# 100% Duty Cycle PWM

I recently had to build a filter to go from PWM to 0-5Vcc, and it got me thinking...

At 100% duty cycle, the PWM used was 100% 5V output, DC voltage...

If i used this PWM to, for example, switch a MOSFET on/off to drive a transformer's primary coil (like on a SMPS model).. the 100% Duty Cycle would actually be the same as 0%, (the transformer won't work for a Non-Oscilating voltage on primary), and so there would be no output on the supply...

I assume that this is considered on every PWM circuit, and in cases like the one i mentioned the max would be set to 99.9%-

So, both cases (100% Duty Cycle => DC) and (100% Duty Cycle => 99.9% Duty Cycle) seem ok for different circuits, my question is...

When an engineer refers to 100% Duty Cycle, does he mean one or the other?

• In this circuit with a transformer, what would be the practical difference(s) between a 20% duty cycle signal and a 80% duty cycle signal? – Dampmaskin Feb 8 '17 at 19:28
• Not sure what you are asking but 100% PWM is as oxymoron as organic water: if the duty cycle is 100%, it is a DC signal and it cannot switch anything. the mosfet controlled by it would be either on or off. – dannyf Feb 8 '17 at 19:30
• You have to divide this question in the controlling of converters with a single coil (like Buck) and transformer (like Flyback). A Buck does not care about a PWM duty cycle of 100%, with DC current the coil looses it's function and just forwards the electrical energy. If the converters uses a transformer (2 coils on 1 core) you're right, there will be no more power transmitted in the case of DC current. Therefore this circuit needs more than one switching element. – auoa Feb 8 '17 at 20:25

100% PWM duty cycle means the PWM pulse is high 100% of the time. This is the same as the flat level of whatever the pulse high voltage is. For a typical microcontroller PWM output, this would be its supply voltage.

In many cases, the average voltage of PWM is what matters. The pulses are deliberately fast enough so that their effect is averaged out. This is the case, for example, when driving LEDs, solenoids, and motors.

However, when driving a transformer, only some function of the AC component makes it to the secondary. DC just causes heating without any benefit, and brings the AC peaks of one direction closer to saturating the core.

Therefore, when driving a transformer with PWM, the maximum result is obtained from 50% duty cycle. 100% yields the same result as 0%, except for the extra heating. Of course this is taken into account when driving a transformer. The firmware, or whatever is adjusting the duty cycle, must be limited to not exceed 50%.

You might consider this the maximum drive level, but you'd still call it 50% PWM duty cycle.

Consider that to drive a transformer properly, you want to drive it with pure AC (because a DC component across the primary can saturate the core, reducing its efficiency.

There are two common ways : you can use an H-bridge, or a centre tapped primary with a switch at each end, I'll consider the latter but the same principle (reversing the drive direction in each AC half cycle) applies to both.

Then 0% PWM means neither switch is turned on. Typically 100% PWM means each switch is turned on alternately for the whole PWM period, which corresponds to a half cycle of the AC waveform.

• In other words, for an AC circuit, your full duty cycle range is really 0 to 50% as 50% to 100% will produce the same result at AC, just backwards (i.e. 0% = 100%, 10% = 90%, etc.). – alex.forencich Feb 8 '17 at 20:47

Duty cycle is well defined as $$DC=\frac{T_{high}}{T_{period}}$$ Here $T_{high}$ is the time your signal is high out of the total period of length $T_{period}$. In percents it will be $$DC_{\%}=100\frac{T_{high}}{T_{period}}$$

So $DC_{\%}=0$ implies $T_{high}=0$, i.e. the signal is always low. And $DC_{\%}=100$ implies $T_{high}=T_{period}$ - the signal is high during the whole period time, i.e. always.

100% duty cycle means that the voltage is always high (on), 0% means that is is always low/off. Although both are DC, they are certainly not the same thing.

Take for example a light source: 0% duty cycle means that it is dark/off and 100% means that it is maximum bright.

When talking about switch mode power supplies, the duty cycle might be changed to regulate the output, but it doesn't make sense to have a duty cycle higher than 50%, as it would just waste power. The whole point of SMPS is to be more energy efficient.