Description

AN4407

4/19

DocID025572 Rev 1

Figure 2. Demo board of PFC 2 kW using MOSFETs in TO247-4 (4 pin)

1.1

Approach to the study

Usually, in electronic power systems, a feedback network is implemented to create a control

of the electrical parameters (for the DC/DC converter, this can involve both the output

voltage and output current or only one of these). In order to control the current on the

system, a resistor is often connected in series to the line under control, forming a common

path between the input and output network. The information coming from this element,

(voltage across the sensing resistor) is used as the control signal and sent to the pin of the

driver. This pin is connected to an input of the internal comparator inside the driver; it’s

output signal is used to regulate the conduction time of the power switch.

When the power switch requires particular source/sink current values, the drivers are

created with a push-pull on the output (Figure 3). The feedback type is known as current

sampling series-mixing, which offers the main benefit of increasing the input impedance,

while the drawback lies in the impact of the output net on the driving circuitry. A possible way

to reduce the latter phenomenon is to separate the input circuit from the output one, which is

along the lines of the solution proposed by STMicroelectronics.

GIPG211120131727FSR