Application of aluminum substrate and multilayer PCB in switching power supply
Then the application of aluminum substrate in switching power supply and multilayer PCB in switching power supply circuit are discussed.
Aluminum substrate by its own structure, with the following characteristics: thermal conductivity is very good, single-side bound copper, device can only be placed in the bound copper surface, can not open electrical wiring hole so can not be placed as a single panel jumper.
On the aluminum substrate, a patch device, a switch tube and an output rectifier tube are generally placed to conduct heat through the substrate, with low thermal resistance and high reliability.The transformer adopts planar patch structure, which can also dissipate heat through the substrate, and its temperature rise is lower than that of the conventional one. The transformer of the same specification adopts aluminum substrate structure, which can get larger output power.Aluminum base plate jumper wire can be used to bridge the way.Aluminum substrate power supply is generally composed of two printed boards, the other board placed control circuit, the two boards through physical connection between the synthesis of one.
Due to excellent thermal conductivity aluminum plate, difficult in small amount of manual welding, solder cooling too fast, easy problems existing a simple and practical method, will be a common electric iron used ironing temperature adjustment function is), turn it over, ironing, oriented fixed well, the temperature to 150 ℃ or so, put the aluminum plate on top of the iron, heating time, and then affix components and welding according to the conventional method, the iron temperature with device is easy to weld advisable, high device damage when possible, copper or aluminum plate, low temperature welding effect is bad, be flexible.
In recent years, with the multilayer circuit board used in switching power supply circuit, making it possible to print line transformer, because of the sandwich plate, layer spacing is small, also can make full use of the transformer window section, can add one or two on the main circuit board piece composed of multilayer printed coil to use window, reduce the line current density as a result of printed coil, reduce manual intervention, transformer good consistency and flat structure, the leakage inductance is low, good coupling.Open core, good heat dissipation condition.
Due to its many advantages, it is conducive to mass production, so it is widely used.However, the initial investment in research and development is large, which is not suitable for small-scale production.
Switch power supply is divided into two forms: isolated and non-isolated. Here, we mainly talk about the topological form of isolated switch power supply.Isolated power sources can be divided into two categories according to the structure: forward excitation and back excitation.Flyback type refers to the auxiliary edge cut off when the original side of the transformer is switched on, and the transformer accumulates energy.When the primary side is cut off, the secondary side conducts and energy is released to the working state of the load.The forward excitation type refers to the voltage output from the primary side of the transformer to the load induced by the secondary side, and the energy is directly transmitted through the transformer.According to the specification can be divided into conventional forward excitation, including single tube forward excitation, double tube forward excitation.Half - bridge and bridge circuits belong to positive excitation circuits.
The forward and back excitation circuits have their own characteristics. In order to achieve the best cost performance, they can be used flexibly.Generally in the case of low power can choose flyback.Slightly larger can be used for single tube forward circuit, medium power can be used for double tube forward circuit or half bridge circuit, low voltage use push-pull circuit, and half bridge working state.High power output, generally use bridge circuit, low voltage can also use push-pull circuit.
Flyback power supply is widely used in small and medium power supply because of its simple structure, which saves an inductor about the same size as transformer.In some of the introduction that flyback power supply power can only do a few watts, output power more than 100 watts will not have advantages, it is difficult to achieve.I think that is the case in general, but also can not be generalized, PI has an article on the back power supply can do up to the kilowatt, but did not see the real thing.The output power is related to the output voltage.
Flyback power supply is the leakage inductance of the transformer is a very key parameter, with the need of flyback power supply transformer store energy, to make full use of the transformer core, typically KaiQi gap in the magnetic circuit, the purpose is to change the core of hysteresis loop of the slope, the transformer to withstand large pulse current shock, into a state without iron core saturation nonlinear, magnetic circuit gas gap under high magnetic resistance, magnetic flux leakage is greater than in magnetic circuit completely closed magnetic circuit.
The coupling between the transformer's primary electrodes is also the key factor to determine the leakage inductance. To make the primary electrode coil as close as possible, sandwich winding method can be adopted, but this will increase the distributed capacitance of the transformer.Choose the iron core with a longer window to reduce leakage, such as EE, EF, EER, PQ core than EI type better effect.
As for the duty ratio of flyback power supply, in principle, the maximum duty ratio of flyback power supply should be less than 0.5, otherwise the loop is not easy to compensate and may be unstable, but there are some exceptions. For example, the TOP series chips launched by PI company in the United States can work under the condition that the duty ratio is greater than 0.5.Duty cycle is determined by the ratio of turns of the primary and secondary side of the transformer. My opinion on flyback is to determine the reflected voltage (the output voltage is reflected to the primary side through the transformer coupling) first. If the reflected voltage increases within a certain voltage range, the duty cycle of the work will increase and the switch tube loss will decrease.When the reflected voltage decreases, the duty cycle decreases and the switch loss increases.
