Basic topology of common switching power supplies
1. Buck step-down
Drop the input to a lower voltage.
Probably the easiest circuit.
An inductor/capacitor filter smoothes the switched square wave.
The output is always less than or equal to the input.
The input current is discontinuous (chopped).
The output current is smooth.
2. Boost
Raise the input to a higher voltage.
Same as buck, but rearranged inductor, switch and diode.
The output is always greater than or equal to the input (neglecting the forward voltage drop of the diode).
The input current is smooth.
The output current is discontinuous (chopped).
3. Buck-Boost buck-boost
Another arrangement of inductors, switches, and diodes.
Combining the disadvantages of buck and boost circuits.
The input current is discontinuous (chopped).
The output current is also discontinuous (chopped).
The output is always the opposite of the input (note the polarity of the capacitor), but the magnitude can be smaller or larger than the input.
A "flyback" converter is actually a buck-boost circuit isolation (transformer coupled) form.
4. Flyback flyback
Works like a buck-boost circuit, but the inductor has two windings and acts as both a transformer and an inductor.
The output can be positive or negative, determined by the polarity of the coil and diode.
The output voltage can be greater or less than the input voltage, determined by the turns ratio of the transformer.
This is the simplest of the isolation topologies.
Multiple outputs can be obtained by adding secondary windings and circuits.
5. Forward
A transformer-coupled form of a step-down circuit.
Discontinuous input current, smooth output current.
Because of the transformer, the output can be larger or smaller than the input, and can be of any polarity.
Multiple outputs can be obtained by adding secondary windings and circuits.
The transformer core must be demagnetized during each switching cycle. A common practice is to add a winding with the same number of turns as the primary winding.
The energy stored in the primary inductance during the switch-on phase is released through the additional winding and diode during the switch-off phase.
6. Two-Transistor Forward
Both switches work simultaneously.
When the switch is off, the energy stored in the transformer reverses the polarity of the primary, causing the diode to conduct.
Key advantages: Voltage on each switch never exceeds input voltage; no need to reset winding tracks.
7. Push-Pull
The switches (FETs) are driven out of phase and pulse-width modulated (PWM) to regulate the output voltage.
Good transformer core utilization - power is delivered in both half-cycles.
Full-wave topology, so the output ripple frequency is twice the transformer frequency.
The voltage applied to the FET is twice the input voltage.
8. Half-Bridge
Very common topology for higher power converters.
The switches are driven out of phase and pulse width modulated to regulate the output voltage.
Good transformer core utilization - power is delivered in both half-cycles. And the utilization of the primary winding is better than that of a push-pull circuit.
Full-wave topology, so the output ripple frequency is twice the transformer frequency.
The voltage applied across the FET is equal to the input voltage.
9. Full-Bridge
Most common topology for higher power converters.
The switches are driven in diagonal pairs with pulse width modulation to regulate the output voltage.
Good transformer core utilization - power is delivered in both half-cycles.
Full-wave topology, so the output ripple frequency is twice the transformer frequency.
The voltage applied to the FETs is equal to the input voltage.
At a given power, the primary current is half that of the half bridge.
