Important Characteristics ? Power Supply Characteristics

So far we have covered the important properties of the switching regulator as part of a discussion on switching regulator basics. In this article, we explain the important properties of a switching regulator as a “power supply” in connection with the previous topic, “IC Specifications.”

As noted previously, the design of newer switching regulators depends significantly on the power supply IC that is used in the regulator. Consequently, an important prerequisite to satisfying the required specifications as a power supply is the selection of a proper IC. For this purpose, some tradeoff may be needed between IC and power supply specifications.

For example, if an IC is selected based upon the assumption that as a power supply unit an over-current protection function is needed, the IC may include over-voltage and thermal protection functions in addition to the over-current protection. While some ICs permit the disabling of specific features, many of them do not provide such an option. In such a case, a good choice might be to change the specifications to power supply specifications that include the missing feature, if the presence of the feature does not present an adverse impact. Conversely, an over-current protection function could be added by selecting an IC lacking such a feature and by providing an appropriate external circuit. However, the work needed in circuit design, the cost of additional components, and the need to test the operation could turn out to be not cost effective in terms of time, dollar cost, and required mount space. Absent functional issues and cost increases, tradeoffs that produce an improved output as a power supply unit might be a desirable approach.

As important properties of a power supply, at least the following properties should be understood and carefully examined:

Line Regulation

Line regulation refers to fluctuations of the output voltage relative to the variation of the input DC voltage. This may be expressed in percentage points or a specific fluctuation in a given input range, such as 12 mV. For power supply ICs, and in particular for linear regulators, in most cases they have the same name specification. In terms of semantics, it is identical. Input voltage conditions for line regulation of a power supply are based on a presumed input voltage range of the power supply. In the case of line regulation, the property to be addressed means static output voltage fluctuations, that is, non-transient fluctuations.

Although newer power supply ICs provide excellent its line regulation performance, in terms of circuitry as a power supply, we need to look beyond IC capabilities, but also we must study the capability of input capacitor to be used to ensure sufficient line regulation.

Load Regulation

Load Regulation refers to fluctuations in the output voltage relative to the variation in load current. Similar to line Regulation, the load regulation is expressed in terms of percentage points and fluctuations between a given set of load variations. As in the case of line regulation, load regulation specifications apply to the IC itself. However, when the IC is viewed as a power supply, we need to focus on the fact that voltage levels differ between power supply outlet and load input as the voltage declines, due to the resistive components of the output wires. At the outlet for power output, when the load current fluctuates, changes occur in a manner dependent upon the load regulation of the power circuit itself. At the load inlet, however, there is an additional decrease in voltage due to the resistance component of the interconnect. For this reason, many situations can arise where the voltages at the power supply pins for the load requiring large currents decline unexpectedly. A more detailed discussion on this topic will be presented in the section on “Evaluating the Switching Regulator”.

One of the load fluctuations is a transient fluctuation. As in the case of line regulation, however, load regulation is not a property on transient phenomena. To address load transients, we invoke a separate concept of transient response.

Efficiency

Efficiency is defined as the ratio (%) of the output power to the input power. In simple terms, efficiency is a value that can be arrived at by measuring the power (current x voltage) pulled in at the input end and the power extracted from the output end. While the importance of efficiency is obvious, remember that minimizing losses directly translates to reducing heat generation. Heat generation represents a critical evaluation item because not only it limits the amount of output power that can be utilized, but it also requires the space and devices for heat dissipation and cooling, and can even be a factor that reduces the reliability of power supply circuits and of add-on circuits.

Input/Output Ripple Voltage

Ripple Voltage, which refers to pulsation, occurs on both the input and output ends. On the output end, since the device of interest is a switching regulator, there always exists a ripple voltage stemming from switching operations. Although the term Switching Noise may also be used to describe Ripple Voltage, the former generally encompasses both harmonics and spikes.

In terms of ripples, the ripple voltage, which is the height of a pulse, and the frequency, need to be evaluated. In cases where a low power supply voltage, such as 1V or less, is used, as in the case of an FPGA, situations may arise where the required power supply voltage accuracy cannot be satisfied due to the ripple voltage. In addition, ripples, including harmonics and spikes, tend to reduce the system S/N.

Although output ripples can be reduced by means of an output filter, in situations where the frequency fluctuates, such as in PFM, methods for reducing the output ripple requires a careful analysis.

Input ripples arise when the switching transistor pulls in a large current by switching operations. Because spikes can occur by the switching (on/off) of the current and by the parasitic inductance of the input, elimination of spikes requires a careful circuit layout design. In concrete terms, the input capacitor should be connected right next to the input pins for the IC to eliminate parasitic inductance.

Transient Response

The transient response characteristic describes the rate of response from the time the output load current changes suddenly until the output voltage returns to the set value. Critical factors affecting the transient response characteristic include the response performance of the IC itself, in addition to the output capacitor and the equivalent serial resistance (ESR). In the current-mode power supply IC, the transient response characteristic can be optimized by adjusting the phase characteristics. Also, hysteresis (ripple) control provides highly favorable transient response characteristics.

Allowable Dissipation

Allowable dissipation refers to the extent of direct loss that can be tolerated by the devices (ICs and transistors) used in a power supply circuit. Specifically, it means the quantity of allowable power loss that can be calculated from Tjmax (the maximum junction temperature rating) and the package thermal resistance. In the case of power elements (switching transistors), the term refers to the allowable loss, and for built-in power devices, the term refers to the allowable loss inherent in the IC itself. In terms of circuits, because newer power devices are surface-mounted on a circuit board, in most cases the PCB can be used as a heat sink (it goes without saying that in the case of large-power circuits a separate heat sink is provided); consequently, pattern layout is an important consideration. At any rate, since thermal dissipation and allowable dissipation must be evaluated carefully, sound heat calculations are an important step.

The table below summarizes the main points covered in the above discussion:

• Switching regulator