Alternating PWM signal with delay

Thread Starter

Daan Stevenson

Joined Jan 28, 2018
2
I'm trying to design an alternating PWM signal that has a delay (dead band) built in.

This is to switch power between a HV reed relay and another component. Since there is some delay in the reed relay, I need the dead band so that the input to the components does not to overlap.

I first generated the 50% duty cycle with a 555 timer, and then realized that it can be made simply with a transistor based astable multivibrator. Question is - how do I reliably generate the delay (ok if only on one end). Seems like it should be possible with an extra resistor/capacitor somewhere in the circuit.

Size and simplicity is key (no microcontrollers, etc.). Thanks for your help!
 

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crutschow

Joined Mar 14, 2008
25,980
Below is the LTspice simulation of a non-overlapping clock circuit.
The non-overlap time is determined by the RC time-constant.
Is that what you want?

1587615276595.png
 

danadak

Joined Mar 10, 2018
4,057
For future reference modern processors with PWMs usually have dead band programmable
in their configuration.

One distinct advantage is their timing is faily precise, unlike delays created with RC networks
relying on thresholds of CMOS gates varying all over the map. If you are making a one off you
can use pots to trim delays, but the T dependence of values and CMOS gate thresholds simply
still awful.


1587639600987.png


Regards, Dana.
 

Thread Starter

Daan Stevenson

Joined Jan 28, 2018
2
Thanks for the great responses guys.

This is for a wearable, so the size of the comparator and Schmitt trigger ICs are a tough pill to swallow, though it looks to be a reliable solution.

danadak - what do I look for to determine whether a microcontroller is capable of these dead bands? I might be able to get away with one of the smaller options. I also need to be able to try different frequencies.
 

ronsimpson

Joined Oct 7, 2019
1,121
TL494 is a very old part that I would not use but it one of a large family of parts that do what you want.
PWM with two output that alternate. The Flip-Flop in the center chooses which output is next. Dead time and all is there inside.
1587654529049.pngTL494
 

danadak

Joined Mar 10, 2018
4,057
Thanks for the great responses guys.

This is for a wearable, so the size of the comparator and Schmitt trigger ICs are a tough pill to swallow, though it looks to be a reliable solution.

danadak - what do I look for to determine whether a microcontroller is capable of these dead bands? I might be able to get away with one of the smaller options. I also need to be able to try different frequencies.
What else is in design besides the PWM ? PSOC has a lot of analog, and DSP as well as simple logic gates
to complex timers/counters/PWM/COM/LUT.....

The part I showed is PSOC. Attached is a catalog of the onchip resources inside the part. If you
look at the right hand window of the screenshot I posted you can see resources used in chip
and resources left to be used by designer inside chip.

There are low end parts, PSOC 4, Mid Range PSOC 5LP, high end dual core blue tooth targeted
solutions PSOC 6.

There are super small packages available - https://www.cypress.com/packaging

The IDE (PSOC Creator) and compiler free - https://www.cypress.com/products/psoc-creator-integrated-design-environment-ide


Regards, Dana.
 

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