the problem needs to consider during the design module power supply

A power module is a power supply that can be mounted directly on a printed circuit board (Figure 1). It is characterized by providing power for asICS, DIGITAL signal processors (DSP), microprocessors, memory, field programmable gate arrays (FPGas), and other digital or analog loads. Generally, such modules are referred to as point-of-load (POL) power supply systems or point-of-use power supply systems (PUPS). Because of the advantages of modular structure, high-performance telecommunication, network connection and data communication systems are widely used in various modules. Although modules have many advantages, reliability and measurement issues are often ignored when engineers design power modules and even most on-board DC/DC converters. This paper will discuss these problems in depth and propose relevant solutions respectively.

Advantages of the power module

At present, different suppliers have launched a variety of different power modules in the market, and the input voltage, output power, function and topology of different products are different. Power modules can save development time and bring products to market faster, so power modules are superior to integrated solutions. The power module also has the following advantages:

1. Each module can be rigorously tested separately to ensure a high degree of reliability, including power testing to weed out substandard products. In contrast, integrated solutions are more difficult to test because the entire power supply system is closely linked to other functional systems on the circuit.

2. Different suppliers can design modules of the same size according to existing technical standards, providing engineers designing power supplies with many different options.

3. Each module is designed and tested in accordance with standard performance requirements, helping to reduce the risk of adopting new technologies.

In the case of an integrated solution, the whole motherboard needs to be replaced once the power supply system fails. If the modular design is used, simply replace the problem module, which can save cost and development time.

A power module design problem that is easily overlooked

Although the modular design has many advantages above, but modular design and even on board DC/DC converter design also has its own problems, many people do not understand these problems, or do not give enough attention. Here are some of the questions:

1. Measurement of output noise;

2. Design of magnetic system;

3. Breakdown phenomenon of synchronous step-down converter;

4. Reliability of printed circuit board. Each of these issues is discussed below, along with a variety of simple techniques that can be used to solve them

Measurement of output noise

All power supplies in switching mode have noise output. The higher the switching frequency, the greater the need for proper measurement techniques to ensure the accuracy and reliability of the measured data. When measuring output noise and other important data, the Tektronix probe shown in Figure 2 (commonly known as a cold nozzle probe) can be used to ensure that the measurements are accurate, reliable and in line with predictions. This measurement technique also ensures that ground loops are kept to a minimum.

We should also take into account the possible propagation delay of the measuring instrument when making measurements. The propagation delay of most current probes is greater than that of voltage probes. Therefore, a measurement that must display both voltage and current waveforms cannot ensure the accuracy of the measured figures unless the different delays are equalized by hand.

The current probe also feeds the inductance into the circuit. A typical current probe will input a 600nH inductor. For high frequency circuit design, the inductance of the circuit can not be more than 1mH, so the inductance input through the probe will affect the accuracy of the DI/DT current measurement, and even make the measurement number appear large error. If the inductor is saturated, a more accurate method of measuring the current may be used. For example, we may measure the voltage of the small shunt resistor serial with the inductor.

Magnetic design

The reliability of magnetic cores is another problem that is often overlooked. Most output inductors use iron cores because iron is the lowest cost material. About 95% of the iron core is pure iron particles, which are held together by an organic binder. These adhesives also separate each iron particle, filling the inner and outer core with breathable space.

Iron powder is the raw material used to form magnetic cores. However, iron powder contains small amounts of impurities such as manganese and chromium. These impurities can affect the reliability of magnetic cores depending on the amount of impurities contained. The cross section of the core can be examined by spectroelectron microscopy (SEM) to determine the relative distribution of impurities. The reliability of magnetic cores depends on whether the material is predictable and whether its supply is stable and reliable.

If iron powder cores are exposed to high temperatures for a long period of time, core losses may increase and can never be recovered due to the molecular breakdown of organic adhesives, resulting in increased eddy current losses. This phenomenon is called thermal aging and may eventually lead to thermal runaway of the magnetic core.

The size of core loss is affected by many different factors, such as alternating current flux density, operating frequency, core size and material type. At high frequencies, for example, most of the losses are eddy current losses. Operating at low frequencies, hysteresis loss is the largest loss.

Eddy current losses heat up the core, causing efficiency to decline. The reason for eddy current loss is that the object caused by ferromagnetic material is affected by different magnetic flux at different times, so that the object produces an endless cycle of current. The eddy current losses can be reduced by selecting pieces of ferromagnetic flakes instead of solid ferromagnets as core materials. Metglas made of magnetic tape, for example, are such cores. Other ferromagnetic product vendors such as Magnetics also produce magnetic cores wound from magnetic tape.

Suppliers of magnetic core products, such as Micrometals, provide engineers designing magnetic products with up-to-date information and calculation methods on thermal aging of magnetic cores. The iron powder magnetic core with inorganic adhesive will not be heated and aged. Such cores are already on the market and Micrometals' 200C series cores are of this type.

Breakdown phenomenon of synchronous buck converter

Synchronous step-down converters are widely used in power supply systems such as load-point power supply systems (POL) or point-of-use power supply systems (PUPS) (Figure 3). The synchronous step-down converter uses high - and low-end MOSFEts to replace the clamp diodes of conventional step-down converters to reduce load current losses.

