“In the process of power supply research and development, we will always encounter problems of one kind or another. Here are Daniel’s many years of research and development of power supply problems and answers, let’s learn together!

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In the process of power supply research and development, we will always encounter problems of one kind or another. Here are Daniel’s many years of research and development of power supply problems and answers, let’s learn together!

Without further ado, let’s go straight to the topic.

Question one

We use the most flyback power supplies for low power. Why do we often choose 65K or 100K (near these frequency bands) as the switching frequency? What are the constraints? Or in which cases can we increase the switching frequency? Or reduce the switching frequency?

Why do switching power supplies often choose the range of 65K or 100K as the switching frequency? Some people will say that IC manufacturers produce such ICs. Of course, there are reasons for this. What does the switching frequency of each power supply determine?

The reason should be considered from here. Some people will also say that EMC is not easy to deal with when the frequency is high. Generally speaking, this is the case, but this is not inevitable. EMC is related to frequency, but it is not inevitable. Imagine that the switching frequency of our power supply has increased. What is the direct impact? Of course, the MOS switching loss increases, because the number of switching per unit time increases. What happens if the frequency is reduced? The switching loss is reduced, but the energy provided by our energy storage device in a single cycle will increase, which is bound to require a larger transformer magnetism and a larger energy storage inductance. Selecting around 65K to 100K is a more appropriate empirical compromise, and the power supply is a reasonable compromise.

If in a special case, the input voltage is relatively low, and the switching loss is already very small, do you not care about this switching loss, then we can increase the switching frequency to reduce the size of the magnetic device.

The key to this post: How to choose the switching frequency of the appropriate IC? Why is the switching frequency of mainstream ICs in such a range? What is the switching frequency related to? It is about the general situation, not the different frequencies of a lot of ICs. More want to diverge everyone’s thinking to pay attention to these problems!

The general situation I want to talk about here is mainly about what the switching frequency is related to, how to choose a suitable switching frequency, why there are so many mainstream ICs and switching frequencies, note that it is not a certain, it is a common situation, let the novice area understand the general behavior , Of course, the switching power supply can do whatever you want, as long as it can be used reasonably.

1. How do you know that 65 or 100KHZ is generally selected as the switching frequency of the switching power supply? (Investigate the mainstream ICs of major manufacturers, these two will be more, of course, some are near here, and some are adjustable switching frequency)

2. How to find out that the switching frequency of the switching power supply does work at 65KHZ, or 100KHZ. (From a design point of view, common power supplies use this range)

3. Are there more than two pictures for testing the frequency of 65KHZ100KHZ? (more than two pictures, meaningless)

4. Do you know that the switching power supply can work at 1.5HZ. (Do you think it is necessary to talk about it, there is nothing wrong with work, skilled drills, remember to drill the horns when doing technology, then you can talk about why the general power supply does not work at 1.5HZ, say This only makes sense, it is completely meaningless for you to make a 1.5HZ power supply)

Reminder: As a technician, remember to dig into the horns, we are not a campus research school, we need to combine theory with practice, and the products made are meaningful products!

Question two

Why do we often design the switching frequency in the second zone in LLC? Why not one and three districts? What are the constraints? Or what would be the consequences if the first and third regions were chosen as the switching frequency?

The principle of LLC is to use the inductive load to increase the inductive reactance with the increase of the switching frequency to adjust the output voltage, that is, PFM modulation. And the turn-on loss ZVS of the MOS tube is smaller than that of ZCS, and the first area is a capacitive load area, which is naturally not desirable. Then in the third area, the switching frequency is greater than the resonant frequency, this is still the inductive load area, and it is reasonable to say that there is no problem with MOS implementing ZVS, which is true. But we can’t ignore the output diode turn off on the secondary side. That is, when the primary side MOS tube is turned off, the resonant current is not reduced to be equal to the excitation current, and the secondary side rectifier diode is softly turned off. That’s why we don’t usually choose the three districts either.

We can’t just design according to the experience of predecessors, but we must know that there is an inevitable reason for this design!

Question three

What happens when our flyback duty cycle is greater than 50%? What are the good aspects? What are the bad aspects?

What does it mean that the duty cycle of the flyback is greater than 50%, and what factors are affected by the duty cycle? First: the design of the duty cycle is too large, which will first bring about an increase in the turns ratio, and the stress of the main MOS transistor will inevitably increase. Generally, the flyback selects a MOS tube below 600V or 650V, and the cost is considered. Too large a duty cycle is bound to be unbearable.

