Technical characteristics of full-bridge push-pull converter

　　Technical characteristics of full-bridge push-pull converter

　　Bridge converter

　　A full-bridge push-pull converter requires four power transistors and additional drive components, which makes the converter more expensive than a flyback or half-bridge converter, so this converter is usually used in high-power applications.

　　This technology has many useful features, especially its main transformer requires only one primary winding, which makes it bear the full charger voltage in both directions. In this way, when the output uses full-wave rectification, the transformer core and winding have an excellent utilization rate, which makes the design of a high-efficiency transformer possible.

　　The second advantage is that the power switch works in a very good environment. The maximum voltage stress does not exceed the charger voltage under any conditions. The forward clamping action of the four energy recovery diodes eliminates any transient voltage usually caused by leakage inductance.

　　The disadvantage of this circuit is that it requires four switching transistors. Since two transistors work in series, the power loss during effective saturation and conduction is slightly greater than that of using two transistors to push-pull. However, in high-voltage off-line switching systems, these losses are relatively small and acceptable.

　　Finally, the circuit provides flyback energy recovery through four recovery diodes, without any energy recovery winding.

　　Stepped saturation

　　The forward and reverse volt-second conditions imposed on the transformer are inevitably somewhat unbalanced. This may be due to the difference in the storage time of the transistors or the imbalance of the forward voltage of the output rectifier diode. As the work cycle progresses, this imbalance will cause stepped saturation of the magnetic flux density in the transformer core.

　　The magnetic core will not recover during the turn-off period. This is because during this period due to the forced action of L1 (the load current is greater than the critical current of L1), the diodes D5 and D6 are both turned on, and their clamping action short-circuits the secondary side.

　　When the magnetic core reaches saturation, there is a compensation effect. The on-current of the transistor during the saturation period is relatively large. As its storage time is reduced, the balance will be restored to a certain extent. However, there are still problems with the operation of transistors, which will be discussed in subsequent chapters.

　　Instant saturation effect

　　Assuming that the mobile phone charger has been working for a period of time under light load conditions, stepped saturation has appeared, and a pair of transistors are working near the saturation point. If a load is suddenly applied, the control circuit will quickly increase the pulse width to compensate for the loss and increase the current flowing in L. The magnetic core immediately saturates in one direction, and a pair of transistors will withstand overcurrent, which may cause catastrophic results.

　　If the power transistor has an independent fast-acting current limit, the on-pulse will be terminated before the overcurrent flows to avoid component damage. This is not an ideal solution because this method reduces transient response performance.

　　Another method is to reduce the slew rate of the control amplifier so that the pulse width increased per cycle is less than 0.28. Under these conditions, the storage self-compensation effect of the power transistor can usually prevent oversaturation. But again, the transient response performance will decrease a lot. In any case, these two techniques are commonly used.

　　SAA power adapter forced magnetic flux density balance

　　A better solution to the stepped saturation problem is the push-pull bridge circuit shown in the figure.

　　If two identical current transformers are connected to the emitters of Q3 and Q4, the peak currents flowing through the two pairs of transistors (also flowing through the primary winding) can be compared with each other every half cycle.

　　If it detects that the two currents are unbalanced, it acts on the ramp comparator to differentially adjust the width of the power transistor drive pulse. This can keep the average working magnetic flux density of the transformer near the center of the B/H characteristic, detect the DC bias, and differentially adjust the drive pulse to maintain balance.

　　It should be noted that this technique is only used where there is a DC path through the transformer windings. In order to block the DC current, a capacitor is sometimes connected in series with the primary winding to avoid the DC saturation of the transformer. However, under unbalanced conditions, the capacitor will retain the net charge, so that the alternating primary voltage pulses have different voltage amplitudes. This leads to power loss and sub-harmonic ripples in the output filter. Furthermore, maintaining the transformer current balance and maintaining the capacitor charge will cause the system to diverge and cause the system to lose control. Therefore, the use of capacitors to block DC is not recommended.

　　If a forced current balance system is used, capacitor C must not be connected in series. This is because the capacitor eliminates the detectable DC component and makes the system unable to work.

　　If the transformer's operating point can be maintained close to its center point, the magnetic flux density has the best operating range, which can improve transient performance and eliminate the possibility of transformer saturation and power component damage.

　　When current-type control is used in the primary pulse adjustment, the magnetic flux balance is automatically generated, and there is no need to connect Cx in series.

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