What Is a Power Supply Circuit Board?

The power module is a power supply that can be directly mounted on a printed circuit board. Its characteristics are that it can be an application specific integrated circuit (ASIC), a digital signal processor (DSP), a microprocessor, a memory, and a field programmable gate array (FPGA). ) And other digital or analog loads. Generally, such modules are called point-of-load (POL) power supply systems or point-of-use power supply systems (PUPS). Due to the many advantages of the modular structure, the module power is widely used in communication fields such as switching equipment, access equipment, mobile communications, microwave communications, and optical transmission, routers, and automotive electronics, aerospace and so on.

Power module

The power module is a power supply that can be directly mounted on a printed circuit board.

Generally, such modules are called point-of-load (POL) power supply systems or point-of-use power supply systems (PUPS). Due to the many advantages of the modular structure,

DC / DC conversion converts a variable DC voltage into a fixed DC voltage, also known as DC chopping. There are two working modes of the chopper, one is that the pulse width modulation mode Ts is unchanged, ton (general purpose) is changed, and the other is frequency modulation (

(1) Buck circuit-step-down chopper, whose average output voltage U0 is less than the input voltage Ui, with the same polarity.

(2) Boost circuit-boost chopper, whose average output voltage U0 is greater than the input voltage Ui, with the same polarity.

(3) Buck-Boost circuit-a buck or boost chopper, whose average output voltage U0 is greater than or less than the input voltage Ui, with opposite polarity and inductive transmission.

(4) Cuk circuit-a buck or boost chopper, whose average output voltage U0 is greater than or less than the input voltage Ui, the polarity is opposite, and the capacitance is transmitted. There are Sepic and Zeta circuits.

The above is a non-isolated DC-DC converter circuit. The isolated DC-DC converter includes a forward circuit, a flyback circuit,

AC / DC conversion is to convert AC to DC. The power flow can be bidirectional. The power flow from the power source to the load is called "

Due to the high working efficiency of the switching power supply, which can generally reach more than 80%, in the selection of its output current, the maximum absorbed current of the electrical equipment should be accurately measured or calculated, so that the selected switching power supply has a high

The switching power supply must have protection functions such as overcurrent, overheating, and short circuit in the design. Therefore, a switching power supply module with complete protection functions should be selected during design, and the technical parameters of its protection circuit should match the working characteristics of the electrical equipment. Avoid damaging electrical equipment or switching power supplies.

Power P = UI is the product of output voltage and output current.

There are two types of input voltage: AC input and DC input.

The output voltage is usually DC output, but there are also AC output.

Operating temperature

Isolation voltage: Isolation is the circuit separation of the output from the input. It has the following functions:

First, current conversion;

Second, in order to prevent input and output interference;

Third, the signal characteristics of the input and output circuits are too different, such as using weak signals to control

According to the application field of modern power electronics, we divide the power module as follows:

The electromagnetic interference level of the power supply is the most difficult part of the design. The most a designer can do is to fully consider it in the design, especially in the layout. Since DC-to-DC converters are very common, hardware engineers are more or less exposed to related work. In this article, we will consider two common compromises related to low electromagnetic interference design ^[1] .

In the design of power supplies, even common DC-to-DC switching converters have a series of problems, especially in high-power power supply designs. In addition to functional considerations, engineers must ensure the robustness of the design to meet cost targets, thermal performance, and space constraints, while at the same time ensuring design progress. In addition, due to product specifications and system performance considerations, the electromagnetic interference (EMI) generated by the power supply must be sufficiently low. However, the level of electromagnetic interference from a power supply is the most difficult item to accurately predict in a design. Some people even think that this is simply impossible. The most a designer can do is to fully consider the design, especially when it comes to layout.

Although the principles discussed in this article are applicable to a wide range of power supply designs, we only focus on DC-to-DC converters here, because it is widely used, and almost every hardware engineer will be exposed to the work related to it, maybe Whenever it is necessary to design a power converter. In this article, we will consider two common compromises related to low electromagnetic interference design; thermal performance, electromagnetic interference, and the size of the scheme related to PCB layout and electromagnetic interference. In this article we will use a simple buck converter as an example, as shown in Figure 1.

Ordinary buck converter

Figure 1. A common buck converter

Measuring radiated and conducted electromagnetic interference in the frequency domain is a Fourier series expansion of a known waveform. In this article, we focus on the performance of radiated electromagnetic interference. In synchronous buck converters, the main switching waveforms that cause electromagnetic interference are generated by Q1 and Q2, that is, the current di / dt from the drain to the source of each field-effect transistor during its respective conduction period. The current waveforms (Q and Q2on) shown in Figure 2 are not very trapezoidal, but we have greater freedom of operation. Because the transition of the conductor current is relatively slow, Henry Ott's classic work "In Electronic Systems Equation 1 in Noise Reduction Techniques. We found that for a similar waveform, its rise and fall times directly affect the harmonic amplitude or Fourier coefficient (In).

