Silanna Semiconductor’s Active Clamp Flyback Controllers Combine Four ACF Subsystems

January 16, 2020 by Gary Elinoff

The SZ1101 (33W) and SZ1105 (65W) incorporate four key ACF subsystems to simply design and reduce BOM count.

Silanna Semiconductor has announced new active clamp flyback (ACF) controllers that the company claims are the first to combine four key ACF subsystems. These include an adaptive digital ACF controller, an active clamp FET, an active clamp FET driver, and a start-up regulator.


Four ACF components

The SZ1101 and SZ1105 include four ACF components. Image used courtesy of Silanna Semiconductor

This configuration allows the new flyback PWM controllers to recapture the flyback transformer’s leakage inductive energy and limit the FET’s drain voltage spike during turn off. 


Designed for Adapters

The (PDF) SZ1101 can source 33W and is aimed at applications such as mobile phone travel adapters. 


SZ1101 PCB front side (left) and backside (right side)

SZ1101 PCB front side (left) and backside (right side). Image used courtesy of Silanna Semiconductor

The (PDF) SZ1105, rated at 65W, is targeted at power adapters designated for notebooks, video game consoles, and multi-port wall chargers. At its full 65W output, SZ1105 is designed for a 25 watt-per-inch power density. This can be achieved while using a standard, avalanche-rated silicon MOSFET main switch.


SZ1105 PCB front side (left) and backside (right side)

SZ1105 PCB front side (left) and backside (right side). Image used courtesy of Silanna Semiconductor


Silanna Semiconductor states that the devices can work with inputs ranging from 90VAC to 265VAC and are capable of 93% efficiency. In addition to USB-PD output ports, the units can also be used to implement programmable power supplies (PPS). 

During no-load periods, the units draw less than 50mW. Both devices are available in 16-pin SOIC packages.


OptimodeTM Digital Control Architecture

Using Silanna's OptimodeTM Digital Control Architecture, the internal operations of both ACF PWM controllers can be optimized on a cycle-by-cycle basis. This is said to allow high-power efficiency as well as low EMI. 


Application diagram of active clamp flyback controller

Application diagram of SZ1105 active clamp flyback controller. Image used courtesy of (PDF) Silanna Semiconductor

According to Silanna Semiconductor, the devices aim for fast, dynamic load regulations even with variations in the line voltage and with fast-changing output demands.


Switching Frequency and EMI Control

Both the SZ1101 and the SZ1105 feature switching frequencies that can range up to 140kHz. The actual switching frequency is tightly controlled. This simplifies the task of EMI control.

They can also be put into Quasi-Resonant Valley Mode switching, which is said to contribute to EMI suppression. 


Efficiency vs load current (230VAC) of SZ1101 and SZ1105

Efficiency vs load current (230VAC) of SZ1101 (top) and efficiency vs load current (230VAC) of SZ1105 (bottom). Image used courtesy of Silanna Semiconductor

Furthermore, adaptive digital control of active clamp operation allows users to turn on near-zero voltage switching (ZVS) in the primary FET. This also serves to clamp the drain voltage during the turn-off. This improves efficiency even further and also helps to further reduce EMI.

The new units achieve a 6dB margin for EMI. Silanna Semiconductor claims that this is an industry best for an all-silicon solution.


Electrical and Temperature Hazards

The new flyback PWM controllers provide protection against the hazards often encountered in power applications. Some of these protections include: 

  • OTP (overtemperature protection)
  • OVP (overvoltage protection)
  • OCP (over-current protection) 
  • OOPP (over-power protection)
  • OSCP (short circuit protection)


Typical Application Circuit

The diagram below applies to both the SZ1101 and the SZ1105. As depicted, the flyback PWM controller is used to implement a USB-PD active clamp flyback converter.


Typical application circuit of SZ1101

Typical application circuit of SZ1101. Image used courtesy of (PDF) Silanna Semiconductor

Unlike standard ACF designs, there are no requirements for tight tolerances of the clamp capacitor and leakage inductance values in order to assure proper operation of the circuit in high volume production. 

Additionally, a 3.3nF clamp capacitor can help users realize the benefits of ACF operation.

As Director of Marketing Ahsan Zaman explains, “Silanna’s all-silicon design combines the simplicity of traditional flyback controllers with the efficiency and power density enabled by ACF controllers. The SZ1101 and SZ1105 reduce BOM cost not only by integrating the ACF circuit but also by enabling designers to use a lower-cost transformer, SR FET, clamp capacitor, and input filter. Our proprietary OptiModeTM control maximizes performance on a cycle-by-cycle basis, giving the best performance under all operating conditions.”