How to design power supply easily?

How to design power supply easily?

This article provides an overview of power supply design possibilities and introduces basic and common isolated and unisolated power topologies and their advantages and disadvantages. Electromagnetic interference (EMI) and filtering considerations will also be introduced.

In conclusion, the purpose of this article is to help you understand and appreciate the art of power supply design.

Most electronic systems require some kind of conversion between the power supply voltage and the circuit voltage that needs to be supplied. When the battery loses its charge, the voltage drops. Some DC-DC conversions ensure that more stored energy in the battery is used to power the

circuit. In addition, if we use 110 V AC lines, we cannot directly power semiconductors such as microcontrollers. Because almost every electronic system uses voltage converters (also known as power supplies), they have been optimized for different purposes over the years. Of course, optimization goals are often solution size, conversion efficiency, EMI, and cost.

Simple power supply: LDO

One simple form of power supply is a low voltage differential (LDO) regulator. LDO is a linear regulator opposite the switching regulator. The linear regulator places an adjustable resistor between the input voltage and the output voltage, which means that no matter how the input voltage

changes, which load current is passing through the device, the output voltage is fixed. Figure 1 shows the basic principles of this simple voltage converter.

Figure 1

For many years, a typical power converter consisted of a 50 Hz or 60 Hz transformer connected to the grid to produce an unstable output voltage at a certain winding ratio, several volts higher than the supply voltage required in the system. This voltage is then converted to the stable

regulated voltage required for electronic products using a linear regulator. Figure 2 shows a block diagram of this concept.

figure 2

Switch mode power supplies help

The problem with the basic setup in Figure 2 is that the 50 Hz/60 Hz transformer is relatively large and expensive. In addition, linear regulators emit a lot of heat, so the total efficiency of the system is low, and due to the high power of the system, it is difficult to eliminate the heat generated.

In order to avoid the disadvantages of the power supply shown in Figure 2, the switching mode power supply was invented. Instead of relying on 50 Hz or 60 Hz AC voltages, they use DC voltages, sometimes rectifier AC voltages, to produce higher frequencies of AC voltages to use smaller transformers, or in nonisolated systems, LC filter rectifier voltages to produce DC output voltages. The advantage is that the solution is small in size and relatively low in cost. The resulting AC voltage does not need to be a sine voltage waveform. Simple PWM signal waveforms work well

and can be easily generated using PWM generators and switches.

Until 2000, bipolar transistors were commonly used switches. They perform well, but switching speeds are relatively low. The power efficiency is also not high, the switching frequency limit is 50 kHz or 100 kHz. Today, we use switchING instead of bipolar transistors, and switching is much

faster. In turn, switching losses are lower, with switching frequencies up to 5 MHz. This high switching frequency supports the use of very small inductors and capacitors for power stages.

Isolation in the power supply

Switching regulators offers many advantages. They typically provide high-efficiency voltage conversion, allowing boost and buck, and offer a relatively compact and low-cost design. The disadvantage is that the design and optimization process is complex, and switching and switching

frequencies also produce EMI. The difficult design process is greatly simplified by the market of switching mode power regulators and power design tools such as the LTpowerCAD ? and the LTspice ?. With these tools, the circuit design process for switching mode power supplies is semi-automated.

When designing a power supply, the first question to answer is whether electrical isolation is required. There are several reasons for using electrical isolation. It improves the safety of the circuit, allows floating system operation, and prevents noisy ground currents from spreading through

different electronic devices in one circuit. The two common isolation topologies are the reflus converter and the positive shock converter. However, for higher power, other isolation topologies such as push-pull, half-bridge, and full-bridge are used.

If electrical isolation is not required, a nonisolated topology is used in most cases. Isolation topology always requires transformers, which are often expensive and cumbersome, and off_theshelf devices that meet the exact needs of a custom power supply are often difficult to obtain.

