Because the energy storage device is on the rear BUCK inductor, the forward transformer is not as complicated as the Flyback transformer. Its function is mainly voltage, current conversion, electrical isolation, energy transfer and so on. Therefore, when we calculate the forward transformer, we usually first study the BUCK inductor of the transformer secondary back end. The input voltage of the BUCK inductor is the secondary output voltage of the forward transformer minus the forward voltage drop of the rectifier diode. So we also call the forward power supply an isolated version of BUCK. In the following, from the aspects of forward switching power supply 1, the choice of primary and secondary turns, magnetic reset 3, on the duty cycle and turns ratio 4, other reset modes 5, loss and EMI6, forward transformer design.
Taking the third winding reset forward transformer as an example, once the turns ratio is determined, the next step is to calculate the number of primary and secondary turns. The engineers in the forum believe that the forward transformer is in the case of full load unsaturation. The smaller the number of turns, the better.
In fact, this is a misunderstanding. The number of turns determines the primary inductance (without opening the air gap or opening the same air gap), and the magnitude of the inductance determines the magnitude of the primary excitation current. It does not participate in the transmission of energy, but it also needs to consume energy. Therefore, the smaller the excitation current is, the higher the efficiency of the power supply is. In addition, the too small number of turns will cause the deltB to become larger. If the air gap is not balanced, the transformer is easily saturated. .
Magnetic reset
Whether it is single-tube forward or double-tube forward, there is a problem of magnetic reset. And can be seen as a passive reset. The reset current is very important. If it is too small, the reset effect will be affected by the transformer's own distribution parameters (mainly uncontrollable capacitance, leakage inductance). The reset current is because the inductor current cannot be abrupt. After the primary MOSFET is turned off, the flyback of the primary winding is reversed, and the reset winding has the opposite phase to the primary winding. Therefore, there is a reset current in the reset winding to generate a reset current. The demagnetization reset, its importance is self-evident; when the transformer does not add air gap, its primary inductance is large, and the reset current is naturally small.
However, in the practical application of high-power single-tube forward and double-tube forward, it is often necessary to add a small air gap, otherwise the design is extremely unreliable, the high-power power supply, the primary side current is large, and the leakage inductance is caused. The magnetic induction intensity changes, B=I*Llik/nAe, which is large, and the air gap is added to reduce the leakage inductance Llik.
About duty cycle and turns ratio
The forward duty cycle is mainly determined by the input and output of the secondary freewheeling inductor, the secondary is a BUCK circuit, and the CCM BUCK line Vo=Vin*D is independent of the secondary current.
Vo=Vin*D
Vo: output voltage, Vin: BUCK input voltage, that is, the output voltage of the forward transformer minus the forward voltage drop of the rectifier, D: duty cycle here, the output voltage is known, we only need to determine a suitable Empty ratio, you can calculate the Vin of the BUCK inductor, which means that the output voltage of the transformer is basically fixed. In this case, we must remind everyone that the value of the duty cycle D has a great relationship with the reset mode. It is recommended that the value of D should not exceed 0.5.
After knowing the output voltage Vs of the transformer, the ratio of the transformer can be calculated according to the input voltage. Here, the lowest input DC voltage is used to calculate the turns ratio, because the minimum input DC voltage corresponds to the maximum duty cycle. The voltage of this Vs is also an important data for selecting the withstand voltage of the secondary rectifier diode.
Please pay attention when choosing Debbie, because the calculated value is usually one or even a few digits after the decimal point, and when we actually wind the transformer, it is very difficult to wrap around a few turns, so try to take an integer. The ratio of times is greater; of course, if the transformer is calculated, the primary to secondary turns ratio of the transformer does not exclude the case where it is just a decimal.
The forward transformer plus a small amount of air gap can empty the residual magnet in the electro-magnetic conversion, and the actual utilization of the magnetic core increases. At the same time, the added little no-load current accounts for a small proportion of the high-power current, and the efficiency is not affected too much. Large impact, this can make the transformer not easy to saturate, the reliability of the power supply increases, and the primary turns can be reduced, the internal resistance of the transformer is reduced, and the power can be reduced in a small volume. The air gap is also equivalent to the increase of the transformer core, but The practical benefits (especially the ability to resist saturation) are better than the increase in the core. After the air gap is added, the reduced inductance will be supplemented by the increased core utilization, and more than enough, it is cost-effective to worry.
