Switch-Mode Power Supplies for Beginners: An Efficiency Primer Part 2

May 27, 2008 3:28 PM
By Daniel Wagner, Associate Member of the Technical Staff, Applications, and Roger M. Kenyon, Director, Customer Applications Engineering, Maxim Integrated Products, Chandler, Ariz.

Obviously, to minimize capacitor power loss, low-ESR capacitors are chosen. SMPS with larger ripple currents especially benefit from low-ESR capacitors. Also, since ESR is the main contributor to output-voltage ripple, selecting a low ESR capacitor offers much more of a benefit than improved efficiency alone.

In general, different dielectric-type capacitors are characterized by certain levels of ESR. As a rule of thumb, for a given capacity and voltage rating, aluminum electrolytic and tantalum capacitors are distinguished by higher values of ESR than their ceramic counterparts. Polyester and polypropylene capacitor ESR values usually fall in between.

For a given type of capacitor, observing the ESR equation above, larger capacitances and lower DFs offer lower ESR. Larger case sizes often reduce ESR as well, but at the expense of performance due to increased series inductance. Additionally, lower capacitor voltage ratings will tend to reduce ESR.

SMPS IC Tradeoffs

The selection of an SMPS IC can bring about good opportunity for exceptional efficiency numbers, especially if that particular IC has efficiency enhancements included in the package, design or control architecture.

The integration of switching devices into the IC package not only offers the advantage of eliminating the time required for MOSFET or diode selection, but it can improve efficiency by making the circuit area more compact, thereby reducing trace losses and other parasitics generated by a looser circuit design. Depending on the power level and process limitations, the MOSFET, diode (or synchronous MOSFET), or both can be integrated into the product.

One important IC specification that demands attention is quiescent current (I_Q), which is the current required to support the device itself. The efficiency effects of I_Q are relatively unseen for heavier loads (greater than about one or two magnitudes of I_Q), since load current swamps IQ. However, as load current decreases, efficiency begins a downward trend, since power loss due to I_Q is a larger percentage of overall power transfer from the source.

While ICs are generally designed for low I_Q, some products tout extremely low I_Q, and are geared toward portable or battery-powered applications. Some ICs offer selectable operating modes that reduce I_Q, making them more suitable for applications with sleep or other low-power modes.

The control architecture of an SMPS has a significant effect on the potential efficiency of the SMPS. This was seen with synchronous rectification control as discussed in part 1 of this article, where the larger power loss exhibited by the switching diode was replaced with a much lower loss of a MOSFET.

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Another common control technique that is important for converters operating in light-load regions is pulse skipping. Unlike pure PWM switching, where the regulation scheme requires a constant switching frequency regardless of heavy or light loads, pulse skipping allows the controller to skip switching cycles. This action prevents unnecessary energy transfer that would ultimately reduce efficiency.

When pulses are skipped, the inductor is allowed to discharge for a longer period of time, and more energy is transferred from the inductor to the load to maintain the output voltage. Naturally, the output voltage bleeds down according to the current draw of the light load. Once the low-voltage regulation threshold is reached, a new switching cycle is initiated to recharge the inductor and refresh the output voltage from the input supply.

Keep in mind that pulse skipping creates output noise dependent on the load. This makes noise more difficult to filter, since switching noise does not occur at constant intervals as with constant PWM control.

Advanced SMPS ICs often combine the benefits of constant-frequency PWM at higher loads with the enhanced efficiency of pulse skipping at light loads. The IC depicted in Fig. 1 is such a device.

As loads increase to higher active values, pulse skipping waveforms transition to constant PWM, with noise easily being filtered during the normal operating load. The overall effect is maximum efficiency over the entire operating range, as demonstrated in the example efficiency curves of a typical stepdown converter with selectable pulse-skipping and PWM modes (Fig. 3).

The constant PWM operation of curves D, E and F shown in Fig. 3 exhibit low efficiency at lighter loads, but offer tremendous efficiency (up to 98%) for higher loads. For light loads, the IC switches whether or not the load requires it, thus wasting power and yielding the low efficiencies indicated. For higher loads, the energy penalty of maintaining PWM switching is small when compared to the load, causing power losses to be overshadowed by the output power, forcing efficiency to be high.

On the other hand, the pulse-skipping “idle mode” efficiency of curves A, B and C maintain a high value of efficiency even down very light loads since switching occurs only as required by the load. For the 7-V curves, that’s more than a 60% efficiency improvement for 1-mA loads.

Maximizing SMPS Potential

Although switch-mode power supplies are popular for their very high efficiencies, the efficiency is ultimately limited by intrinsic losses present throughout the SMPS circuit. But by carefully considering fundamental SMPS losses while pouring over the data sheets of selected SMPS ICs and supporting components, the engineer can expect to make well-informed choices that maximize SMPS efficiency to its full potential.

For Additional Reading

Eichhorn, Travis, “Estimate Inductor Losses Easily in Power Supply Designs,” Power Electronics Technology, April 2005.

Application Note, “Equivalent Series Resistance (ESR) of Capacitors,” Quadtech Inc., www.quadtech.com.

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