Second point: It is very important that many people know that slope compensation is required, otherwise the loop will oscillate. However, this is also conditional. The generation of the zero point of the right plane needs to work in the CCM mode. If the design is in the DCM mode, this problem does not exist. This is also one of the reasons why the low power is designed in DCM mode. Of course, we can also overcome this problem by designing a good enough loop compensation.

Of course, in special cases, it is also necessary to design the duty cycle to be greater than 50%, and the energy transferred per unit cycle can increase, which can reduce the switching frequency and achieve the purpose of improving efficiency. If the flyback is to be more efficient, this method can be considered. .

Question four

If the flyback power supply wants to achieve a certain efficiency, what aspects need to be started? Quasi-resonant? Synchronous rectification?

A major disadvantage of flyback is efficiency. What are the ways to improve efficiency? It is inevitable to reduce the loss. The point of loss is the switch tube, the transformer, and the output rectifier tube. These are the three main parts.

For switching tubes, we know that flyback is mainly PWM-modulated hard switching, and switching loss is a major difficulty for us. Fortunately, there is hope for the emergence of soft switching. The flyback cannot achieve full resonance like LLC, it can only develop towards quasi-resonance (resonance in some time periods). There are also many such ICs. Our company mostly uses NCP1207. After the MOS tube is turned off, Before the next turn-on, the 1 pin detects that the VCC voltage crosses zero, and then turns on the next cycle after a set time.

How to minimize the loss of the transformer, the problem behind the perfect use of the transformer will be involved.

Synchronous rectification is generally in the case of outputting a large current, the secondary side rectifier current diode, even if the Schottky loss is still large, at this time, the synchronous rectification MOS is used instead of the Schottky diode. Some people will say that it is better to use an LLC or a forward due to the high cost. Of course, there is no best, only more suitable ones.

Question five

How is the conduction of power formed? What are the pathways of transmission? Commonly used means? What are the effects of radiation from the power supply? How to do high power EMC.

The power conduction measurement method is to receive high-frequency interference from the power supply (generally 150K to 30M) through the input ports L, N, PE.

Addressing conduction requires figuring out how to attenuate the interference received by the port.

As shown in the figure: There are generally two modes: L, N differential mode components, and common mode components through the PE ground loop. Some frequencies are differential common mode.

Filtering method: Generally, the second-level common mode is used with Y capacitors to filter out. The selection method and skills are also very important, and the layout has a great influence. Generally, low U inductance is placed close to the port, preferably nickel-zinc material, which is specially designed for high frequency. The winding method adopts double-wire winding to reduce the differential mode component. The post-stage generally has a large sense of placement, around 4MH to 10MH, which is only an empirical value, and needs to be matched with the Y capacitor. The differential mode filter of the X capacitor also needs to be close to the port, and is generally placed in the middle of the secondary common mode. Place Y capacitors. When the capacitors are laid out, the traces need to be thicker and cannot be externally mounted, otherwise the effect will be poor. (These are just a fuss on the input filtering network)

Of course, you can also start from the source. Conduction is the result of radiation coupling into the line. Reducing the switching radiation can also bring benefits to conduction. Several places that affect the radiation generally include the turn-on speed of the MOS tube, the turn-on and turn-off of the rectifier tube, the transformer, and the PFC inductance and so on. The design on these circuits requires compromises with other aspects that are not detailed.

Some experience and skills: For high-power EMC, it is generally necessary to add shielding, which is immediate, and there are generally several options for shielding parts:

First: the shielding between the input EMI circuit and the switch tube, which has a great effect on EMC, and this method is generally very effective for many people who rely on the filter to be ineffective.

Second: the primary and secondary shielding of the transformer. Generally, if there is space for the transformer, it is better to add shielding.

Third: The position of the radiator can act as a shield, and the layout of the board is reasonable, and the choice of grounding of the radiator is also very important.

Fourth: There are generally several simple methods for judging the position of the radiation source, which may not be completely accurate. You can refer to it. If the magnetic ring of the input line is good for EMC, it is usually the primary side MOS tube. If the magnetic ring of the output line is good for EMC Effective, generally the secondary side output rectifier, especially the high frequency greater than 100M. You can consider adding capacitors or common mode inductors to the output.

