Distributed capacitance of multi-stage high voltage transformer

The distributed parameters of high-voltage transformers are mainly leakage inductance and distributed capacitance. In high-voltage transformer applications, the distributed capacitance of a single secondary transformer is very large, which seriously affects the working performance of the circuit. In order to reduce the distributed capacitance, the single secondary winding is wound in sections and then connected in series, followed by a rectifier and filter circuit. If the transformation ratio is still large after the sections, the distributed capacitance will still be large when the number of secondary turns is still large. When the distributed capacitance is not used, we can only find ways to reduce it.

This article compares the two processes of traditional winding and PCB winding, and uses the resonance method to measure the resonant frequency and calculate the distributed capacitance. Finally, the measured waveform is used to illustrate the impact of distributed capacitance on circuit performance.

1 Distributed capacitance generation mechanism

In high-voltage transformers, distributed capacitance is composed of inter-turn distributed capacitance and inter-layer distributed capacitance. Distributed capacitance exists between any two turns of coils. The expression of "the capacitance of a flat plate capacitor is proportional to the plate area and inversely proportional to the plate spacing" is expressed as the distributed capacitance per unit length when there are only two adjacent turns of coils:

where C is the distributed capacitance per unit length; ε is the dielectric constant of the medium between the two coils; s is the equivalent opposite area with a length of unit length and a width of wire diameter; and d is the center spacing of the two coils. To reduce the distributed capacitance between the two wires, reduce ε and s and increase d.

2 Comparison between traditional winding coil and PCB coil

For the convenience of comparison, the transformers wound by the two processes use the same parameters as follows:

working mode: full-bridge topology; working frequency: 150 kHz; input voltage/current: 50 V/3A; primary turns Np: 4 turns; 8 output secondary turns Ns1~Ns8: 68 turns, 68 turns, 68 turns, 68 turns, 62 turns, 62 turns, 62 turns, 62 turns; secondary wire diameter/line width: 0.2mm; magnetic core: PQ40; insulation level: the withstand voltage between primary and secondary and between secondary and magnetic core is greater than 8kV DC; insulation treatment: 0.05 mm thick polyester film adhesive tape is used.

2.1 Wire wrapping process

The wire wrapping process of high-voltage transformers is as follows:

(1) A cylindrical winding frame with a diameter of 16 mm is used, and all windings should have a margin distance of more than 4 mm from the top and bottom of the frame;

(2) The primary uses a copper foil with a width of 6 mm and a thickness of 0.05 mm, and the secondary uses an enameled wire with a wire diameter of 0.2 mm. After winding the primary, the secondary is wound in turn, and all secondary are wound in one layer;

(3) The polyester film required for insulation between the primary/core column is 24 mm×0.05 mm×2 layers, and the polyester film required for insulation between the primary/secondary group, between the secondary groups, and between the Ns8 secondary and the outer core is 24 mm×0.05 mm×6 layers;

(4) All secondary should be wound in one layer, and the primary and secondary outlet terminals should extend about 50 mm;

(5) The primary and secondary outgoing wires are wound around the two sides of the center column respectively, and the Ns1~Ns4 outgoing wires and the Ns5~Ns8 outgoing wires are located at the upper and lower parts of one side of the magnetic core respectively.

Figure 1 shows a physical picture of the wire package wound by the traditional method.

2.2 PCB coil drawing

The line width is 0.2 mm and the line spacing is 0.3 mm. Since the PQ40 core window width is 11 mm, a maximum of 17 turns of coils can be arranged on each PCB layer with sufficient insulation space reserved, so each secondary winding requires 4 layers. If all primary and secondary windings are printed on the same PCB board, there will be 34 layers (4 layers × 8 + 2 layers), which is not only too expensive, but also does not reflect the advantage of the "thinness" of multi-layer PCB boards. Therefore, the PCB board uses a single-layer double-sided polytetrafluoroethylene board with a thickness of 0.5 mm, and double-sided coils are wound. Each set of secondary coils requires two PCB boards. The PCB coil diagram drawn by DXP2004 software is shown in Figure 2. Due to space limitations, Figure 2 only shows the front and back of a PCB board in Ns1.

2.3 Determination of the size of distributed capacitance

From the perspective of the transformer primary, the distributed capacitance and primary inductance form a parallel resonant circuit, so the resonant frequency of this resonant circuit can be measured by a network analyzer, and then the size of the distributed capacitance can be determined by the following formula:

Where f is the resonant frequency, L is the primary inductance, and C is the distributed capacitance. Table 1 lists the static parameter test results of the above transformer.

From the data in Table 1, it can be seen that the PCB coil transformer is superior to the wire-wound transformer in terms of distributed parameters. This is attributed to the fact that the PCB coil is easy to control the line spacing and layer spacing.

3 Primary voltage waveform and analysis when the secondary is fully loaded

The test circuit adopts a phase-shifted full-bridge topology, the control chip uses UCC3895, the switch tube is IRFP460, and the chip output switch tube drive frequency is 146.8 kHz. The input voltage is 50 V and the output full-load power is 160 W. Figure 3 shows the actual picture of the 8-secondary high-voltage transformer.

FIG4 shows a full-load waveform diagram of the primary of a transformer wound with wire, and FIG5 shows a full-load waveform diagram of the primary of a transformer using a PCB coil.

By comparing the two waveforms shown in Figure 4 and Figure 5, it can be seen that the distributed parameters have already affected the performance of the circuit. The larger the primary leakage inductance, the larger the peak voltage amplitude; the larger the distributed capacitance, the worse the integrity of the primary waveform. This is because the distributed capacitance and the parasitic parameters in the circuit (such as leakage inductance and parasitic capacitance of the switch tube, etc.) produce attenuated oscillations; at the same time, the switch tube loss increases, making it difficult to improve the efficiency of the conversion loop. In the process of slowly raising the input voltage from 0 V to 50 V for debugging, the wire-wound transformer will emit a "squeaking" noise. This phenomenon is eliminated after replacing the PCB coil transformer.

4 Conclusion

This article uses transformers wound by two different processes to illustrate the impact of distributed capacitance on circuit performance. At the same time, the PCB coil winding method also effectively reduces the distributed capacitance and improves the working performance of the circuit, achieving the purpose of optimized design.