ML4818 View Datasheet(PDF) - Fairchild Semiconductor

Part Name

Description

Manufacturer

ML4818

Phase Modulation/Soft Switching Controller

Fairchild Semiconductor 

PRODUCT SPECIFICATION

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 sec-

ondary. 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.

3. B2 now goes high after tDELAY (set by RDELAY). Since 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 leak-

age current discharges the drain-to-source capacitance

on Q4 until there is 0V across it.

5. A1 will remain low for a period defined by tDELAY, 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 oppo-

site polarity on all outputs (see Figure 2).

The above sequence is then repeated but with the opposite

polarity on all outputs.

TB

A2

B2

ML4818

B

A1

B1

ILIM

+VIN

Q3

Q1

LLEAKAGE

TRANSFORMER

Q2

Q4

RSENSE

TA

A

Figure 1. Simplified diagram of Phase Modulated power Outputs

CT

CLOCK

A2

tDELAY

A1

tDELAY

B1

tDELAY

tPD1

B2

tDELAY

tPD1

tDELAY

tPD1

tDELAY

B

A

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

REV. 1.0.3 6/21/01

6

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