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). 

Output range: 0 to infinity.

Power Losses

Three common factor of power losses:

Conduction loss

Switching loss

AC core loss


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

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.

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.



Low Droput (LDO) Linear regulator 

Working principle: this linear device operate PMOST in it's ohmic or linear region by adaptively change its internal on-state resistor (Rdson) through negatvie feedback network as shown to the right. Hence the transistor regulates the voltage from input to output by comparing to the reference voltage source.

Efficiency Equation


load capability typically can handle current up to 200 mA



Summary & Conclusion

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