Of course, there are many other detailed skills, especially the layout loop, which will be explained separately for LAYOUT later.

Question six

What factors do we need to consider when choosing a topology? What are the usage environments and advantages and disadvantages of various topologies?

The first step in designing a power supply, I don’t know what everyone will think of? I think so, I carefully study the customer’s technical index requirements, convert it into the specification of the power supply, and communicate with the customer about the indicators. Different indicators mean design difficulty and cost. It has a great influence on the question I raised. When choosing a topology, we consider it according to our power supply index and cost. What are the characteristics of several commonly used topologies?

The isolation type is mainly discussed here. The non-isolation type has limited applications and of course the lowest cost.

Flyback characteristics: It is suitable for less than 150W. In theory, it is rarely used when it is greater than 75W, not to mention very special cases. The flyback is a bit low-cost and easy to debug (compared to half-bridge and full-bridge), mainly due to unidirectional excitation of the magnetic core, limited power, and low efficiency, mainly due to hard switching, large leakage inductance and so on. The efficiency of the full voltage range (85V-264V) is generally below 80%, and it is easy to reach 80% for a single voltage.

Forward excitation characteristics: moderate power, can be used for small and medium power, the power is generally below 200W, of course, it can be used for high power, but it is not often done, because the forward excitation and flyback are the same as unidirectional excitation, and the volume of high-power magnetic cores is required to be large. , Of course, there are also two transformers in series and parallel, pay attention to only talk about the general situation, do not mislead newcomers. The forward excitation is somewhat better, the cost is moderate, of course, it is higher than the flyback, and the advantage efficiency is higher than that of the flyback. In particular, the active clamp is used for primary side absorption, and the leakage inductance energy is reused.

Half-bridge: At present, the most popular is the LLC resonant half-bridge, which is medium and small power, and high-power take-all type. (Generally more than 100W and less than 3KW). The characteristic cost is higher than that of the flyback forward excitation, because one more MOS tube (bidirectional excitation) and one rectifier tube are used, the control IC is also expensive, and the loop design industry is complicated (op amps are generally used, especially the current loop). Advantages: Soft switching, good EMC, extremely high efficiency, higher than forward excitation, I have done 960W LLC, the efficiency can reach more than 96% (full voltage) (of course, PFC adopts bridgeless method). I don’t recommend other half bridges, at least I won’t use them. The older asymmetric bridges are difficult to achieve soft switching. LLCs were used a lot before they matured, but they are rarely used now. At least big companies such as Emerson prefer LLCs. Going with the mainstream is generally not wrong.

Full bridge: generally used for more than 2KW, the first phase shift full bridge, features, bidirectional excitation, MOS tube stress is small, half less than LLC stress, high power especially when the input voltage is high, the phase shift full bridge is generally used, the input voltage Low use LLC. The cost is particularly high, and it uses 2 more MOSs than LLC. This is not the most important, mainly because the driving is complex, and the general IC driving capability cannot be achieved. To amplify the driving and use an isolation transformer to drive, this is the other side of the high cost.

Push-pull: It is used in high-power applications, especially in high-power applications with low input voltage. It features high voltage stress, and of course, small current stress. High-power full-bridge or push-pull generally depends on the input voltage. The transformer has one more winding, and the stress of the tube is high. Of course, the often mentioned magnetic bias also needs to be overcome. I really haven’t used this one, it doesn’t involve power supply, it’s hard to use it.

Question seven

When considering power costs, where do we start?

Designing a power supply requires cost assessment. At present, customers keep the cost of the power supply very low. All major competitors are in a price war. Everyone can make a power supply. , which aspects will benefit us Chen Ben:

First: technical indicators. The higher the technical indicators of the power supply, the higher the cost. If your power supply cost is high, then you can sell your performance indicators. The more performance requirements, the more circuits, the higher the cost. It is also the capital of talking with customers.

Second: the cost of material procurement, why are the profits of large companies’ power supplies high? It is nothing more than that they have a superior procurement platform, large procurement volume, low material cost, and of course lower cost. If you do not consider purchasing, as an engineer, you must figure out the corresponding costs of different materials, such as using SMD, less plug-in, (for example, the cost of plug-in resistors is higher than that of SMD), can use domestic, no Taiwanese, can use Taiwanese Japanese, the price difference here is not cheap. (For example, the price of Japanese capacitors is several times higher than that of domestic capacitors!!! Of course, the quality is also different;)

Third: Important devices that affect the cost: transformers, inductors, MOS tubes, capacitors, optocouplers, diodes and other semiconductor devices, ICs, etc.The price of transformers wound by different transformer manufacturers varies greatly, the choice of MOS tube stress and thermal resistance is enough, the cost of IC solutions, etc.

