LTC3722-1/LTC3722-2
13
372212fb
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occur, independent of load current as long as energy in
the transformer’s magnetizing and leakage inductance is
greater than the capacitive energy. That is, 1/2 • (LM + LI)
• IM2 > 1/2 • 2 • COSS • 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 throughout 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 ZVS 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 available
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 do not 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:
2
LC
When the voltage on the passive leg nears GND, MOSFET
MB is commanded “ON” by the ZVS 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 opposite secondary winding
becomes forward biased and a new power pulse is initi-
ated. The time required for the current reversal reduces
the effective maximum duty cycle and must be considered
when computing the power transformer turns ratio. If
ZVS is required over the entire range of loads, a small
commutating inductor is added in series with the primary
to aid with the passive leg transition, since the leakage
inductance alone is usually not sufficient and predictable
enough to guarantee ZVS over the full load range.
State 4 (Power Pulse 2)
During power pulse 2, current builds up in the primary
winding in the opposite direction as power pulse 1. The
primary current consists of reflected output inductor cur-
rent and current due to the primary magnetizing inductance.
At the end of State 4, MOSFET MC turns off and an active
transition, essentially similar to State 2 but opposite in
direction (high to low), takes place.
Zero Voltage Switching (ZVS)
A lossless switching transition requires that the respective
full bridge MOSFETs be switched to the “ON” state at the
exact instant their drain-to-source voltage is zero. Delaying
the turn-on results in lower efficiency due to circulating cur-
rent flowing in the body diode of the primary side MOSFET
rather than its low resistance channel. Premature turn-on
produces hard switching of the MOSFETs, increasing noise
and power dissipation.
LTC3722-1/LTC3722-2 Adaptive Delay Circuitry
The LTC3722-1/LTC3722-2 monitors both the input supply
and instantaneous bridge leg voltages, and commands
a switching transition when the expected zero voltage
condition is reached. DirectSense technology provides
optimal turn-on delay timing, regardless of input voltage,
output load, or component tolerances. The DirectSense
technique requires only a simple voltage divider sense
OPERATION
