LTC1922-1
9
OPERATIO
U
Each full cycle of the transformer has two distinct periods
in which power is delivered to the output, and two “free-
wheeling” periods. The two sides of the external bridge
have fundamentally different operating characteristics that
become important when designing for ZVS over a wide
load current range. The left bridge leg is referred to as the
“passive” leg, while the right leg is referred to as the
“active” leg. The following descriptions provide insight as
to why these differences exist.
State 1 (Power Pulse 1)
Referring to Figure 1, State 1 begins with MA, MD and MF
“ON” and MB, MC and ME “OFF.” During the simultaneous
conduction of MA and MD, the full input voltage is applied
across the transformer primary winding and following the
dot convention, V
IN
/N is applied to the left side of LO1
allowing current to increase in LO1. The primary current
during this period is approximately equal to the output
inductor current (LO1) divided by the transformer turns
ratio plus the transformer magnetizing current (V
IN
• t
ON
/
L
MAG
). MD turns off and ME turns on at the end of State 1.
State 2 (Active Transition and Freewheel Interval)
MD turns off when the phase modulator comparator
transitions. At this instant, the voltage on the MD/MC
junction begins to rise towards the applied input voltage
(V
IN
). The transformer’s magnetizing current and the
reflected output inductor current propels this action. The
slew rate is limited by MOSFET MC and MD’s output
capacitance (C
OSS
), snubbing capacitance and the trans-
former interwinding capacitance. The voltage transition
on the active leg from the ground reference point to V
IN
will
always occur, independent of load current as long as
energy in the transformer’s magnetizing and leakage in-
ductance is greater than the capacitive energy. That is,
1/2 • (L
M
+ L
I
) • I
M
2
> 1/2 • 2 • C
OSS
• V
IN
2
— the worst case
occurs when the load current is zero. This condition is
usually easy to meet. The magnetizing current is virtually
constant during this transition because the magnetizing
inductance has positive voltage applied across it through-
out the low to high transition. Since the leg is actively
driven by this “current source,” it is called the active or
linear transition. When the voltage on the active leg has
risen to V
IN
, MOSFET MC is switched on by the LTC1922-
1 DirectSense circuitry. The primary current␣ now flows
through the two high side MOSFETs (MA and MC). The
transformer’s secondary windings are electrically shorted
at this time since both ME and MF are “ON”. As long as
positive current flows in LO1 and LO2, the transformer
primary (magnetizing) inductance is also shorted through
normal transformer action. MA and MF turn off at the end
of State 2.
State 3 (Passive Transition)
MA turns off when the oscillator timing period ends, i.e.,
the clock pulse toggles the internal flip-flop. At the instant
MA turns off, the voltage on the MA/MB junction begins to
decay towards the lower supply (GND). The energy avail-
able to drive this transition is limited to the primary leakage
inductance and added commutating inductance which
have (I
MAG
+ I
OUT
/2N) flowing through them initially. The
magnetizing and output inductors don’t contribute any
energy because they are effectively shorted as mentioned
previously, significantly reducing the available energy.
This is the major difference between the active and passive
transitions. If the energy stored in the leakage and com-
mutating inductance is greater than the capacitive energy,
the transition will be completed successfully. During the
transition, an increasing reverse voltage is applied to the
leakage and commutating inductances, helping the overall
primary current to decay. The inductive energy is thus
resonantly transferred to the capacitive elements, hence,
the term passive or resonant transition. Assuming there is
sufficient inductive energy to propel the bridge leg to
GND, the time required will be approximately equal to
π • √LC/2. When the voltage on the passive leg nears GND,
MOSFET MB is commanded “ON” by the LTC1922-1
DirectSense circuitry. Current continues to increase in the
leakage and external series inductance which is opposite
in polarity to the reflected output inductor current. When
this current is equal in magnitude to the reflected output
current, the primary current reverses direction, the oppo-
site secondary winding becomes forward biased and a
new power pulse is initiated. The time required for the
current reversal reduces the effective maximum duty cycle