Other aspects lead to cost issues: device heat sinks, the right size, too much is a waste of money. For PCB layout, it is a waste of money to use a single panel into a double panel. The PCB layout process, choosing a reasonable process, has low processing costs and high production efficiency.

Question eight

The loop design of the power supply, which parts of the power supply affect the loop of the power supply? What are the indicators of a good loop?

The loop design of the power supply has always been a difficult point. Why do I say this? Because there are too many main influencing factors, the theoretical calculation is difficult to be accurate. The simulation is also based on an idealized model. Understand the loop and how to do loop compensation from a qualitative point of view.

When the loop is based on input and output fluctuations, it needs to pass feedback, and the loop informs the control IC to adjust accordingly to maintain the stability of the output. The power supply loop is generally series negative feedback, some are voltage series negative feedback (in CC mode), and some are current series negative feedback (in CV mode).

What places will affect the loop? Zeros and poles in a circuit. A zero will generally cause the gain to rise, causing a 90 degree phase shift (right half plane zero will cause a -90 degree phase shift). The poles generally cause the gain to drop, causing a -90 degree phase shift, and the left half-plane poles cause the system to oscillate. Therefore, we need to use the zero-pole compensation method to reasonably regulate our loop. For the low-frequency part, zero compensation is generally introduced to meet sufficient gain, and pole compensation is generally introduced for high-frequency interference to cancel and reduce high-frequency interference.

The principles of loop stability are: 1. At the crossover frequency (that is, the frequency when the gain is zero dB), the phase margin of the system is greater than 45 degrees.

2. When the phase reaches -180 degrees, the gain margin is greater than -12dB. 3. Avoid entering the crossover frequency too quickly, and the slope of the curve near the crossover frequency is -1.

For general flyback circuits: 1. There is an output filter capacitor that generates a zero point: it can increase the loop gain. (Generally around the mid-frequency 4K, it is good for the gain, no need to compensate)

2. If working in CCM mode, the zero point of the right half plane will also be generated. At high frequencies, pole compensation can be used. This is generally difficult to compensate, try to avoid it, let the crossing frequency be less than the zero frequency of the right half plane (about 15K, which will change with the load), select 3. The load will produce a low frequency pole. Use low frequency zero to compensate. 4. The LC filter will produce a low frequency pole, which requires zero compensation. Be clear in your mind which zeros and poles are pros and cons, and make targeted compensation.

The compensation circuit is relatively simple for the power loop. Generally, type 2 compensation is used for the op amp, and some use type 3 compensation, which is rarely used.

Question nine

What are the soft-switching forms for various topologies? How is soft switching implemented?

Soft switching is currently used very frequently, which can improve secondary efficiency and benefit EMC. Many topologies have begun to use soft switching. Even the flyback introduces quasi-resonance to achieve soft switching in order to achieve high efficiency. This has been discussed in the previous question. The soft switching of LLC also mentioned the realization conditions in the previous question, and the specific realization process is not detailed. Here I share my understanding of soft switches.

Realization conditions and process: Two elements are required to use soft switching, one is C and the other is L to achieve resonance (of course, it can also be multi-resonant). The resonance will generate a sine wave, and the sine wave can achieve zero-crossing. If the series resonance belongs to the voltage resonance, the parallel resonance belongs to the current resonance.

Secondly, the difference between soft switching and hard switching is that the voltage and current overlap during the hard switching process, and the soft switching is either zero current (ZCS) or zero voltage (ZVS). The soft switching of the MOS tube can use the junction capacitance or parallel capacitance, and then the series inductance realizes the series ZVS, such as quasi-resonant flyback, active clamping absorption circuit, and moving to the soft switching of the full bridge. There is also LC parallel ZCS, but it is rarely used, because the loss of MOS tube ZVS is smaller than ZCS. LLC is a series-parallel type, but we are using the ZVS region. (When the resonant current crosses zero in the dead zone, before the upper tube is softly turned on, the lower tube junction capacitor is charged first, and the upper tube is softly turned on)

question ten

What kind of transformer is the most suitable for it? What does the transformer determine and what does it affect?

