Datasheet LTC1922-1 (Analog Devices) - 8

LTC1922-1
U OPERATIO Phase Shift Full-Bridge PWM
5) Optimized current mode control architecture. Conventional full-bridge switching power supply topolo- Benefit: eliminates glue circuitry, less overshoot at start- gies are often employed for high power, isolated DC/DC up, faster recovery from system faults. and off-line converters. Although they require two addi- 6) Proven reference circuits and design tools. tional switching elements, substantially greater power and higher efficiency can be attained for a given transformer Benefit: substantially reduced learning curve, more time size compared to the more common single-ended forward for optimization. and flyback converters. These improvements are realized As a result, the LTC1922-1 makes the ZVS topology since the full-bridge converter delivers power during both feasible for a wider variety of applications, including those parts of the switching cycle, reducing transformer core at lower power levels. loss and lowering voltage and current stresses. The full- bridge converter also provides inherent automatic trans- The LTC1922-1 controls four external power switches in former flux reset and balancing due to its bidirectional a full-bridge arrangement. The load on the bridge is the drive configuration. As a result, the maximum duty cycle primary winding of a power transformer. The diagonal range is extended, further improving efficiency. Soft switch- switches in the bridge connect the primary winding be- ing variations on the full-bridge topology have been pro- tween the input voltage and ground every oscillator cycle. posed to improve and extend its performance and The pair of switches that conduct are alternated by an application. These zero voltage switching (ZVS) tech- internal flip-flop in the LTC1922-1. Thus, the voltage niques exploit the generally undesirable parasitic ele- applied to the primary is reversed in polarity on every ments present within the power stage. The parasitic switching cycle and each output drive signal is 1/2 the elements are utilized to drive near lossless switching frequency of the oscillator. The on-time of each driver transitions for all of the external power MOSFETs. signal is slightly less that 50%. The actual percentage is adaptively modulated by the LTC1922-1. The on-time LTC1922-1 phase shift PWM controller provides enhanced overlap of the diagonal switch pairs is controlled by the performance and simplifies the design task required for a LTC1922-1 phase modulation circuitry. (Refer to Block ZVS phase shifted full-bridge converter. The primary and Timing Diagrams) This overlap sets the approximate attributes of the LTC1922-1 as compared to currently duty cycle of the converter. The LTC1922-1 driver output available solutions include: signals (OUTA to OUTF) are optimized for interface with an 1) Truly adaptive and accurate (DirectSense technology) external gate driver IC or buffer. External power MOSFETs ZVS switching delays. A and C require high side driver circuitry, while B and D are ground referenced and E and F are ground referenced but Benefit: higher efficiency, higher duty cycle capability, on the secondary side of the isolation barrier. Methods for eliminates external trim. providing drive to these elements are detailed in the data 2) Internally generated drive signals for current doubler sheet. The secondary voltage of the transformer is the synchronous rectifiers. primary voltage divided by the transformer turns ratio. Similar to a buck converter, the secondary square wave is Benefit: eliminates external glue logic, drivers, optimal applied to an output filter inductor and capacitor to pro- timing for highest efficiency. duce a well regulated DC output voltage. 3) Programmable (single resistor) leading edge blanking.
Switching Transitions
Benefit: prevents spurious operation, reduces external filtering required on CS. The phase shifted full-bridge can be described by four primary operating states. The key to understanding how 4) Programmable (single resistor) slope compensation. ZVS occurs is revealed by examining the states in detail. Benefit: eliminates external glue circuitry. 8