Engineers often neglect the problem of "breakdown" when designing buck converters. When both high - and low-end MOSFets are fully or partially activated at the same time, the phenomenon of "breakdown" occurs, allowing the input voltage to transfer current directly to the ground.

The breakdown phenomenon will cause the current to spike at the moment of switching, preventing the converter from working at its maximum efficiency. We cannot use a current probe to measure the breakdown because the inductance of the probe would seriously interfere with the operation of the circuit. We can check the gate/source voltage of both feTs to see if any spikes appear. This is another way to detect breakdown. (The gate/source voltage of the upper MOSFET can be monitored using differential mode.)

We can use the following methods to reduce the occurrence of breakdown phenomenon.

Adopting controller chip with "fixed dead time" is one feasible method. This controller chip ensures that there is a delay after the upper MOSFEts are shut down before the lower MOSFEts are restarted. This is a simple method, but one that needs to be carried out carefully. If the dead zone time is too short, it may not prevent the breakdown phenomenon. If the dead zone time is too long, the conductivity loss increases, because the diodes in the underlying MOSFET are activated throughout the dead zone time. Since the diode conducts electricity during the dead zone, the efficiency of the system using this method depends on the characteristics of the diode embedded in the underlying MOSFET.

Another way to reduce breakdown is to use a controller chip with an "adaptive dead-time". The advantage of this approach is that the gate/source voltage of the upper MOSFEts can be continuously monitored to determine when to start the lower MOSFEts.

When the high-end MOSFET is started, the gate of the low-end MOSFET will have a DV/DT spike through inductive induction, so as to push up the gate voltage (Figure 4). If the gate/source voltage is high enough to turn it on, breakdown will occur.

The adaptive deadzone time controller is responsible for monitoring the gate voltage of the MOSFET outside. Therefore, any additional external gate resistance will take some of the voltage from the controller's built-in pull-down resistance, so that the gate voltage will actually be higher than the voltage monitored by the controller.

Predictive gate drive is another possible solution that uses a digital feedback circuit to detect the conductivity of the built-in diode and adjust the deadzone time delay to minimize the conductivity of the built-in diode and ensure the maximum efficiency of the system. With this approach, more pins need to be added to the controller chip, which increases the cost of the chip and power module.

It is important to note that even with predictive gate drive, there is no guarantee that the MOSFET will not be turned on by the dv/ DT inductance.

Delaying the start of high-end MOSFEts also helps reduce breakdown. Although this method can reduce or eliminate breakdown, the disadvantage is that switching losses are high and efficiency is reduced. If we choose a better MOSFET, also can help reduce the dv/ DT inductance voltage amplitude in the bottom MOSFET gate. The higher the ratio between Cgs and Cgd, the lower the inductance voltage at the MOSFET gate.

The breakdown test situation is often overlooked, such as the narrowband pulse that the controller continuously generates during load transients -- especially whenever the load has been lifted or suddenly reduced. Most of the current high current systems use multi-phase design, using a driver chip to drive MOSFET. But using driver chips can complicate the breakdown problem, especially if the load is transient. For example, interference from a narrow-frequency drive pulse, coupled with a driver propagation delay, can lead to breakdown.

Most driver chip manufacturers specify that the pulse width of the controller must not fall below a certain minimum, below which there will be no pulse input to the MOSFET gate.

In addition, the driver chip has been added with a configurable dead zone time (TRT) function to enhance the accuracy of adaptive switching timing. The method is to add a resistance between the settable dead zone time pin and the ground to set the dead zone time, in order to determine the high and low end conversion process of the dead zone time. This deadzone time setting and propagation delay turn off complementary MOSFEts during conversion to prevent breakdown of synchronous step-down converters.

reliability

Any module must pass rigorous testing at an early stage to ensure that the design is sound and reliable, so as not to have unexpected problems at the end of the production process. Modules must be available for testing in the customer's system to ensure that all factors that may lead to system failure, such as fan failure and fan downtime, are fully considered. Engineers who use distributed structures want to design systems that can last for many years with few or even no failures. With test numbers showing MTBF of power modules in the millions of hours, this is not too difficult to achieve. But what is often overlooked is the reliability of printed circuit boards. The current trend is that the PCB area is getting smaller and smaller, but the current needs to be handled more and more, so an increase in current density may cause concealed or other through-holes to fail to function properly.

Some of the hidden holes in the printed circuit board must carry a lot of current. For these hidden holes, there must be sufficient copper protection around them to ensure that the design is more reliable and durable. The guard also inhibits z-axis thermal expansion, otherwise concealed through holes can be exposed if there is any change in PCB ambient temperature during production and during product use. The engineer must thoroughly review the PCB design with reference to the PCB manufacturer's expertise, which can provide expertise on PCB design reliability based on their manufacturing capacity.

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Conclusion:

If we want to build a reliable power supply system with power modules, we must solve the problem of design reliability. The main issues discussed above include the reliability of iron powder cores, the characteristics of magnetic systems, the breakdown phenomenon of synchronous step-down converters, and the reliability of printed circuit boards in high current systems.