Transformer design is one of the core points of various topologies. The quality of the transformer design affects all aspects of the power supply. Some cannot work, some are not efficient, some EMC is difficult to do, some temperature rises, and some extreme situations It will be saturated, and some safety regulations cannot be passed. It is necessary to design the transformer by integrating various factors.

Where to start designing a transformer? Generally speaking, the size of the magnetic core is selected according to the power. Those who have experience can refer to the ones they have designed. Those who are inexperienced can only calculate according to the AP algorithm. Of course, there must be a certain margin. Finally, the experiment will check the quality of the design. .

Generally, low-power flybacks are recommended for more EE type, EF type, EI type, ER type, and medium and high power PQ. There are also everyone’s habits and platform differences of different companies, and the power is very large. Yes, there is no suitable magnetic core, it can be done in the way of two transformers in series and secondary.

Different topologies have different requirements for transformers, such as flyback. What needs to be considered is what mode it needs to work in and how to adjust the inductance moderately. In particular, the multi-channel output must pay attention to the load regulation rate to meet the requirements, and the coupling effect is better, such as using parallel winding, uniform winding, and increasing the number of secondary turns as much as possible. The withstand voltage of the MOS tube determines the turns ratio, how to choose the appropriate duty cycle, and how much Bmax to choose (usually less than 0.35, of course, 0.3 is better, even if the short circuit is not too serious), some need to increase the shield to rectify the EMC, the original Generally, 2 layers of secondary shield are added, and 1 layer of outer shield is enough.

High-power transformers generally pay more attention to loss, which requires a balance between copper loss and magnetic loss, and also considers air-cooled natural cooling, the current density is appropriate, and the general current density with slightly larger power (greater than 150W) is relatively small ( 3.5-4.5), small power (5.0-7.0).

It is also necessary to know what safety regulations the power supply has passed, whether the retaining wall is sufficient, and whether the interlayer tape is set reasonably can not be ignored. Once certification is required, changing the transformer will also affect the progress.

question eleven

Do we really need to get to the point where we are obsessed with design tools and rely on simulation?

Power supply design tools are mainly used in the following aspects: 1. Select magnetic cores and design transformers 2. Loop simulation design 3. Main power topology simulation 4. Analog circuit simulation 5. Thermal simulation (for high power) 6. Calculation tools (calculation book) and so on.

For newcomers, my suggestion is to use less tools and more calculations, and master the design process yourself, because tools are made by people, and different people have different design habits, so you cannot use a fixed design pattern to design different power supplies.

Some simulations can be combined with the design: for example, it is difficult to directly meet the design requirements after the loop is designed. The simulation can be well verified before the test, but the simulation is not exactly the same as the test, at least not too far.

It is also necessary to be proficient in using Mathcad and Saber, but there are many aspects that we need to understand the principle, and the tool only needs to be used as a calculator, which is faster, more convenient and more efficient to meet our design. It’s a huge misunderstanding.

Question twelve

When judging the quality of a power board LAYOUT, what can be found in blood?

What kind of PCB is a good PCB must meet at least one of the following aspects: 1. There is little interference in electrical performance, the key signal lines and bottom lines are reasonable, and the performance in all aspects is stable (provided that the circuit is free of defects). 2. Conducive to EMC, low radiation, reasonable loop. 3. Meet the safety regulations, and the safety distance meets the requirements. 4. Satisfy the process, mass production manufacturability, and reduce production costs. 5. Beautiful, regular and orderly layout (devices are not crooked), the wiring is beautiful and beautiful, and there are no twists and turns.

How can I do the above points and share my experience in layout:

1. Before the layout, understand the specifications of the power supply, the specifications of the power supply, whether there are special requirements, and the safety standards to be passed.

Whether the structural input conditions are accurate, as well as the confirmation of the air duct, the confirmation of the input and output ports, and the main power flow direction.

The process route selection, according to the density of the device, and whether there are special devices, select the corresponding process route.

2. In the layout, pay attention to a reasonable layout, ensure that the four major loops are as small as possible, and predict in advance whether the subsequent routing is easy to go. The placement of the transformer basically determines the overall layout, so it must be carefully placed in the best position. The layout of the EMI section has a clear flow and has a clear isolation band from other main power sections. Reduce interference from main power switching devices. The area of ​​each absorption circuit should be as small as possible, and the length and position of the radiator should be reasonable without blocking the wind channel.

