High-frequency inverter pre-stage circuit design

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The pre-inverter circuit of the inverter generally adopts a push-pull structure. Here, the problems of open-loop and closed-loop are mainly explained. The circuit for analysis is as follows:

1. Design of the turns ratio of closed-loop pre-stage transformer

The front stage of the inverter, whether open-loop or closed-loop, is only the difference between the turns ratio of the transformer and the feedback loop. For example, it is necessary to design a high frequency modified square wave inverter with an input of 12V, a variation range of 10.5-15V, and an output voltage of 220V 50Hz. If the front stage adopts a closed-loop structure, the DC voltage after 12V boosting is stable at 270V. Therefore, in order to output 270V when the input is 10.5V, the transformer ratio is approximately (270+2VD)/(10.5-VDS). ) / D, where VD is the voltage drop of the high voltage rectifier, VDS is the voltage drop of the previous MOS tube, and D is the maximum duty cycle. The calculated result is approximately 28.

Special attention is paid to the fact that when the current stage is in the closed-loop state, for example, if the input voltage is relatively high, the peak value of the pulse rectified at the positive end of D1 and D3 will exceed 270V, and the duty ratio is less than 1 requires L1, C11 smoothing filtering, so L1 cannot be omitted. It must be large enough, otherwise the MOS tube will have a large heat loss. The specific calculation can be based on the calculation of the output filter inductor of the forward switching power supply, which will not be described here.

2. Design of the turns ratio of the pre-open loop pre-transformer

In fact, the inverter front stage often omits L1. From the circuit point of view, it is closed-loop voltage regulation. The voltage is also fed back through R1. What is going on? It can be seen from the calculation of the closed-loop voltage regulation above that in order to keep the output stable, the transformer has a larger ratio design. It is better to have the voltage regulator before and after the inverter, but it can only be the post-stage voltage regulator. The post-stage voltage regulator is at AC220V. We can design the DC voltage of the pre-stage to a minimum of 220V. The duty cycle is 50. %. If the DC voltage of the front stage is greater than 220V, we can automatically reduce the duty cycle to a small point, so that the output AC power is also stable at 220V.

In this way, our transformer ratio can be designed to output 220V according to the input 10.5V, and the calculated result ratio is about 22. When the input 10.5-15V changes, the range of the high voltage of the front stage is about 220-320V.

If L1 is directly short-circuited, R1 is removed, so that it is a pure open-loop circuit, but there is a leakage peak of the transformer. When the inverter is unloaded, the DC output of the pre-stage output will be artificially high, and the high-voltage filter capacitor and The safety of the post-stage high voltage MOS tube is unfavorable. We can also connect R1 to make a shallow closed-loop feedback to limit the no-load high voltage at 320V. When the voltage exceeds 320V, the duty cycle will be controlled to be small, so that the safety of the high-voltage filter capacitor and the high-voltage MOS transistor is guaranteed. The load current is also reduced. If the front stage is designed in this way, the front stage duty ratio is immediately pulled to the maximum as long as the load is small, and the front stage DC high voltage drops below 320V.

In the pre-stage circuit of the sinusoidal ratio inverter, it can also be designed in such a way that for the inverter with input 12V output of 220V, the transformer ratio can be designed to be about 32, so that the range of the front-stage DC high voltage is about 320- 420V, by changing the modulation degree of the latter stage SPWM can also ensure the stability of the rear stage output 220V voltage.