AN2872

Description

Doc ID 15305 Rev 1

7/23

The regulator of the circuit measures the output voltage, compares it with the reference

voltage and modifies the duration of the ON time to keep the output voltage constant.

In cases where the inductor current is operating in the continuous mode (the current does

not cross zero at full load) the duty cycle can be obtained using Equation 2. This formula

follows from Equation 1. Another method to obtain Equation 2 is to consider the buck

pass filter generates a mean value.

Equation 2

3.1.2

Practical aspects of a buck converter dedicated for mains and 3-phase

input

The application of a mains or 3-phase buck converter using a simple monolithic device

results in several special conditions. A few of the most important are described in the

following paragraphs.

The operation of a buck converter such as that of the diagram in Figure 2 requires an active

high side switch. Therefore, the monolithic device (with integrated N-channel MOSFET) is

also connected on the high side (between + of bulk capacitor and inductor). The GND of the

controller connected to the source of the MOSFET refers to the high side of the inductor

(see Figure 3). This wiring of circuit causes the feedback signal not to be directly sensed

from the output due to the shift of the GND of output voltage and controller. Basically, there

are two ways to move the information regarding the output voltage from the output to the

controller. The first way is to apply an optocoupler between the output and the converter.

The additional error amplifier and reference (typically TL431 or a simple Zener diode) must

be assembled to drive the LED of the optocoupler. This method gives high precision of the

output voltage level and low load regulation. However, it also increases the cost and space

requirements. The second principle is to use a replica of the output voltage stored in the

auxiliary capacitor during OFF time. The schematic in Figure 3 shows the principle

the voltages are not exactly the same. This difference is caused by the difference in the

discharge current of capacitors, different capacitance and different voltage drop on diodes

theoretically unlimited increase in the output voltage at light or no load, because the energy

delivered during the ON time is higher than the total energy required by the load. Therefore,

an additional load (resistor) or voltage limiter (Zener diode) is required on the output to

protect the output capacitor and the load against overvoltage at light load.

δ

V

2

V

1

------

=