3. In the wiring part, whether the wiring of the input EMI circuit meets the safety regulations, the distance between the primary and secondary sides, and the distance between the input and output to the ground must meet the safety regulations. Whether the thickness of the trace satisfies enough current, whether the key signal (such as driving signal, sampling signal, ground wire is reasonable), the driving signal should not interfere with the sensitive signal (high-frequency signal); whether the sampling signal is accurate, and whether it will be interfered; Whether the ground wire is properly pulled (sometimes single-point grounding is required, and sometimes multi-point grounding is related to actual needs), the main power ground and the signal ground are strictly separated, and the primary chip ground is taken from the sampling resistor, not from the large electrolysis (especially When the distance between the sampling resistor and the large electrolytic ground is long), the ground of the VCC ground is returned to the large electrolytic ground, the secondary capacitor ground is connected to the chip, the feedback signal is also connected to the IC at a single point, and the ground is connected to the IC at a single point. The ground of the radiator must be connected to the main power ground and cannot be connected to the signal ground and many other detailed requirements.

Question Thirteen

How much do you know about the components of the power supply? How big is the junction capacitance of the MOS tube, which affects? What is the relationship between RDS and temperature? What does Schottky reverse recovery current affect? What is the impact of capacitor ESR?

There are many types of devices designed in the power supply, mainly semiconductor devices such as: MOS tubes, triodes, ICs, operational amplifiers, diodes, optocouplers, etc.; magnetic devices: inductors, transformers, magnetic beads, etc.; capacitors: Y capacitors, X capacitors, Ceramic capacitors, electrolytic capacitors, chip capacitors, etc.; each device has its specifications and limit parameters.

Conventional parameters are easy to grasp in our selection. For example, when selecting a MOS tube, the withstand voltage parameters will definitely be considered, the rated current will also be considered, and the on-resistance will be considered, but there are also some parasitic parameters and some parameters that change with temperature. But they seldom pay attention, or only go to them when they find a problem. The resistance value of the on-resistance Rds(on) increases as the temperature increases, and the ambient temperature of its operation should be considered when designing the loss of the MOS tube. Junction capacitance affects our turn-on losses and also affects EMC.

Schottky diode withstand voltage, rated current is generally good to pay attention to, some parameters such as the on-state voltage drop will decrease when the temperature rises, the reverse recovery time is short, but the leakage current is large (especially considering the effect of leakage current at high temperature) larger), parasitic inductance can cause high turn-off spikes.

ESR, an important parameter of capacitors, is usually considered when calculating ripple. The relationship between ESR and C is generally very large, but the influence of quality factors of different manufacturers is also huge, so it must be clearly distinguished.

Generally, the company can refer to: ESR=10/(0.73 power of C), the life of the capacitor will be shortened at high temperature, the capacity will be reduced at low temperature, and the leakage current will also increase, etc.;

Of course, the difference in the characteristics of the device in special cases is a problem worth thinking about. Please think about it a lot, it is very helpful for us to solve problems in special cases.

Question Fourteen

How much do you know about magnetic materials, what are the differences between magnetic rings and magnetic cores? What are the low magnetic rings and high magnetic rings used for?

The importance of magnetic devices to switching power supplies is self-evident, and can be said to be the heart of the power supply. There are also many types of magnetic materials. Generally, ferrite materials are commonly used for transformers, which are mainly cheap. The maximum switching frequency can reach 1000K, which is sufficient for general use. Ferrite cores can be used as both main transformers and inductors, such as PFC inductors (generally made of iron-silicon-aluminum materials, which are cost-effective), and energy storage inductors. Of course, in the case of high requirements, especially high-power magnetic rings are generally used, the main reason is that the inductance can be increased and it is not easy to saturate. Compared with ferrite cores, the disadvantage is that the price is expensive, especially for large currents, winding The process is more difficult. The magnetic ring is also divided into high U value and low U value, mainly due to the different materials of the magnetic ring. The appearance of the high U ring magnetic ring is green. Generally, the common mode inductance of the EMI circuit is selected, and the inductance will be relatively large to filter low frequencies. The gray ones are low U-rings, with very low inductance and high frequency filtering. Generally, for EMC, the effect is generally better when used together!

Question fifteen

How is the power loss distributed? MOS tube loss? Transformer loss? In addition to the DC loss of the transformer, how to calculate the AC loss?