Is the position of the reset winding close to the primary winding, or is it sandwiched between the primary and secondary? If you go around, of course, the coupling with the primary is the best, but the pressure on the enameled wire is a test! Of course, this will not be directly broken down.
Regardless of the EMC point of view or the process point of view, it is better to place the reset winding on the innermost layer, which is the majority of the actual mass production.
Single-tube forward, if it is commercial power or has PFC output voltage as input, the minimum withstand voltage of MOSFET is 2 times DC bus voltage, plus the leakage inductance factor, MOSFET is recommended to choose 800V or even 900V tube.
In the high-power power supply, considering the reliability, the margin of the general transformer is large. In order to avoid the saturation of the transformer, the deltB is generally selected to be smaller, generally 0.2 or less; due to the switching loss of the EMC and the MOSFET, the frequency is set. Lower, generally below 40KHz; high-power power supplies generally have active PFC circuits, so the bus voltage of a single-tube or dual-tube forward topology is about 400V.
For the above three reasons, according to the transformer formula Np=Vin*Ton/(deltB*Ae), it can be seen that the transformer has more primary and secondary turns, and more turns will make the distribution parameters (leakage, distributed capacitance). ) becomes larger, so that the AC loss of the winding, especially the DC loss, becomes larger. In addition, the heat dissipation characteristics of the internal winding of the high-power transformer are very poor, so the temperature rise of the winding is considerable, and the core heat dissipation area of ​​the high-power transformer is added. Small, the middle column heat is more serious than the two side columns, and the heat dissipation is even worse, so the temperature rise caused by the loss of the iron core is also considerable. Larger iron loss and copper loss cause the temperature of the core to rise, which causes the saturation point of the transformer's magnetic flux density to drop. If the design margin is not enough, when the transformer is under the impact of high temperature and large load, it may immediately saturate and cause the explosion tube. ! Adding a small air gap can reduce the remanence of the transformer, so as to avoid saturation of the transformer under the impact of high temperature and large load.
Other reset methodsWhy do some transformers reset normally without adding a reset winding? External reset RCD, LCD, active clamp, etc. can be used.
The resonant reset forward converter is a resonant reset using the transformer magnetizing inductance and the MOSFET junction capacitance, but the required inductance and capacitance are required to be calculated in detail, and it is usually necessary to open the air gap to the forward transformer. The reset current is generally small, so the heat generated by the reset winding is also small, and it can be bypassed in the inner layer to facilitate the control of the process. The transformers I make are generally reset, primary, secondary, and auxiliary.
If the secondary winding is inside, the length of the copper wire used for this winding is small, and the DC loss is low, but the heat dissipation is a little worse. If it is outside, the opposite is true.
For a forward power supply, the turns ratio affects the duty cycle, the primary and secondary peak currents, the number of turns, and the inductance of the secondary storage inductor.
There is no bias and straight-through fault, the main advantage is high reliability. At the same frequency, the heat of the forward transformer core is only 1/3 of the bridge type. 200W-500W forward transformer can be added 0.05-0.1 The air gap of MM can reduce the number of primary turns, and can also increase the frequency appropriately, further reducing the number of turns to reduce the heat generation of the wire.
What are the components of the primary and secondary currents when the positive power supply is turned on and off? After the steady state? The two clamped diodes are turned on during reset, so that the voltage across the clamp MOSFET is approximately equal to the DC bus voltage. The reset diode is preferably used for ultra-fast recovery. The most ideal choice is BYV26C. The tube, UF series is also available.
Loss and EMIHard-switched circuits, theoretically analyzed, the benefits of increased frequency: allow for fewer turns or use smaller transformers (the same type of transformer outputs the same power, the iron loss will be significantly reduced), reducing the size of the power supply, Increase the power density of the power supply. Of course, there is also a bad side: increasing the frequency will increase the switching loss of the MOSFET, the skin depth of the transformer winding will decrease, the oscillation of the distributed parameters will be more severe, and the EMI will deteriorate. Therefore, reliability and frequency are not necessarily related. As long as the circuit is processed well, especially the thermal design is completed, the general reliability is still relatively high.
The size of the turns ratio is related to the input voltage range and duty cycle. Forward and flyback are different, the flyback "inductance" transformer is before, and the forward inductance is after the transformer, so the secondary and flyback transformer secondary output voltages are different under the same duty cycle. The secondary can be regarded as a BUCK circuit. The input voltage of the BUCK circuit is the secondary output voltage of the transformer minus the voltage drop of the rectifier. As long as the duty ratio is determined, the input voltage of the front end of the inductor can be calculated. The secondary voltage is then scaled through the duty cycle. After selecting the transformer, the primary turns can be calculated and the secondary turns calculated by the turns ratio.
