25 V

L

2

1 mH

R

1

22

Ω

R

4

9.1 k

Ω

R

3

24 k

Ω

2

J

1

85 - 500 VAC

C

2

150 nF / X2

C

6

10 µF

/ 35 V

1

2

3

J

2

Output

C

8

100 nF

D

1

1N4148

3

1

2

VOUT

VIN

GND

U

2

L78L05

D

4

1N4007

Drain

7

Drain

8

S

1

VDD

2

FB

4

COMP

5

LIM

3

U

1

VIPer16

C

4

10 µF /

450 V

R

7

N.A.

AM00361