Dr. FAE: Voltage Feed-Forward Feature for Power Supply Controllers

September 21, 2009

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 N^n-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.

Tags: DC-DC converter, power supply controllers, voltage feed-forward

Cell Phone Connection Failed – AGAIN!?

July 17, 2009

Standing in the park, I have been checking emails, making phone calls and taking a few precious pictures of my darling dog. Wishing to send the pictures to a friend’s cell phone, I press the button and stare at the cell phone display. Suddenly, it says “connection failed.” What! I try again and realize that the cell phone battery is too low with barely one bar showing.

Does this sound familiar to you?

We all want to be connected via cell phone whenever and wherever we are. Unfortunately, our cell phone battery does not always keep up with our busy lifestyle.

Inside of every cell phone is a radio frequency (RF) transmitter. Every time you talk, text or send data, the transmitter drains power from the battery. Often the transmitter consumes only 10% of the phone’s total power. However, if you’re in a poor coverage location, surf the net or up-load photo/video, the transmitter’s battery consumption can top six times, sapping 60% of the battery’s charge. This transmitter can be a huge power drain and dramatically reduce your talk time on the cell phone.

Today’s feature-rich 3G phones require more power than ever before. The design challenge facing engineers today is how to improve efficiency in existing areas such as RFPA, display and increased talk time so more power is available for new features.

One solution is to install a step down DC-DC converter with dynamic voltage scaling to control V_OUT between the battery and the RF Power Amplifier (RFPA). This reduces current consumption by 30-50%, and makes power available for other functions (such as MP3, GPS and video). In addition to reducing current consumption, a step down DC-DC converter also reduces heat and increases talk time.

If you would like to check out Fairchild’s solution, please watch a short video demonstrating a solution to extend battery power for a 3G phone by improving the efficiency of the RFPA. More information on RF Power is available at www.fairchildsemi.com/rfpower.

Tags: battery life, cell phone, DC-DC converter, Fairchild, Fairchild Semiconductor, power, RF Power Amplifier, RF transmitter