The power loss is generally concentrated in the following aspects: 1. The turn-on loss and conduction loss of the MOS tube. 2. Copper loss and iron loss of transformer; 3. Loss of secondary rectifier; 4. Loss of bridge rectifier. 5. Loss of sampling resistance; 6. Loss of absorption circuit; 7. Other losses: loss of PFC inductance, loss of resonant inductance of LLC, loss of MOS tube of synchronous rectification. etc. . .

For these losses, appropriate reduction can improve efficiency. 1. For the MOS tube, the switch with fast switching speed and low on-resistance can be selected, and the soft switch is used in the circuit class. 2. For transformers: choose the appropriate size of the magnetic core. If the magnetic core is too small, the loss will be large, and it is difficult to balance the copper loss and the iron loss. In particular, copper loss includes not only DC loss but also AC loss. AC loss is generally twice as large as DC loss, because the AC impedance of copper wire at high frequency is much larger than DC impedance, so it must be fully estimated in the calculation.

Question sixteen

Thermal design in the power supply, how to choose the heat sink? What to Consider in Heatsink Design?

The design of the radiator is a key point of the switching power supply. The radiator is mainly for the high temperature rise of our heating devices. It is necessary to use a radiator to reduce the thermal resistance to achieve the effect of reducing the temperature rise!

Main heating devices: rectifier bridge, MOS tube, rectifier diode, transformer, Inductor, etc.

The size of the radiator is generally selected according to the power loss and the required temperature rise to calculate the thermal resistance, and the radiator of the corresponding area is selected according to the thermal resistance.

Of course, some auxiliary methods are also needed, such as applying thermal paste between the device and the heat sink, which will have some effects. Profiles can be used for heat dissipation in relatively small spaces, with small volume and large heat dissipation area.

Special devices have special treatment: for example, the transformer can hollow out the PCB board under the transformer to dissipate heat, or use the heat-conducting mud to paste the heat sink on the transformer. Inductors can also add copper rings to dissipate heat and so on. . .

Question seventeen

How is the output filter capacitor of the LLC determined? Influenced by what factors?

The output filter capacitor is very important to the output ripple. The selection of the appropriate filter capacitor needs to consider the cost and ripple requirements. Of course, the selection of each topology filter capacitor is based on the output ripple requirements and the ESR value corresponding to the ripple current. To select the corresponding capacitor, of course, the relationship between the capacitance of the capacitor and the ESR also has a very important relationship with the quality of the capacitor. The relationship has been discussed before. When the ripple voltage is our demand, generally according to the demand of 50mv, the design has a margin of 10mv. (Considering the filtering effect of the PCB board, the low temperature capacitance value of the capacitor is reduced), the ripple current calculation formula is as follows:

question eighteen

What is the driver of the phase-shifted full bridge? What is phase shift? What does phase shift bring?

The phase-shifted full bridge is currently used in medium and high power applications, and is also very popular, and its popularity is second only to the LLC resonant half bridge. The use of different topologies has been compared before, and here we will introduce the characteristics of the down-shift full bridge.

Phase-shifted full-bridge feature 1: The drive is relatively complex, resulting in complex control circuits and high cost. The reason is that the phase-shifted full-bridge generally has 4 MOSs, which requires high driving ability. It is amplified and used by an external MOS tube, and an isolation transformer is generally used to drive the MOS tube in order to enhance reliability.

Phase-shifting full-bridge feature 2: phase-shifting, why is phase-shifting, what does phase-shifting bring, and what is the difference between it and ordinary full-bridges. The phase shift is aimed at the same group of MOS tubes, so that the two MOS tubes are turned on in sequence, which can reduce the switching loss. At the same time when the super front arm bridge realizes ZVS, the secondary side is in freewheeling, the primary side current is shared by the diode, the current of the MOS tube is also very small, almost zero current conduction, and the lagging arm bridge can be turned on at zero voltage.

Phase-shifted full-bridge feature 3: The working process is complex, with two output power states (supplying energy by the primary side), two freewheeling states (supplying energy by the secondary side inductance and capacitor), and four dead zones (to achieve each MOS tube soft turn-on I)

Just to give the novice an understanding of the phase-shifted full bridge, as a more important part of the topology of the switching power supply, where is its focus and difficulty.

question nineteen

If high power pursues efficiency, how is bridgeless PFC realized? What is the principle?

