Pulse Skipping Modulation
Green – Input AC
Yellow – Output AC after Pulse Skipping Modulation
Pink – Gate Drive Signal
Pulse skipping modulation (PSM) or cycle control or burst fire is a method of power control where whole cycles of voltage are applied to the load. Here I’ll talk about PSM involving a thyristor, specifically a triac. The triac connects the AC supply to the load for a given number of cycles and then disconnects the AC supply for another given number of cycles. It has of course become quite obvious from the title that the purpose of PSM is to control or limit power to the load.
We know that the thyristor is a latching device – when the thyristor is turned on by a gating signal and the current is higher than the holding current and the latching current, the thyristor stays on, until the current through it becomes sufficiently low (very close to zero). The thyristor turns off when current through it becomes zero, as happens at the AC mains zero crossing. This is the natural line commutation. (Another method of turning the thyristor off is by forced commutation. I won’t go into that now.) The assumption here is that the load is resistive and has little to no inductance. Of course, this is not always the case, as inductive loads are often used. However, I’ll work with this assumption for now.
I’ve added the circuit, code and simulation of an example later in this article. And that uses a triac as the power device. So, from now on, I’ll just refer to the triac instead of talking about a thyristor in general.
So, in pulse skipping modulation (PSM) or cycle control, the load receives power for a number of cycles, during which time the triac is on, and does not receive power for another number of cycles, during which time the triac is off. When the triac is to be turned on, the gating signal to the triac is given right after zero-crossing. The triac then stays on until the current through it becomes zero (natural line commutation). This is at the next zero crossing. For simplicity’s sake and as usually should be, assume that the current through the triac (when on) is larger than the latching current and the holding current. If you didn’t already know this, the latching current is the current that must pass through the triac right after it is turned on to ensure that it latches. The holding current is the current level through the triac below which the triac will turn off. So, the assumption that current through the triac is higher than the latching current and the holding current means that the triac stays on once it is fired on. It stays on until the current through it is zero.
Let’s say that we are going to control over 5 complete cycles, that is, 10 half-cycles. My AC input voltage is 220V RMS, is sinusoidal in shape and has 50Hz frequency.
So, the principle is that we are going to supply voltage to the load for a certain number of cycles. Let’s say we’ll supply voltage for 3 cycles and will shut supply to the load off for 2 cycles. This is a duty cycle of 60%. The duty cycle is defined as the ratio (expressed in percentage) of the number of on cycles to the total number of cycles we are controlling.
The output RMS voltage is related to the supply voltage by the relationship:
where Vout = Output RMS voltage
Vs = Supply RMS Voltage
k = duty cycle
The output is related to the voltage by the relationship P = V2/R. So, assuming a constant resistance, power is directly proportional to the square of the voltage. So, if you half the voltage, the power is not halved, but is reduced to one-fourth the original power! One-fourth power with half the voltage! But the general idea is that, by controlling the output voltage, you can control the output power.
So, using the previous values, where we have a duty cycle of 60% and a 220V RMS input voltage, the RMS output voltage is:
Now let’s consider the ratio of the powers. P = V2/R. Therefore, by using pulse skipping modulation, the power changed by changing the voltage is related by:
This means that if you had a resistance R as load, and if the power was P when the voltage supplied was Vs, using PSM with a duty cycle of 0.6 decreased the power to 0.6P – using PSM we changed the output voltage and thus output power.
Now let’s now go on to the design part – how we’re actually going to do this.
For the microcontroller, I’ve chosen the extremely popular PIC 16F877A. However, since this application requires only a few pins, you can easily use any other small microcontroller for this purpose, such as PIC 12F675.
The zero-crossing is done using the bridge-optocoupler method as I had previously shown.
Now, let’s take a look at the code: