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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
@kabiru,
The table pointer keeps increasing by 1 to increase the duty cycle and so, the output voltage of the inverter.

@picgak,
IR2110 and IR2113 are good, but there are a lot of fake IR2110 and IR2113 chips as well. I personally use and like the L6385E driver.
 
kabiru
I think i understand how it works now,but i still need to know how this table pointer increment by 1 or decrement ,and the most important thing i will like to know is this 13*32 duty cycle which give us 416 counts and how this 416 count affect the 32 variable pulse when the table pointer incr or decr, if possible with little example.


thanks
 
Tahmid
At the beginning of program execution, the table pointer is zero and the microcontroller starts with 65% duty cycle. Let's assume we have a 8V (primary) to 256V (secondary) transformer and a full bridge converter is being used to drive the transformer. The bridge is being driven by signals from the microcontroller - these SPWM signals.

The RMS voltage to the transformer primary will be (0.65 * 12/1.4142135)V = 5.5V.
Turns ratio of the transformer (primary:secondary) is 1:32.
So, secondary voltage (assuming 100% transformer efficiency) will be 32*5.5V = 176V.
As we require an output of 220V, this is lower than required. So, output voltage must be increased. So, table pointer is incremented to 1.
Then, duty cycle is 67.5% and output voltage is 183V.
Table pointer is incremented to 2.
Duty cycle is 70% and output voltage is 190V.
Table pointer is incremented to 3.
Duty cycle is 72.5% and output voltage is 197V.
Table pointer is incremented to 4.
Duty cycle is 75% and output voltage is 204V.
Table pointer is incremented to 5.
Duty cycle is 77.5% and output voltage is 210V.
Table pointer is incremented to 6.
Duty cycle is 80% and output voltage is 217V.
Table pointer is incremented to 7.
Duty cycle is 82.5% and output voltage is 224V.
Table pointer is decremented to 6.
Duty cycle is 80% and output voltage is 217V.
Table pointer is incremented to 7.
Duty cycle is 82.5% and output voltage is 224V.
Table pointer is decremented to 6.
Duty cycle is 80% and output voltage is 217V.
.
.
.
.
And so on.

This is how regulation is achieved.

This is all at 12V. But, you can see how the table pointer will be changed to maintain a constant output.
 
picgak
Tahmid TLP350 datasheet claims 50Khz , whereas TLP152 claims 250Khz, cant we use it for 19.2 Khz
 
Tahmid
I have never used them. Yes you should be able to use them, but remember to add (auxiliary) isolated power supplies or maybe you can make your own bootstrap circuit.

Thank you picgak for letting me know about them. I didn't know about them since they're not available in Bangladesh. But they will be very useful and I'll try to collect them.

I have now checked at Mouser and it's quite expensive. Take that into consideration. Many high/low side drivers are cheaper than that.
 
K
hello tahmid, how can i implement charging system on spwm inverter ,i just want to know the charging system since it works like boost converter?
 
K
second question is, in the above code you provide (416) the table pointer is to increment by % so how will the table pointer move to the next % if the set value needed is not acheived? is the (416) code writting in % when writting the code so that the table pointer will increment by % just the way you give an example or in an array like the 32 variable duty cycle (LUT) ,meaning how do i increment with duty cycle and decrement.
 
picgak
Tahmid I dont know costwise, I am also looking for a cheap and reliable alternative. Today I experimented with 6n137 very very cheap in india the pulse out was good datasheet claims(High speed: 10 MBd), you can use the enable disable feature also.
TLP250 andTLP350 is not of much difference in india
 
A
Dear Tahmid,

I just stumbled across this thread and would like to ask and confirm a few details (Forgive my ignorance if I have skipped something during this fast read, Inverters have been my involvement sometimes back):

1. As per your correction method the ADC measures an average rectified output so, Sine Amplitude is corrected every Full Cycle (Since Half Cycle corrections may call for Flux Imbalance).

2. That means for a 50Hz System it is around 20mS period for the task schedule.

3. Your method provides a pre-computed sine table with different amplitude values so error control will be stepped.

Please look here : http://www.piclist.com/techref/microchip/math/mul/8x8u.htm

That should be around 40 more instructions around each lookup entry load to 10BIT PWM and a PIC doing 20MHz takes 0.2uS for each instruction so with housekeeping interrupt context saving the overall instructions should be around 70 x 0.2 = 14uS, probably enough for a proportional control with around 312uS between interrupts (for 64 point full cycle LUT)

if you tend to retain portions of your old ADC values you can have a dirty I, so a PI control.

Regards
Sougata
Kolkata
India
 
S
hi Tahmid
i still cant get my table pointer to increment or decrement in %. i guess i have exhausted my little programming knowledge. need your help like the way you did in your blog demystifying table pointer. that gives a detail explanation on how the sine wave inverter works. kindly provide a code and an explanation for the feedback control too. a lot of us small programmers are waiting.
 
S
Hi Tamid,
i am still trying to solve the feedback control of my inverter. can u please explain how the following works in the code or give me a reference materials.
adder = FB_Step << 5;
TMR1L = 0;
TMR1H = 0;
T1IF_bit = 0;
 
embpic
ok if i have 25 VDC then is it possible me get sine wave of approximately 50 Vac peak to peak and control through 8051.
thanx.
 

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Tahmid
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