Design and application of perforated plate magnetic devices for DC/DC module power supply

1 Introduction
With the continuous increase in the power of DC/DC module power supplies, higher requirements are placed on the volume and efficiency of magnetic devices in power supplies. At present, planar magnetic devices with low-profile magnetic cores and planar winding structures are common components of 1/8 to 1/2 brick DC/DC modules. Flat matrix transformers with multi-core structures are also used in module power supplies because they can automatically balance the secondary current, have flexible structural design, and are easy to dissipate heat. However, the magnetic cores of the above two structures occupy a large part of the PCB surface area, making it difficult to arrange power devices and control devices. At the same time, the thickness of the magnetic core also determines the height of the module, making the space utilization of the module in height insufficient. In
order to provide a larger layout area for the power devices and control devices of the module, reduce the thickness of the module, and increase the power density, this paper attempts to use perforated flat magnetic devices and install the magnetic devices parallel to the PCB to achieve the above purpose. For the application of an active clamp DC/DC power module, the structure and magnetic circuit characteristics of perforated flat transformers and perforated flat inductors are first analyzed. Then the specific parameters of the magnetic devices are designed, and the magnetic devices are manufactured after finite element simulation verification, and their effects are confirmed by actual measurements.

2 Active clamping module power supply and its structure
The designed DC/DC module power supply is an isolated forward circuit with input low-end active clamping and output synchronous rectification. Its main circuit is shown in Figure 1.

In order to improve the power density of the module power supply and improve the problem of tight layout area of ​​power devices, the PCB of the module power supply is installed in parallel with the perforated flat magnetic device, as shown in Figure 2. It can be seen that the perforated flat magnetic core is placed above the PCB and does not occupy its surface, so that more power switches and control devices can be placed on the PCB to achieve more complex functions or higher power levels.

3 Analysis and design of the flat magnetic device of the module
power supply The transformer and inductor of the module power supply adopt the perforated flat magnetic device structure, which is essentially a deformation of the matrix transformer. The design is introduced in detail below.
3.1 Design and analysis of perforated flat transformer
The perforated flat transformer consists of a flat magnetic core with through holes and a flat copper wire winding. Among them, the structure of the perforated flat magnetic core is quite different from that of the usual magnetic core. On the flat magnetic core, with the through hole as the center, each through hole and the surrounding magnetic material constitute a magnetic core unit. The primary and secondary windings of the transformer pass through the through hole, that is, they are interlinked to form a transformer conversion unit. In the design, considering the limited area of ​​the through hole and the large output current of the secondary winding, a 1:1 turns ratio primary and secondary winding design is adopted in the transformer conversion unit. In addition, the core unit adopts an I-type design with opposite excitation current directions of adjacent core units, so that the magnetic circuit coupling between different core units is small enough to be ignored, thereby simplifying the analysis and design.
When designing a perforated flat core, it is necessary to determine parameters such as the number of perforations, the diameter of the perforations, the area and thickness of the plate, and these parameters need to be based on the basic constraints of unsaturated core, winding passability, and appropriate volume. The following introduces the process of determining the parameters in the design. For the core unit, an obvious difference from ordinary transformers is that its internal magnetic induction intensity is unevenly distributed. The excitation inductance of the core unit is:

where: μ is the magnetic permeability of the core, in H/m; h is the thickness of the flat core, in m; Rmax is the radius of the core unit, in m; Rmin is the radius of the core through hole, in m.
It can be seen from formula (1) that the maximum value of the magnetic induction intensity in the core unit appears at the inner wall of the through hole, and the minimum value appears at the outer boundary of the core unit. When designing, it is necessary to first ensure that the core on the inner wall of the through hole is not saturated. Since the primary winding is connected in series, the overall magnetizing inductance of the transformer is the sum of the series units, that is:

