The simplified principle circuit of EPWM chopper type AC regulated power supply is shown in Figure 1. It is composed of the main circuit and the control circuit. The main circuit is composed of EPWM bridge chopper V1～V4 and its output transformer Tr, DC rectifier power supply VD1～VD4 and output AC filters LF, CF. The bridge chopper is connected in series between the mains power supply and the load through the secondary of its output transformer Tr, so as to perform positive and negative compensation for the fluctuation of the mains voltage. The harmonics in the output voltage of the bridge chopper are filtered out by the filter LFCF. The DC power supply required by the bridge chopper is supplied by the commercial power supply from the output end of the regulated power supply through the rectifiers VD1-VD4. It should be pointed out here that the EPWM bridge choppers V1 to V4 do not work in the inverter state, but in the bridge chopper state. This is distinguished by its EPWM working mode, DC power supply voltage waveform and DC capacitor Cd value and its function. As shown in Figure 2, the DC voltage of the bridge chopper does not filter the rectified voltage into a constant smooth DC voltage through the capacitor Cd, but is still the waveform of the single-phase bridge rectified voltage. The DC capacitor Cd no longer has the function of DC filtering, but is only set up to create a freewheeling path. For inductive loads, the energy of the freewheeling current in one chopping switching cycle is very small (due to the high chopping frequency), so the value of Cd is also very small, and the charging and discharging speed of Cd is very fast, which will not affect the rectified voltage. Rise or fall speed so that the voltage across Cd has the same waveform as the unfiltered rectified voltage. That is to say, because the value of the capacitor Cd is very small, it only allows the freewheeling current to pass, and no longer has the function of DC filtering, so it has no effect on the rectified waveform. This shows that the bridge chopper is working in the EPWM chopper state, not in the inverter state.
The control circuit of the chopper type AC stabilized power supply is composed of the mains input voltage rectification detection circuit, the comparison circuit, the EPWM circuit and the switching and trigger circuit of the bridge chopper switches V1~V4. In the mains voltage rectification detection circuit, the voltage detection on the filter inductor LF is added to reduce the influence of the reactance of the filter inductor LF on the voltage stabilization accuracy.
The working principle of the EPWM chopper-type AC regulated power supply is shown in Figure 1. When the mains voltage fluctuates, the voltage signal US.L is obtained through the rectification detection circuit of the mains input voltage us and the voltage on the filter inductor LF, and the US, L is compared with the reference voltage Ur to obtain the error voltage ΔU. When US, L>Ur (the mains voltage fluctuates), +ΔU and +ΔU make the comparator U2 in the EPWM modulator unable to work, only the comparator U1 can work, and +ΔU is compared with the triangular wave uc in U1, The EPWM pulse signal is generated in the part where +ΔU is greater than the triangular wave. This signal controls the switches V1-V4 in the bridge chopper through the "state switching trigger circuit", and generates a negative compensation voltage on the secondary side of the output transformer Tr -uco , so that the load voltage UL=US-Uco=Ur; when US, L The positive and negative compensation for the mains voltage is realized by switching the state switching trigger circuit and switching the working order of the switches V1-V4 in the bridge chopper. If V1 and V4 are turned on corresponding to the positive half cycle of the mains, and V2 and V3 are turned on corresponding to the negative half cycle of the mains, the mains voltage is positively compensated, as shown by the dotted path in Figure 2. Corresponding to the positive half cycle of the mains, V2 and V3 are turned on, and corresponding to the negative half cycle of the mains, V1 and V4 are turned on, which is to negatively compensate the mains voltage, as shown by the dot-dash line in Figure 2. There are two key points to use the main circuit shown in Figure 2 to compensate the mains voltage fluctuation: one is EPWM; the other is that the value of the capacitor Cd should be so small that it does not affect the change of the rectified voltage ucd, even if Cd is so small that it no longer has a DC filter Function.
