Dr. FAE: Voltage Feed-Forward Feature for Power Supply Controllers
September 21, 2009
-By Dr. F.A.E.
Dear Dr. Fred A. Engleberry. Could you please explain the advantage of the voltage feed-forward feature of many of your power supply controllers? - - Puzzled in Peoria
Greetings to you in Peoria.
My IQ is 181 so, certainly, I could explain the advantage of voltage feed-forward. In anticipation of your next question, I will elaborate…
This morning, over a warm, half-caf, double-short, non-fat latté at the local coffee shop, I was enjoying a recreational review of Kreck and Lück’s Novikov Conjecture (Geometry and Algebra) which says the following:
Finally, we indicate the proof of Theorem 16.2 for arbitrary n. The idea is to work inductively. If f is a diffeomorphism on Tn X P with P a 1-connected manifold, one can isotope it so that it preserves Nn-1 X P.
My thinking might be illustrated more clearly with a transfer function from Erickson and Maksimovich, Fundamentals of Power Electronics:
This formula clearly shows that input voltage is not a variable. It contains a built-in assumption that the input voltage is invariant. Adding input voltage greatly complicates the transfer function.
Dear Doctor FAE, pardon me, but I do not recognize that answer as plain English.
- PiP
Very well, I shall explain without the crystal clarity of the simple equation. The control loop of a DC-DC converter operates by sampling the output voltage and adjusting the pulse width modulation of the power train. The control loop acts as follows: if the output voltage changes, then we adjust power supply to counteract the change and keep the output stable.
However, if the input voltage changes, the power supply must respond to this change too. We could wait for the effect of the input change to appear at the power supply output, but wouldn’t it be glorious if we could monitor the input voltage and adjust the PWM immediately without waiting for the output voltage to change…if we provided some direct control method that did not complicate the feedback loop?
That’s the advantage of voltage feed-forward.
We do this by allowing the input voltage to directly modulate the slope of the PWM ramp. With an increased input voltage, the slope of the ramp increases and crosses the feedback signal sooner, giving a shorter output control pulse. Get it? Thus, increasing Vin reduces the PWM control signal outside of the output voltage control loop.
Now, if you’ll excuse me… I’ll be seeking a refreshing nap.
About the author:
Dr. Fred A. Engleberry (also know as Dr. F.A.E.) holds a PhD from MIT (Muckton Institute of Talknology). Fred has a world-class intellect and rarely neglects an opportunity to mention it. Argumentative and contentious, he's often irritatingly correct when answering questions and discussing semiconductor technology, real world design trade-offs and engineering issues. Dr. F.A.E's core competencies include using big words when small ones would suffice and providing long-winded, indirect answers to simple questions.
Tags: DC-DC converter, power supply controllers, voltage feed-forward
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