Buck and boost converter topologies
The two fundamental building blocks of every DC/DC converter on the car.
You'll learn
- Explain buck and boost converter topologies at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
Almost every DC/DC conversion in an EV, from stepping the HV bus down to 12 V to boosting a charger's rectified voltage, is built from combinations of two simple circuits: the buck and the boost.
In depth
Buck vs. boost
| Buck (step-down) | Boost (step-up) | |
|---|---|---|
| Output vs input voltage | Lower | Higher |
| Switch position | In series with the load | Across the input, with a diode/switch to the output |
| Energy storage element | Inductor smooths current to load | Inductor stores energy each cycle, dumps it at higher voltage |
| Automotive example | HV-to-12V DC/DC | 800V battery boosting to charge on a 400V charger |
Formula
Buck: Vout = D × Vin ; Boost: Vout = Vin ÷ (1 − D)
D is duty cycle, the fraction of each switching period the main switch is on. A buck's output is always below input; a boost's is always above.
Example: A boost converter at D = 0.5 doubles its input voltage.
Key takeaways
Remember this
- A buck converter always steps voltage down; a boost always steps it up.
- Both rely on an inductor storing and releasing energy each switching cycle.
- Isolated automotive converters add a transformer on top of these basic principles.
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