Replacing inefficient low-power chargers and adapters
for low-power applications linear-regulated power supplies have historically been a popular choice thanks to their simplicity and low cost. Linear supplies require only a few components (see Figure 1), and are easier to design and manufacture than comparable switched-mode power supplies (SMPS) designed with discrete components.
While SMPS offer key advantages over linears—smaller size, worldwide operation and better energy efficiency—neither manufacturers nor consumers have been willing to pay a premium for these benefits.
However, linear power supplies are now falling out of favour as a result of two recent developments. First, many linears are sold as external power supplies (EPS) alongside such products as PDAs, cordless phones and cell phones.
EPS are now subject to stringent new energy-efficiency standards that all but rule out the use of linear supplies. As shown in Figure 2, linear supplies generally fail to meet the standards for both operating-efficiency and no-load consumption.
Supplies not meeting the standards will effectively be banned from sale. In Australia , after 1st April 2008, importation or local manufacture of non-compliant External Power Supplies will not be allowed.
Second, integrated circuits now enable the design of low-power SMPS that have significantly fewer components than in the past.
In fact, the most advanced low-power SMPS actually rival the cost and simplicity of linear supplies. This article explores the shortcomings of rudimentary low-power SMPS implementations, and proposes an approach that enables designers to cost-effectively meet the new EPS efficiency standards with minimal design time and effort.Low-power SMPS: The old way
Until recently, the least expensive way to implement a low-power SMPS was as a ringing choke converter (RCC) like the one shown in Figure 3. However, RCCs have a number of drawbacks that have prevented them from displacing linears; these drawbacks should be cause for concern when developing designs to conform to the new EPS efficiency standards.
First, RCCs are not inherently energy-efficient, nor do they contain integrated thermal protection. Each of these features must be added to a basic RCC design, which increases cost and design cycle time.
Also, a typical RCC contains five to ten times more components than a comparable linear supply. While most of these components are fairly inexpensive, their sheer number results in higher design and manufacturing costs.
The higher the parts count, the more complex the PCB trace network is and the longer it takes to optimise the layout. Errors in component placement also become more likely as the parts count rises.
Also, because the board size allotted for low-power chargers and adapters is typically very small, a double-sided board is often needed to accommodate surface-mounted devices (SMD) and make all of the connections. Moreover, the installation of SMD parts requires extra manufacturing steps, which increases production time and costs.
Finally, since the performance of RCCs depends on the interaction between hard-to-control parasitic component values and the combined tolerances of numerous discrete parts, constant monitoring and adjustment are required during manufacturing to keep yield rates at acceptable levels. A detailed examination of the RCC circuit in Figure 3 illustrates a number of these drawbacks.
Inefficient start-up circuit
A typical start-up circuit (R1, R2, R3 and VR1 in Figure 3) provides initial operating current to the MOSFET switch drive. However, current flows through the start-up circuit continuously, even after normal operation begins, and the power loss (I2R) in the voltage-dropping resistors (R1 and R2) prevents many SMPS (not just RCCs) from meeting the no-load power consumption limits of the EPS efficiency standards.
Parts can be added to inhibit current flow once the supply is working normally, but doing so would increase the component count, complexity and cost of the design. Any practical solution must eliminate the post start-up losses without increasing the parts count or cost of the supply.
Switching frequency / MOSFET gate drive
Since RCCs are self-oscillating, their switching frequency (FSW) depends on a number of factors including the inductance of the transformer and its part-to-part variance, resistor and capacitor value tolerances and stability, the amount of load current being drawn from the supply, and the ambient temperature in which the supply operates.
The FSW of a basic RCC largely depends on the time it takes to reset the magnetic flux in the transformer core, which means that FSW will be lowest at full load and highest at no-load.
However, to meet EPS efficiency standards, a power supply’s FSW must decline significantly as the load decreases. This problem cannot be resolved without increasing the design complexity, the parts count, and the cost of the RCC supply.
Controlling the switching of MOSFET Q1 requires eight discrete parts (Q2, C3, C4, C5, R4, R5, R7 and VR2 in Figure 3) and a winding on transformer T1. The imprecision of this method allows variations in MOSFET performance, power supply efficiency, and EMI generation that can easily shut down a production line.
Using a PWM control IC would solve many issues while reducing the component count, but such ICs are rarely cost-competitive in supplies that deliver less than 10 watts of output power.
Furthermore, very few control ICs currently have a FSW reduction function that works automatically as the load on the output drops. Most only have a burst-mode that works at or near no-load.
