The power necessary for controller operation is generally lower than that for designs using discrete parts, thanks to the device's gate-voltage-level clamp and high integration. Independent of the VCC supply, the gate charges to 10.7 V. As the trend toward greater switching frequencies continues, particularly with the increasing adoption of resonant controllers, and as paralleling devices boost the gate charge, gate-switching losses account for an increasingly significant amount of power.
The IR1167's gate clamp can reduce those losses. With a 1-nC gate charge, the effect is small ? even at relatively high frequencies. However, at 15 and 30 nC, this gate-drive modification saves in excess of 0.7 and 1.8 W, respectively, of switching power at 500 kHz. The power losses in synchronous FETs are often less than 1 W; this simple reduction in gate-drive level can easily save a comparable amount of power.
The IC also has a sleep mode, which further reduces current consumption. All of these factors help meet the upcoming stringent standards on power consumption under standby, extremely light load conditions.
Experimental Results The following comparison characterizes two commercially available universal-input supplies using flyback topology before and after implementation of the new control technique on the secondary side. The comparison measures efficiency and thermal performance in a thermal chamber at a 45ºC ambient temperature.
24-V, 150-W LCD TV open-frame power supply operating at full load (6.5 A) at 45 ° C ambient: This design provides an active front-end power-factor corrector with two bus voltage-levels, depending on the mains. The secondary circuit's synchronous rectifiers are two 10.5- mΩ MOSFETs in TO-220 packages. The SR drive circuit is a discrete voltage-based controller. The system retrofit consists only of an IR1167 controller, which replaces the original discrete voltage-based contoller; the SR devices and their associated thermal system remain unchanged.
The retrofit improved the supply's overall efficiency by between 0.6 and 0.9% over the input voltage range (100 to 240 V) while reducing the SR FET's tab temperature by between 15.9 and 17 ° C over the same voltage range.
19V, 90W Laptop Adapter operating at full load (4.62A) at 45 ° C ambient: This design provides an active-front-end boost-follower power-factor corrector. The secondary circuit's rectifiers are two 100-V, 30-A, Schottky diodes in TO-220 packages. The system retrofit consists of an IR1167 and two 100-V, 18- mΩ IRF7853 MOSFETs in SO-8 packages.
The retrofit improved the laptop adapter's overall efficiency by between 1.0 and 1.3% over the test input voltage range (120 to 240V), while reducing the SR FET's case temperature by between 6.1 and 8.2 ° C over the same voltage range.
As these efficiency and thermal data indicate, standard flyback-system designs can easily implement SR functions or enhance the efficiency of their current implementations.
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