Design of high frequency AC signal sampling circuit

灞波儿奔

Design of high frequency AC signal sampling circuit [Copy link]

When making high-frequency power supplies, sampling of high-frequency AC signals is often involved. The traditional sampling circuit is as follows:

This circuit is suitable for voltage sampling and current sampling. The transformer in the figure is a transformer for voltage sampling, and a current transformer for current sampling. The transformer and the transformer can be considered to be used in opposite ways. For example, the transformer is generally used to reduce voltage with a turns ratio of 50:1, while the transformer reduces current with a turns ratio of 1:50. The output of the transformer generally requires a closed-loop terminal, such as a small resistor, which the reader can adjust by himself. For voltage sampling, when the primary input voltage is too low, after the voltage is reduced by the transformer, due to the voltage drop of the rectifier diode, the sampled voltage will be low, and the error will be large under low voltage. For current sampling, when the primary current is small, the presence of the diode will cause phase error. The first generation of high-frequency induction heating power supply needs to automatically lock the phase of voltage and current. In actual use, the phase waveform is biased under different powers. Because the diode is introduced, errors are introduced in the low-voltage and low-current section. In order to better solve the problem of the diode, a high-speed op amp is used to deal with it, as shown in the following figure:

This is to use a single-power high-speed op amp to obtain half-wave voltage or current. Note that a high-speed op amp is required. The first generation of high-frequency power supply operates at 1MHz. In order to obtain a better phase, a 100MHz high-speed comparison op amp is selected to obtain the phase. In many cases, transformers or mutual inductors are made of small magnetic rings, and the primary and secondary sides are wound with enameled wire. However, due to the small size of the magnetic ring and the ring structure, the enameled wire is repeatedly wound, which is very time-consuming and can easily damage the paint of the enameled wire, eventually causing the magnetic ring to short-circuit. The general operation method is often to dip in paint or fill in epoxy resin, which is more troublesome to operate. Of course, you can also buy professionally made magnetic rings. The second-generation high-frequency power supply voltage sampling uses 50:2 turns. At present, it has been found that several magnetic rings have burned out due to short circuits. I have seen a high-frequency sampling circuit from the UK recently, as shown below:

This circuit caught my eye. It uses two large resistors and a 2:2 turn low-turn current sampling loop to achieve voltage sampling. This is not surprising, because I do the same thing for 220VAC 50Hz AC signal sampling, which is relatively traditional. However, because the sampling transformer of the second-generation high-frequency power supply has the phenomenon of burning magnetic rings, this circuit needs to be used. Because the current transformer turns ratio can be 2:2 turns, there is no problem of enameled wire damage. In addition, its output circuit design is quite clever. Sampling is achieved through two diodes and two capacitors. This circuit can better obtain the peak value of the AC signal, and the diode has little effect. Looking at other people's designs and summarizing them more can make you understand more deeply.