Miniature isolated DC/DC converters for space-saving designs

The growth of distributed power architectures has led to the development of miniature, low-power (<2 W) DC/DC converters. As the name implies, these devices minimize the board space occupied by the converter. Measuring just 0.71 x 0.29 x 0.09 inches (18.1 x 7.6 x 2.35 mm), the devices provide point-of-use isolated power conversion for analog circuits in industrial and safety-critical applications such as telecommunications and medical equipment. In addition, miniature DC/DC converters are convenient for designers who need electrically isolated output power or need to reduce noise in analog circuits.

For most new noise-sensitive circuits, system designers often need to ensure that a few components are completely isolated from the input power supply. Loads and noise on local power rails should be isolated from the main power rails of the entire system. For example, mixed-signal IC designs often fail to work properly due to noise issues. Large amounts of digital noise combined with sensitive analog circuits often cause interference noise [1].

Miniature DC/DC converters with galvanic isolation enable extremely low

Output noise and very high accuracy. Galvanic isolation can help reduce system noise by creating a floating ground on the secondary side of the converter [2]. Therefore, we can achieve a simple isolated output power supply through input-to-output isolation, and we can also use it to generate different voltage rails, bipolar rails, and/or non-standard voltages. In addition, as a noise reduction technique in analog circuits, the isolation barrier can prevent the digital ground bus from affecting sensitive analog circuits.

Standard brick-type converters are not suitable for this requirement. In addition to their high cost, the output power of brick-type converters is not suitable for applications with power less than 2W. In addition, the mounting area of ​​brick-type converters is too large. Designers want to miniaturize devices to save valuable board real estate. Since micro DC/DC converters are even smaller than the new 1/16 brick converters (0.29 square inches compared to 1.17 square inches, 18,709.64 square millimeters compared to 754.84 square millimeters), they are particularly suitable for generating on-board voltages in space-constrained designs.

So why not "build your own" DC/DC power supply? Discrete component on-board converter design is a good alternative to brick converters and off-the-shelf micro DC/DC converters. However, fewer and fewer people are doing discrete design. The disadvantages are that it is difficult to implement additional functions (such as device protection and module-to-module synchronization), reliability is low, and it is difficult to reduce the size of the power supply. In addition, for many advanced products, the DC/DC converter is just one of the components, and it is difficult for designers to have the time to specialize in converter design and become experts in related equipment. In view of these difficulties, more and more designers are choosing to apply micro DC/DC converters. This converter greatly reduces risk, significantly accelerates time to market for new products, and greatly saves development costs.

In addition to their advantages over brick converters and discrete designs, micro DC/DC converters provide a small power solution for point-of-load (POL) power conversion. POL converters help designers address the challenges posed by the high peak current requirements and low noise margin requirements of new high-performance semiconductor devices. The converter can be placed close to its load. This minimizes losses due to voltage drops, helps overcome noise susceptibility and EMI interference issues, and ensures tight regulation under dynamic load conditions.

Availability

Designers who require isolation and output currents from 20 mA to 500 mA are aided by micro DC/DC converters from C&D Technologies, Texas Instruments (TI), and Wall Industries. The converters take 5 V, 12 V, 15 V, or 24 V inputs. Single-output converters range from 3.3 V to 24 VDC. Dual-output converters range from ±3.3 to ±24 VDC.

The micro DC/DC converters provide regulated or unregulated outputs with input-to-output isolation ratings of 1000, 1500 or 3000 VDC. Operating temperature range is -40 °C to +100 °C. Due to their high switching frequency (400 MHz), the converters can achieve efficiencies as high as 85%.

