Issues to consider when choosing a power module

DCDC means DC to DC (conversion of different DC power values). Anything that meets this definition can be called a DCDC converter. Specifically, it means converting the input DC into AC through a self-excited oscillation circuit, and then converting it into DC output after changing the voltage through a transformer, or converting AC into high-voltage DC output through a voltage doubler rectifier circuit.

DC/DC module power supply is increasingly widely used in the fields of communication, network, industrial control, railway, military, etc., with its remarkable features of small size, excellent performance and convenient use. How to correctly and reasonably select DC/DC module power supply? The author will talk about this issue from the perspective of DC/DC module power supply development and design, for reference by system designers.

1 Several aspects to consider when selecting a power module

Rated Power

Package

Temperature range and derating

Isolation voltage

Power consumption and efficiency

2 Rated power

It is generally recommended that the actual power used is 30-80% of the rated power of the module power supply (the specific ratio is also related to other factors, which will be mentioned later.), and within this power range, the performance of the module power supply in all aspects is relatively full and stable and reliable. All module power supplies have a certain overload capacity, but it is still not recommended to work under overload conditions for a long time. After all, this is a short-term emergency measure.

3 Packaging

The dimensions and output forms of DC/DC converters vary greatly. Low-power products use sealed housings with very slim appearance; high-power products often use quarter-brick or half-brick forms, with circuits either exposed or wrapped in housings. When choosing, you need to pay attention to the following two aspects: first, whether the pins are on the same plane; second, whether it is easy to solder. SMT converters must comply with the requirements of the IEC191-6:1990 standard, which strictly limits the coplanarity of SMT device pins. If the converter cannot meet this requirement, a special welding assembly process needs to be designed for it, which will increase assembly time and increase production costs.

There are various packaging forms for modular power supplies, some of which meet international standards, and some are non-standard. For the same company's products, the same power products have different packaging, and the same packaging has different powers. So how to choose the packaging form? There are three main aspects: ① The volume should be as small as possible under certain power conditions, so that more space and more functions can be given to other parts of the system; ② Try to choose products that meet international standard packaging, because they have good compatibility and are not limited to one or two suppliers; ③ They should be scalable to facilitate system expansion and upgrades. All of them meet international standards and are widely used in the industry. Half-brick and full-brick packaging is fully compatible with famous brands such as VICOR and LAMBDA, and the power range of half-brick products covers 50~200W, and full-brick products cover 100~300W.

4 Temperature range and derating

Generally, there are several temperature ranges for the power modules of manufacturers: commercial grade, industrial grade, military grade, etc. When choosing a power module, you must consider the actual required operating temperature range, because the prices of different materials and manufacturing processes vary greatly with different temperature grades. Improper selection will also affect the use, so you have to consider it carefully. There are two ways to choose: one is to choose according to the power used and the packaging form. If the actual power used is close to the rated power under the condition of a certain volume (packaging form), then the nominal temperature range of the module must strictly meet the actual needs or even have a slight margin. The second is to choose according to the temperature range. If a product with a smaller temperature range is selected due to cost considerations, but sometimes the temperature is close to the limit, what should I do? Use it at a reduced rating. That is, choose a product with a larger power or package, so that the temperature rise will be lower when the "big horse pulls a small cart", which can alleviate this contradiction to a certain extent. A compromise should be considered.

Commercial grade (0 ℃ to +70 ℃)

Industrial grade (-40 ℃ to +85 ℃)

Military grade (-55℃ to +125℃)

5 Variable frequency and fixed frequency

Like all switching devices, DC/DC converters generate noise when working, so the filtering performance is also an important selection basis. Integrated DC/DC converters usually use variable frequency switching technology or fixed frequency switching technology. Fixed frequency switching converters are much simpler in this regard, and even LC filters can be used.

6 Operating frequency

Generally speaking, the higher the operating frequency, the smaller the output ripple noise, and the better the dynamic response of the power supply. However, the requirements for components, especially magnetic materials, are also higher, and the cost will increase. Therefore, the switching frequency of domestic module power supply products is mostly below 300kHz, and some are even only around 100kHz. This makes it difficult to meet the requirements of dynamic response under load variation conditions. Therefore, high-switching frequency products should be considered for applications with high requirements. On the other hand, when the switching frequency of the module power supply is close to the signal operating frequency, it is easy to cause beat oscillation, and this should also be considered when selecting.

7 Isolation

Most circuits must be isolated, that is, the load and the noise it makes to the local power supply must be separated from other loads and noise on the power grid. Only isolated converters can meet this requirement. In addition to achieving the above requirements, isolated converters can also achieve differential output and bipolar output (see figure). In addition, connecting the output high voltage end of the isolated converter to the power ground of the load forms a negative power supply. Since the voltage reference point is not the ground, the load can obtain a higher voltage. The highest voltage that the converter can withstand and applied between the input and output ends within a certain time limit (usually 1 second) is called the isolation strength of the converter. Therefore, when designing a low-noise power supply, a DC/DC converter with high isolation strength and low isolation capacitance should be selected to reduce leakage current.

Usually, a very high isolation voltage is required in medical equipment, so that the leakage current is small and the harm to the body is small. The isolation voltage requirement for the module power supply in general occasions is not very high, but a higher isolation voltage can ensure that the module power supply has a smaller leakage current, higher safety and reliability, and better EMC characteristics. Therefore, the current isolation voltage level in the industry is generally above 1500VDC.

