Design play/challenge: Phase Shift PWM

Tim Williams · 2016-02-16T19:56:44+00:00

It finally occurred to me, I've been using these things for so long, yet never built my own. And therefore, never fully understood nor trusted what the documentation says (or hides!). So here's the challenge: Design a phase shift PWM controller. Example applications: Full bridge forward converter (SMPS, UPS, motor control, etc.) Frequency-locked, resonant systems (resonant SMPS, induction heating): PWM provides linear, continuous power control, independent of frequency. Requirements: - Constant 50% +/- 3% duty cycle per output (complementary outputs with dead time for full bridge gate driving are easily added on) - Variable frequency, using a control voltage/current, or single variable resistor (duty cycle and phase shift remain ~constant) - Variable phase shift, from a control voltage input - Phase shift adjustable from 0-180 degrees or more (or 180-360, same thing) - Must use continuous-time methods, not sampled/divided. (Or, ENOB > 12, which will rule out most digital methods for rather modest output frequencies anyway.) - Uses discrete transistors, comparators, logic gates (no more than MSI), that sort of thing. ("Transistors" includes the hollow FET kinds, if you like.) Homework: The basic prototype is an astable multivibrator (makes 50% duty cycle reference clock), followed by a delay and ramp circuit. The tricky parts are: - The multivibrator has to be timed from the same global (frequency control) bias current (e.g., using a current steering triangle-wave oscillator, like you'd find in an analog function generator) - Essentially, an oscilloscope's triggered sweep circuit is required for the delay generator. Multivibrator triggers it on rising edge; cap charges for almost the whole cycle time (again, charged by the global bias current), making a sawtooth wave; comparator picks a point along that ramp, thus implementing the delay. - Thus, delay is adjustable anywhere along the sawtooth ramp, which might be, say, 10-350 degrees. It...

Feb 16, 2016 63 Replies

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Jim Thompson 10 years ago

What sort of counting method do you use for your abortion ?>:-} ...Jim Thompson

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whit3rd 10 years ago

Oh, not just one. I had in mind TWO PLLs. As to overkill, if the problem gets dead, and stays dead...

It would look nice on an oscilloscope, dialing the "Vref" control and seeing the second square wave shift smoothly through several cycles of phase. Kinda like alternate delayed-sweep, but done in 'chop' mode.

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Tim Williams 10 years ago

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Well, I photographed the breadboard, unlike some others here. I count two PDIPs. Did you miss the links? ;-)

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com

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Jim Thompson 10 years ago

I saw so many parts I became nauseous >:-} ...Jim Thompson

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

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