The DC to DC Converter and its Applications

Abstract

This paper is for amateur engineers who are introduced to the design and development of a project at a component level abstraction. In this paper, we will discuss briefly about a DC-to-DC converter and its uses. By looking at the simplified model of the converters and linear regulators, it would be possible to foresee areas in which efficiency can be greatly enhanced. We will also discuss and examine the selection criteria for some applications.

Ricky Ramdani, Anil Rajpatei, Oscar Agudelo, Rocky Maharjan, and Sandhya Sapkota

Every electrical system requires some sort of input power requirement. These power requirements can rarely be obtained at the point of installation. For direct current (DC) systems, the accessible voltage at the location may be higher or lower than required, depending on the sub-system to be installed. In some cases, the sub-system to be installed may need to be connected to two-point, where the differential voltage may be kilovolts high. In other cases, the supply may come from a battery in which voltage is very limited and varies over time.

In the first case where the accessible voltage is higher than required by the subsystem and power loss is not a major concern, a simple linear regulator may be used. However, there is a tradeoff for using a linear regulator. The disadvantage is, the losses incurred would be the voltage across the regulator multiplied by the current passing through it. For example, a 1 amp 5 volt regulator operating from a 12 volt supply would dissipate 7 watt of power while only supplying the needed 5 watts to the load as shown in the figure.

 

Therefore, linear regulators are found in systems when the required input voltage to the regulator should be higher than the required output voltage, when power loss is not a major concern, and when isolation is not required.

The DC-to-DC Converter (Switching Regulator)

The DC-to-DC converter, also called a switching regulator, is classified in two major groups – isolated and non-isolated. In the isolated group, the input is galvanically isolated from the output. These converters are used when direct current flow from the output to the input is not allowed. Other uses of these converters include isolation from higher voltages, floating current and voltage sense, isolation from high power, and inductive and noisy systems – some of them will be discussed later.

The non-isolated converters are used when isolation is not required and efficiency is a major concern. They are also used when the required input voltage to the regulator is lower than the required output voltage. Note: The isolated family has a capability to step-up voltage.

The operation of the DC-to-DC converter can be understood by looking at the structure of a simplified model and the cycle operation. The simplified converter consists of a switch, inductor, capacitor, and a diode as shown in the figure.

The inductor and the capacitor are used to store energy. A transformer can be introduced between the switch and the filter in order to provide higher voltage than that is being supplied and to galvanically isolate the system. The time it takes for a signal to complete one on-and-off state is called the duty cycle and it is used to adjust the energy supplied to the load. When the switch is closed, energy is supplied to the storage elements and the load. When the switch is open, the stored energy is released to the load. By adjusting the duty cycle (on vs. off time of the switch) the amount of power going to the load can be adjusted. It is good to note that the power loss in this system is close to zero assuming ideal components. That is, by treating the inductor and capacitor as lossless components and by having the instantaneous V*I product (wattage) across the ideal switch and diode approach zero.

The Losses Associated with the Linear Regulator

All linear regulator contain a series pass element and/or shunt element. A simplified series regulator is shown in the figure below.

 

The base of the transistor is held at the reverse voltage of the zener diode and the emitter is held at the zener diode plus the base emitter voltage. As a result, the gross power loss in the pass transistor is the emitter collector voltage times the emitter current.

The table below lists the uses for linear regulators, isolated and non-isolated DC-to-DC Converters.

A Closer Look at the Linear Regulator vs. the Converter from a Performance Perspective

In the not too distant past, DC-to-DC converters were only considered in extreme cases where linear regulators could not be used. This was due to the infant stage of converter technology. These days, the converter has matured to a point where linear regulators are only used when their advantages are highly needed. The advantages of the linear regulators are fast response, simplicity, and clean power. These advantages are rapidly being met by the switching converters. Comparing these advantages with current DC-to-DC converters, we see that the converters are currently made as a plugin component which is as simple as the linear regulator. The response time for a well-designed DC-to-DC converter can approach somewhere in the µS range and with a single LC filter it can perform as good as the linear regulators. The same LC circuit can also be tailored to meet the noise requirements of the system. These LC filters can be added as glue circuit and in many cases the manufacturer could introduce a variation in the model to meet the requirement. These advances are quickly replacing the linear regulators in most applications.

