Power Electronics

Created: April/2020Last Updated: 5/22/2020


Efficient and low noise power supply design is key to elongate battery life and reliable functionality for consumer electronics. The key message of this text is to present the fundamental principles of different types of voltage regulators and provide insight to design trade offs and examples of application.


Power electronics become popular due to GaN MOSFET allow MOSFET to operate closer to ideal switch with very low conduction resistance.


The main objective of power electronics design is to equip engineers with fundamental theory to design, debug, and make right trades offs in real world applications.


Switch mode power supply

Following key concepts to should be memorized:

Power in = Power out

Average voltage across a inductor in a cycle Must be zero, else there will be energy stored in inductor which violates principle #1

Average current going into the output capacitor Must be zero else there will be energy stored in capacitor which violates principle #1

Inductor acts as a current source, its current flow cannot suddenly change.

Capacitor acts as a voltage source, its voltage cannot suddenly change.

Step Down (Buck) Converter

Working principle

Input voltage square waveform is seen by a low pass LC filter, which is an analog averaging circuit, hence output is the average of the input signal.

The average of a square waveform is the product of duty ratio * inputvoltage.

Output range: Vin to 0.

Step Up (Boost) Converter

Working principle

Energy get charged up inside the inductor when low side fet is on, and energy is injected to the output load (cap+load resistor) when low side switch is off, hence output voltage is Vin+Inductor Voltage, resulting a boost in voltage.

Output range: Vin to infinity.

Step down and up (Buck and Boost)

Working principle

Energy gets charged up inside the inductor when high side fet is on, and energy is injected to the output load load (cap+load resistor) when high side switch is off, hence output voltage is VL (inductor voltage).

    • Note: load is parallel to inductor so the output voltage equals to VL when high side switch is on.

Output range: 0 to infinity.

Power Losses

Three common factor of power losses:

Conduction loss

  • power Loss through MOSFET internal resistance when it's on, Rdson and Inductor internal resistance (ESR)

Switching loss

  • power loss due to imperfect MOSFET switch on/off time, MOSFET parasitic capacitance. It's proportional due frequency. In other words, higher switching frequency increase power loss.

AC core loss

  • power loss due to AC current ripple going through inductor ferrite/irons cores.


Excessive capacitance at output reduce phase margin of the feedback network and it is likely that power supplies can be come unstable under high load transients. One Must follow recommended output capacitance and ESR selection as well as recommended compensation feedback network

Capacitor Selection

  • Capacitor has a voltage derating current meaning that the its effective capacitance decreases inverse proportionally to applied DC voltage. A rule of thumb is to select the capacitor rated voltage at least 2x of operating voltage.

Common Modulation Techniques

PWM Pulse Width Modulation, change the duty cycle of a fixed frequency SMPS resulting change in output voltage.

PFM is used to a light load to reduce power consumption of the power supply by have a burst of switching action periodically to keep the output voltage within a boundary. In this mode, voltage deviation is large.


Change in current loop area produces varying magnetic flux, which induce noise ( e.g. ground bounce, electromagnetic interference, etc.) on the circuit. The main layout objective is to reduce change in current loop path. Therefor all passives (e.g. input caps, output caps, and inductor) Must be placed as close as possible to input, switching node, and ground pins.

    • keep all passives closer to the ICs pins.
    • use wide and short power planes
    • multiple stitching vias for ground planes
    • at least two ground vias per input and output bulk capacitors.
    • feedback trace must be short and isolate from switching nodes copper and route quietly back to the feedback node.

Current loops are the ac current paths of the voltage regulator when during switching on and off cycle. For buck converter, the input capacitor sees a discontinuous current whenever the high side switch is turned off, as we know fast switching current di/dit induces voltage spikes on input voltage node due to parasitic loop capacitance. Hence to reducing the current loop would reduces voltage ringing due to lower parasitic inductance.

Switch Cap regulator

working principle:

it uses only capacitors to regulate output voltage by a rearranging capacitor charging and output paths using MOSFET switch. it can used to both boost and buck input voltage.


    • high efficiency (up to 95% efficiency) when conversion ratio is an integer
    • low power and low cost
    • Small footprint (i.e it's inductorless)


    • use for alkaline battery such as a remote

Low Droput (LDO) Linear regulator

  • Efficiency
    • n= vout/vin*100, where n is power efficiency
    • Note: efficiency of LDO does not depend on load current!


  • high noise rejection
    • metric PSRR
  • low cost
  • small footprint

load range up to 200 ma


    • keep all passives closer to the ICs pins.
    • wide and short power planes
    • multiple stitching vias for ground planes
    • at least two ground vias per input and output bulk capacitors.


    • power sensitive analog rails that requires low noise
    • power low power digital circuits

Summary & Conclusion

    • understand working principles of SMPS, switching caps, and LDO regulators
    • understand layout rules for these type of voltage regulators
    • understand trade-offs and applications of these type of voltage regulators

We see that that the working principle of these common type of regulators following simple rule of circuit analysis. The design of a regulator is always based on the load requirement as well as cost and board areas. A good designer chooses the most appropriate regulator for the design with best trade-offs.

Reference and Further Reading

"Power Supply Layout and EMI", https://www.analog.com/media/en/technical-documentation/application-notes/an139f.pdf