Design For Power Efficiency Like A Symphony Conductor

Power passives—inductors and capacitors—are big in today’s consumer products. That’s big in physical size compared with the tiny thin phones and tablets appearing everywhere today. What’s an engineer to do? We have to be smart and apply scientific methods. Every week, it seems like someone suggests reducing the switching power-supply size and increasing efficiency, which usually means adding more battery operational time. Sure, it’s a good idea. But we still need to reduce the physical space.

A promising approach is to attack the power passives because of their relatively large size, which requires many contradictory and incongruous engineering tradeoffs. Symphony conductors must know the sound and the capability of each instrument and each musician to direct a harmonious production. Their clever intertwining of the music adds to the audience’s delight and creates return customers. Similarly, a good designer must have a wide range of knowledge to ensure a well-integrated, power-efficient solution.

Why Concentrate On Switching Supplies?

Appliances are getting smaller, lighter, and thinner, consuming less power while offering lots of cool features. This is true for portable consumer devices, ultra-thin big-screen TVs, and even white goods like dishwashers and clothes washers and dryers. Consumers are demanding longer lasting batteries for operation and faster charging. The issues, then, really crystallize into power management and batteries. Battery technology has promised breakthrough advances but they are delayed because the problems in chemistry are hard to solve quickly. So, we must address power management. Linear analog regulator power supplies must turn excess voltage into heat where switching supplies can transform the voltage with minimum loss.

The initial thought is to save power. First, we must account for nanoamperes, so we won’t need as much power. While there are many techniques for saving power in discrete components, many more can be used inside ICs. Second, we must place the passives inside a hybrid IC package. We might get away with placing one or two discretes inside, but not 10. Really? Well, let’s try. Third, we must ask how small we can get, which is logically the same as how high we can go in frequency without sacrificing efficiency. Can we get high enough in frequency so we could actually use a bond wire or an on-chip capacitor?

There are many types of switching supplies, but we will concentrate on the buck supply, which takes a higher voltage and reduces it to a lower voltage. Our focus is on small supplies, less than a few amps and with voltages under 10 V. Small, light, efficient power supplies are the hallmarks of battery-powered consumer and portable devices.^1,2

Understanding Power Switching Losses

The power losses (mainly heat) are in two areas: switching and filtering losses. Switching losses are in conduction and in the actual switch time. Filter losses happen when reducing ripple to an acceptable level.

Conduction loss is the power lost in transistors because they have a small resistance in the on-condition. A short circuit with no resistance is not possible, because even a mechanical switch has some resistance. Bipolar (BJT) and metal oxide (MOS) transistors could be used in different applications. We will concentrate on complementary MOS transistors, as they are the most commonly integrated components.

One tradeoff is that a CMOS with a low drain source on-resistance (R_DS(ON)) tends to be physically larger and switch slowly. Small-pattern CMOS transistors are fast but have higher R_DS(ON). Inside an IC we can parallel many small CMOS transistors to reduce R_DS(ON) but, of course, we never get something for nothing. Soon the capacitance, resistance, and inductance of the parallel devices start slowing the switching speed.

Switching losses are associated with the time that it takes to switch the transistors on and off, and the losses can be considerable. Obviously, we must switch faster, but how? There are limits because it is necessary to charge and discharge inductance and capacitance. Eventually as the switching speed reaches its shortest time and the frequency rises, we reach a point where we have all switch, rise, and fall times, and no “on time.” By that time, efficiency is pretty well gone (see the figure).

A simplified CMOS switching supply output stage illustrates energy and efficiency loss during switch operation. Let’s compound the problem by introducing dead time and shoot-through prevention.^3,4 The top and bottom transistor in the figure cannot be on simultaneously without shorting the power supply to ground, so great care is taken to ensure that the gate-drive signals are well timed to prevent it. Because of process variations, designers force a short dead time in the waveform where both transistors are off. This time is subtracted from the available time period, further reducing the highest possible switching frequency.

