*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 works by varying the duty cycle of pulses input to a filter. Let's break down why this is the correct answer and why the other choices are incorrect.

### Correct Answer: B

- **Explanation**: A switchmode voltage regulator controls the output voltage by rapidly switching the input voltage on and off. The key to its operation is the **duty cycle**, which is the ratio of the "on" time to the total cycle time. By adjusting the duty cycle, the average voltage output can be controlled. This pulsed input is then smoothed out by a filter (usually an inductor-capacitor combination) to produce a stable DC output.

- **Useful Formula**:  
  The output voltage $V_{\text{out}}$ can be approximated by:  
  $$V_{\text{out}} = D \times V_{\text{in}}$$  
  where:
  - $D$ = duty cycle (a value between 0 and 1)
  - $V_{\text{in}}$ = input voltage

### Why the Other Choices are Incorrect:

- **Choice A**: "By alternating the output between positive and negative voltages"  
  This describes an AC inverter or a type of waveform generator, not a voltage regulator. A switchmode regulator maintains a DC output, not alternating voltages.

- **Choice C**: "By varying the conductivity of a pass element"  
  This describes a linear voltage regulator, which uses a pass transistor to drop excess voltage. It is less efficient than a switchmode regulator because it dissipates excess power as heat.

- **Choice D**: "By switching between two Zener diode reference voltages"  
  This is not a method used in switchmode regulators. Zener diodes are used for voltage reference or regulation in linear circuits, not for switching operations in switchmode designs.

In summary, the switchmode voltage regulator's efficiency and effectiveness come from its ability to adjust the duty cycle of the input pulses, which is then filtered to provide a stable DC output. This method is more efficient than linear regulation, especially for large voltage differences.