Built-in features

For system designers looking to implement a micro DC/DC converter, the devices have several built-in features that simplify system integration and design. Many of the available products include overtemperature protection, short-circuit protection, and internal filtering. Some of the more advanced converters also allow synchronization between devices. If an application uses more than one converter on a PC board, beat frequencies or other electrical interference may be generated. The DCP010512B and DCR010505 converters (Figure 1) address this problem with built-in synchronization control, which allows multiple converters to be synchronized with each other. This feature is very convenient and allows designers to synchronize up to eight devices by simply connecting the SYNCIN pins together, which eliminates electrical interference caused by switching frequencies.

structure

DC/DC converter technology continues to advance toward higher density, higher efficiency, and smaller packaged converters. One innovative converter is manufactured in the same way as standard integrated circuit (IC) packages, including dual in-line (DIL) and small outline (SO) packages (Figure 2). It uses the IC lead frame as the medium to interconnect the silicon chip device and the magnetic components in the IC package (Figure 3), resulting in an isolated DC/DC converter that achieves high reliability, excellent thermal management, a small form factor, and is compatible with standard board assembly processes. The standard IC format also allows tape and reel assembly, which helps reduce manufacturing costs.

Different configurations

Output galvanic isolation of micro DC/DC converters Non-standard voltage rails can be generated by connecting multiple converters in series. This is achieved by simply connecting the positive output of one converter to the negative output of another converter, thereby generating a variety of voltage variations. The outputs of some dual-output converters can also be connected in series, thereby doubling the amplitude of the output voltage that can be provided. Figure 4 shows the connection diagram of a dual ±15 V converter that can provide a 30 V voltage rail [3].

If a single converter cannot provide the required output power, then connecting multiple converters in parallel can solve the problem. If parallel connection is adopted, a good design principle is to ensure that the converters in parallel are of the same type. Figure 5 shows two converters connected in parallel [2].

Solving Voltage Isolation Problems in Low-Power Applications

Low output power micro DC/DC converters with input-to-output isolation provide innovative solutions for high-density power supply designs in a variety of applications. Typical examples include industrial process control, DC motor drives, measurement and testing, power transmission, medical equipment, and data acquisition. The following figures show examples of DC/DC converters and operational amplifiers in common power supply designs.

Positive and negative voltage conversion

Positive-to-negative voltage conversion is common in applications such as micro DC/DC converters. High-precision op amps are optimized for higher speed applications. The devices achieve very low offset voltage and drift and are often used in data acquisition, telecommunications equipment, professional audio equipment, and portable applications that require high accuracy. Some op amps require an auxiliary power rail. The isolated output of the converter in Figure 6 is used to generate a negative supply voltage. The isolated negative output is referenced to the positive input of the op amp.

Positive and negative voltages from a single power supply

Tiny DC/DC converters in standard IC packages reduce the board area required to generate positive and negative output voltages to op amps. Figure 7 shows how a single isolated converter can efficiently generate positive and negative output voltages for signal conditioning circuits with differential inputs. Dual-output converters are used to generate unregulated output voltages greater than their +5 V input voltage. The converters operate from an input voltage range of 4.5 V to 5.5 V and feature a boost function to generate a +12 V output and a buck-boost function to generate a -12 V output.

Power supply isolation for AC or DC instrumentation amplifiers

When it comes to AC or DC instrumentation amplifier signal conditioning circuits, the challenge for system designers is to eliminate ground loops that affect measurement accuracy. Miniature isolated DC/DC converters achieve this goal. The circuit shown in Figure 8 uses a dual-output DC/DC converter to generate isolated voltages. The converter is combined with a high-precision isolation amplifier to completely isolate the instrument from noise generated by other parts of the circuit. This isolation barrier property does not affect the integrity of the signal, so it has excellent reliability and good high-frequency noise immunity.

Summarize

When designing noise-sensitive circuits, system designers often need to completely isolate a small number of components from their input power supply. Micro DC/DC converters can effectively isolate the load and noise of the local power rail from the main power rail of the entire system.

The devices available on the market are designed with highly integrated packages, which make them widely used in space-compact designs. Compared with standard brick converters, they are smaller in size and lower in price. Compared with discrete designs, micro DC/DC converters not only reduce risks, greatly speed up the time to market, but also save development costs.