, of course, this also is a prerequisite for, when the duty ratio increases, means that the output diode conduction time, to maintain stable output, more time will be guaranteed by the output capacitor discharge current, output capacitance will be under more high frequency ripple current scour, and exacerbated the fever, it is not allowed in many conditions.Duty ratio increase, change the transformer turn ratio, make the leakage inductance of the transformer, make its overall performance, when the leakage inductance energy to a certain extent, can be fully offset switch tube big duty with low loss, when it no longer increases the meaning of duty cycle, may even because of the leakage inductance of the high peak voltage and breakdown switch tube.
As a result of large leakage inductance, may make output ripple, and some other electromagnetic indicators worse.When the duty ratio is small, the RMS of the switching tube through the current is high, and the RMS of the transformer primary current is large, which reduces the efficiency of the converter, but can improve the working conditions of the output capacitor and reduce the heating.How to determine the reflected voltage (i.e. duty cycle) of the transformer.
Some netizens mentioned the parameter setting of the feedback loop of the switching power supply and the analysis of the working state.Because in the school of higher mathematics is poor, "automatic control principle" almost make-up examination, for this door now also feel fear, up to now, do not write the full closed-loop system transfer function for the system, the concept of zero and pole feel very fuzzy, see the bode diagram is just about to see is divergent or convergent, so for the feedback compensation is not nonsense, but there are some Suggestions.
If you have some basic knowledge of mathematics, and some learning time, you can find out the textbook "automatic control principle" of the university and carefully digest it, and combine with the actual switching power circuit to analyze according to the working state.

Sixth,Duty ratio of flyback power supply
Finally talk about the duty ratio of the flyback power supply (I focus reflected voltage, consistent with the duty ratio), the duty ratio is associated with the pressure of selection switch tube, there are some early flyback power supply using low pressure switch tube, such as 600 v or 650 v as input ac 220 v power switch tube, may be about when the production technology, high pressure pipe, not easy to make, or low pressure pipe has a more reasonable conduction loss and switch characteristic, like this line reflected voltage cannot be too high, otherwise, to make the switch tube work safety within the scope of absorbing circuit is a considerable loss of power.
Practice has proved that the reflected voltage of 600V pipe should not be more than 100V, and the reflected voltage of 650V pipe should not be more than 120V. When the peak voltage of leakage inductance is clamped at 50V, the pipe still has working margin of 50V.Now due to the improvement of MOS tube manufacturing technology, the general flyback power supply is 700V or 750V or even 800-900v switch tube.
Like this kind of circuit, the ability of resisting overvoltage is strong some switch transformer reflection voltage can also do some higher, the maximum reflection voltage is more appropriate at 150V, can obtain better comprehensive performance.PI's TOP chip is recommended to use transient voltage suppression diode clamp for 135V.But his panels typically reflect less than that, around 110 volts.Both types have advantages and disadvantages:
The first category: weak overvoltage resistance, small duty cycle, transformer primary pulse current.Advantages: the transformer leakage inductance is small, the electromagnetic radiation is low, the ripple index is high, the switch tube loss is small, the conversion efficiency is not necessarily lower than the second kind.
The second kind: the fault switch tube loss is some big, the transformer leakage feeling is some big, the ripple is some bad.Advantages: strong overvoltage resistance, high duty cycle, low transformer loss, high efficiency.
Flyback power reflected voltage and a certain factor, reflected voltage of the flyback power supply is also associated with a parameter, the output voltage, output voltage is lower, the greater the transformer turn ratio, the larger the leakage inductance of the transformer, switch tube to withstand voltage is higher, is likely to breakdown the consumed power, the greater the switch tube, absorption circuit has the potential to absorb circuit power component permanent failure (especially the transient voltage suppression diode circuits).In the design of low-voltage output low-power flyback power supply optimization process must be carefully handled, the treatment methods are several:
1. The magnetic core with a large power level is adopted to reduce leakage inductance, which can improve the conversion efficiency of low-voltage flyback power supply, reduce loss, reduce output ripple, and improve the crossover adjustment rate of multi-channel output power supply. It is commonly used in switching power supply for household appliances, such as cd-rom drive and DVB set-top box.