where: Lm is the primary magnetizing inductance.
It can be seen that the number of series connected core units n in the perforated flat transformer is similar to the number of turns N of the winding of an ordinary transformer. By selecting the number of series connected primary and secondary core units, the required primary and secondary turns ratio can be achieved. For an active clamped DC/DC power supply, when a main switch is turned on, the maximum primary flux linkage ψ=UinDT in the perforated flat transformer, where Uin is the DC input voltage, D is the duty cycle, and T is the switching period. The flux linkage of each core unit is 1/n, so the maximum magnetizing current of the core unit is:

where: f is the operating frequency of the switch tube.
According to Ampere's loop law, the maximum magnetic induction intensity of the perforated flat core appears at the inner wall of the through hole when the main switch tube is about to be turned off, and its value is:

It can be seen that when designing a perforated flat transformer, it is necessary to select n, Rmin, h, etc. to achieve the unsaturated requirement. The larger n is, the less likely it is to saturate, and when Rmin is too large or too small, the core is easy to saturate. Therefore, when designing a perforated flat transformer with basically determined external dimensions, it is necessary to balance n and Rmin.

As can be seen from the figure, when Rmin is between 0.5 and 1 mm, Bpeak is small. In order to facilitate winding production, Rmin=1 mm is designed. The primary winding of the transformer is laid out in the horizontal direction, and the secondary winding is laid out in the vertical direction to reduce mutual interference between the windings.
3.2 Design and analysis of perforated flat inductor
The basic structure and materials of the designed perforated flat inductor are similar to those of the perforated flat transformer mentioned above. The main structure of its core unit is shown in Figure 4. It can be seen that an air gap l0 is opened on the through hole of the inductor core unit. Since μ is much larger than the magnetic permeability of air, the magnetic flux of the inductor winding mainly falls on the air gap, so the magnetic induction intensity in the air gap is almost the same, and the magnetic induction intensity at various locations in the magnetic body of the inductor core unit is also basically the same.

As shown in Figure 4, the inductance of a single core unit is:

The entire inductor has NL series core units, and the total inductance is:

Therefore, when designing a perforated flat inductor, the product of each parameter can be obtained by formula (6) according to the required inductance, and then Rmin, l0, NL and the size of the core can be considered in turn. For this DC/DC module power supply, the output filter inductor is 0.63μH, the core is a manganese-zinc ferrite core of 25 mmx16 mmx4 mm, 6 perforations of Rmin=1 mm, and the winding uses Litz wire.

In order to test the design of perforated flat plate transformer and perforated flat plate inductor, transformer and inductor prototypes were made and experiments were carried out on active clamp DC/DC module power supply.

Figure 6a shows the voltage waveforms of the primary and secondary windings of the perforated transformer when the input voltage is 48 V and the output full load current is 15 A. It can be seen that the primary and secondary voltage waveforms are similar, and their voltage amplitude ratio meets the designed 6:1 relationship, and the secondary voltage waveform phase delay is also small. The experiment shows that the transformer design can meet the switching conversion requirements. Figure 6b shows the ripple condition of the power supply output voltage at this time. It can be seen that the peak-to-peak value of the ripple is only 70 mV, which has a good filter effect. Of course, due to the limitations of manufacturing process and other conditions, there are non-ideal high-frequency oscillations in the secondary voltage waveform, but it does not affect the transformer as a whole.

5 Conclusions
The perforated flat core combines some characteristics of the planar core and the matrix transformer, and is suitable for high-power density DC/DC module power supplies. Aiming at the requirement of reducing the volume of a DC/DC active clamping module power supply, the magnetic field distribution characteristics of the perforated flat transformer and the perforated flat inductor are analyzed, and the specific relationship between the inductance of the magnetic device, the maximum magnetic induction intensity and the design parameters is derived, and then the actual prototype of the perforated flat transformer and the perforated flat inductor is designed. The finite element software simulation results of the above magnetic devices verified the rationality of the analysis, and the operation of the actual module power supply also demonstrated the effectiveness of the design.