Many people have heard of bridgeless PFC, but it is not very common to use it. The reason is that the efficiency of bridgeless PFC is improved compared with ordinary bridged PFC, which is generally 1-2%. If it is not the pursuit of high efficiency, Generally not used, the cost is too high. According to the characteristics of the bridgeless PFC, in fact, the rectifier bridge is not really unnecessary, but is used as the isolation of the positive and negative half-shafts of the AC input. The inductance will also increase by one, the MOS tube will also increase by one, and the driver IC will be more complicated. For high power, in order to achieve high efficiency, the detection resistor is made of transformer windings, which can reduce losses. I have contacted a 960W bridgeless PFC+LLC with an efficiency of 96.5%, but in the end, because the customer requires that the input voltage can be met by both AC and DC, at this time, the bridgeless PFC cannot play a good role in DC and was rejected.

question twenty

Why is three-phase power used in power supply? How is three-phase three-level realized, and what does three-level bring?

Three-phase electricity is used more in power supply, generally in the case of high power above 1KW or tens of thousands of W. Three-phase electricity generally adopts three-phase four-wire, one of which is the zero wire, and the four wires are equivalent to three times the power of ordinary two-phase electricity, and the greater transmission power is its biggest advantage; The most common three-phase asynchronous motor can be easily and conveniently produced.

What is going on with three-phase three-level, because three-phase electricity cannot directly supply power to some electrical equipment, and needs to be converted into ordinary two-phase electricity. In the general process, three-phase PFC is used to convert to direct current, and the direct current is then inverted to two-phase alternating current. This involves three-level technology. The rectified three-phase electric PFC is not an ordinary positive and negative DC, but a three-level, that is, positive DC, zero, and negative DC. It can also be seen from this that the stress of the three-level device is reduced, the harmonic content is low, and the switch loss is also low, so the advantages are very prominent in high-voltage and high-power applications.

question twenty one

There are many protection circuits in the power supply, how many protections can you say at most? How to achieve this?

The reliability of the power supply is inseparable from the protection circuit. What protection circuits are there usually?

1. Input undervoltage and overvoltage are very common, sampling the AC signal.

2. Output overvoltage protection, once the power switch can be locked, it will also help the reliability of the power supply.

3. Overcurrent protection, some use constant current to overcurrent, some use limited power to overcurrent, of course, it can also be done by locking the machine, the purpose is reliability, and there are many methods. The most reliable protection must be to lock up instead of hiccup!

4. Over temperature protection, using thermal detection on the transformer or ambient temperature, etc., to feedback to the IC lock or hiccup.

5. Short circuit protection, short circuit can hiccup, and it can also lock the machine.

These are commonly used in general power supplies, and some can be said to be necessary protection circuits. So look at the specifications and choose the appropriate IC to do the protection circuit with more convenient protection function. I have used an LD7522 as a flyback, and these functions can be very good and can be easily done.

question twenty two

What is the difference between a normal LDO and a high PSRR LDO?

This question is very typical. In fact, the general LDO plays the role of stabilizing the voltage. The control suppression caused by the temperature wave is basically concentrated below 10K. In a typical LDO data sheet, the PSR below 10K or 100K is usually It is below 40DB, because the LDO error amplifier has basically lost its amplifying ability at this time. For practical needs, the temperature wave frequency of many DCDC power supplies is hundreds of K or even megabytes. If it is an ordinary LDO, it has no ability to suppress such noise. It only has the ability to suppress the audio frequency range. Where RF applications are required, LDOs are usually powerless, and high PSR LDOs can provide this suppression, so this is also a fundamentally different difference.

question twenty three

Do not understand the power supply market? Where is your power supply going? Has it been developed? It is useful to make money for the boss.

Finally came to the last question. Generally, engineers may pay less attention to power supply market issues, and it is a mistake to focus on R&D. The success of the project is not done, but to earn less money.

For example: You work hard for three projects a year and earn 1 million. Another person does one project a year, which is much easier than doing three projects and earns 10 million a year. Boss Which do you like?

Some people say that the project is not our choice, how do we know whether to make money or not, but we should be familiar with the characteristics of profitable projects, what kind of power supply is popular in the market, do you know? If you develop it according to your own company’s existing model, is there a design gap between it and big companies? It’s not whether the project can be made, but whether it can be made optimally. In fact, from the perspective of research and development, it is how to choose the optimal topology and make a provincial plan.