ml4818 Microsemi Corporation, ml4818 Datasheet - Page 5

• Manufacturer Part Number: ml4818
• Description: Phase Modulation/soft Switching Controller
• Manufacturer: Microsemi Corporation

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
2. After the
3. B2 now goes high after t
the power conversion cycle.
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).
MOD comparator trips, B1 goes low
A2
A1
ML4818
Figure 1. Simplified diagram of Phase Modulated power Outputs.
I
DELAY
LIM
B2
B1
CLOCK
A2
A1
B1
B2
C
(set by R
B
A
T
T
B
DELAY
t
). Since
PD1
t
DELAY
B
t
DELAY
Q3
Q2
The above sequence is then repeated but with the
opposite polarity on all outputs.
L
4. The CLOCK now goes high turning A2 off. During
5. A1 will remain low for a period defined by t
6. The previous sequence is now repeated with the
TRANSFORMER
LEAKAGE
t
PD1
t
DELAY
the voltage across Q3 is now 0V, B2 turns Q3 on at
zero voltage.
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.
opposite polarity on all outputs (see Figure 2).
+VIN
R
t
SENSE
DELAY
Q1
Q4
t
PD1
t
DELAY
t
DELAY
A
T
A
ML4818
DELAY
,
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