AN1606 APPLICATION NOTE

4/18

The L4981A/B current loop is designed to handle this negative signal. This type of resistor current sense can

easily be achieve in medium power applications. For high power PFC circuits it is necessary to use a magnetic

current transformer for improved efficiency as shown in Figure 5b.

In the bridgeless PFC configuration since an input rectifier bridge is not used, the current is continuously chang-

ing its direction and the complexity of current sensing with a simple resistor can increase. Also in high power

applications, resistor sensing may dissipate too much power. In these cases, current sensing with a current

transformer is the preferred approach.

A current sense transformer core is typically high permeability ferrite (toroidal or a small core set). The primary

of the transformer is a single turn of wire through the core. The secondary typically consists of 50 to 100 turns.

Figure 5. .

This type of sense transformer cannot operate at low frequency and for this reason it must be connected where

the current is switched at high frequency. The magnetic core must be allowed reset.

This is normally accomplished by using a diode. In order to reproduce the inductor's current in boost topology,

two of magnetic sense sections are needed and the simplified schematic is shown in figure 5b.

When the sense transformer solution is applied in the bridgeless topology, the simple sense as in fig5b, is no

longer valid.

v

s

R

s

IL

Iret.

Fig.5a

Standard Sensing

Inductor

Magnetic sensing for high power

Fig.5b

v

s

R

s

Iret.

IL

Inductor

Lm_p

Rs

1:n

Typical sense transformers