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Feedback in sine wave inverter (PIC16F series based)

  • Author Tahmid
  • Create date
  • Updated
  • Blog entry read time 4 min read
I have previously shown how to implement SPWM in PIC16: https://www.edaboard.com/blog/1800/
Now I will show how to implement feedback for SPWM.

Due to various limitations in PIC16, such as ADC speed, instruction time and the ALU, it is extremely difficult, if not impossible, to calculate in real time the values required for feedback in sinusoidal pulse width modulation (SPWM). Thus, to implement feedback, a different approach must be used. That approach would be to retrieve the values from a sine table that contains the duty cycle values for a specific duty cycle. Here is one sine table I used, for example:

Code: 
const unsigned char sin_table[416]={
0, 16, 32, 47, 62, 77, 91, 103, 115, 126, 136, 144, 151, 156, 160, 162, 163, 162, 160, 156, 151, 144, 136, 126, 115, 103, 91, 77, 62, 47, 32, 16, //65%

0, 17, 33, 49, 65, 80, 94, 107, 120, 131, 141, 149, 156, 162, 166, 168, 169, 168, 166, 162, 156, 149, 141, 131, 120, 107, 94, 80, 65, 49, 33, 17, //67.5%

0, 17, 34, 51, 67, 82, 97, 111, 124, 135, 146, 154, 162, 167, 172, 174, 175, 174, 172, 167, 162, 154, 146, 135, 124, 111, 97, 82, 67, 51, 34, 17, //70%

0, 18, 35, 53, 69, 85, 101, 115, 128, 140, 150, 160, 167, 173, 178, 180, 181, 180, 178, 173, 167, 160, 150, 140, 128, 115, 101, 85, 69, 53, 35, 18, //72.5%

0, 18, 37, 55, 72, 89, 104, 119, 133, 145, 156, 166, 174, 180, 184, 187, 188, 187, 184, 180, 174, 166, 156, 145, 133, 119, 104, 89, 72, 55, 37, 18, //75%

0, 19, 38, 56, 74, 91, 108, 123, 137, 150, 161, 171, 179, 186, 190, 193, 194, 193, 190, 186, 179, 171, 161, 150, 137, 123, 108, 91, 74, 56, 38, 19, //77.5%

0, 20, 39, 58, 77, 94, 111, 127, 141, 155, 166, 176, 185, 191, 196, 199, 200, 199, 196, 191, 185, 176, 166, 155, 141, 127, 111, 94, 77, 58, 39, 20, //80%

0, 20, 40, 60, 79, 97, 114, 131, 146, 159, 171, 182, 190, 197, 202, 205, 206, 205, 202, 197, 190, 182, 171, 159, 146, 131, 114, 97, 79, 60, 40, 20, //82.5%

0, 21, 42, 62, 82, 100, 118, 135, 151, 165, 177, 188, 197, 204, 209, 212, 213, 212, 209, 204, 197, 188, 177, 165, 151, 135, 118, 100, 82, 62, 42, 21, //85

0, 21, 43, 64, 84, 103, 122, 139, 155, 169, 182, 193, 202, 210, 215, 218, 219, 218, 215, 210, 202, 193, 182, 169, 155, 139, 122, 103, 84, 64, 43, 21, //87.5%

0, 22, 44, 65, 86, 106, 125, 143, 159, 174, 187, 198, 208, 215, 221, 224, 225, 224, 221, 215, 208, 198, 187, 174, 159, 143, 125, 106, 86, 65, 44, 22, //90%

0, 23, 45, 67, 88, 109, 128, 147, 163, 179, 192, 204, 213, 221, 227, 230, 231, 230, 227, 221, 213, 204, 192, 179, 163, 147, 128, 109, 88, 67, 45, 23, //92.5%

0, 23, 46, 69, 91, 112, 132, 151, 168, 184, 198, 210, 220, 228, 233, 237, 238, 237, 233, 228, 220, 210, 198, 184, 168, 151, 132, 112, 91, 69, 46, 23 //95%

//0, 25, 49, 73, 96, 118, 139, 159, 177, 193, 208, 220, 231, 239, 245, 249, 250, 249, 245, 239, 231, 220, 208, 193, 177, 159, 139, 118, 96, 73, 49, 25, //100%
};
Each set of values corresponding to one duty cycle has 32 values.

A table pointer is used to retrieve the values for a given duty cycle. So, when the value of the table pointer is 0, the program reads the first 32 values (65% duty cycle), then the next 32 values when value of table pointer is 1 and so on.

The microcontroller first starts with the lowest duty cycle and then analyses the output voltage.

If the output voltage must be increased, the value of the table pointer is incremented and so, the next set of values is retrieved, increasing duty cycle and thus output voltage. If output voltage must be decreased, the value of the table pointer is decremented so that the previous set of values is retrieved, lowering duty cycle and thus output voltage.

Here is how the table pointer is updated:
Code: 
                    FBV = ADC_Get_Sample(FBCh);
                    if (FBV < 512){
                       FB_Step++;
                       if (FB_Step > 12) FB_Step = 12;
                    }
                    else{
                         if (FB_Step > 0){
                            FB_Step--;
                         }
                    }
                    adder = FB_Step << 5;
                    TMR1L = 0;
                    TMR1H = 0;
                    T1IF_bit = 0;

The reference value of the ADC is 5V, so 512 represents a voltage of 2.5V, which is the feedback reference voltage in this example. When voltage on ADC pin is >2.5V, table pointer value is decremented and when it is <2.5V, table pointer value is incremented.

