Yesterday, Silanna Semiconductor expanded its line of fully integrated active clamp flyback (ACF) PWM controllers to include the 60W SZ1105 flyback PWM controller. The new device is targeted at designs for compact power adapters for notebooks, video game consoles, and multi-port wall chargers and has a power density of 20W/in3.
Its 30W sibling, the SZ1101 is meant for devices such as mobile phone power chargers.
Image from Silanna Semiconductor
ACF controllers serve two important functions in today’s mobile world. Firstly, they enable designers to keep the size of the external power supply “bricks” down to a small size so that they don’t become burdensome to consumers. Second, the power efficiency that they enable vastly cuts down on the heat generated by the units.
As pictured in the diagram below, the SZ1105 is an ACF pulse width controller combining four key ACF components:
- The flyback controller
- The active clamp driver
- The active clamp FET
- Startup low-dropout (LDO) regulator
Block diagram of the SZ1105. Image from Silanna Semiconductor. Click to enlarge.
The device augments the simplicity of a flyback controller the added benefits afforded by an ACF design. These benefits include the recapture of the leakage inductance energy of the flyback transformer and also limiting the primary FET drain voltage spike during the turn-off events.
Silanna’s OptiModeTM digital control architecture allows the SZ1105 to adjust the device’s mode of operation on a cycle-by-cycle basis. This allows the unit to maintain its high efficiency, low EMI, and fast dynamic load regulation in response to variations in both the line voltage and the changing power demand presented by the load.
The device switches within a tight range in the area of 140 kHz, which makes for simplified EMI filtering. Furthermore, adaptive digital control of active clamp operation enables near zero voltage switching (ZVS) turn-on of the primary FET, and clamps the drain voltage during the turn-off. This further improves efficiency and also serves to reduce EMI.
Application diagram for the SZ1105. Image from Silanna Semiconductor
The unit operates over an input range of 90 – 265VAC. The 60W unit’s outputs of 3.3V, 5V, 9V, 11V, 12V, 15V or 20V are available at a maximum of 3A. It meets DoE and CoC efficiency and no-load standby power requirements, and when the load draws no power, the SZ1105 consumes less than 50mW of power.
Silanna plans to offer a 60-W USB-PD evaluation board using the SZ1105.
The device comes in a 16-pin SOIC package that takes up about three square inches.
The Field of Active Clamp Devices
Not surprisingly, the SZ1105 and SZ1101 are not the only entrants in this important ecosystem.
Texas Instruments offers an active-clamp flyback controller, UCC28780, that operates at high-frequency (1 MHz). According to TI, the controller enables high-density AC-to-DC power supplies to comply with global efficiency standards. If you're curious, TI provides an excellent video describing the operations of active clamp flyback controllers.
Beyond these entries from TI, ON Semi, and Silanna, there don't seem to be many more flyback converters that utilize the active clamp design, though the topology has also been used in reference designs, including a GaN-focused design from Navitas released in early 2018.
Do you know of other devices that compare? Have you designed power supplies using an active clamp flyback converter? Share your thoughts and experiences in the comments below.