In the EV industry, power supply devices such as on-board chargers, DC-DC converters, and DC-AC inverters are essentially switching power supply products. Focusing on switching power supplies, I will launch a 3-part series to systematically break down the full process of switching power supply circuit analysis and debugging—from an in-depth analysis of underlying working principles to a detailed explanation of key testing methods, aiming to cover the core knowledge system of switching power supply technology.
As the opening article, this piece will focus on the Buck converter, one of the three basic topologies (Buck, Boost, and Buck-Boost) of switching power supplies.
These three topologies are the atomic structures of switching power supplies. Their importance lies not only in enabling engineers to quickly decouple complex circuits (e.g., the flyback topology is equivalent to a Buck-Boost converter plus a transformer) but also in laying a solid theoretical foundation for core engineering tasks such as power efficiency optimization and electromagnetic compatibility (EMC) design. Skipping basic topologies to directly study isolated topologies often leads to the cognitive misunderstanding of "focusing only on circuit topology while ignoring the essence of energy conversion".
1. Voltage Reduction via Resistor Voltage Division
For those with basic circuit knowledge, the voltage division characteristic of series resistors (derived from Ohm’s Law) can be used to achieve voltage reduction. Let us first review the relevant fundamentals.
Based on Ohm’s Law:Uout=Uin×R1+R2,R2=5V
However, voltage reduction using resistor voltage division has the following drawbacks:
1. Low efficiency and severe energy waste
2. Unstable output voltage with poor load adaptability
3. Limited current-carrying capacity, creating a trade-off between heat dissipation and cost
· To reduce power consumption, the resistance value of voltage-dividing resistors needs to be increased (e.g., to the kΩ range), but this limits the output current to the μA~mA level, making it unable to drive power loads such as motors and LEDs.
· Forcing higher current output (by reducing resistance) leads to a sharp increase in power consumption: for example, when the output current is 1A, the power consumption of the voltage-dividing resistors can exceed 10W, requiring expensive, bulky power resistors paired with heat sinks.
4. Narrow application scope, suitable only for signal sampling and reference voltage generation.
2. Design of the Buck Converter
In applications where voltage reduction is achieved through the periodic switching action of power devices, PWM duty cycle is defined as the ratio of the on-time of the switch to the total period of one switching cycle.
We aim to output a stable 5V voltage, but the actual output waveform deviates from the ideal waveform.
When the switch (MOSFET) turns on or off rapidly, the voltage at circuit nodes changes drastically (e.g., jumping from 0V to 12V instantaneously). This abrupt change generates high-frequency oscillations and voltage spikes. How can we filter out these unwanted voltage components?
When it comes to filtering, capacitors are the first components that come to mind. Connecting a capacitor to the circuit should theoretically play a filtering role. However, the actual output waveform still fails to meet the requirement of a stable output.
In practical scenarios, the output current of the circuit is as follows. Why is this the case?
Due to the characteristic that voltage across a capacitor cannot change instantaneously, a capacitor behaves like a short circuit at the moment of power-on, resulting in a large inrush current that can even damage the MOSFET.
The first intuitive solution is to connect a resistor in series to provide a path for energy dissipation.
This method can indeed achieve a 5V output, but the introduction of a resistor brings a new problem: power loss.
Is there a component that can limit current without consuming energy? The answer is an inductor. Simply replacing the resistor with an inductor enables voltage stabilization.
But the inductor current cannot change abruptly, limiting the capacitor’s charging current. The resulting current waveform is as follows:
When the MOSFET turns off, the path through the MOSFET is interrupted, leaving one end of the inductor "floating". However, since the inductor current cannot change abruptly, a freewheeling path must be provided to maintain current flow; otherwise, the inductor will generate a high voltage due to the sudden current change, damaging circuit components.
Freewheeling via Diode
A diode provides a freewheeling path to dissipate the energy stored in the inductor and prevents reverse current from flowing back through the MOSFET, which would cause additional power loss.
With this configuration, the design of a complete Buck converter is finalized.
3. Step-Down Principle of the Buck Converter
The Buck converter, commonly referred to as a step-down converter, features DC input and DC output. It does not incorporate a transformer, meaning the input and output share a common ground.
1. When MOSFET Q is Turned On
The current flow is shown by the red arrow in the diagram.
Current flows from the positive terminal of the input power supply Vin, through the MOSFET Q, and then through the inductor L. At this time, an induced electromotive force with left positive and right negative polarity is generated across the inductor, which stores energy. The current then supplies power to the load Vout, and simultaneously, Vin charges the capacitor C. Finally, the current returns to the negative terminal of Vin. During this phase, the diode D has no current flowing through it and remains in the off state.
2. When MOSFET Q is Turned Off
The current flow is shown by the arrow in the diagram.
A reverse electromotive force with left negative and right positive polarity is induced across the inductor L. At this point, the inductor releases the stored energy, and the current forms a closed loop through the load Vout and the diode D. The diode D plays the role of a freewheeling diode in this phase. Meanwhile, the capacitor C discharges to supply power to the load Vout.
4. Duty Cycle
What is Duty Cycle?
Duty cycle is a key parameter describing the characteristics of periodic signals. It is defined as the ratio of the duration of the high level (Ton) to the total period of the signal (T). The formula is as follows:
For example, if the high-level duration is 3 seconds and the signal period is 10 seconds, the duty cycle is 30%.
Inductors and magnetic cores are all magnetic materials. A hysteresis loop is a closed magnetization curve that describes the hysteresis phenomenon of ferromagnetic materials when the magnetic field strength changes periodically. It represents the relationship between magnetic flux density B and magnetic field strength H.
Volt-Second Product
The volt-second product refers to the product of the voltage V across an inductor and the duration T of that voltage.
Volt-Second Balance
Volt-second balance is a principle commonly applied in switching circuits. When a switching circuit operates stably, the total change in inductor current over one switching cycle is zero. In other words, the increase in inductor current during the switch-on phase is equal to the decrease in current during the switch-off phase, expressed as:△i*Ton=△i*Toff
From the inductor voltage formula:V=L*di/dt=L*△i/△T
We can derive the volt-second balance equation:Von×Ton=Voff×Toff
In a Buck converter, the magnetic core is magnetized when the switch is on, causing the inductor current to rise. When the switch is off, the magnetic core is demagnetized, and the inductor current decreases.
As seen in the figure above, volt-second balance enables the derivation of the output voltage-duty cycle relationship:Based on volt-second balance, we can further derive the relationship between output voltage and duty cycle:
(Vin-Vo)*Ton=Vo*Toff;
Vo=Vin*D→D=Vo/Vin。
Actual Duty Cycle
D1=Ton/T
· Ton = switch-on time
· T = switching period
· Theoretically, the duty cycle of a Buck converter can vary between 0% and 100%.
· Note: The actual duty cycle is slightly larger than the theoretical duty cycle. This is because the theoretical calculation does not account for the voltage drops across the MOSFET during conduction and the freewheeling diode during operation.
