1
\$\begingroup\$

These days it seems all power inverters use variable PWM and an H-bridge to generate the signal which drives the output transformer. Why do we not use a pure sine wave oscillator and amplifier instead?

It seems to me the technical complexities of PWM are much greater than a simple sine-wave. But I cannot find a single example of an inverter which uses a sine wave oscillator. Therefore, I'm assuming there are very good reasons for not using such a design. Can anyone explain what those reasons might be?

\$\endgroup\$

4 Answers 4

5
\$\begingroup\$

To generate a true sine wave from a DC bus requires a linear amplification system and, this adds up to a lot of power losses (maybe around 35% of the power output). It's as simple as that.

PWM mimics a true sinewave with a fraction of the losses (maybe 5 to 10% of the power output) albeit with some harmonic distortion that is removed using filters (lossless types in the main).


Just compare the class AB amplifier (a linear amplifier with very little distortion) with a class D amplifier (using hard switching MOSFETs and PWM): -

enter image description here

Extracts from this website by MAXIM.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you. I figured there must be a good reason PWM is used instead of analog sine waves. \$\endgroup\$
    – spinjector
    Sep 6, 2020 at 16:52
3
\$\begingroup\$

PWM allows the output switches to be fully on or fully off. The power dissipated in the switch is given by \$ P = VI = I^2R \$ where V is the voltage across the switch, I is the current through it and R is the switch resistance.

Let's look at an output stage using IRLZ24N MOSFETS (chosen at random) and running on a 20 V supply. The datasheet says that the on-resistance of the device is about 60 mΩ. Assume an output load is 2 Ω so 10 A peak current.

  • Switch open: \$I = 0\$ so \$P = 0\$.
  • Switch closed: \$P = I^2R = 10^2 \times 0.06 = 6 W\$.

Now consider your sine generator. When the sine is have way up the curve there will be 10 V across the switch and 5 A through the load.

\$ P = VI = 10 \times 5 = 50 \ \text W \$.

This means wasted power and a big thermal management problem.

We use PWM for efficiency.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you. That makes sense. \$\endgroup\$
    – spinjector
    Sep 6, 2020 at 16:53
2
\$\begingroup\$

It's basically because we have no solid-state way to reduce voltage without dissipating heat. When a solid state switch is fully on it conducts lots of current at almost no voltage drop, and when it is fully off it blocks full voltage at nearly zero current so losses are low.

Anything in between it behaves like a resistor. So to avoid lot's of heat and power loss we only use solid-state switches in full on or off in power applications. This rules out the simplest method of producing a sine wave for high power applications which requires a gradient of output voltages.

If you want to make a sine-wave while still using the switch only ever in full no or full off, then it becomes more complex than simple PWM because you are still PWMing to maintain efficiency in the solid state switches, but now you are adding on extra filtering afterwards to turn it into a sine wave.

If you can come up with a solid-state method to reduce energy coupling between two points while maintaining high efficiency then you can reduce voltage without incurring losses, similar to how the coils move past each other in a generator and overlap to varying degrees producing different amounts of coupling. If you figure that out, then you can produce a graduated output without incurring losses much the same way a spinning generator is able to make a sine wave without incurring losses. Then we will have efficient, simple, straightforward high power sine PWM. You will also probably win a Nobel prize.

\$\endgroup\$
2
  • \$\begingroup\$ Than you. I suppose that's why audio amps get so warm. Years ago I was a roadie, and one day during a show I was fixing some speaker wiring, and I touched the bare wires and got rapped pretty hard. I was so shocked (literally and figuratively), that I put my meter on the subwoofer wiring, and it was about 90v..! This was on an 8kw Crest Aduio amplifier (the entire rig that was 90kw, lol). So this idea of a pure sine-driver inverter has been stuck in my head for years. \$\endgroup\$
    – spinjector
    Sep 6, 2020 at 16:55
  • 1
    \$\begingroup\$ @spinjector Yes, that is exactly why a linear amp has less distortion but also produces much more heat. A class D amps works by switching the transistors full on/off which is efficient but can only produces square waves so those square waves are filtered down to "average" things out and produce the in-between outputs. But since it ultimately comes from a square wave, still more distortion. \$\endgroup\$
    – DKNguyen
    Sep 6, 2020 at 21:22
1
\$\begingroup\$

The losses in an amplifier are significantly higher than in switching schemes like PWM. For high quality sinusoidal output one can combine the two approaches e.g. like shown in this paper:

G. Gong, S. Round and J. W. Kolar , "Design, Control and Performance of Tracking Power Supply for a Linear Power Amplifier"

\$\endgroup\$
1
  • \$\begingroup\$ Thank you. That's been an interesting read (so far). Lots of excellent info in there. A bit above my head in a lot of it, but learning is good. \$\endgroup\$
    – spinjector
    Sep 6, 2020 at 16:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.