Datasheet LTC1922-1 (Analog Devices) - 9

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