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AC-DC flyback power supplies have a very simple circuit configuration that consists of a minimal part count. They are suitable for low-power power supplies up to 100W.
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When the switch is closed, the primary inductance of a transformer stores energy. When the switch is opened, the stored energy is transferred from the transformer to the output load. Therefore, the output voltage is not a function of the transformer turns ratio.
The output voltage is determined by the on/off ratio of the transistor, the primary inductance of the transformer, and the load resistance (R):
where, Lp is the primary inductance, Ton and Toff are the "on" and "off" periods of the transistor, and R is the load resistance.
A flyback converter consists of a small number of parts. However, since the core size must be increased to obtain high power output, flyback converters are utilized only for low-wattage power supply applications.
Compared with a forward converter, a flyback converter can dispense with an output inductor and a flywheel diode, making it suitable for low-cost power supplies.
Since the flyback converter does not have an output inductor, it provides excellent load transient response.
For high-frequency switching, a small, lightweight transformer can be used.
When Q1 is turned on, energy is stored in the primary winding. When Q1 is turned off, it is transferred to the secondary winding and supplies the output load.
A voltage (Vdc) is applied across LP.
The primary current in the transformer increases as shown according to its current gain.
where, VDS1 is the Q1 voltage, and Lp is the primary inductance.
Hence, the peak current at the end of the "on" state is calculated as follows:
The electromagnetic energy stored in the primary winding is calculated as:
E=Lp × Ip2 /2
The voltage polarity of the secondary winding reverses
As a result, the energy stored in the secondary winding is released. At this time, the secondary current linearly decreases as shown in the following equation:
dIs/dt=VOUT/Ls where, Ls is the secondary inductance.
If Is becomes zero before Q1 is turned on again, all the energy stored in the primary (E) is delivered to the output load. The power supplied to the output load is calculated as follows:
P=E/T＝［(VS-VDS1)Ton］2/(2T×Lp) ≈ (VS×Ton)2/(2T×Lp)
Then, since the power delivered to the load is Vo2/RL, VOUT is calculated as follows:
(VS×Ton)2 / (2T/Lp) = VOUT2/RL
VOUT = (VS×Ton)√［RL/(2T×Lp)］