The flyback converter is the isolated version of the buck-boost converter. It is safe to call it the "absolute monarch" of low-power, small-capacity power supplies under 100W, as it is widely used in our daily lives. In fact, it's the very circuit powering the smartphone charger I am using right now.
Beyond the theory, here are some practical questions I had about this circuit and the answers I discovered.
1. Why Bother with "Isolation"?
The biggest reason is Safety. It physically separates the ground (GND) of the primary side (AC input) from the secondary side (DC output). Thanks to this isolation, we don't get electrocuted by the 220V AC mains even if we touch the end of a charging cable or the secondary circuit bare-handed.
2. Why is it the "King" Despite High Voltage and Current Stress?
A distinct disadvantage of the flyback converter is the massive voltage stress across the switch when it turns off, reaching Vin + nVo. Nevertheless, it dominates the market because of its overwhelming economic efficiency.
[1]Reduced Component Count and Footprint: A single transformer acts as both a coupled inductor and a voltage transformer. With fewer components, manufacturing costs drop, and charger sizes can be radically miniaturized.
[2]Universal Input Support: Because it is based on the buck-boost topology, you can control a vast range of input voltages simply by adjusting the duty cycle (D) and the turns ratio (n:1). This flexibility is why a charger used in Korea (220V) can be plugged straight into an outlet in Japan (100V) with just a simple plug adapter.
[3]Easy Multi-Output: Adding another winding to the secondary side instantly creates a multi-output system. A single charger can simultaneously output various voltage lines, such as 5V and 20V.
3. What is Reflected Voltage (VR) and Why Set it to 100V?
In a flyback converter, when the primary main switch turns off, the secondary diode turns on, transferring energy to the output. At this moment, the voltage across the secondary winding (output voltage Vo + diode forward voltage VF) is reflected back to the primary side according to the transformer turns ratio. This is called the Reflected Voltage (VR).
- If VR is set too high: The primary switch must handle a higher voltage, requiring a bulkier and more expensive component.
- If VR is set too low: The primary voltage stress decreases, but the voltage stress on the secondary diode significantly increases, leading to higher conduction losses.
Therefore, in universal designs using 220V commercial AC power, setting VR around 80V ~ 120V (typically 100V) is the industry's "golden rule" for utilizing the most cost-effective and stable components.
4. The Secret Behind the 650V Power Semiconductor Market
Understanding the flyback converter's topology reveals why so many semiconductor companies worldwide risk everything to develop switches (MOSFETs, GaN, etc.) with a "650V rating."
The universal adapters we use are designed to support up to 264V (accounting for a 10% voltage fluctuation in 240V countries). When this AC voltage passes through a bridge diode and charges the smoothing capacitor, it jumps by a factor of sqrt{2} to reach its peak.
- Vin,max = 264 * sqrt{2} = 373V
Now, let's add up all the voltages across the switch the moment it turns off:
- Max DC Input Voltage: 373V
- Reflected Voltage (VR): 100V
- Snubber Voltage (due to leakage inductance): Approx. 50V
The sum is approximately 523V. In practice, engineers apply a derating margin to protect the component, typically operating it at 80% of its maximum limit. Thus, the required voltage rating for the switch is calculated as 523V / 0.8 = 653V.
As a result, switches capable of withstanding around 650V fit perfectly into the flyback converters that dominate our daily lives. Because the market share is so massive, any company that produces a smart, highly efficient 650V switch will ultimately win the power semiconductor market.
Massive power transmission systems are incredible, but dissecting the hidden world of power conversion inside electronics smaller than the palm of my hand is quite a thrilling joy as an engineering student!
