*Question E7D02:
How does a switchmode voltage regulator work?*  

*Correct Answer is  B: By varying the duty cycle of pulses input to a filter*  

### AI Response: (may contain errors)  

A switchmode voltage regulator (SMPS) is a type of power supply that efficiently converts electrical power by switching the input voltage on and off rapidly. Let's break down the correct answer and the other options.

### Correct Answer: B
**By varying the duty cycle of pulses input to a filter.**

- **Explanation**: 
  - In a switchmode voltage regulator, the output voltage is regulated by adjusting the duty cycle of the switching signal. The duty cycle is the ratio of the time the switch is on to the total time of one cycle. 
  - By increasing the duty cycle, more energy is transferred to the output, raising the output voltage. Conversely, decreasing the duty cycle reduces the output voltage.
  - The rapid switching creates a series of pulses that are then smoothed out by an output filter (usually an inductor and capacitor) to provide a stable DC output.

### Why the Other Choices are Incorrect:

- **A: By alternating the output between positive and negative voltages**  
  - This describes an AC signal rather than the operation of a switchmode power supply, which typically provides a stable DC output. SMPS does not alternate the output voltage in this manner.

- **C: By varying the conductivity of a pass element**  
  - This describes linear voltage regulators, which use a pass transistor to control the output voltage by varying its resistance. In contrast, switchmode regulators use rapid switching to control energy transfer, not by varying conductivity.

- **D: By switching between two Zener diode reference voltages**  
  - While Zener diodes can be used in voltage regulation, they are not the primary mechanism in switchmode power supplies. SMPS does not switch between Zener voltages; instead, it uses a feedback loop to adjust the duty cycle of the switching element.

### Summary
- The key to understanding switchmode voltage regulators is the concept of duty cycle and how it affects the output voltage through rapid switching and filtering. This method allows for high efficiency and compact design compared to linear regulators.