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  1. #1
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    How to you generate 50hz o/p using PWM?

    Hi, wondered if someone can help with this. I'm trying to generate a 240v 50Hz o/p, using a dsPIC33 and PWM from a sine LUT. After many wks work I've managed to get some basic code running that is giving what looks like the correct PWM o/p ie. a PWMh and a complementary PWMl signal.

    Now the tricky bit. From what I can see for 240v I need to control 2pairs of IGBTs in a H-bridge and switch them on independently in pairs, but I only have one PWM pair of signals?

    Is there an easier way than this?
    Thanks

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    Re: How to you generate 50hz o/p using PWM?

    Desing your IGBT driver circuits to turn off fast and turn on with some small delay. This hardware will prevent shoot-through (top & bottom of totem pole transistors both on).



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    Re: How to you generate 50hz o/p using PWM?

    Ok thanks, but what I just need to understand is what is required at a more system level. I think if I connect up a simple RC filter to my PWMh o/p hopefully this should give a sine wave o/p, but with no power.

    But if I want to supply power I will need to use IGBT's connected as a H-bridge? The code I have written just o/ps one complementary pair PWMh and PWMl so if I want to supply power to my sine wave o/p is this the way to do it? I want to eventually supply all 3ph's but single phase is the first step. For me at the moment its just trying to undersatnd the concept of how to achieve each step..

    Thanks again for your help...



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    Re: How to you generate 50hz o/p using PWM?

    I have never done quite what you are looking for, but I would use your 2 PWM phases as Left & Right, and let the IGBT driver circuits generate Top & Bottom of each.

    What do you want for an output? Are you generating standard 3 phase 50Hz AC using a transformer? If you are just driving a 3 phase motor couldn't you use 3 half-bridges instead of 3 full bridges? That would reduce switching losses and only take 3 PWM signals instead of 6.



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    Re: How to you generate 50hz o/p using PWM?

    The most versatile method would be to use a motor control PiC with 4 (for single phase) respectively 6 (for three phase) separate PWM outputs that can be configured as complementary outputs with deadtime. If you don't have it, the deadtime can be generated in hardware, basically by RCD circuits, delay turn-on, turn-off immediately. Some IRF gatedriver ICs also have a single input and built-in deadtime generation with complementary outputs. But it's not that flexible if you want to tune the switching behaviour.
    P.S.:
    couldn't you use 3 half-bridges instead of 3 full bridges?
    Yes, everyone does it that way. I don't think the original poster intended something different.



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  6. #6
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    Re: How to you generate 50hz o/p using PWM?

    Thanks for the replies..

    Iam using a dsPIC33 which does seem to have all the attributes for PWM control incl complementary outputs and deadtime setup. My system is a 400v dc bus that will be connected through an inverter to a 3ph 415v 50Hz bus, with the inverter being controlled through my dsPIC33.


    What Iam trying to understand is that I can see for a 3ph 50Hz output I will need 3 Look Up Tables, offset by 120deg and this will give the requires 3 complementary pairs to drive the 6off IGBTs. Bus for a 240V 50Hz output I only need the one Look Up Table, which gives one pair of complementary siganls. Wont I need two for a H-bridge configuration?

    Also how do I add the 120deg phase offset for the 3ph version ie. how do you convert 120deg to a value I can use in the code?

    Sorry for asking all these questions, but any help is always most appreciated....



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    Re: How to you generate 50hz o/p using PWM?

    Most inverter designs are using one or two quadrants of the sine function, anything else is just arithmetics. There are some inverter reference applications available from Microchip. A three phase inverter needs to balance the output voltages from the table to increase the available output swing, see below:




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    Re: How to you generate 50hz o/p using PWM?

    Ive been doing alot of searching through the microchip website, but nothing so far is similar to what Im doing. Just have to keep looking.

    Any idea how do I add the 120deg phase offsets for the 3ph version ie. how do you convert 120deg to a value I can use in the C code?

    Thanks...



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