Analysis Can SPWM signal affect high-frequency transformers?

Different from PWM, SPWM is a more mature and widely used technology. An important theoretical basis of SPWM is to use different narrow pulses to impact the inertial links. In this process, can the SPWM signal be transformed by a high-frequency transformer? This article will introduce this possibility.

First of all, it should be made clear that the transformer can only transform AC signals. PWM can be divided into AC PWM and DC PWM, while PWM is just a square wave. SPWM is a PWM wave whose duty cycle changes according to the sine law, but it is still a PWM wave in essence. AC SPWM can be transformed by a high-frequency transformer. But the effect must be bad. The square wave can be regarded as a superposition of countless sine waves.

The sine wave is the fundamental wave in the square wave, but the bandwidth that a general transformer can transmit is very narrow and cannot transmit high-order harmonics. Therefore, it is difficult for the secondary of the transformer to output a square wave, not to mention a square wave with a changing duty cycle.

In addition, it should be noted that the high-frequency transformer works at a high frequency, and the sine wave is generally the one that changes. The square wave can be changed into a pulse transformer. The pulse transformer has a wide bandwidth and can be used to change into a square wave.

After reading this, I believe everyone has a clearer understanding of the voltage transformation of the high-frequency transformer on SPWM. The transformer can only change the AC signal, which is what everyone needs to pay attention to. And different transformers can act on different waveforms, so they need to be analyzed according to different situations.

High-frequency transformer winding method

First of all, I believe everyone knows about high-frequency transformers. It is an important component in switching power supplies. It is mainly used as a transformer in high-frequency switching power supplies. In other work tasks, it will also be used as a high-frequency inverter power transformer in high-frequency inverter welding machines.

However, many people are not standardized when winding high-frequency transformers. The following is an explanation of the basic winding method of high-frequency transformers.

Two basic winding methods of high-frequency transformers: sequential winding and sandwich winding.

Sequential winding

(1) For a general single-output power supply, the transformer is divided into three windings, the primary winding Np, the secondary winding Ns, and the auxiliary power winding Nb. The winding order is: Np–Ns-Nb.

(2) This winding method has a simple process, is easy to control various parameters of the magnetic core, has good consistency, low winding cost, and is suitable for mass production. However, the leakage inductance is slightly larger, while the coupling capacitance is small, and the EMI is relatively good. Therefore, it is suitable for low-power occasions that are not sensitive to leakage inductance. Generally, this winding method is generally used in power supplies with a power of less than 30~40W.

The sandwich is divided into two winding methods: primary clamped secondary, and secondary clamped primary.

(1) The winding method of primary clamped secondary (also called primary average winding method) has the advantage of large quantity, which is conducive to the coupling of primary and secondary, reducing leakage inductance; it is also conducive to the flatness of the winding; the last advantage is that the voltage change of the power supply winding is less affected by the secondary load and is more stable. The disadvantage is that since the primary and secondary have two contact surfaces, the winding coupling capacitance is relatively large, so EMI is more difficult.

(2) The secondary-to-primary winding method (also called secondary average winding) is generally used when the output is low voltage and high current. The advantage is that it can effectively reduce the temperature rise caused by copper loss and reduce the high-frequency interference of the primary coupling to the transformer core.

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