Thursday, December 05, 2013

IC Design Interview 7: Buck Regulator

Buck DC-DC Switch Mode Power Supply (SMPS) come up in interviews quite a bit these days. Here is my quick review sheet. You may want to be careful it; I bombed the interview I prepared it for.

The obvious things about a buck regulator need to be said. A buck regulator can only translate an input voltage to a lower voltage, never a higher voltage; that requires a boost regulator. The current through an inductor may not change instantly. The voltage across an inductor may change "instantly". 

I sometimes think of a buck regulator as a filter placed on a pulse width modulated (PWM) signal. The output is then simply the average value the voltage before the inductor/capacitor filter. This analogy is not acceptable to most engineers who actually design these circuits. 

Figure 1. Buck regulator overview.

The diode is almost always implemented as a transistor. The transistor that replaces the diode is turned on when the other transistor is turned off. They must not both be on at the same time. If they are both on, significant current shoots through from the input power straight to ground through them. 

Figure 2. Current through and voltage across the inductor.
V = L di/dt        the basic inductor equation
V/L dt = di        just pretend you can do that with derivatives
∫ V/L dt = ∆I     the integration is to ton
(Von-Vn)/L ton = ∆I

The area under the voltage curve (Vn-Vo)*ton is the same same as the area over the rest of the voltage curve -Vo*(toff-ton). That is the same as saying the energy into and out of the inductor is the same.