Designers of mobile phone chargers must prevent many of the problems that may occur with such conver

　　A major difficulty is the stepped saturation of the transformer core. If the average volt-second of all positive pulses added to the primary winding is not strictly equal to that of all negative pulses, the transformer flux density will increase with each cycle (stepped) and enter saturation. The same effect will occur if the voltage of the secondary diode is unbalanced. Since storage time and saturation voltage are rarely equal in two transistors or diodes, this effect is difficult to avoid unless effective measures are taken to prevent them. Adding a small air gap in the transformer core will improve the resistance to this effect, but it cannot be eliminated.

　　Fortunately, as the transformer approaches saturation, there is a natural compensation effect. When the core begins to saturate, the collector current on a transistor will tend to increase towards the end of conduction. This results in a shortened storage time, so the on-time of the transistor is shortened, and a certain natural balancing effect occurs.

　　However, if very fast switching transistors or power MOSFETs with low storage time are used, the storage time is not enough for this natural correction effect.

　　To prevent stepped saturation, current-type control can be used. For the 110V voltage doubler input rectifier connection, there is a DC path on the primary side, and no special DC recovery circuit is required. However, for 220V bridge operation, when using current-type control, a DC path through the primary side must be provided, and a specific recovery circuit must be used.

　　At lower power, another alternative is to select transistors with approximately equal storage times. The output diodes D3 and D4 should be selected to have an equal forward voltage drop under the operating current. The transistor type and drive circuit should have a reasonable storage time during the turn-off period.

　　Finally, the slew rate of the control amplifier must be slow so that the pulse width does not change significantly between the two cycles (otherwise the transistor that is working close to saturation will immediately saturate). Such a design makes the transient response worse, but this may not be important. This mainly depends on the actual application.

　　Although the local saturation of the magnetic core caused by stepped saturation may not be a major problem under steady-state conditions, more serious problems may occur during transient load changes.

　　It is assumed that the charger has been working under a lighter load and stable working conditions have been established. The ladder-shaped saturation trend of any transistor has made the transformer work very close to saturation. Because the inductor current does not change immediately, when the output current suddenly increases, it will initially cause the output voltage to decrease. The control circuit will respond to the drop in the output voltage and adjust the drive pulse width to the maximum. The transformer will saturate immediately within half a cycle, which may cause damage to the switching elements.

　　Therefore, it is necessary to introduce some other control methods, such as primary side current limit or amplifier slew rate limit, to prevent damage to the charger. Both of these methods will strictly limit the transient response time. More effective methods to deal with step-shaped saturation (current-type control method).

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