Boost Converter
Let’s start with a short inductor review:
Faraday’s Law drives inductors: current through a wire creates a magnetic field.
The voltage across an inductor is V= L ⋅ (dI/dt), proportional to the rate of current change and inductance (L).
Faster current shifts result in a bigger voltage spikes
If there is steady current (rate of change is 0), the output voltage from an inductor = 0
In the layout of a boost converter the voltage applied to the inductor is equal to the Vin, which results in a linear current rise that builds the magnetic field.
Typically a coil, an inductor stores energy in its magnetic field, often enhanced by a ferromagnetic core like ferrite—though air-core versions exist too
Boost Converter Design
Hand Calculations Passive Components
At work I am used to high-power equipment, so for this boost converter, I chose 5 V in, 12 V out, and 20 A for 240 W of power output. Big designs like this punish sloppy math, and my first attempt proved it. I picked unrealistically tight ripple targets and messed up the input current, using Vout in the denominator Vin. This resulted in a on oversized capacitor by a factor of 10, which did not produce a pretty output.
To fix that I used some common percentages—1% voltage ripple (0.12 V), 30% current ripple, and calculated a 53.33 A input current. With 90% efficiency my spice model output came together.
LTspice Model
In designing my boost converter circuit, I encountered a few practical challenges that underscored the importance of component selection and compatibility. One oversight occurred when I initially entered the capacitor’s equivalent series resistance (ESR) as 0.02 ohms instead of the intended 0.002 ohms. This seemingly small error significantly amplified the output voltage ripple, as ESR directly influences how effectively the capacitor smooths the boosted voltage. Correcting this brought the ripple under control, but I noticed the average output voltage remained slightly above my 12V target. To address this, I reduced the switch’s duty cycle from 61% to 58%, which aligned the output closer to 12V without noticeably affecting the ripple amplitude.
I was able to reduce the duty cycle partly because I used an inductor larger than the ideal calculated value. A bigger inductor stores more energy in its magnetic field, which let it keep boosting the voltage even with a lower duty cycle. This ties into a core idea with boost converters: you don’t want the inductor’s magnetic field to fully collapse when the switch is off, since it needs enough energy to add to the next cycle to increase the voltage. The output voltage settles when the energy going into the inductor matches what’s delivered to the load and capacitor (minus losses, of course). This shows how critical it is to pick components that work together, especially since the switching frequency heavily influences the inductance needed to avoid dropping into discontinuous mode.
For the diode, I opted for a Schottky type due to its efficiency advantages, well-suited to my moderate output voltage. Unlike traditional PN junction diodes, Schottky diodes feature a metal to N-type semiconductor junction, reducing the forward voltage drop to ~0.3V (vs. 0.7V for PN diodes) and eliminating recovery time by relying solely on majority carriers (electrons). This enables faster switching and lower power losses—critical for a boost converter’s high-frequency operation. The trade-off is an increased reverse leakage current when reverse-biased, due to the reduced Schottky barrier height, though this posed no significant issue for my 12V design. In applications with significantly higher output voltages, however, a different diode or an active switch might be necessary to handle reverse bias conditions.
This experience reinforced how interconnected component choices are in power electronics. From capacitor ESR to inductor sizing and diode characteristics, each decision ripples through the system, shaping efficiency, stability, and performance.
Vout, V at Inductor, Current through diode, Current though Inductor
To break down my boost converter’s operation, let’s define two phases: T1, when the switch is on and the inductor current (I_L1, blue trace) rises, and T2, when the switch is off and the current falls.
T1: Switch On
During T1, the input voltage (Vin) drives the inductor (L), causing its current to increase linearly as the magnetic field builds. This follows V = L * (di/dt), visible in the blue trace’s upward slope.
When the switch is closed, the voltage at the switch node (Vn002, green trace) falls to near zero because of the minimal resistance in the switch and inductor, while the voltage across the inductor remains equal to Vin.
Meanwhile, the capacitor (C) sustains the output voltage (Vout) by supplying current to the load (R).
You’ll notice a slight dip in the Vout trace, evidence of voltage ripple as the capacitor discharges.
The diode, reverse-biased here, blocks any backflow from the output to ground, as confirmed by the red trace showing zero diode current.
T2: Switch Off
When the switch opens in T2, the inductor’s magnetic field collapses, driving current through the diode to the capacitor and load at a steadily declining rate. This induces a voltage across the inductor (V = L * (di/dt)), which adds to Vin to produce the “boosted” Vout.
The green trace Vn002 spikes as the inductor’s voltage reverses polarity (voltage adds to Vin), driving the boost effect. This process transfers energy to the capacitor, which stores it to stabilize Vout, smoothing the output and supplying current to the load.
The red trace now shows forward biased diode current flowing, matching the inductor’s discharge.
These phases highlight how the inductor and capacitor work together: L stores energy in T1, then releases it in T2, while C ensures a stable output.
Websites and References Used:
