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.
3. B2 now goes high after t
DELAY
(set by R
DELAY
). 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 leakage 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 t
DELAY
,
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.
+VIN
T
B
A2
B2
Q3
Q1
T
A
ML4818
B
Q2
A1
B1
I
LIM
L
LEAKAGE
A
TRANSFORMER
Q4
R
SENSE
Figure 1. Simplified diagram of Phase Modulated power Outputs.
C
T
CLOCK
A2
t
DELAY
A1
t
DELAY
B1
t
PD1
B2
t
DELAY
B
t
PD1
t
DELAY
t
DELAY
t
DELAY
t
PD1
A
Figure 2. Phase Modulation control waveforms (Shaded areas indicate a power cycle).
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
