Knowledge about DC-DC converters

DC-DC converter modules are widely used in consumer electronic products, but how do they work and what are the most important criteria when choosing converter products? In this article, we will introduce DC-DC converters to help understand some of the technical challenges faced in creating compact and reliable energy-efficient designs.

DC-DC converters are divided into different categories according to application: isolated and non-isolated, step-up and step-down, or both buck-boost types. The market can also provide modules with specific power levels and functions, many of which have become general "commercial" components.

Let us first define the meaning of "isolation": If there is an electrical connection (usually ground) between the input and the output, the part is non-isolated. The isolation component has an internal transformer that magnetically transfers energy between the input and the output, so the output can "float" with respect to the input. We will consider the isolation level later.

Basic design knowledge of DC-DC converter

Non-isolated DC-DC converter

The simplest non-isolated DC-DC converter is a series of series resistors, but since the voltage drop changes with the load current, the actual circuit uses a transistor to reduce the voltage, which is controlled by a feedback circuit to keep the voltage constant. This is a "linear" regulator, usually available in a three-pin TO-220 package. But the problem is that the voltage can only be lowered but not increased, and the device itself consumes a lot of power, the magnitude of which is the difference between the input and output voltage multiplied by the load current. The solution to the above problems is a "switch mode" regulator, whose transistors are completely turned on or off, and in both cases consume very little power. After flowing through the inductor and capacitor, the voltage pulse is delivered to the output, and then the pulse is "Smooth" back to DC, and control the output voltage by changing the pulse width. Compared with linear circuits, switch-mode regulators have significantly improved efficiency. A good feature of switch-mode regulators is that they can also be configured to increase the voltage and become a "boost" converter. When it is only used to reduce the voltage, it is a "buck" converter.

Basic design knowledge of DC-DC converter

There are other types of regulators, such as "SEPIC" circuits that can generate negative output voltages (buck-boost) and "SEPIC" circuits that can generate output higher or lower than the positive input voltage. This is very useful in battery-powered applications, where the load voltage needs to be kept constant while the battery is discharging.

Basic design knowledge of DC-DC converter

In many circuits, achieving the highest efficiency with the smallest size is a key consideration for product development, and it is very important for DC-DC converters. When increasing the pulse frequency of the switch-mode converter, smaller inductors and capacitors can be used. However, another influencing factor is that the transistor will dissipate some power at each switching edge. The more switching edges (frequency) per second, the more power is dissipated. Therefore, efficiency and size are often opposed. Magnetic technology has hardly improved in these aspects recently, but the latest wide bandgap semiconductor technologies such as silicon carbide (SiC) and gallium nitride (GaN) make the loss of each switch edge smaller, so higher frequencies and volume can be used. Smaller magnetic components. This technology enables non-isolated DC-DC converters to have an efficiency higher than 95%, and the output current can reach 100A or even higher. For example, Texas Instruments (TI) PTH04040W has a rated current of 60A, its output is adjustable from 0.8V to 2.5V, its input voltage is from 2.95V to 5.5V, and its package size is only 51.94 x 26.54mm.

A modular boost converter is not very common. An example here is ABXS002A3X41-SRZ from GE Critical Power, which can convert 816V input into 1634V output, the output current is 2.3A, and module size is 27. 9 x 11.4mm.

Non-isolated DC-DC converters are often called "point of load (PoL)" converters because they can be used directly on typical loads of processors or FPGAs to make the load voltage as accurate as possible. The most complex type has many features such as digital control, so performance can be adjusted "on the fly" to adapt to changes in load conditions, usually using standard protocols (such as PMBusTM) I2C lines for setting. Voltage Regulator Module (VRM) is a special type of PoL that can meet the specific requirements of IC manufacturers, such as similar products from Intel.

Isolated DC-DC converter

For various reasons, isolation may be required. It is often convenient to separate the input and output grounds so that the current paths are separated without interaction. A common application is to power circuits for the RS485 interface. The isolated power rail for the driver can prevent the current flow between the host ground and the connected device.

Basic design knowledge of DC-DC converter

With a "floating" output, the load ground point can be connected to any DC-DC output terminal. For example, by connecting the positive output to the ground, the floating 12V can be configured as -12V. Similarly, 12V can be "stacked" on another voltage, such as inputting 12V to provide a total voltage of 24 volts, which may be useful for small motor drives.

Basic design knowledge of DC-DC converter

Power coupling through the transformer can also provide a certain degree of anti-EMI performance, especially for the common mode noise generated on the local ground line.

An important reason for isolation is usually the need for security. You might think that if a DC-DC converter has a lower input and output voltage, safety is not a problem. However, the isolation layer in DC-DC is often used as part of a wider isolation system to achieve overall safety ratings. An extreme example is the "patient connection" application in the medical market (Figure 6). In this application, the DC-DC converter must have full medical-grade rated isolation for the highest system voltage (possibly 230VAC). The reason is that other equipment connected to the patient may fail due to this applied dangerous voltage. DC-DC can make dangerous current flow back to the original equipment through the "unspecified connection path", so it must have a high level of isolation.

Under the appropriate system voltage, a DC-DC converter that can fit this medical application specification should be marked with two "patient protection measures (MOPPS)", such as the THM 30WI series from Traco Power.

Basic design knowledge of DC-DC converter

Many promotional materials for DC-DC converters simply call an isolation rating a voltage value, such as 3kVDC. Users should be especially careful because this is only a factory "test" voltage, although this is an indicator of isolation robustness. Unless a specific safety agency limits its isolation level to "basic", "enhanced" or medical "protective measures", DC-DC converters should not be used in safety isolation systems. Even if standards such as "EN60950 enhanced" are cited, the system voltage must be specified. The enhanced version at 30VAC system voltage will be of no value if used in a 230VAC system as a safety barrier.

There are many types of isolated DC-DC converters. The simplest low-power type usually has no regulation but is just a "proportional" converter, that is, the output changes proportionally with the input. These components are low-cost and can be used to provide "spot" voltages for interfaces or analog circuits. However, since their no-load output voltage may be much higher than the rated value, they usually have a minimum load requirement. These components are very efficient at rated load, but are much lower at light load, and may even be less than 50%. After careful design, these types of DC-DC converters can provide high performance and good mechanical rated isolation, such as Murata's 2W surface mount NXJ2 series.

The DC-DC converter with active regulation function can withstand input and load changes while maintaining strict output voltage regulation. 2:1 input changes used to be a standard, such as 18-36V, but now components can be used for a wider range of 5:1 input, or even larger, such as CM1901-9RG from BEL Power, this product is suitable for extreme Power rail applications for input drops and surges.