Switching power supply principle and design (serial 46) advantages and disadvantages of full-bridge transformer switching power supply

1-8-3-5. Advantages and disadvantages of full-bridge transformer switching power supply

The full-bridge transformer switching power supply is the same as the push-pull transformer switching power supply. Since the two sets of switching devices work alternately, it is equivalent to two switching power supplies outputting power at the same time. Its output power is approximately twice the output power of a single switching power supply. Therefore, the full-bridge transformer switching power supply has a large output power and high working efficiency. After bridge rectification or full-wave rectification, the voltage ripple coefficient Sv and current ripple coefficient Si of its output voltage are very small. Only a very small energy storage filter capacitor or energy storage filter inductor is needed to obtain an output voltage with very small voltage ripple and current ripple.

The biggest advantage of the full-bridge transformer switching power supply is that the withstand voltage requirement for the four switching devices can be reduced by half compared to the withstand voltage requirement for the two switching devices of the push-pull transformer switching power supply. This is because the four switching devices of the full-bridge transformer switching power supply are divided into two groups. When working, the two switching devices are connected in series. When turned off, the voltage borne by each switching device is only half of the voltage borne by a single switching device. Its maximum withstand voltage is equal to half of the sum of the operating voltage and the back electromotive force, which is exactly half of the withstand voltage of the two switching devices of the push-pull transformer switching power supply.

The full-bridge transformer switching power supply is mainly used in situations where the input voltage is relatively high. When the input voltage is very high, the full-bridge transformer switching power supply is used, and its output power is much greater than that of the push-pull transformer switching power supply. Therefore, most of the high-power switching power supplies with a power grid voltage of 220 volts AC use full-bridge transformer switching power supplies. When the input voltage is low, the output power of the push-pull transformer switching power supply is much greater than that of the full-bridge transformer switching power supply.

The power utilization rate of the full-bridge transformer switching power supply is lower than that of the push-pull transformer switching power supply, because the two sets of switching devices are connected in series, and the total voltage drop when the two switching devices are turned on is twice as large as the voltage drop when a single switching device is turned on; but the power utilization rate is much higher than that of the half-bridge transformer switching power supply. Therefore, the full-bridge transformer switching power supply can also be used in occasions where the working power supply voltage is relatively low.

Like the half-bridge switching power supply, the full-bridge transformer switching power supply only needs one winding for the primary coil, which is also its advantage. It brings some convenience to the coil winding of the small-power switching power supply transformer. But it has no advantage for the coil winding of the high-power switching power supply transformer, because the coil of the high-power switching power supply transformer needs to be wound with multiple strands of wire.

The main disadvantage of the full-bridge transformer switching power supply is that the power loss is relatively large. Therefore, the full-bridge transformer switching power supply is not suitable for occasions with low working voltage, otherwise the working efficiency will be very low. In addition, the four switching devices in the full-bridge transformer switching power supply are connected without a common ground, which makes it more troublesome to connect with the drive signal.

The biggest disadvantage of the full-bridge switching power supply is that when the two sets of control switches K1, K4 and K2, K3 are in the alternating switching working state, the four switch devices will simultaneously appear in a very short semi-conduction area, that is, the two sets of control switches are in the on state at the same time. This is because when the switch device starts to conduct, it is equivalent to charging the capacitor, and it takes a transition process from the off state to the fully on state; and when the switch device switches from the on state to the off state, it is equivalent to discharging the capacitor, and it also takes a transition process from the on state to the fully off state.

When the two groups of switch devices are in the on and off transition process respectively, that is, when the two groups of switch devices are in the semi-on state, it is equivalent to that the two groups of control switches are turned on at the same time, and they will cause a short circuit to the power supply voltage; at this time, a large current will appear in the series circuit of the four control switches, and this current does not pass through the transformer load. Therefore, when the four control switches K1, K4 and K2, K3 are in the transition process at the same time, the four switch devices will generate a large power loss. In order to reduce the loss generated by the control switch transition process, generally in the full-bridge switching power supply circuit, the turn-on and turn-off time of the two groups of control switches are intentionally staggered for a short period of time.