5

ML4818

FUNCTIONAL DESCRIPTION

PHASE MODULATOR

Power is controlled by modulating the switching phase on

sides A and B of the full H-bridge converter (Figure 1).

Power is delivered to the output through the transformer

secondary. The power conversion process is described by

the following sequence and illustrated by the timing

diagram of Figure 2:

1. A2 and B1 are high (Q1 and Q2 are on), beginning

the power conversion cycle.

2. After the

Φ MOD comparator trips, B1 goes low

turning off Q2. The parasitic drain-to-source

capacitances of Q2 and Q4 charge to +VIN. This

forces the drain-to-source voltage across Q3 to 0V.

Figure 2. Phase Modulation control waveforms (Shaded areas indicate a power cycle).

B2

B1

A2

A1

Q3

Q2

Q4

Q1

TRANSFORMER

B

A

+VIN

ML4818

Figure 1. Simplified diagram of Phase Modulated power Outputs.

the voltage across Q3 is now 0V, B2 turns Q3 on at

zero voltage.

4. The CLOCK now goes high turning A2 off. During

this period, Q1 and Q2 and Q4 are off. The

transformer leakage current discharges the drain-to-

source capacitance on Q4 until there is 0V across it.

then it goes high. The voltage across Q4 is now 0V

as A1 turns it on at zero voltage.

6. The previous sequence is now repeated with the

opposite polarity on all outputs (see Figure 2).

The above sequence is then repeated but with the

opposite polarity on all outputs.

CLOCK

A2

A1

B1

B2

B

A