Description

AN2872

8/23

Doc ID 15305 Rev 1

Figure 3.

Standard implementation of a buck converter with a monolithic device

The second important behavior of a buck converter dedicated for mains applications follows

from the ratio between the input and output voltage. As derived from Equation 2 the duty

cycle is very low when the ratio between the input and output is very high. This also means

the ON time must be very short. It is possible for the ON time required for correct operation

to be shorter than the minimum ON time of the controller defined in the datasheet (the

maximum duration of the minimum ON time of the VIPer16 is 450 ns. see Section 6: 3). In

this case the SMPS delivers more energy than the controller, which is compensated by the

skipping of some pulses. This leads to higher ripple of the output voltage. This effect can be

compensated by applying a higher inductance value and a lower switching frequency.

The last technical point discussed in this paragraph concerns the input bulk capacitor in

applications dedicated to very high input AC voltage. In this case, the input voltage is up to

500 VAC. The DC voltage can reach voltage levels of up to 707 VDC. The input bulk

capacitor must be able to sustain such voltage levels. Unfortunately, there are no standard

aluminum capacitors on the market suitable for this voltage, so two capacitors connected in

series have been applied.

The series connection of two capacitors has two effects on the voltage drop over each

capacitor. The first is an influence over the tolerance of the capacitance and the second is

an influence over the leakage current.

Regarding the tolerance, the worst case is that one capacitor’s capacitance is at the

maximum upper limit and the second at the lower limit. Equation 3 defines the voltage

distribution on each capacitor for this configuration:

Equation 3

where

δ is the tolerance of the capacitors expressed as a percentage

If the voltage bus is 707 V and the tolerance is 20%, the highest possible voltage across the

capacitor is 424 VDC. Therefore, for this voltage range, the use of 450 V capacitors is

recommended.

The leakage current can cause additional unbalancing of the voltages and an increase in

the voltage drop of one capacitor over the allowed limit. If the leakage currents of both

T1

L1

Control

V1

V2

C1

C2

D1

C3

D2

FB

GND

AM00360

V

C1 C2

,

V

MAX

2

-------------- 1

δ

±

()

=