A triac phase control using a PWM circuit can be useful only if it's implemented using a time-proportional format, otherwise the response could be haphazard and inefficient.
In a few of of my earlier articles as given below:
I discussed regarding using PWM for initiating a triac phase control circuit, however since the designs did not include a time-proportional technology the response from these circuits could be erratic and inefficient.
In this article we learn how to correct the same using time-proportional theory so that the execution is done through a well calculated manner and much efficiently.
What is Time-Proportional Phase Control using Triacs or Thyristors?
It is a system in which the triac is triggered with calculated lengths of PWM pulses allowing the triac to conduct intermittently for specific lengths of the mains 50/60 Hz frequency, as determined by the PWM pulse positions and time periods.
The average conduction period of the triac subsequently determines the average output for which the load may be powered or controlled, and which executes the required load control.
For example, as we know that the mains phase is comprised of 50 cycles per second, therefore if the triac is triggered to conduct intermittently for 25 times with a rate of 1 cycle ON and 1 cycle OFF periods, then the load could be expected to be controlled with 50% power. Similarly other ON OFF time-proportionals could be implemented for generating corresponding amounts of higher or lower power inputs to the load.
Time-proportional phase control is implemented using two modes, synchronous mode and asynchronous mode, wherein synchronous mode refers to the switching ON of the triac at zero crossings only, while in the asynchronous mode the triac is not specifically switched at zero crossings, rather instantaneous at any random locations, on the respective phase cycles.
In the asynchronous mode, the process may induce a significant levels of RF, while this may be significantly reduced or absent in the synchronous mode due to the zero crossing switching of the triac.
In other words, if the triac is not specifically switched ON at zero crossings, rather at any random peak value then this may give rise to RF noise in the atmosphere, therefore it is always advised to use a zero crossing switching so that RF noise could be eliminated during the triac operations.
How it Works
The following illustration shows how a time proportional phase control may be executed using timed PWMs:
1) The first waveform in the above figure shows a normal 50Hz AC phase signal consisting of a sinusoidal rising and falling 330V peak positive, and negative pulses, with respect to the central zero line. This central zero line is termed as the zero crossing line for the AC phase signals.
The triac can be expected to conduct the shown signal continuously if its gate DC trigger is continuous without breaks.
2) The second figure shows how a triac can be forced to conduct only during positive half cycles in response to its gate triggers (PWM shown in red) at every alternate positive zero crossings of the phase cycles.This results in a 50% phase control.
3) The third figure shows an identical response wherein the pulses are timed to produce alternately at every negative zero crossing of the AC phase, which also results in a 50% phase control for the triac and the load.
However producing such timed PWMs at different calculated zero crossing nodes can be difficult and complex, therefore an easy approach for acquiring any desired proportion of phase control is to employ timed pulse trains as shown in the 4rth figure above.
4) In this figure bursts of 4 PWMs can be seen after every alternate phase cycle which results in around 30% reduction in the triac operation and the same for the connected load.
It may be interesting to notice that here the middle 3nos of pulses are useless or ineffective pulses because after the first pulse the triac gets latched and therefore the middle 3 pulses have no effect on the triac, and the triac continues to conduct until the next zero crossing where it is triggered by the subsequent 5th (last) pulse enabling the triac to latch ON for the next negative cycle. After this as soon as the following zero crossing is reached, the absence of any further PWM inhibits the triac from conducting and it's cut OFF, until the next pulse at the next zero crossing which simply repeats the process for the triac and its phase control operations.
In this way other time-proportional PWM pulse trains can be generated for the triac gate so that different measures of phase control can be implemented as per preference.
In one of our next articles we will learn about a practical circuit for achieving the above discussed triac phase control using time proportional PWM circuit