Dead time isn’t always enough to prevent all shoot-through. Alternatively, the dead time needed is so long in the worst case that switching efficiency suffers. Shoot-through events occur when both output devices are on together (e.g., with circuit faults or transients such as sudden load changes). Shoot-through is most common near crossover (dead time) when the transistors are almost completely on or off. A transient at this time can drive both transistors partially or completely on.

Now let’s pile on yet another conflicting requirement: where the “cure” wastes energy and actually makes heat. Switching glitches primarily form impedance mismatches at the output. With general-purpose supplies where the load is highly variable, it is sometimes difficult to remove switching glitches. Thus, transient snubbers are added.

A transient snubber usually comprises a combination of resistors, inductors, capacitors, and sometimes a diode. It is used to reduce switch transients that might damage the output transistors and other components. However, snubbers aren’t good at returning the extra energy back into the circuit to add to efficiency. Instead, they make it into heat.

Filtering losses are usually defined by the amount of ripple that the load can tolerate. Digital circuits can withstand more ripple because of their digital thresholds.5 The analog circuits cannot withstand even a small amount of ripple.

Inductor and capacitor size is related to frequency. As the frequency increases, the inductor and capacitor shrink. Imagine drinking a large soda drink in big gulps, which might be the “manly” thing to do, but a child could do the same job in many smaller sips. The total power transferred will be the same, but the small inductor and capacitor can do it at a higher frequency and, with proper design, less ripple and less power lost to the needed low-pass filtering.

Starting Power-Supply Design

Now some good news. These switching losses are very challenging for large supplies with multiple discrete components intended for general use. Those supplies must accommodate a wide range of conditions. But well-defined, highly integrated supplies inside ICs let the designer control, simulate, and custom fit some special circuit topologies. It’s not that difficult to design, if you know some tricks of the trade.

First, we must gather information about the design and thoroughly define the issues. Included in the exercise are details about each voltage and current required. What are the tolerances, accuracy, load, and line regulation? Most important, what voltage is really required? If the voltage does not require tight tolerances, would a resistor-divider and an impedance converter such as a transistor or operational amplifier do the job?

Heat is a major concern for several reasons. The heat must be dissipated to prevent a device from overheating. This can be a very difficult challenge when, for example, a cell phone is left in the sunlight and sealed in a closed automobile. Moreover, any heat is wasted battery life from overheating and will cause the battery to be charged more often. Nobody wants that. Time between charges is the paramount consumer concern. Thus, efficiency counts.

Switched Or Linear?

Now we will decide how to partition the supplies into switched and linear configurations. The best way to make a switched supply efficient is to understand both the supply voltage (usually the battery and battery under charge voltages) and the load variations. The most efficient switcher is one that has little change in load current. One easy way to shrink the inductor and capacitor physical size is to increase the switching frequency.⁶

Consider a specialized computer that has large differences in power required in different operating modes. We can think of two applications with similar requirements: a satellite receiver and a computer monitoring a physical process perhaps inside a factory. Both utilize a hard disk intermittently for low-cost storage. To save energy and increase the disk life, the disk can despin.

We will specify that the disk can spin up and be ready to read or write within 20 seconds. This means that the satellite receiver must provide a minimum of 20 seconds of semiconductor memory so any command from the operator appears seamless. The process monitoring device has to have enough memory so data can be intermittently recorded on the hard disk, while minimizing the necessity of spinning the disk up to speed.

Another operating mode might be a deep sleep with minimum power. Here, the satellite receiver or factory process is not in use during the night. In either case, each operating mode must be accommodated to maintain top efficiency.

Think “Inside The IC”

Generic switching and linear three-terminal voltage regulators need to operate over wide extremes of current draw because designers cannot predict how or where they will be used. When we know the exact application, we can tailor a circuit and optimize the efficiency. For example, the easy way to make a generic regulator is to confine it to low bandwidth to prevent oscillation. Then we depend on the decoupling capacitors to supply the fast high-frequency transient currents. The regulator supplies the slower average current. This is usually a good design tradeoff because the small local capacitors near the ICs mitigate the resistance and inductance losses in the connections back to the regulator.

