Quality and Efficiency – The Cornerstones for the Connected Appliance
The latest smart appliances require multiple power rails for the increasingly complex electronics, displays and motor controllers.
Don’t be surprised if your next refrigerator arrives with a 20-inch high resolution color LCD display, touch screen interface and WiFi connectivity. More and more appliances are integrating sophisticated user interfaces along with the ability to communicate with their owners or the internet as part of a “smart home.” In addition to this increased level of complexity manufacturers are also expected to reduce the energy consumption and improve efficiency to meet new European and international standards.
High performance microprocessors, LCD displays and wireless interfaces used in these smart appliances require high quality, well-regulated power supplies that are efficient and cost effective. In response to these requirements power conversion and control chip manufacturers are incorporating new technologies into their devices which improve voltage regulation, power conversion efficiency and reliability whilst reducing manufacturing costs and overall power supply size.
Smart homes, offices, and factories are evolving to provide easier ways of living and working while reducing energy consumption. A number of manufacturers have released appliances which link to the internet via intelligent hubs or directly to customer’s smart phones. These appliances can be monitored and controlled remotely to improve power grid efficiency, reduce consumer’s energy costs and simplify their control interface.
Traditional appliances required very little electronics. Control was often provided mechanically via a small motor based mechanism and simple switches, which limited functionality. The next generation added basic electronic control using a 4-bit microcontroller and enabled appliance manufacturers to add features and improve reliability. Future generations moved to 8-bit microcontrollers capable of driving alpha numeric displays to indicate options available and provide feedback to the user, enabling more complex features to be added. These control units were much more flexible. Their 4-bit and 8-bit microcontrollers and simple alpha numeric displays did not require well-regulated supplies and efficiency was not a driving requirement. Where isolation was not a requirement low cost capacitive droppers could be used to convert from the AC line voltage to low voltage DC supply, these were followed by linear regulators to provide stable power rails. These were inherently unreliable, had very poor overall efficiency, often below 30% and required large expensive X-Caps. Where safety isolation was a requirement several power supply topologies were available to the designer. As power requirements increased above 3 W the most popular isolated integrated power supplies used a switching regulator design known as a flyback converter. A flyback design is similar to a buck converter with a transformer replacing the inductor. In addition to an energy storage element, the transformer provides a galvanic isolation barrier to provide the safety isolation required in these applications.
Initial flyback designs had the feedback control signal taken directly from the secondary side. This signal also had to be isolated to maintain the integrity of the safety isolation barrier with opto-isolators used in the majority of designs. Taking the feedback directly from one of the secondary supply rails improved output voltage regulation and transient response but opto-isolators proved to have a poor reliability (MTBF) compared to the other active components in these designs. These circuits worked well for single rail power supplies and are still used in many high volume applications.
The lower cost option introduced around 2005 used Primary Side Regulation (PSR) where an auxiliary winding was added on the primary (live) side of the supply to provide an analog of the secondary side voltage. This was used as a feedback signal to the switching element controller to regulate the secondary side supply voltages. Although these supplies were reliable and had reasonable efficiency they lacked the supply voltage regulation required by the newer generation of multi output voltage rail designs. A simple fix was to introduce linear regulators on the secondary side to generate multiple supplies but this pushed cost up and efficiency down.
Figure 1 illustrates a typical high efficiency flyback converter design for a single 12V supply with primary side regulation. During the OFF time of the primary side high voltage MOSFET switching element integrated into regulator U3 the auxiliary winding voltage is proportional to the output voltage and the turns ratio between the output winding and the sense and bias winding. The potential divider formed by R19 and R20 feeds a proportion of this voltage back to the FB (feedback) pin of controller U3 where it is compared against an internal reference voltage to generate an error signal which is used to control the high voltage primary side switching element. As the output voltage rises and the error increases the ON time of the MOSFET is decreased to reduce the energy stored in the transformer and transferred to the secondary windings while keeping the circuit operating in the Continuous Conduction Mode (CCM), which is the most efficient mode of operation. Primary side regulation is ideal for single voltage supplies where it can provide good regulation, reduce the system cost and improve the long term reliability of such supplies by removing the optocoupler used in isolated secondary side voltage sensing designs. Power supplies of this type were, and still are, used in high volume appliances ranging from washing machines and microwaves through to shavers and toothbrushes, where a simple single rail supply is required.
