Advantages of Transformer/Two-Tube Single-Ended Forward Converter

(1) The transformer has a release circuit for energy storage, so there is no need to set up a magnetic reset circuit or magnetic reset winding. Although the transformer has only one primary winding, due to the presence of a freewheeling diode, this winding can be used to transmit electrical energy and feedback excitation electrical energy. The structure is simple, and the transformer energy storage feedback process is as follows: When the switch tubes V1 and v2 are turned off at the same time, the transformer primary voltage up is reversed to slightly greater than or approximately equal to the input voltage ui, and the current with magnetic field energy feeds back electrical energy to the DC power supply Ui through the freewheeling diodes D3 and D4. In addition, since the working winding voltage (-uP) is directly clamped on Ui by the freewheeling diodes D3 and D4, the voltage applied to the switch tube has almost no overvoltage spikes, which is safer for the switch tube (if there is a demagnetization winding for feedback energy, the leakage inductance energy between the working winding and the energy recovery winding will cause the working winding to generate a corresponding overvoltage, affecting the clamping effect. In order to avoid this effect, an additional absorption circuit is required).

(2) The voltage borne by the semiconductor device in the primary circuit of the transformer is equal to the input voltage Ui of the converter. For example, the switch tube V1 and the diode D3 are connected in series to withstand the voltage Ui. When the switch tube V1 is turned on, the voltage borne by D3 is Ui. When D3 is turned on, the voltage borne by V1 is Ui. Therefore:

Where Uvmax, UDmax are the maximum peak voltages that the corresponding semiconductor devices can withstand.

When the output voltage of the current stage is 400V, that is, Ui = 400V, the switch tubes V1, V2 and the freewheeling diodes D3, D4 can use 500V devices (the corresponding single-tube forward circuit requires 1000V devices). It can be seen that the dual-tube forward converter requires the device to have a lower withstand voltage. Because the 500V withstand voltage device has a large current, a large number of products, and a low price, and the switching speed of the relatively low withstand voltage device is relatively fast, the dual-tube single-ended forward converter is suitable for high voltage input (such as 800V or 1000V) and high power output (such as 10kW).

(3) The two switching tubes of the dual-tube forward converter have no risk of direct short circuit. The two switching tubes are connected to the diagonal lines of the bridge. When working normally, the two tubes are turned on and off at the same time. The primary winding of the transformer bears the voltage and there is no direct short circuit. However, the upper and lower switching tubes of the full-bridge converter and the half-bridge converter have the risk of direct short circuit.