2. If it is not allowed to increase the magnetic core, the reflected voltage and duty cycle can only be reduced.To reduce the leakage inductance of the reflected voltage can be reduced but is likely to reduce the power conversion efficiency, both of which is a contradiction, there must be an alternative process to find a suitable point, in the process of transformer replacement experiment, can detect the peak inverse voltage transformer primary side, try to reduce the peak inverse voltage pulse width, and amplitude, can increase the work safety margin of the converter.Generally, the reflected voltage is suitable at 110V.
3, strengthen coupling, reduce loss, adopt new technology, and winding process, transformer to meet the safety specification will take insulation measures between the original side and the side, such as insulation tape pad, insulation end empty tape.These will affect the leakage inductive energy of transformer. The winding method of primary winding around secondary winding can be used in practical production.Or secondary with triple insulation wire winding, eliminating the insulation between the first stage, can enhance the coupling, or even use wide copper skin winding.
The low voltage output refers to less than or equal to 5 v output, like this kind of small power sources, my experience is that the power output is more than 20 w output can be used normal shock type, can get the best price, of course it is not definitely right, and personal habits, and application environment have relations, next time to talk about a flyback power supply with magnetic cores, magnetic circuit KaiQi gap of some understanding, I hope you tall person to give directions.
The flyback power transformer core is operating in a unidirectional magnetized state, so the magnetic circuit needs to open an air gap, similar to the pulsating dc inductor.Part of the magnetic circuit is coupled through air gaps.
Principle I understand is why KaiQi gap: due to the power ferrite also have similar to the rectangular working characteristic curve (hysteresis loop), on the working characteristic curve of the Y axis magnetic induction intensity (B), now the production process of general saturation point in more than 400 mt, this value in the design values should be in commonly 200-300 mt is more appropriate, the X-axis represents the magnetization of the magnetic field intensity (H) this value and current intensity is proportional to the relationship.
Magnetic circuit open air gap is equivalent to the magnet hysteresis loop to the X-axis tilt, under the same magnetic induction intensity, can withstand greater magnetization current, is equivalent to the magnetic core storage more energy, this energy in the switch tube cut-off through the transformer secondary discharge into the load circuit, flyback power core open air gap has two roles.One is to transfer more energy, and the other is to prevent the core from becoming saturated.
The transformer of flyback power supply works in a unidirectional magnetized state, which not only transfers energy through magnetic coupling, but also performs multiple functions of voltage transformation input and output isolation.Therefore, the handling of air gap needs to be very careful. If the air gap is too large, leakage inductance will increase, hysteresis loss will increase, iron loss and copper loss will increase, and the overall performance of the power supply will be affected.The small air gap may saturate the transformer core, resulting in power supply damage.
The continuous and discontinuous mode of flyback power supply refers to the working state of the transformer. The transformer works in the full load state in the mode of full or incomplete transmission of energy.Generally, it should be designed according to the working environment. The conventional flyback power supply should work in the continuous mode, so that the loss of switch tube and line is relatively small, and the working stress of input and output capacitor can be reduced. However, there are some exceptions.
Need to point out in particular: because of the characteristics of flyback power supply is more suitable for the design of high-voltage power supply, and high-voltage power supply transformer generally works in the interrupted mode, I understand that because of the high-voltage power supply output needs to use high-voltage rectifier diode.
Due to the manufacturing process characteristics, the high back pressure diode reverse recovery time is long, low speed, in current continuous state, where it is forward biased diode is restored, when the reverse recovery of the energy loss is very big, is not conducive to the improvement of the performance of the converter, light conversion efficiency, rectifier serious fever, or even burned rectifier.Since the diode is reverse-biased at zero bias in discontinuous mode, the loss can be reduced to a relatively low level.Therefore, the high-voltage power supply works in intermittent mode, and the operating frequency cannot be too high.
There is a flyback power supply work in the critical state, general this type of power supply work in frequency modulation mode, or wide frequency and dual mode, some low cost self excitation power supply (RCC) often use this form, in order to ensure the stable output, transformer with working frequency, output current and input voltage change, close to full load when the transformer keep between continuous and intermittent, the power supply only suitable for small power output, otherwise the emc characteristics of processing can be a headache.
The flyback switching power supply transformer should be working in continuous mode, the bigger requires winding inductance, of course, is also a certain degree of continuously, it is not realistic to excessive pursuit of absolutely continuous, may need a great deal of magnetic core, a huge number of coil number of turns, accompanied by a large leakage inductance and distributed capacitance, may do more harm than good.
Then how to determine this parameter? Through many practices and analysis of the design of peers, I think it is appropriate for the output of 50%-60% transformer to transition from intermittent to continuous state when nominal voltage input. Or at the highest input voltage state, full load output, the transformer can transition to a continuous state.

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