The required set of values is retrieved and applied by something like this:
Code: 
        TBL_POINTER_NEW = TBL_POINTER_OLD + SET_FREQ;
        if (TBL_POINTER_NEW < TBL_POINTER_OLD){
           P1M1_bit = ~P1M1_bit;
        }
        TBL_POINTER_SHIFT = TBL_POINTER_NEW >> 11;
        DUTY_CYCLE = TBL_POINTER_SHIFT + adder;
        CCPR1L = sin_table[DUTY_CYCLE];
        TBL_POINTER_OLD = TBL_POINTER_NEW;
        TMR2IF_bit = 0;

Now that I've shown how to generate a sine table manually and with "Smart Sine", implement SPWM in PIC16 and now, how to implement feedback, you can easily make a sine wave inverter using the information I've provided. All you need to do is make sure you've understood what I've said and do some research on your own to make the project complete.

Comments

Tahmid
Yes. 32 for each half cycle.
The 2nd half cycle just uses the same 32 values for the first half cycle but "in the opposite direction" of current (different MOSFETs driven).
 
picgak
Tahmid can you please insert comments after every line of code , I use JAL for PIC programming my knowledge of C is minimum. I am designing a pure sine wave inverter using an output H bridge at 310vdc , is there any need for feed back control if I keep the 310 Vdc steady
 
Tahmid
I will insert the comments.

If you can keep the 310VDC steady, then there is no need for feedback of SPWM. Your output will be relatively stable, since the DC bus is regulated.
 
kabiru
@tahmid why the feedback not necessary? since there will be a voltage drop when their is increase in load,remember the code is not incrementing or decrementing at this time, now take an example of your normal modified sine inverter using SG3524 or any pwm ic, the output voltage drop when their is increase in load but the battery voltage doesn't change, therefore their will be a need to control the code also to my own understanding.
 
Tahmid
When there is increase in load, you will notice that the battery voltage always decreases due to the internal resistance. Measure the battery voltage of the inverter under load and you will see that it decreases as load increases. A battery that measures about 12-13V open circuit may drop to 11V or lower under load. Plus, there is a significant drop by the transformer under load.

Also, think about this: in the H-bridge converter, when load increases (and so current increases) but the +ve bus voltage is constant, where will the voltage be dropped? There is no transformer to deal with, as the primary side PWM will take care of drops by the transformer (and battery voltage drop) and keep the output fixed. The only place for voltage drop is the MOSFETs and there will be a drop in voltage in the MOSFETs, but not so large that the output voltage will be beyond acceptable bounds.

I hope I could make this clear. :)
 
picgak
Thank you for making clear the feed back part. my sine table is 192 steps per half cycle, so what type of H bridge mosfet driver/ isolator do you suggest from the following TLP 350, IR2110, or is it good to do with 6N137. or is there any cheap and best solution.
 
Tahmid
There maybe cheaper alternatives, but I am always in support of using high/low side drivers. So, I would recommend IR2110 or any other high/low side driver instead of attempting to create a high/low side driver with optoisolators such as TLP250 or 6N137. However, at 3.5kHz, you may get away with using a TLP250, but I would still recommend a high/low side driver.
 
kabiru
pic gak ,i thought we use 32 set of duty cycle in an array , how can 192 steps will be implimented?
 
Tahmid
You can use any number of duty cycle values in an array. 32 is usually sufficient, but with 192, greater precision can be had, especially since the frequency is 3.5kHz and the time period is much longer as compared to 16kHz. So, the SPWM is more accurate. However, just keep in mind that all of the values are stored in program memory. So, it will consume more program memory. You can use the software Smart Sine to generate the sine table: http://www.edaboard.com/entry1798.html
 
kabiru
what about this unsigned charact i can see 416 code what are they used for?
 
Tahmid
32 values correspond to the sine wave at one duty cycle. I have set 13 duty cycle settings. 13 * 32 = 416
There are total 416 values in the array, 13 sets, and each set (containing 32 values) corresponds to one duty cycle setting.
 
kabiru
i think in this part i need special tutorial, i understand everything from the beginning but at this point i am lost,32 values correspond to sine wave at one duty cycle,now let me say a complete cycle=360degrees, now half cycle =180degrees, at this half cycle we have 32 duty cycle, the how the 13 duty cycle come in?
 
Tahmid
Each set of values corresponding to one duty cycle has 32 values.

A table pointer is used to retrieve the values for a given duty cycle. So, when the value of the table pointer is 0, the program reads the first 32 values (65% duty cycle), then the next 32 values when value of table pointer is 1 and so on.

The microcontroller first starts with the lowest duty cycle and then analyses the output voltage.

If the output voltage must be increased, the value of the table pointer is incremented and so, the next set of values is retrieved, increasing duty cycle and thus output voltage. If output voltage must be decreased, the value of the table pointer is decremented so that the previous set of values is retrieved, lowering duty cycle and thus output voltage.
 
kabiru
ok if i get you right, you mean the table pointer keep incrementing by % until the required voltage is obtained, this means the 416 code varies the 32 values duty cycle by % to get the right voltage? correct me if i am wrong.
 
picgak
Tahmid can you kindly suggest me a robust as well as reasonable high/low side driver in your opinion
 

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