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I'm quite interested in all kind of inverter especially sinusoidal inverter. A sine wave output needs SPWM, there are two ways to generate SPWM wave: 1.Hardware(A circuit with a lot of operational amplifier, very complex, I try this method before and found it really hard to add more function) 2.Software(Use a MCU, circuit is simple but the program is.......)
This time I try the second way.I use STC12C5410AD as the processor, it has 7 AD converters and 2 PWM modules. The idea of the program is not complex.I'm going to explain how to generate a unipolar SPWM wave. First putting a series of number to a array to represent half period of a sine wave, then use a timer to set the duty cycle for both PWMs periodically, Finally we can get SPWM wave from PWM pins of MCU. How to regulate the output voltage?It's one of the difficulties in this project.Currently my idea is, check the feedback from output per 50Hz period, if the feedback is too far away from setting value,then change the duty cycle(if feedback<setting value,increase duty cycle otherwise decrease).Looks quite simple and it works properly now. For protection circuit, I decided to use the method that detect the VDS of MOSFET.We all know that when a MOS opens,it still have a very small resistance, if a current pass through it,there will be voltage between Drain and Source(Follow this rule:V=IR),If a short occurs the DS voltage will be much bigger than usual, so wo can use this to detect short and close mosfet. My program already support this but i havent finish designing this part of circuit yet.I dont know how to design this part of the circuit.Some one can help me? The Spwm wave from MCU The SPWM wave before LC filter Output wave form Push-pull 12V to 380VDC
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
What you call "SPWM" (I know it as unipolar PWM) will make driving reactive loads difficult. I suggest using bipolar PWM instead, it simplifies the control greatly.
SPWM = Sinusoidal PWM I think that in unipolar system, half of the full bridge can work in low frequency(50Hz), it will reduce the switching lost and increase the efficiency a little bit. Why you said unipolar PWM will make driving reactive loads difficult? I haven't met this kind of problem.I had used my inverter to drive a 70W fan and a 20W transformer, no problem........... Maybe in next version of my sinusoidal inverter I'll use bipolar PWM......... This is my full bridge driving waveform:
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Trying to built a sinusoidal inverter
