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I was interested in making an analogue to pwm circuit. I want to feed the output to a differential amplifier that will then drive the gates of transistors. The differential amplifier==>isolation transformer==>gates works just fine. I have studied the 555 timer pulse width modulator circuit, and I would be fine using it if not for the fact that I would strongly prefer two outputs that swing from (Vcc, Ground) to (Ground,Vcc). This way I can be sure the push pull differential amplifier goes negative during the "output low" portion of the duty cycle.

Could anyone suggest such a circuit? I thought there might be a way using a 556 timer in a similar fashion, but I do not know how.

EDIT: Here is a quick sketch of what the circuit looks like: pwm

So I want to have analogue inputs, and pwm outputs fed to a pair of differential amplifiers that boost the signal to higher current (I have a one amp dual op amp). This is then fed to an isolation transformer that switches an igbt.

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  • \$\begingroup\$ With a 556 you will not get both outputs fully in sync. To drive two transistors, sync of the outputs is extremely important to prevent both from conducting at the same time and practically shorting your power supply. Some microcontrollers have this functionallity by default. I've seen it in the ATtiny45 datasheet, but I bet there are PIC controllers that offer this functionality too. It is called 'dead time' and prevents the two outputs from being active at the same time. \$\endgroup\$ – jippie Jul 5 '12 at 7:16
  • \$\begingroup\$ I'm not quite clear on what you are trying to do here - can you maybe draw a little diagram or something? \$\endgroup\$ – Oli Glaser Jul 5 '12 at 10:02
  • \$\begingroup\$ Ok, I added a picture and hopefully a better explanation. \$\endgroup\$ – Alex Eftimiades Jul 5 '12 at 16:36
  • \$\begingroup\$ Re your edit: you're using the opamps without feedback, that means a high gain and a very low bandwidth, way too low to reproduce the PWM's edges properly. BTW, what opamp is that? And can't you use a fast optocoupler instead of a transformer? They're made for digital signals. \$\endgroup\$ – stevenvh Jul 5 '12 at 17:03
  • \$\begingroup\$ The op amp is tca0372. The op amps were supposed to just flip the output's sign at a high current. The reason I use an isolation transformer is to ensure I can produce enough current to drive the gate of an igbt. Besides, I use a large autotransformer to supply power to both the logic and the load, so I need to use an isolation transformer at some point to keep the logic side isolated from the gates of the igbts I am switching. \$\endgroup\$ – Alex Eftimiades Jul 5 '12 at 17:29
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Whole tribes are fighting religious wars over the microcontroller vs. 555 debate. I'm on the microcontroller side.

So the first question is: does it have to be a 555, and what are the arguments for it? (that's two questions.) There may be good reasons for it, but I'd like to know them. "My wife wants me to do it this way" is not a good reason (unless maybe when I see her). Without a good reason I'd have a look at the microcontroller.

The main advantages of the microcontroller are simplicity and accuracy. Simplicity as in you only need a microcontroller and a potmeter to set the duty cycle. Two parts for the complete circuit, or one if your analog control voltage comes from elsewhere in the circuit; then you can drop the potmeter. Accuracy as in 10 bits ADC translates in 10 bit PWM, that's 0.1 % precision at a 1 % accurate clock. Make that a 20 ppm clock if you want to add a crystal. Accuracy is also reproducibility.

OTOH the 555 can work at 15 V if necessary, which the microcontroller can't, and doesn't need programming. The latter is again one of those religious things, but in practice writing a program for this can be done as quickly as designing and calculating the 555 circuit: setup two identical PWM channels but one inverted, the other non-inverted, and in a loop read the ADC and copy the value to both PWM registers.
So the microcontroller outputs complementary signals without external components.

The 555 circuit relies on a capacitor and if you want that at 1 % tolerance you'll pay a few times more than what the microcontroller costs. And even then the frequency will vary with the duty cycle.

All in all there are few 555 applications which a microcontroller can't do better.

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I appreciate the answer(s) and suggestions, but I recently came up with a simple way to modify my existing dual op amp arrangement to pulse width modulate a signal: op amp pwm

To me at least, this is the simplest way to integrate pwm into my existing circuit. The period should be 2*RC*ln(2) (if I did my calculations right), with R being 10k and C being 100nF. Pretend the 100ohm resistor is a speaker (as was the case when testing the circuit), or a gate drive transformer, as was the case for the final product. Finally, the 40Hz sine wave represents the analogue input signal.

I will note that this circuit is unexpectedly noisy. If anyone sees any inherent weaknesses in the circuit, please leave a comment.

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  • \$\begingroup\$ Can you explain 'noisy' a bit? \$\endgroup\$ – jippie Jul 6 '12 at 19:36
  • \$\begingroup\$ Well, I tried audio input, and it there was just a lot of static. Interestingly, when the 10k resistors are replaced with wires, the sound is relatively clear. I suppose I was left with the internal resistance of the op amps. However, the audio quality was still not great. \$\endgroup\$ – Alex Eftimiades Jul 7 '12 at 0:55
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I was also looking for a simple dual op-amp PWM circuit so I examined yours just with my eyeballs and I think I see a problem. First, I think the idea of getting push-pull out is clever. The "green" op-amp is apparently running at a different gain; the impedance of your source would affect that. So, what is probably happening is that the two stages are trying to produce non-related frequencies (even FM) so that you are getting a mixture of sidebands. I imagine it sounded sorta like an SSB reception with the BFO not correctly set. This idea is reinforced by your statement that taking the resistors out made it better thereby reducing the gains and making them run more together. And of course, even PWM can be over modulated. I'd look for another method to inject the audio.

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