8 What is a power surge?

A surge is also called a transient overcurrent, which is a short-term current or voltage fluctuation in a circuit, usually lasting about one millionth of a second. A voltage fluctuation of 5,000 or 10,000 volts that lasts for a moment (one millionth of a second) in a 220-volt circuit system is a surge or transient overcurrent.

Sources of surges: In simple terms, they come from two aspects: external surges and internal surges. External surges: The main source is lightning. When charges accumulate in the clouds and the ground below the clouds accumulates equal charges of opposite polarity, lightning discharge occurs. The charge potential between the clouds and the ground is as high as several million volts. When lightning strikes, several kiloamperes of current are discharged through lightning strikes, passing through all equipment and the ground and returning to the clouds, thus completing the path of electricity. Unfortunately, the path often passes through important or valuable equipment.

Another source of external surges is overvoltage generated on the power lines by the power company's utility grid switches.

Internal surges: 88% of surges are generated by equipment inside buildings. A few years ago, a square centimeter of computer chips had 2,000 transistors, while today's Pentium chips have more than 10,000,000 transistors, which increases the probability of computers being damaged by surges.

Due to the design and structure of the computer, it should work within a specific voltage range. When the surge exceeds the level that the computer can withstand, the computer will have garbled data, damaged chips, and premature aging of components. These symptoms include: unexpected data errors, failure to receive/transmit data, loss of documents, malfunctions, frequent maintenance, unexplained failures and hardware problems, etc.

Lightning surges far exceed the level that computers and other electrical equipment can withstand, and in most cases, they cause immediate destruction of computers and other electrical equipment, or permanent loss of data. Even the start or shutdown of a small 20-horsepower induction engine can generate a 3,000-5,000-volt surge, causing computers that share the same distribution box to be damaged or interfered with every surge, which occurs very frequently.

9. Which electrical equipment can be damaged by power surges?

Electrical equipment containing microprocessors is extremely vulnerable to damage from power surges, including computers and computer auxiliary equipment, program controllers, PLCs, fax machines, telephones, answering machines, etc.; program-controlled switches, radio and television transmitters, microwave relay equipment; and household appliances, including televisions, stereos, microwave ovens, video recorders, washing machines, dryers, and refrigerators. Survey data from the United States shows that 63% of electrical products that have problems during the warranty period are caused by power surges.

10 Sources of Power Surges

Surges can come from outside the electrical device or from inside the electrical device, that is, from the electrical equipment inside the electrical device. Surges from the outside are caused by lightning or the switching of public power grid switches. Both types of harmful power disturbances can disrupt the operation of computers and microcomputer information processing systems, causing shutdowns or permanent equipment damage.

When there is charge accumulation on the cloud layer, and an equal amount of charge with opposite polarity is generated on the lower surface of the cloud layer, lightning discharge will be caused. The situation afterwards is like the discharge of a large battery or a large capacitor, and the charge potential between the cloud layer and the ground is as high as several million volts. When a lightning strike occurs, a current of several kiloamperes is discharged through the lightning strike, passing through all equipment and the earth and returning to the cloud layer, thus completing the electrical path. Unfortunately, this lightning path often takes important or expensive equipment. The key concept of surge protection is to provide a short and effective path to the earth for the lightning induced current. Surges from the inside Surges from the inside are common, such as surges from air conditioners, air compressors, arc welders, electric pumps, elevators, switching power supplies and some other inductive loads.

11 Mean time between failures

Many DPA systems require high reliability, which puts forward requirements for the mean time between failures (MTTF). Here, readers should be reminded that the reliability of a product cannot be evaluated based on the data in the product manual alone. The reason for this problem is that there is currently no recognized definition and calculation standard for the MTTF index in the world. Manufacturers generally use the "general situation" reliability prediction method in the US military standard MIL-HDBK-217F and the telecommunications equipment model in the Bellcore standard TR-NWT-000332. However, even MTTF indicators that claim to be calculated in accordance with the same standard are often inconsistent. Before the converter is put into use, any MTTF index is meaningless. Temperature has a significant impact on reliability. The empirical formula is: for every 10°C increase in ambient temperature, the device life will be shortened by half.

Relevant statistics show that the main reason for module power failure within the expected effective time is damage under external fault conditions. The probability of failure in normal use is very low.

12 Power consumption and efficiency

According to the formula, Pin, Pout, and P consumption are the module power input, output power, and self-power loss respectively. It can be seen that under certain output power conditions, the smaller the module loss P consumption, the higher the efficiency, the lower the temperature rise, and the longer the life. Of course, the smaller the loss, the more in line with the energy-saving requirements.

Soft switching technology: In order to improve the conversion efficiency of the converter, various soft switching technologies are applied, the representative ones are passive soft switching technology and active soft switching technology, mainly including zero voltage switching/zero current switching (ZVS/ZCS) resonance, quasi-resonance, zero voltage/zero current pulse width modulation technology (ZVS/ZCS-PWM) and zero voltage transition/zero current transition pulse width modulation (ZVT/ZCT-PWM) technology, etc. The use of soft switching technology can effectively reduce switching loss and switching stress, which helps to improve the conversion efficiency of the converter.