When calculating the transformer, it is often adjusted because of the decimal ratio or the number of turns, so that the first calculated output inductor margin is not to be left larger? Yes, generally in the actual circuit design, there will be a certain margin compared with the calculated value, and when the value is used, the reverse value verification is needed to ensure the working state of the circuit. Among the controls. The forward transformer has three currents when the switch is turned on. 1. Excitation current, I1=VIN*Ton/Lp; that is, the ramp current in Ip. This part of the current does not transfer energy, only the electromotive force of the transformer is maintained. 2. The platform current I2 in Ip, this part is the transfer of energy. 3. Secondary induced current I3 = n * I2. Since the magnetic fields generated by I3=n*I2, I2, and I3 cancel each other out, they are not considered in the calculation of the forward transformer.
The switching loss is a hard fault in the hard-switching circuit, and the switching loss can be significantly reduced unless the soft switch is applied. The method of reducing the switching loss of the hard switch is to reduce the switching frequency, speed up the turn-on and turn-off (to make the rising and falling edges of the waveform steeper), but it will make the EMI worse. The MOSFET with small input capacitance will improve the driving ability of the circuit. Wait.
The two-tube forward excitation is exactly the same as the single-tube forward transformer. In fact, the primary current of the forward transformer can be obtained by the equivalent model of the transformer. In the text, when the Ton is turned on, the rectifier is turned on, and the freewheeling tube is turned off (ignoring the influence of the reverse recovery time and the leakage inductance, etc.) The influence of the factor), the secondary energy storage inductor current rises linearly, di(L)/dt=(Us-Uo)/L, and this current is fed back to the current waveform of the transformer primary through the turns ratio. Of course, under the action of the input voltage Uin, the primary magnetizing inductance of the transformer also has a linearly rising excitation current, di(m)/dt=Uin/Lm, both of which flow through the primary coil of the transformer. So the current we tested is the superposition of these two currents. This also explains why the wire diameter of the reset coil is much smaller than the wire diameter of the primary coil.
The value of the value limits the loss of the transformer core. The smaller the B-value transformer, the less likely it is to saturate, but on the contrary it requires more winding turns, and sometimes the copper loss is increased even because the window area is too much.
Forward is generally working in CCM mode, with a large DC component, if you want to use a larger deltB, you need to add a little air gap to reduce the residual magnetism, to balance the impact of DC components, but this will make As the excitation current increases, the copper loss of the transformer increases, and the current stress of the switching tube also increases accordingly.
Because the positive duty cycle is typically less than 0.5, the secondary freewheeling diode has a longer turn-on time. In addition to the effect of the capacitor, the average current of the rectifier diode and the freewheeling diode should be the same. Forward is rarely used in the full voltage range because the duty cycle changes too much? Yes, too much change in duty cycle can make secondary inductor design troublesome. Positive problem with a minimum duty cycle
Forward transformer designThe first thing that needs to be faced is the choice of transformer skeleton and core. There are too many factors to consider. Let us discuss some of them:
First, use the Ap method (core area product method) to calculate the AP value of the transformer:
AP=AW*Ae=(Ps*10^4)/(2ΔB*fs*J*Ku)
AW: the window area of ​​the core. ( cm^2);
Ae: core effective cross-sectional area. ( cm^2);
Ps: Transformer delivers apparent power (W)
Ps=Po/η+Po (forward);
ΔB: magnetic induction increment ( T );
Fs : transformer operating frequency (HZ);
J : current density ( A ). According to different heat dissipation methods, 300~1000 A/cm^2;
Ku: Core window coefficient. It can be 0.2-0.4.
The value obtained by the above-mentioned Ap algorithm is far from the actual AP value of the transformer, so it is widely criticized. In fact, the root cause of the error is that the above formula is basically obtained by optimizing the approximation in engineering applications, so some parameters are ideal, and many parameters in actual use are changed, and even some distribution parameters are "Disorder", so caused a deviation, in the actual use is also to consider the margin, so it is reasonable to multiply the calculated Ap value by a factor of 1.5-2.