Most common topologies when isolation is not required

A common nonisolated switch mode power topology is a buck converter, also known as a buck converter, which accepts a positive input voltage and generates an output voltage below that input voltage. It is one of three basic switching mode power topologies that require only two

switches, one inductor and two capacitors. Figure 3 shows the basic principles of this topology. The high-end switch emits pulsed current from the input, generating a switching mode voltage that alternates between the input voltage and the ground voltage. The LC filter obtains the pulse

voltage on the switch node to generate a DC output voltage. According to the duty-on ratio of the PWM signal that controls the high-end switch, the DC output voltage at different levels is generated. This DC-DC buck converter is highly efficient, relatively easy to build, and requires very few components.

figure3

The buck converter emits pulsed current at the input, which has a continuous current from the inductor. This is why buck converters are noisy at the input and less noisy at the output. It is important to understand this when designing a low noise system.

In addition to buck topology, the second basic topology is boost topology. The boost topology uses the same five basic power elements as the buck topology, but has been rearranged to place the inductor on the input and the high-end switch on the output. The boost topology is used to raise

an input voltage above the input voltage.

fiture 4

When selecting a boost converter, it is important to note that the boost converter always specifies the maximum rated switching current in the data sheet, not the maximum output current. In a buck converter, the maximum switching current is directly related to the maximum achievable

output current, regardless of the voltage ratio between the input voltage and the output voltage. In boost regulators, the voltage ratio directly affects the maximum possible output current based on the fixed maximum switching current. When selecting the right boost regulator IC, you need

to know not only the required output current, but also the input and output voltages designed in development.

The boost converter has very low noise at the input because the inductor consistent with the input connection prevents a rapid change in current. At the output, however, the noise of this topology is significant. We only see pulsed current flowing through the external switch, so we pay more

attention to the output ripple than the buck topology.

A specialized topology

The third basic topology is the inverting buck-boost converter, which consists of only five basic components. The converter takes the positive input voltage and converts it to a negative output voltage, and the name comes from this. In addition, the input voltage may be higher or lower than

the absolute value of the inverting output voltage. For example, a -12 V output voltage may be generated from 5 V or 24 V at the input. This can also happen without any special circuit modifications. Figure 5 shows the circuit concept of the inverting buck-boost converter.

fiture 5

In the inverted buck-boost topology, the inductor is grounded from the switch node. The converter has pulse currents at both the input and output, so the input and output of this topology are noisy. In low-noise applications, this feature is compensated by adding additional input and output

filtering.

One beneficial aspect of the inverse buck-boost topology is that any buck switch regulator IC can be used for this converter. Simply connect the output voltage of the buck circuit to the system ground. Buck IC circuit ground becomes the adjusted negative voltage. This feature makes the

choice of switching regulator ICs on the market wide.

Common isolation topology

In addition to the three basic non-isolated switch mode power topologies discussed earlier, there are many topologies available. However, they all require additional power components. This typically increases costs and reduces power conversion efficiency. Although there are some

exceptions, adding additional components to the power path usually increases the wear and tear. Some common topologies include SEPIC, Zeta, Sauuk, and 4-switch buck-boost. They all have three basic topologies that do not have the capabilities. Here is a list of important features for each

topology:

01

SEPIC: SEPIC produces a positive output voltage from a positive input voltage above or below the output voltage. Boost regulator ICs can be used to design SEPIC power supplies. The disadvantage of this topology is that a second inductor or a coupled inductor and a SEPIC capacitor are

required.

02

Zeta: This Zeta converter is similar to SEPIC, but produces positive or negative output voltages. Also, it does not have a right half plane zero point (RHPZ), which simplifies the adjustment loop. Buck converter ICs can be used for such topologies.

03

The Suk: The Suk converter converts the positive input voltage to a negative output voltage. It uses two inductors, one at the input and one at the output, so the noise at both the input and output is low. The disadvantage is that there are not many switching mode power conversion ICs that

support this topology, because the adjustment loop requires a negative voltage feedback pin.

04

4 Switch Buck-Boost: This type of converter has become very popular in recent years. It provides a positive output voltage from the positive input voltage. The input voltage may be higher or lower than the regulated output voltage. This converter replaces many SEPIC designs with more

efficient power conversion and requires only one inductor.

In addition to non-isolated topology, some applications require an electrically isolated power converter. The reason may be safety concerns, the need for floating grounding in large systems where different circuits are connected, or the need to prevent ground current loops in noise-sensitive

applications. Common isolation converter topologies are backflammer converters and positive converters.