MOSFET (primary side) current sense
Current sense resistors (R6 in Figure 3) must have tight tolerances and good temperature stability, which makes them expensive. Besides that, sense resistors effectively add to the RDSON of the MOSFET, which can lower the efficiency of the supply by as much as 1–2%.
Current sense transformers are prohibitively costly in this power range, and the only other means of sensing MOSFET current requires patented integration techniques. Nevertheless, finding a way to eliminate the current sense resistor would reduce component count and cost while increasing efficiency.
Voltage sense and feedback
Four components (R12, R13, VR3 and U1-A in Figure 3) on the secondary side of the supply sense the output voltage and pass a feedback signal to the primary side, where it is used to control the duty cycle of Q1.
The parts count on the secondary side is already low and cannot be reduced without losing voltage regulation accuracy. However, supporting the collector of U1-B on the primary side requires a diode and RC filter (D5, C6 and R8). Eliminating those components would simplify the primary-side PCB trace network.
Drain-node clamp
One last place where components could be eliminated from a low-power SMPS is the drain-node clamp (D6, C7, R9 and R10 in Figure 3). Eliminating the clamp would reduce the amount of space required on the primary side of the PCB and make trace layout faster and easier.
Lack of thermal protection
RCCs have no inherent over-temperature protection, a function that has become an industry-wide standard for EPS, thanks to the thermal fuse present in most linear supplies (see Figure 1).
Adding a temperature sensor and shutdown circuit would only increase the design time, materials and manufacturing costs of RCC supplies.
A solution: integrated SMPS ICs
All of the drawbacks of designing and manufacturing RCC supplies can be eliminated by using a highly integrated power conversion IC that has a controller, a power MOSFET, and protection functions incorporated on a single chip.
This approach keeps component count low, which not only minimises time, labour and material costs, but also reduces PCB layout and fabrication costs. In fact, commercially available, highly integrated power conversion ICs make it possible to design low-power SMPS that meet the cost, parts count, and design simplicity benchmarks of the linear supplies that are becoming obsolete.
Additionally, supplies designed around such ICs typically provide superior end-user safety, field reliability and energy-efficiency performance compared to linears or RCCs.
Figure 4 is the circuit diagram of a 2-watt SMPS designed around a highly integrated power conversion IC. The circuit contains only half as many components as the RCC circuit shown in Figure 3, making it far easier and less expensive to design and manufacture.
In fact, when materials, design time, manufacturability and shipping logistics are all accurately compared, this circuit can be produced at a cost equal to or lower than that of the equivalent linear supply.
The reduction in parts count is enabled by the IC, which features a high-voltage (700 V) power MOSFET and a low-voltage controller integrated onto a single chip.
The IC’s ON/OFF control scheme enables quick start-up with no output overshoot, and requires no control-loop frequency compensation components. The controller is self-biased from an internal high-voltage current source connected to the DRAIN pin, which eliminates external start-up and bias supply circuitry.
This further reduces component count while also reducing no-load power consumption. The controller skips switching cycles to regulate the output voltage of the power supply; cycle skipping reduces the effective FSW as the current demanded by the load drops, which further reduces no-load consumption and increases active-mode efficiency.
The IC contains a built-in, auto-recovering hysteretic thermal shutdown function, which improves user safety and field reliability without adding to the parts count. The chip’s integrated auto-restart function protects the supply against output short-circuits and open feedback loops, again without any additional components.
Finally, proprietary chip design and innovative transformer winding techniques eliminate the need for a drain-node clamp circuit, which further reduces component count, design time and PCB layout time.
Summary
Until now, RCCs have been the lowest-cost type of SMPS in the sub-four-watt power range. However, their high parts count, design and manufacturing difficulties, and inability to meet energy-efficiency standards without substantial redesign make them undesirable candidates for replacing the linear supplies that are becoming obsolete.
In this article, various aspects of an RCC supply were examined to see what could be done to reduce its parts count. Integrating many of the circuit functions onto a single chip would be an economical way to lower component count while enabling a power supply to meet EPS energy-efficiency standards. Power conversion ICs that include these circuit functions are already commercially available to power supply designers.
John Jovalusky is a Technical Marketing Engineer at Power Integrations. John has worked as a power-supply designer since 1993, and has designed power supplies for Lambda Electronics, C&D Technologies and Power Integrations. For more information, contact Braemac on (02) 9550 6600.
16-Apr-2007