As appliances began to integrate more sophisticated electronic control circuits and graphics displays they required multiple supply rails. Secondary side regulation was required due to its superior output regulation and efficiency in these new multi rail applications. Traditional flyback designs continued to use opto-isolated secondary side regulation with Schottky rectifier diodes which initially meet the multi rail voltage regulation and efficiency requirements but have proved the weak link in a lot of appliances due to the poor reliability of the optocoupler and the Schottky diode.
To address these requirements power supply controller manufacturers have developed new technologies and improved the level of integration to ensure they can meet the new performance and cost targets required by the appliance manufacturers. Figure 2 shows the integration levels achieved by the latest generation of AC-DC power supplies based on the new generation of switching controllers. Controllers from several manufacturers combine the high voltage primary side switching power FET, the switching controller, high speed digital isolated link and a host of safety features to protect the sensitive electronic circuits of the appliance. The internal isolation barrier could be a micro-transformer integrated in the package, a transformer integrated on the silicon wafer or new magneto-inductive coupling techniques based around the package leadframe. This new class of controller introduces new conversion techniques which improves both regulation and efficiency while providing reinforced isolation.
The latest smart appliances require multiple power rails for the increasingly complex electronics, displays and motor controllers. For example, 3.3 V for the microcontroller, 5 V for the analog and sensors, 12 V for the LED lighting relays and fans and 18 V for IGBT drivers. A single rail output followed by multiple linear regulators adds cost and significantly reduces the overall efficiency making it impossible to meet the ErP European Energy Directive. Especially important in new designs is the light load efficiency required in standby mode where a limit of 1 W is now becoming a legal requirement. Good cross regulation is also required across the range of output rails to ensure the voltages remain stable under all load conditions.
If we examine the schematic of a typical design shown in Figure 2 we can see that the design uses the familiar flyback topology used in previous generations. However there are important differences which improve voltage regulation, efficiency, transient response and reduce the bill of materials cost. The control IC U1 contains two synchronized switching controllers, one for the primary side controlling the internal high voltage power FET and one for the secondary side controlling the external synchronous rectifying FET’s Q1 and Q2. These controllers are linked by an internal isolated high speed digital link, with the transmitter on the secondary side and the receiver on the primary side. The output voltages are set via the potential divider formed by R8 and R6 for the 12 V rail and R6 and R14 for the 5 V rail. These voltages are then summed and the value applied to the feeds back (FB) pin where it is compared against an internal reference. An internal oscillator generates a continuously running switching cycle clock. At the beginning of each clock cycle the feedback voltage is compared with the internal reference and the controller decides whether or not to implement a switching cycle. At peak loads it will switch continuously while at slightly lower loads it will begin to skip cycles. In continuous conduction mode MOSFETs Q1 and Q2 are turned off just prior to the secondary side controller commanding a new primary side switching cycle. Secondary side control of the primary side power MOSFET avoids any possibility of cross conduction between the two switching elements.
Synchronous secondary side rectification reduces switching losses associated with a conventional rectification diode increasing efficiency. Multiple output rails can be sensed and a weighted sum applied to the FB Pin to provide excellent output cross regulation in designs which require more than one output voltage rail. The ability to directly sense the output voltage and control the primary side Power MOSFET switching via the high speed isolated digital link removes the need for an external optocoupler improving voltage regulation, response to transient loads and reliability. A design like the one illustrated in Figure 2 is available for evaluation as a Reference Design Report, RDR-469. This circuit achieves output voltage regulation better than +/-3% with efficiencies in excess of 90% making it ideal for the new generation of smart appliances. The switcher ICs can be safely connected across the isolation barrier and has reinforced insulation with an isolation voltage greater than 3500 VAC which is UL1577 and TUV (EN60950) safety approved and EN61000-4-8 (100 A/m) and EN61000-4-9 (1000 A/m) compliant.