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What technology is so powerful, halving the size of mobile phone chargers? May 11, 2018

We are living in an increasingly mobile world. Electronic devices are getting smaller and smaller as technology advances. We foresee this trend will continue.

We want the screens of electronic devices to become bigger and bigger, battery life becomes longer and longer, but we are now at a critical point. This critical point is not from the electronic device itself, but from the battery charger. This often overlooked device is very important because it provides the energy needed for the operation of electronic devices. And the aforementioned long-life requirements for large screens are leading these chargers to become larger and larger, unless there is a new solution, otherwise these bottlenecks will affect the portability of electronic devices.

We need to rethink the working principle of the charger. Most of the current chargers on the market are based on flyback topology. Flyback is indeed a mainstream topology choice in the low power range. Because fewer components are needed to convert the same power, like all other switching power supply topologies, the flyback topology works by switching the switching state of the FET at a switching frequency of several hundred kHz.

The switching frequency of the FET directly affects the volume of the charger. The higher the switching frequency, the smaller the charger. However, the switching frequency will have an upper limit. The first problem comes from the leakage inductance of the transformer. When the main side FET is turned off, the energy stored in the parasitic inductance of the transformer is dissipated in the snubber circuit. If the switching frequency is too high, this part of the power Losses can increase dramatically and cause the power supply to heat up significantly.

The active clamp flyback topology can solve this problem. In the active clamp flyback topology, the energy of transformer leakage inductance is not dissipated, but will be stored in the clamp capacitor and then transferred. To the output. The advantage of the active clamp flyback topology is not limited to this. By intelligently controlling the active clamp circuit, the main-side FET can achieve zero voltage turn-on (ZVS), eliminating the main sources of switching power losses and further improving the efficiency. .

The realization of ZVS allows us to use a higher switching frequency to reduce the size of the charger. The advantage of active clamped flyback topology is not limited to this, if we use GaN instead of silicon based FET to achieve ZVS The energy required will be greatly reduced, so that we can use a higher switching frequency.

The size of the charger will be reduced to one-half. The GaN 30W charger (middle) is much smaller than the traditional 24W charger (right), but reliable control of the active-clamp flyback topology is not simple.

In the past, there was no fast and smart control chip product to achieve this topology, but Texas Instruments' UCC28780 control chip will change this situation by integrating a variety of advanced features such as adaptive, self-adjusting ZVS and pulse mode. .

The application of the active clamp flyback topology in chargers has become a reality. The UCC28780 can control both silicon-based and GaN-based main edge FETs. To achieve strict efficiency standards such as DoE level VI and CoC Tier 2.

Texas Instruments also announced the UCC24612, a synchronous rectifier controller used with the UCC28780. Using this synchronous rectifier controller, the secondary side can use a more efficient synchronous rectifier to replace the diode.