Reaction converters are typically used at power levels of up to 60 W. The circuit works by storing electrical energy in the transformer during on time. When disconnected, the electrical energy is released to the side of the converter to power the output. This converter is easy to build, but

requires a relatively large transformer to store all the power needed for normal operation. This limits the topology to a lower power level. The reverse converter is shown at the top of Figure 6, and the positive converter is shown at the bottom.

figure 6

Advanced isolation topology

In addition to the inverse converter, the positive converter is also popular. It uses transformers in a different way than a reaction converter. During on time, although there is a current flowing through a winding, there is also a current flowing through a second winding. Electrical energy

should not be stored in transformer coils. After each switching cycle, we must ensure that all magnetization of the coil is released to zero so that the transformer does not saturate after several switching cycles. Electricity can be released from the coil using several different technologies. A

common approach is to use active clamps with small additional switches and capacitors.

Figure 7 shows the LTspice simulation environment schematic designed with active clamps of ADP1074. In a positive converter, there is an additional inductor not found in the reaction converter in the output path, as shown in Figure 6. Although this additional component has a associated

spatial and cost impact, it helps produce a lower noise output voltage than a backflammer converter. In addition, at the same power level as the reflaction converter, the transformer size required for the positive shock converter may be much smaller.

figure 7

In addition to the counter-aggressive and positive topology, there are many electrical isolation converter concepts based on different transformers. The following list provides some basic explanations for common converters:

Push-pull: Push-pull topology is similar to positive converter topology. However, the topology requires two active low-side switches, not a low-side switch. A primary transformer with a center tap is also required. The advantage of push-pull converters over positive-shock converters is that

they typically have less noise at runtime and require smaller transformers. The hysteresis of the transformer's BH curve is used in two quadrants instead of one quadrant.

Half/Full Bridge: These two topologies are typically used in higher power designs, starting with a few hundred watts to several thousand watts. In addition to low-end switches, they require high-end switches, but high electrical energy transmission can be achieved with relatively small

transformers.

ZVS: This term is often mentioned when discussing high-power isolation converters. It represents a zero voltage switch. Another term for such converters is LLC (inductive-inductive-capacitor) converters. The purpose of these architectures is to enable efficient transformation. They generate

resonant circuits and switch the power switch when the voltage or current on the switch is close to zero. This minimizes switching losses. However, such designs are difficult to implement, switching frequencies are not fixed, and sometimes EMI problems arise.

Switch capacitor converter

In addition to linear regulators and switching mode power supplies, there is a third set of power converters: switch capacitor converters. Also known as an electric pump. They use switches and capacitor multiplier or inverter voltages. One of the great advantages is that no inductors are

required. These converters are typically used at low power levels below 5 W. However, significant recent advances have allowed for more powerful switching capacitor converters. Figure 8 shows the LTC7820 with a 120 W design with an efficiency of 98.5%, converting 48 V to 24 V.

figure 8

All of the power supplies discussed in this article can be implemented as analog or digital power supplies. What exactly is digital power? The power supply must always pass through the analog power level of switches, inductors, transformers and capacitors. The digital aspect is introduced by two digital building blocks:

Digital power

The first is the digital interface through which the electronic system can communicate with the power supply. Different parameters can be set instantly to optimize the power supply for different operating conditions. In addition, the power supply can communicate with the main processor

and raise a warning or fault flag. For example, the system can easily monitor load currents, exceed preset thresholds, or overheat battery temperatures.

The second digital building block uses a digital loop instead of an analog adjustment loop. This works well, but for most applications, it is best to use a standard analog feedback loop that has a digital impact on some parameters, such as instantly adjusting the gain of an error amplifier or

dynamically setting loop compensation parameters for a stable but fast feedback loop.

An example of a device with a pure digital control loop is ADI's ADP1046A. An example of a digital interface buck regulator optimized by digital impact and with an analog control loop is the LTC3883.

EMI

Electromagnetic interference (EMI) has always been a concern when designing switching mode power supplies. The reason is that the switching mode power supply switches high currents for a very short period of time. The faster the switching speed, the better the total efficiency of the

system. Faster switching speeds reduce the time it takes to partially switch the switch on. Most switching losses occur during this part of the on time. Figure 9 shows the waveform of the switch mode power supply at the switch node. Take buck regulators, for example. High voltage is defined

by the current through the high-end switch, while the low voltage is defined by the high-end switch without the current flowing through the high-end switch.

