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The main transformer in the circuit play on the current transformation, isolation, regulators and so on. Because of the wide use of many types of transformers. This leads to the part when the transformer produces a difficult choice, what should be how to choose a suitable transformer circuit it? Here the choice of forward transformer core skeleton design process, as well as the diameter of the primary winding of the set are summarized, together we look and see.
The first is the need to face the choice of transformer core skeleton, factors to consider too much, we list the next section to discuss.
First with Ap Act (core area product method) to calculate the value of the transformer AP: AP = AW * Ae = (Ps * 10 ^ 4) / (2ΔB * fs * J * Ku)
AW: core of the window area (cm ^ 2);.
Ae: core effective cross-sectional area (cm ^ 2);.
Ps: pass apparent power transformer (W) Ps = Po / η + Po (Forward);
ΔB: magnetic induction increment (T);
fs: transformer operating frequency (HZ);
J: current density (A)
According to the different cooling methods desirable 300 ~ 1000 A / cm ^ 2;
Ku:. Core window desirability factor of 0.2-0.4.
For the value of the type Ap algorithm, the transformer AP value with the actual use of far away, it was widely criticized. In fact, the root cause of the error is that the formula is basically in engineering applications have been optimized approximation, so some parameters are more ideal, but the actual use of a lot of parameters are changing, and even some distribution parameters in "trouble", so the resulting deviation in actual use should also be considered in the margin, so the Ap value calculated multiplied by a coefficient of 1.5 is reasonable.
In fact, here's ΔB (magnetic induction increment) is a relatively important physical quantity, needs attention. ΔB characterization of the core at the time of supply, the range of variation of the magnetic flux density, ΔB = Bmax-Br, Bmax is the maximum magnetic flux density, Br residual magnetic flux density.
Under constant input voltage and operating frequency prerequisite for the same piece of core, ΔB get bigger, the wider the range of variation of the magnetic flux density, the greater the core of iron loss, but the fewer the number of turns required, corresponding copper loss is small. Selection of the core, the need to select as high saturation magnetic flux density, residual magnetic flux density of the core as small as possible, this can be achieved
After obtaining AP value, there may be a lot of transformers are to meet the needs, which is the first to consider the structure size limit, especially limit the height and width.
For example, the value of AP and EI28 EFD30 0.6cm4 are about equally, but a lot of small height EFD30 more suitable power supply and flattened, and EI28 for compact power supply is even more important. Secondly, to reduce leakage inductance and stray capacitance perspective, we should choose the width of the wider framework of the transformer core with skeleton, such single winding turns will be more conducive to reducing the winding layers, thereby reducing the leakage inductance and distributed capacitance, leakage inductance on the issue, we'll discuss in the back again, but also from the versatility and economical point of view, this is the engineering design inescapable reality.
Of course, there are some safety issues, EMI, temperature, and so on around the law need to be considered.
After calculating the turns ratio is good, generally considered secondary rectifier voltage stress, and turns the calculated ratio adjustment or rounding turns ratio, then we can come through the turns ratio of thrust reversers real duty cycle range of the circuit.
Dmax = n (Vo + Vf) / Vin (min)
Dmin = n (Vo + Vf) / Vin (max)
To the rear is based on the true scope of the duty cycle to calculate the parameters thus obtained is reasonable, then you can calculate the maximum and minimum conduction time.
tonmax = Dmax / fs
tonmin = Dmin / fs
Then you can calculate the number of turns of the primary winding.
Np = Vin (min) × tonmax / (ΔB × Ae)
At least the primary winding: Np
Vin (min): Minimum input DC voltage across the primary winding
tonmax: Maximum primary MOSFET conduction time
ΔB: variation of magnetic induction, power forward type according to cooling conditions, the general can take 0.2-0.3.
Ae: Selected core cross-sectional area, in general can be found in the core manual.
Next, calculate the secondary turns, secondary turns Ns = Np / n, of course, is not necessarily an integer value obtained, usually turns to be rounded to the nearest integer, as the fractional turns bad when the winding control processes.
At this point will bring an issue, in order to maintain constant turns ratio, it is bound to be based on secondary turns rounding after the final turn, calculate the primary turns, otherwise change the duty cycle occurs, Np = Ns * n. NP calculated if not an integer, it also requires an approximate value, of course, lead to a slight change in the turns ratio of the duty cycle, but due to the small influence, it is generally no need to inverse the duty cycle again.
Similarly, after determining the final primary turns, you can calculate the magnetic flux density in turn varies the transformer core, verify ΔB is within a reasonable range, ΔB = [Vin (min) × Dmax × Ts] / (Np × Ae).
After getting Np, you can calculate the reset winding turns Nr, and calculate the diameter of the excitation current and the reset winding, taking into account reliable reset MOSFET voltage stress and transformers are generally set Np = Nr, then depending on the selected AL value of the magnetic core, the calculated inductance reset winding Lr = AL * N ^ 2, and then calculate the reset winding of the reset current Ir = Vin (min) × tonmax / Lr, the winding diameter corresponding also be able to be calculated a.
The next job is to calculate the diameter of the primary winding
One thing to note is that we calculate the diameter should be calculated RMS current, rather than the current peak or average!
To calculate the diameter of the primary winding, first to calculate the primary peak current Ip = Pi / VL = Po / (η × Dmax × Vin), and then calculate the peak current Iprms = Ip × √D, finally calculated from the current density needs winding wire cross-sectional area, and finally to the frequency, hasten skin depth and proximity effect factor, transformer skeleton with a width and depth to calculate the diameter of the outer diameter of the single.
The same calculation method of the secondary winding of the same, the difference is used to calculate the average current, Isrms = Io × √D, and then to consider the value of a single diameter, the considerations above.
For the choice of the core skeleton Forward transformer primary winding and diameter, and this article will do a more detailed description. I hope you read this article, the following can accumulate more about the forward transformer design.