Electronic – Easiest way to go from analog input to PWM output

adcmicrocontrollerpwm

Project background: I'm building a plasma speaker. There are several examples online using a TL494 power supply IC and feeding audio into its feedback pin to get sound out. A sample schematic: enter image description here

I'm wondering if there is a better way to do this. I'm a hobbyist, so I'm not terribly interested in the absolute cheapest method since it's a one-off, and I'll also be tolerant of potentially dangerous or sketchy things since I don't have to get certification for anything.

I have access to a few micros, but little experience with them. I did get a Launchpad, and my understanding is one of the MSP430s that came with it has a built-in ADC. While going from analog to PWM seems a little underwhelming for a microcontroller, I do have an additional requirement that I must limit my upper duty cycle to remain below about 95% to prevent saturation of the flyback transformer. Operating frequency is 120-150khz.

To summarize my question:

Are there more straightforward ways to go from a high-impedance 1V P-P input signal to a 150khz output signal with proportional PWM, capped at a maximum 95% duty cycle? Unless built natively into the device, I will be building my own output stage to drive the power MOSFET.

Best Answer

Amusingly enough, I was literally just looking at single-chip PWM generators for my job.

If interest to you might be the Linear LTC6992 - TimerBlox: Voltage-Controlled Pulse Width Modulator (PWM)

You give it a 0-1V input, and you get a PWM pulsetrain out. It can also internally limit the duty cycle to 5%-95%, or 0-100%.

enter image description here

You would need a op-amp or two to scale your input to the chip's input range, but you would need to do that anyways, because very few ADCs are bi-polar (assuming you were going the microprocessor route).

Looking at the reference schematic you provided, I would also recommend using a MOSFET driver of some sort, to improve switching times, if nothing else.
It would also likely provide level-shifting for the gate-voltage.

Related Solutions

Electronic – Making PWM from 2 Analog input in MSP430G2231

You are not waiting for the ADC to finish the conversions. You would have to do something like this:

 for (;;)
{
  ADC10CTL0 &= ~ENC;

  while (ADC10CTL1 & BUSY);                 // Wait if ADC10 core is active
    ADC10SA = 0x200;                        // Data buffer start
    P1OUT |= 0x01;                          // Set P1.0 LED on
    ADC10CTL0 |= ENC + ADC10SC;             // Sampling and conversion start
    __bis_SR_register(CPUOFF + GIE);        // LPM0, ADC10_ISR will force exit
    P1OUT &= ~0x01;                         // Clear P1.0 LED off
 }

}

You should read the user guide - chapter 20 (I think) ADC10 and look at the code examples on ADC10 with DTC

Electronic – Driving Permanent Magnet DC Motor via PWM

A bunch of issues:

Are you sure you have a brushed DC motor? This kind has only two connections. You simply apply voltage and it spins, which is what you are assuming in the rest of your question. All you said is that the motor has permanent magnets and is "DC", which could still leave other possibilities, like a brushless DC motor. Those are a lot more complicated to drive. Those have a lot more than two wires coming out, usually 3 for the drive coils and 5 for the Hall sensors.
$50-$100 sounds like the cheapest way to solve this problem unless you value your time at pennies/hour. If this is a brushless DC motor, then you are in over your head.
Whether you run the motor from 60 V, 120 V, or something else, that DC power still has to come from somewhere. This is independent of how you might switch it by applying something like PWM, for example. In other words, you still need some kind of power supply.
PWM does not somehow supply more consistant torque. PWM is merely a technique for modulating the effective motor voltage while starting with the same DC supply and wasting relatively little power in the process.

Systems that drive motors with PWM also often have speed or position feedback. If the feedback is used to maintain constant motor speed, then the motor can appear to have high torque without running away when the load is removed. However, this is due to the ability to modulate the motor drive and a servo loop with feedback. PWM is one means to modulate the motor drive, but by itself doesn't somehow get more torque from the motor.
Since PWM is a technique to present a lower apparent voltage to the motor than the raw DC power supply it is derived from, you should use the highest voltage supply the motor can handle.

The apparent motor drive voltage is the raw DC voltage times the PWM duty cycle. For example, if you start with a 120 V DC supply but the motor only needs 30 V to turn at the speed you want with the load you are going to present it, then a PWM duty cycle of 30V / 120V = 1/4 will do it, assuming the PWM frequency is high enough so that the motor does not react to individual PWM pulses. Let's say the PWM frequency is 25 kHz, which is a common frequency to run motors at. That means the PWM period is 40 µs. To make effective 30 V with 120 V in, you'd pulse the motor on for 10 µs every 40 µs, or 10 µs on and 30 µs off.
You say you are concerned about torque, but it appears what you really want is constant speed. If so, a servo loop in the micro that controls the PWM duty cycle to whatever it needs to be to maintain the set speed will give more apparent "torque" than brute force gearing the motor down to the point where whatever load variations you give it won't matter much at the motor.

Best Answer

Related Solutions

Electronic – Making PWM from 2 Analog input in MSP430G2231

Electronic – Driving Permanent Magnet DC Motor via PWM

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