A closer look at the power loss also shows that a linear regulator operating at such low efficiencies would require the added cost of heatsinks and would need a larger power supply in their parent systems in order to compensate for the wasted power. With multiple regulators, the cost can exponentially increase. As the non-isolated converter technology matures, it will eventually replace the linear regulators.

The Isolated Converter, where the Linear Regulator or the Non-Isolated Converters cannot be used

Some systems require that the subsystem to be added to be galvanically isolated. Examples of such requirement are medical systems, high power systems where noise is generated, floating measurement systems, and high voltage systems. For medical systems that interface with patients and are connected directly or indirectly to main supply, there should not be any galvanic path between the patients and high potentials as shown in the figure.

The level of protection needed are many folds and can be found in associated regulatory documents. Using medical grade, isolated DC-to-DC converters provided part of the required protection.

Electromagnets being switched on and off in designs containing motors or solenoids can produce large amounts of noise, causing major problems with data converter and sensitive analog circuits. In these systems, the isolated supply can provide true isolation from the electrical noise generated by the high power circuits. Examples of such configuration is shown in the figure.

 

Measurement of the Systems’ Current

Generally, current in a system is measured on the return or the low side of the load. In systems where the return line is directly connected to the ground of the system, a simple linear or non-isolated switching regulator would satisfy the power requirements. A resistor connected in series with the load on the return line would provide the current measurement as shown in the figure. The voltage across the resistor, which is proportional to the current through the load is conditioned and sent to some other parts of the system.

 

Floating Current Measurement

In some cases, the voltage on both nodes of the sense resistor would vary with respect to the system ground due to another circuit connected to the return line of the load. The worst case variation can be as high as tens or even hundreds of volts. For small variations close to system ground, a simple differential amp with a ground reference would provide the level shift. For the larger variations, the isolated DC-to-DC converter would provide the galvanic isolation such that the measurement circuit’s level would shift appropriately with the load shown in the figure.

 

The isolated measurement circuit can provide current measurement on the high side, low side, and practically anywhere in the system being measured. In systems where current measurements are made throughout, the multiple output isolated DC-to-DC converter maybe be used.

DC-to-DC Converter Selection

Selecting a voltage regulator or DC-to-DC converter for your application may not seem like a difficult task at first. However, when given all the combinations of features and non-standard performance specifications, it becomes clear that some research is needed. The initial specifications listed below are the minimum that is needed to start the selection process.

Total Output Power

Output Voltages

Individual Output Current

Isolation

Efficiency

Load and Line Regulation

Caution

The total output power listed should also be reviewed. Some manufacturers list the total output power at an ambient temperature of 25°C. A graph is provided to derate the total power at elevated temperatures. The maximum ambient operating temperature derating factor should be used in conjunction with the estimated temperature rise to when used in systems that contains heat sources. Note that the power entering the converter is the output power added to the power loss.

For systems requiring multiple outputs, the maximum output power on each output should be added to determine power requirements. For transient load, the worst case RMS should be calculated.

The Output Voltages

With single output systems, selection of the output voltage requires that the load and line regulation requirements to be known. The load and line regulation is the amount of error on the output of the supply at various load and line conditions. Another performance specification that may be needed is the maximum output ripple. The regulation and ripple performance are important in systems where precision analog systems are used. However, for analog systems requiring high performance and a good reference voltage, we recommend using a dedicated voltage reference. Lots of troubleshooting and performance loss can be avoided with strategic selection of voltage references and placement of filter capacitor with little added cost to the design.

After reviewing these preliminary specifications and narrowing down the search to a potential product, samples should be purchased and tested. Recommended test procedures are listed here.

Conclusion

The Switching Regulator is an electronic device that converts direct current (DC) from one voltage level to another. There are different models of DC-to-DC converters and linear regulators; each of them possesses different properties and applications. Isolated groups are used in medical field whereas non-isolated groups are used to step-up voltage. If a combination of features and non-standard specifications are given, it is difficult to choose a DC-to-DC converter because there is some specification of the circuit, which should be considered during selection. In addition, to know the converter’s temperature limit, the maximum operating temperature should be tested. Although linear regulators are fast response, simple, and clean power, well-designed DC-to-DC converters provide similar properties without any power loss.