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AN1606 APPLICATION NOTE

The circuitry is more complex than in the boost case because here we have two pair of PowerMOS (M1, M2)

and diodes (D1, D2) alternating.

It is necessary to sense the chopping current of the (PowerMOS + diode) section and to sum the signals to be

applied to Rs.

The sensing of the diode's current can be simply done by placing a magnetic sensor at the common cathode

(L2 in fig.6.). Only one of the two diodes operates each half input cycle.

Figure 6.

For the PowerMOSFET portion of the circuit, the complexity increases because during the half cycle when one

of the PowerMOSFETs is chopping, the other one has to handle the current flowing back to the mains.

Using the configuration of sensors as shown in Figure 6 it is possible to solve the problem without undue com-

plexity. The unnecessary high frequency portion of the current signal is cancelled because of the method M1 is

connected to L1A as shown in Figure 6b. The problem due to the change of polarity during each half cycle is

solved by using a center tapped secondary and two rectifiers.

Since the coupling of the two windings must not permit the demagnetization of L1, an auxiliary transistor Q1 is

used that opens the circuit during the off-time. For the L4981 controller, the off-time is guaranteed not to be less

than 5% of the period. Q1 can be a small signal transistor because its switched current is low due to the fact

that the transformer secondary will have a large number of turns.

To realize the current sensing transformer, a high permeability toroidal core (ur=>5000) has been used. The

secondary has 50 turns as a compromise to reduce secondary current yet not require a large number of turns.

L2

v

s

i

in

L

O

A

D

R

s

M

1

M2

controller

Q1

D

1

D

2

L1

Structure of sense transformers

L2

L1