Full-bridge inverter/bipolar control

The full-bridge inverter is the main component of the full-bridge PWM DC/DC converter. Its main circuit is shown in Figure 1. It consists of four switch tubes V1~V2, their anti-parallel diodes D1~D4, and output transformer Tr, etc. The input DC power supply voltage is Ui, and the output AC voltage is U. The primary winding of the transformer Tr is connected to the midpoints A and B of the two bridge arms. The number of turns of the primary winding of the transformer is w1, the number of turns of the secondary winding is W2, and the transformation ratio K=W1/W2. The full-bridge inverter can adopt bipolar control, limited bipolar control and phase shift control.

1. Bipolar control mode

The working waveforms of the full-bridge inverter using bipolar control are shown in Figure 1 (b) and (c). The switch tubes V1~V2 use PWM control. In the first half of a switching cycle TS, the switch tubes V1 and V4 are turned on, the on time is TON, DU is the duty cycle, and the switch tubes V2 and V3 are turned on in the second half of the cycle, and the on time is also TOn. When the switch tubes V1 and V4 are turned on, if the on-state voltage drop of the switch tube is ignored, the voltage on the primary winding of the transformer is UAB=-Ui; when the switch tubes V2 and V3 are turned on, the voltage on the primary winding is uAB＝-Ui; when the switch tubes V1 and V4 and V2 and V3 are turned off, the voltage on the primary winding is UAB＝0. When the secondary of the transformer is open, the waveform of the voltage UAB on the primary winding of the transformer is a square wave voltage, as shown in Figure 1 (b). By adjusting the on-time of the switch tube, that is, adjusting the duty cycle DU, the pulse width of the voltage uAB can be adjusted, thereby achieving the purpose of adjusting the effective value of the voltage UAB. The waveform of the secondary voltage U. is the same as that of UAB, and its amplitude is Ui/K.

If the secondary is connected to a resistive load RLd, the current flowing through the load is isO. The waveform of the current is, is the same as that of the voltages UO and UAB, and its amplitude is. The waveform of the transformer primary current iP is the same as that of the secondary current is, and its amplitude is. This formula can also be written as where R1 is the secondary load resistance RLd; converted to the primary value. It can be seen that if the transformer is an ideal transformer, the load resistance RLd connected to the secondary of the transformer has the same effect as not using a transformer and directly connecting the resistor R1 at both ends A and B. Therefore, when the switch tubes V1 and V4 are turned on, the current flowing through V1 and V4 is, and when the switch tubes V2 and V3 are turned on, the current flowing through V2 and V3 is also, and at this time, no current flows through the anti-parallel diodes D1~D4 of the switch tubes V1~V4.

Figure 3-29 Bipolar control method

If the inductive load L is connected to the secondary of the transformer, when the switch tubes V1 and V4 are turned on, the primary voltage of the transformer is UAB = Ui, and the secondary voltage UO = K0. Under the action of voltage UO, the load current increases from zero, and the speed of increase is. This current reaches the maximum value when the switch tubes V1 and V4 are turned off. When the switch tubes V1 and V4 are turned off, this current cannot change suddenly and will still flow in the original direction. Therefore, the diodes D3 and D2 will be turned on, so the voltage UAB = -Ui, and the polarity of Uo is reversed. Under the action of reverse voltage, the load inductive current will decrease, and the speed of decrease is the same as the speed of increase when the switch tubes V1 and V4 are turned on. In this case, the voltage waveforms of the primary and secondary of the transformer are very different from those when the load is resistive, and a shaded area appears. The area of ​​this shaded area when the load is purely inductive is the same as the area of ​​UAB: when the load is resistive. Therefore, the waveform of the output voltage Uo is not only determined by the conduction state of the switch tubes v1 and v4, but also related to the nature of the load. When the duty cycle DU≥, that is, the on-time Ton of the switch tubes V1 and v4≥Ts/4, the waveform of the voltage UAB becomes a square wave with a pulse width of 180° electrical angle. That is, when DU changes in the range of ~1, the voltages UAB and Uo are always square waves with a pulse width of 180°, and are not affected by the change of the duty cycle Du. From the above analysis, it can be seen that the full-bridge inverter is not suitable for this pulse width control method when the inductive load is used.