5

ML4828

FUNCTIONAL DESCRIPTION

PHASE MODULATOR

The ML4828 controls the power of a full bridge power

section by modulating the phases of the switches of the A

and B sides (Figure 1). The power cycle starts with A2 and

B1 high, as shown in the timing diagram (Figure 2).

1. With A2 and B1 high, Q1 and Q2 are ON. Current

flows through the primary of the transformer, and

power is delivered to the output through the secondary

winding (not shown).

2. After either the

goes low, turning off Q2. Energy in the primary winding

charges the parasitic capacitances of Q2 and Q3 to

large enough such that the source of Q3 has been

charged to +VIN. At this time, Q3 turns on at zero

voltage. The transformer is now effectively shorted

through Q1 and Q3, with the primary magnetizing

current circulating in the loop formed by the

transformer primary, Q1, and Q3.

4. CLOCK then goes high and A2 goes low, while A1

connected from RA (pin 1) to GND. During this time,

both Q1 and Q4 are OFF. The primary magnetizing

current discharges the parasitic capacitances of Q1 and

Q4 to GND.

Q4 is discharged to GND, and Q4 turns on at zero

voltage. With both Q3 and Q4 ON, a new power cycle

starts, and power is delivered to the output.

The above sequence is then repeated with the roles of

side A and B interchanged.

The ML4828 can also be used in current mode by sensing

the load current on the RAMP input (pin 11).

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

B2

B1

A2

A1

Q3

Q2

Q4

Q1

TRANSFORMER

B

A

+VIN

ML4828

Figure 1. Simplified diagram of Phase Modulated power Outputs.

CLOCK

A2

A1

B1

B2

B

A