As can be seen in Figure 9, the noise generated by the switching mode power supply comes not only from the adjusted switching frequency, but also from the switching speed that is much higher than the frequency. Although the switching frequency typically runs between 500 kHz and 3

MHz, the switching time can be several nanal seconds long. At the 1 ns switch conversion time, the corresponding frequency in the spectrum will be 1 GHz. At least these two frequencies will be considered electromagnetic radiation harassment and conduction radiation. The oscillation of

the adjustment loop or the interaction between the power supply and the filter may also bring other frequencies.

There are two reasons for reducing EMI. The first reason is the ability to protect electronic systems that are powered by a particular power supply. For example, a 16-bit ADC used in a system signal path should not pick up switching noise from a power supply;

EMI takes two forms, radiation EMI and conduction EMI. An effective way to reduce radiation EMI is to optimize the PCB layout and adopt technologies such as ADI's Silent Switcher. Of course, it also works to put the circuit in a shielded metal box. However, this may not be practical and in

most cases expensive.

Conductive EMI is usually attenuated by additional filtering. Next, we discuss the additional filtering to reduce conductive radiation.

filtering

The RC filter is a basic low-pass filter. However, in a power supply design, each filter is an LC filter. In general, it is sufficient to add some inductors in series, as it will form an LC or CLC filter along with the input or output capacitors of the switching mode power supply. Capacitors are

sometimes used only as filters, but given the parasitic inductance of power lines or walklines, we combine capacitors to form an LC filter. Inductor L may be an inductor with a coil or an ferrite bead. The purpose of the LC filter is actually a low-pass effect that allows the DC power supply to

pass through and largely attenuates higher frequency interference. The LC filter has a bipolar point, so high frequency attenuation at 40 dB/ten octaves is achieved. The filter enables a relatively sharp frequency drop. Designing filters is not easy; however, because the parasitic components

of the circuit, such as trace inductors, have an effect, modeling filters also requires modeling the primary parasitic effect. This makes analog filters time-consuming. Many designers with experience in filter design know which filters work well and may iteratively optimize a filter for a new design.

When designing all filters, it is necessary to consider not only the small signal behavior, such as the conversion function of the filter in the Porter diagram, but also the large signal effect. In any LC filter, the power supply passes through the inductor. If the power supply is no longer needed at_the output, the electrical energy stored in the inductor needs to be released somewhere due to the sudden load transient. It charges the capacitor of the filter. If the filter is not designed for this worst_case scenario, stored electrical energy can cause voltage overshoot and damage to the circuit.

Finally, the filter has a certain impedance. This impedance interacts with the impedance of the power converter attached to the filter. This interaction can lead to instability and oscillation. Simulation tools such as ADI's LTspice and LtpowerCAD are useful for answering all these questions_and designing well_designed filters. Figure 10 shows the graphical user interface for filter designers in the LTpowerCAD design environment. Using this tool to design filters is simple.

Silent Switchers

Electromagnetic radiation harassment is hard to stop. Special shielding made of some metal material is required. The cost of doing so is high. Engineers have long been looking for ways to reduce the harassment of electromagnetic radiation from switching mode power supplies. A few_years ago, Silent Switcher made a major breakthrough. By reducing the parasitic inductance in the thermal circuit of the switch mode power supply and dividing the thermal circuit into two circuits, set in a highly symmetrical manner, the electromagnetic radiation disturbance is mostly_offset by each other. Many Silent Switcher devices today offer much lower levels of electromagnetic radiation harassment than conventional products. Reducing electromagnetic radiation harassment increases switching speed without creating a severe EMI. Increasing the switching speed_reduces switching losses and thus increases the switching frequency. An example of this innovation is the LTC3310S, which has a switching frequency of 5 MHz and a very compact design using low-cost external components.

In this article, we discuss many aspects of power design, including different power topologies and their advantages and disadvantages. This information is fundamental for power engineers, but it is helpful for experts and non-professionals to use software tools such as LTpowerCAD and

LTspice during the design process. With these tools, power converters can be designed and optimized in a very short time. Hopefully, this tutorial will help you meet your next power design challenge.

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