When the charger is unloaded, the power tube heating is not serious, because there is no load. The reasons for the power tube heating when the charger is unloaded are analyzed as follows:
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The FET also has a certain voltage at both ends when it is charging, and it cannot be completely ideal to be switched on. If there is a voltage drop, there will be power consumption on it, so it will be hot.
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There is a transformer inside, the copper wire has current through, the heat is normal, the charger heat slow in summer, easy to hot, the charger electrical components aging also heat.