Another philosophy is employed for known loads in audio amplifiers: use wider bandwidth power regulation. The simplest example is fanning out a reference voltage to many stages of amplification. In this approach, one could use op amps to isolate small circuit portions and prevent interactions. Another way to minimize the decoupling capacitor size is to place a relatively small capacitor on the input of an amplifier and use the gain (Miller effect⁷) to increase the effective size of the capacitor.

Think transistors. ICs have the advantage that all the transistors are made at the same time by a photographic process. This ensures close matching of parameters, much closer than discrete transistors made at differing times. The output stage in the figure can have two transistors added when they match well. The top transistor and the bottom each become two transistors in a cascade configuration.

Because the transistors have half the voltage applied to them, they can be controlled more easily and improve switching speed and efficiency. Well-matched transistor circuits on a single die can more easily control parasitic capacitance and, thus, time delays to reduce dead time and control the phase and timing of multiple supplies.

Another “inside the IC” option at first sounds silly—until we do the math and simulation. For example, we have a 5-V supply and want to make 2.5-V and 1.2-V supplies. Conventional thinking says we would build two parallel supplies, 5 V to 2.5 V and 5 V to 1.2 V. At high frequencies (30 MHz to 100 MHz), the filter or ripple losses diminish compared to the switching losses. One must do the math to see if cascaded supplies might have less loss.

Cascading the supplies actually means the 5 V makes 2.5 V and then the 2.5-V supply makes 1.2 V. This is not intuitive, as there is a double conversion and double efficiency loss to two-step the 1.2-V power. Restated simply, the 5-V to 2.5-V supply must pass all the current used at 2.5 V plus all the current used at 1.2 V. Something surprising can happen now because the 1.2-V supply transistors have half the voltage across them compared to the parallel condition: their switching loss drops, especially with loads light enough to make the cascaded pair more efficient.

Designers often will make a clean power supply in an IC and then provide power to multiple circuits with current mirrors. Many designers use identical NPN transistors because they are made at the same time, with the same process, and will have identical VBE voltage drops. By tying the bases together, each emitter can distribute identical voltages to many different circuit stages. Since we are replicating a clean power supply, the number of decoupling capacitors is greatly reduced.

Conclusion

A symphony conductor coordinates the instruments to produce a harmonious, clean sounding, enjoyable musical presentation. The project engineer controls the power parameters that will result in a harmonious, efficient system that pleases the end user. For both the conductor and we engineers, the devil is in the details. Our conductor must be able to recognize a perfect instrumental rendition as well as a poor one. Our design engineer must be able to identify even small deficiencies in the power structure that may add to spoil the whole.

References

For more ideas about battery systems, see Maxim Integrated tutorial 671, “Energy Management for Small Portable Systems”.
Maxim Integrated tutorial 5290, “Higher Integration Drives the Newest Generations of Smartphones”.
For more information on shoot-through, see Maxim Integrated application note 1135, “Small Capacitor Improves Efficiency in High-Power CPU Supply”.
For more information and an example of dead time and shoot-though protection, see Maxim Integrated application note 5424, “Thermoelectric Cooler Control Using the DS4830 Optical Microcontroller,” Figure 6.
For more information on digital thresholds, see Maxim Integrated application note 4345, “Well Grounded, Digital Is Analog”.
For more information on a 4-MHz switcher, see Maxim Integrated application note 3603, “Buck Converters Proliferate in Handhelds as Features and Processing Power Increase”.
See Origin of the Miller Effect, scanned by Kent H. Lundberg; John M. Miller. “Dependence of the input impedance of a three-electrode vacuum tube upon the load in the plate circuit,” Scientific Papers of the Bureau of Standards, 15(351):367{385}, 1920. This paper was written in June 1919 and published in 1920 by the Government Printing Office in Washington, D.C. The copyright on this paper has expired. It is now in the public domain.