Q1 and Q2 waveforms

Figure 2. Waveforms of Q1 and Q2

In = 2IdSin (nd) / nd × Sin (ntr / T) / ntr / T (1)

Among them, n is the harmonic order, T is the period, I is the peak current intensity of the waveform, d is the duty cycle, and tr is the minimum of tr or tf.

In practical applications, it is very likely to encounter both odd and even harmonic emissions at the same time. If only odd harmonics are generated, the duty cycle of the waveform must be exactly 50%. In practice, such a duty cycle accuracy is rare.

The amplitude of electromagnetic interference of the harmonic series is affected by the on and off of Q1 and Q2. This can be clearly seen when measuring the rise time tr and fall time tf of the drain-source voltage VDS, or the current rise rates di / dt through Q1 and Q2. This also means that we can simply reduce the level of electromagnetic interference by slowing the on and off speed of Q1 or Q2. This is indeed the case. Prolonging the switching time does have a great impact on harmonics with frequencies higher than f = 1 / tr. However, a trade-off must be made between increasing heat dissipation and reducing losses. Nonetheless, controlling these parameters is a good way to help balance electromagnetic interference and thermal performance. This can be achieved by adding a small resistance resistor (usually less than 5). This resistor can be controlled in series with the gates of Q1 and Q2 to control tr and tf. You can also connect a gate resistor in series with a "shutdown diode" to independently control Transition time tr or tf (see Figure 3). This is actually an iterative process, and even the most experienced power supply designers use this method. Our ultimate goal is to reduce the on-off speed of the transistor to reduce electromagnetic interference to an acceptable level, while ensuring that its temperature is low enough to ensure stability.

Use associated diodes to control transition time

Figure 3. Associated diodes to control transition time

The physical loop area of the switching node is also very important for controlling electromagnetic interference. Generally, due to the consideration of PCB area, designers want the structure to be as compact as possible, but many designers do not know which part of the layout has the greatest impact on electromagnetic interference. Going back to the previous buck regulator example, there are two loop nodes in this example (as shown in Figure 4 and Figure 5), and their size will directly affect the level of electromagnetic interference.

Buck regulator modelOne

Figure 4. Buck regulator model 1

Buck regulator model 2

Figure 5. Buck regulator model 2

Ott's formula (2) on electromagnetic interference levels in different modes shows the direct linear effect of the loop area on the electromagnetic interference level of the circuit.

E = 263 × 10-16 (f2AI) (1 / r) (2)

The radiation field is proportional to the following parameters: the harmonic frequency involved (f, unit Hz), loop area (A, unit m2), current (I), and measurement distance (r, unit m).

This concept can be generalized to all occasions of circuit design using trapezoidal waveforms, but this article only discusses power supply design. Referring to the AC model in FIG. 4, study the current flow of the loop: the starting point is the input capacitor, then it flows to Q1 during Q1 on-time, then enters the output capacitor through L1, and finally returns to the input capacitor.

When Q1 is turned off and Q2 is turned on, a second loop is formed. The energy stored in L1 then flows through the output capacitor and Q2, as shown in Figure 5. These loop area control is very important to reduce electromagnetic interference, and the layout of the device should be considered in advance when PCB wiring is routed. Of course, there are practical restrictions on how small the loop area can be.

It can be seen from Equation 2 that reducing the loop area of the switching node will effectively reduce the level of electromagnetic interference. If the loop area is reduced by 3 times, the electromagnetic interference will be reduced by 9.5dB, and if it is reduced by 10 times, it will be reduced by 20 dB. When designing, it is best to start by minimizing the loop area of the two loop nodes shown in Figure 4 and Figure 5, carefully consider the layout of the device, and pay attention to the copper connection problem. Try to avoid using both sides of the PCB at the same time, because the vias can significantly increase the inductance and cause other problems.

The importance of proper placement of high-frequency input and output capacitors is often overlooked. A few years ago, my company transferred our product designs to foreign manufacturers. As a result, my job responsibilities have also changed a lot. I became a consultant to help novice power supply designers solve a series of issues that need to be balanced and many other issues mentioned in the article. Here is an example of the design of an off-line switch with integrated ballast: The designer wants to reduce electromagnetic interference in the final power stage. I simply moved the high-frequency output capacitor closer to the output stage, and the loop area was only about half of the original, and the electromagnetic interference was reduced by about 6dB. The designer obviously did not understand the reason. He called the capacitor a "magic hat", but in fact we just reduced the loop area of the switch node.

It is also important to note that the newly improved circuit may cause more serious problems than the original. In other words, although extending the transition time can reduce electromagnetic interference, the thermal effects caused by it also become an important issue. One way to control electromagnetic interference is to replace traditional DC-to-DC converters with fully integrated power modules. The power module is a switching regulator with fully integrated power transistors and inductors. It can be easily integrated into the system design like a linear regulator. The circuit area of the switch node of the module is much smaller than a similarly sized regulator or controller. The power module is not a new thing. It has been available for some time, but until now, due to a series of problems, the module still cannot effectively dissipate heat, and It cannot be changed once installed.