Method to reduce the failure rate of small high-frequency inverter power supply

The power part of the front stage should be "soft"

For the sake of simplicity, low-power inverters generally use push-pull for the front stage, but there is a problem with the push-pull circuit. At the moment of starting up, the duty cycle of the driving pulse has not been fully opened. The MOS tube of the front stage The D pole will have a high recoil, sometimes exceeding the withstand voltage value of the MOS tube. This has buried a hidden danger of failure. Many low-power inverters do not have power-limiting protection in the front stage. If the MOS tube is worn, it will be dangerous and even an open flame will appear.

Therefore, we must find a way to limit the recoil on the MOS tube, so that the waveform on the D pole of the MOS tube is slowly and softly pulled away, and a pre-stage power limiting circuit is added, so that for the pre-stage, The failure rate can be greatly reduced.

The power part of the post-stage should also be "soft"

If the power part of the front stage works in a state of high current, then the power part of the rear stage works in a state of high voltage, and it directly deals with various loads, so the working conditions of the power tube of the latter stage are also harsh. If you want to soften the characteristics of the post-stage power part, you need to start with the following points:

power limit

Also overload protection. First of all, you can't simply set a threshold for the rear stage. If the threshold is exceeded, it will be shut down for protection, making the machine unusable. Once the shock load is applied, the machine will shut down. Secondly, if you set a threshold and add a 2-second delay after the impact, if it still exceeds the threshold, then shut down, so that the impact load can be handled well. But the risk still exists. If the load is very heavy, the power amplifier may burn out before it lasts for 2 seconds. So the above two methods are not perfect.

The safest way is to use the "overload soft compression" circuit. When a heavy impact load is applied, the pulse of SPWM is automatically limited, that is, the top of the output sine wave is compressed. Although there are some high-order harmonics, the output power is low. Be limited, there will be no danger of burning power tubes.

Short circuit protection

There are many kinds of short-circuit protection circuits. Some can be short-circuited for a long time, and they can be shut down after a short-circuit. As long as the design is in place, it is generally possible. When there is a short circuit, there is only the no-load current of the pre-stage circuit, and the power consumption of the whole machine is very small, so it does not generate heat. A machine that can be short-circuited for a long time, if the current is still a few amperes during the short-circuit, but it is not normal to heat up after a long working time. When the short circuit occurs, the rear stage must be completely turned off, and it will automatically resume working when the short circuit is removed.

Do a good job of protecting the SPWM chip

Nowadays, SPWM is generally generated by a single-chip microcomputer, and the single-chip microcomputer is easy to crash, so some measures must be taken on the circuit, and there must be no crash phenomenon.