In fact, ΔB (magnetic induction increment) is a relatively important physical quantity, and everyone needs to pay attention. ΔB characterizes the range of variation of the magnetic induction intensity when the magnetic core is operated, ΔB=Bmax-Br, Bmax is the maximum magnetic induction, and Br residual magnetic induction. Under the premise that the input voltage and the operating frequency are constant, for the same core, the larger the ΔB is, the wider the range of the magnetic induction is, the larger the iron loss of the core is, but the fewer the number of turns required. The corresponding copper loss is small. When selecting a magnetic core, it is necessary to select a magnetic core having a saturation magnetic flux density as high as possible and a residual magnetic flux density as small as possible, so that a small magnetic core can be used for high power.
After getting the AP value, there may be a lot of transformers that meet the needs. This is the first consideration of the size of the structure, especially the height and width limits. For example, the AP values ​​of EFD30 and EI28 are also about 0.6cm4, but the height of EFD30 is much smaller, which is more suitable for flat power supply, and EI28 is more important for compact power supply.
Secondly, from the perspective of reducing the leakage inductance and the distributed capacitance, the transformer core and the skeleton with a wide skeleton width should be selected, so that the number of turns of the single-layer winding will be more, which is beneficial to reducing the number of winding layers and thus reducing the leakage inductance. With distributed capacitance, the issue of leakage inductance, we will discuss later. Again, from the perspective of versatility and economy, this is a real problem that cannot be avoided in engineering design. Of course, there are some issues such as safety regulations, EMI, temperature rise, and winding around.
After calculating the turns ratio, the voltage stress of the secondary rectifier is generally considered. The calculated turns ratio is adjusted or the turns ratio is rounded. Then we can use the turns ratio to reverse the true duty cycle range of the circuit.
Dmax=n(Vo+Vf)/Vin(min)
Dmin=n(Vo+Vf)/Vin(max)
The latter is to be calculated according to the real duty cycle range, so the parameters obtained are reasonable. Then you can calculate the maximum and minimum on-time,
Tonmax= Dmax/ fs
Tonmin= Dmin/ fs
Then you can calculate the number of turns in the primary winding.
Np = Vin(min) × tonmax / (ΔB × Ae)
Np: minimum number of turns in the primary winding
Vin(min): the lowest input DC voltage of the primary winding
Tonmax: maximum on-time of the primary MOSFET
ΔB: the amount of change in magnetic induction, the forward type power supply can generally take 0.2-0.3 according to the heat dissipation conditions.
Ae: The cross-sectional area of ​​the selected core is generally found in the magnetic core manual.
Next, calculate the number of secondary turns, the number of secondary turns Ns = Np / n, of course, the value obtained is not necessarily an integer, generally it is necessary to round off the integer åŒ, because the decimal åŒ is not well controlled when winding.
At this point, there will be another problem. If you want to keep the ratio unchanged, then it is necessary to calculate the final number of turns in the primary according to the number of secondary turns after rounding, otherwise the duty cycle will change, Np= Ns * n
If the calculated NP is not an integer, it also needs an approximate value. Of course, it will bring a slight change in the ratio and duty cycle. However, since the influence is small, it is generally unnecessary to reverse the duty cycle again. Similarly, after determining the final number of primary turns, the range of magnetic induction of the transformer core can be inversely calculated to verify whether ΔB is within a reasonable range, ΔB = [Vin(min) × Dmax × Ts] / (Np × Ae)
After Np is obtained, the reset winding turns Nr can be calculated, and the excitation current and the wire diameter of the reset winding can be calculated. Considering the voltage stress of the MOSFET and the reliable reset of the transformer, Np=Nr is generally set, and then according to the selection. The AL value of the core, calculate the inductance of the reset winding Lr=AL*N^2, and then calculate the reset current Ir=Vin(min) ×tonmax/Lr of the reset winding, and the corresponding winding wire diameter can also be calculated. It is.
The next step is to calculate the wire diameter of the primary and secondary windings. One thing that needs to be noticed is that the calculated wire diameter is calculated as the current rms value, not the current peak or average value!
To calculate the wire diameter of the primary winding, first calculate the primary peak current Ip = Pi / VL = Po / (η × Dmax × Vin), then calculate the peak current Iprms = Ip × √ D, and finally calculate according to the current density The required cross-sectional area of ​​the winding wire, and finally the outer diameter of the single wire diameter is calculated according to the frequency, the skin depth and the proximity effect, the width and depth of the transformer skeleton. The calculation method of the secondary winding is the same, the difference is calculated by the current average value, Isrms=Io×√D, then the value of the single wire diameter should be considered, and the considerations are the same as above.
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