DC-DC｜Application

Important Points in the Design of a Power Supply Using a Floating Type Linear RegulatorHow to determine efficiency and Thermal design for Floating Type Linear Regulator ICs

2025.01.28

table of contents

Where power supplies are concerned, efficiency is an extremely important matter for study. This article concerns the method for determining the efficiency of a linear regulator.

How to Determine the Efficiency of a Linear Regulator

The efficiency of a power supply is expressed as the ratio of the output power to the input power (in percent). This is also the case for linear regulators. The efficiency of the BA1117 can be determined using the following equation. This equation is for floating type linear regulators not having a ground pin, such as the BA1117; for regulators having a ground pin, the (IOUT + IADJ) term becomes IIN (input current). A separate article relating to regulator types having a ground pin will appear in future.

\(\eta = \displaystyle \frac{P_{OUT}}{P_{IN}} = \displaystyle \frac{V_{OUT} \times I_{OUT}}{V_{IN} \times (I_{OUT} + I_{ADJ})} \times 100 \, [\%]\)

VIN：Input voltage［V］
VOUT：Output voltage［V］
IOUT：Output current［A］
IADJ：ADJ pin current［A］

However, when IADJ ≪ IOUT, the following equation can be used.

\(\eta = \displaystyle \frac{V_{OUT}}{V_{IN}} \times 100 \, [\%]\)

From the equation, we see that the smaller the difference between the input and output voltages, the better is the efficiency. However, the minimum value of the input and output voltage difference is specified as the dropout voltage, and it should be kept in mind that a value greater than this must be secured.

Thermal Design for Floating Type Linear Regulators: Estimation of Junction Temperatures

As explained in relation to determination of efficiency in the previous capture, the efficiency of a linear regulator is greatly affected by the difference in the input and output voltages. Put simply, lower efficiency means large losses, which in turn means large amounts of heat generation, and so thermal design is important.
In order to secure a stable and reliable power supply circuit, thermal design must be performed such that the junction temperature TJ of a linear regulator IC does not exceed the stipulated absolute maximum rating TJ(MAX). In order to do so, the junction temperature must be estimated; there are two methods of estimation.

Estimation of the Junction Temperature TJ Using the Thermal Characterization Parameter ΨJT

In cases where measurement of the surface temperature of a linear regulator IC is possible, the thermal characterization parameter ΨJT can be used to estimate the junction temperature TJ. If a thermocouple can be securely affixed to the center of the top surface of the package, the temperature TT at the center of the package top surface can be accurately measured, and so ΨJT can be used to calculate the junction temperature with good accuracy. The calculation equation is as follows.

\(T_j = T_T + \Psi_{JT} \times P \, [\mathrm{℃}]\)

TT：Temperature at the center of top surface of package［℃］
ΨJT：Thermal characterization parameter from junction to center of top surface of package［℃/W］
P：IC power consumption［W］

In the case of a BA1117, the IC power consumption P can be calculated using the following formula. This equation is for a floating type linear regulator that does not have a ground terminal, as is the case for the BA1117; for regulators having a ground terminal, the IADJ term becomes IIN－IOUT. A separate article relating to regulators with ground terminal will appear in future.

\(P = (V_{IN} – V_{OUT}) \times I_{OUT} + (V_{IN} \times I_{ADJ}) \, [W]\)

VIN：Input voltage［V］
VOUT：Output voltage［V］
IOUT：Output current［A］
IADJ：ADJ pin current［A］

However, when IADJ ≪ IOUT, the following equation can be used.

\(P = (V_{IN} – V_{OUT}) \times I_{OUT} \, [W]\)

In addition, the maximum output current that can flow constantly can be calculated using the following equation.

\(I_{OUT(\text{MAX})} = \displaystyle \frac{T_{J(\text{MAX})} – T_{T}}{(V_{IN} – V_{OUT}) \times \Psi_{JT}} \, [A]\)

TJ(MAX)：Absolute maximum rating for junction temperature［℃］
TT：Temperature at the center of top surface of package［℃］
ΨJT：Thermal characterization parameter from junction to center of top surface of package［℃/W］
VIN：Input voltage［V］
VOUT：Output voltage［V］

Estimation of the Junction Temperature TJ Using the Thermal Resistance θJA

A simplified junction temperature TJ can also be calculated using thermal resistance θJA.

\(T_j = T_A + \theta_{JA} \times P \, [\text{℃}]\)

TA：Ambient temperature［℃］
θJA：Thermal resistance between junction and ambient environment［℃/W］
P：IC power consumption［W］

In the case of the BA1117, the IC power consumption P can be calculated using the same equation as that employed for estimates using ΨJT.

Further, the maximum output current that can flow constantly can be calculated using the following equation.

\(I_{OUT(\text{MAX})} = \displaystyle \frac{T_{J(\text{MAX})} – T_{A}}{(V_{IN} – V_{OUT}) \times \theta_{JA}} \, [A]\)

TJ(MAX)：Absolute maximum rating for junction temperature［℃］
TA：Ambient temperature［℃］
θJA：Thermal resistance between junction and ambient environment［℃/W］
VIN：Input voltage［V］
VOUT：Output voltage［V］

The thermal characterization parameter ΨJT and the thermal resistance θJA appear on the IC data sheet, or can be obtained from the IC manufacturer. The thermal characterization parameters ΨJT and thermal resistances θJA shown next are examples of values measured for specific PCBs. Because heat dissipation performance changes due to the effects of PCB characteristics, copper foil layouts, component layouts, housing shapes, the ambient environment, and other factors, the thermal characterization parameters and thermal resistances also change. It must be born in mind that values may differ from the values for actual boards.

Examples of thermal characterization parameters and thermal resistances for TO252-3 packages

PCB type ΨJT［℃/W］ θJA［℃/W］
1 layer（1s） 13 132.2
2 layers（2s） 3 30.2
4 layers（2s2p） 2 23.3

Specifications for the PCBs used in measurements are shown below, in the order of 1-layer boards (1s), 2-layer boards (2s), and 4-layer boards (2s2p).

For further details of thermal design, please refer to “Thermal Design of Semiconductor Components in Electronic Equipment” on Tech Web.

TO252-3 package PCB specifications, 1 layer (1s)

TO252-3 package PCB specifications, 2 layers (2s)

TO252-3 package PCB specifications, 4 layers (2s2p)

    DC-DC

    Basic

    Design

    Evaluation

    Application

    Product Information

    FAQ

    THIS WEBSITE USES COOKIES.
    By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies.
    Privacy Policy
    OK