There are three basic switching regulator configurations from which the majority of present day circuits are derived:
- Step-down, or “buck”, regulator.
- Step-up, or “boost”, regulator.
- Inverting, or “flyback” regulator (which is a variation of the “boost” regulator).
The Step-Down Regulator
Figure 6-2 illustrates the basic step-down or “buck” regulator. The output voltage of this configuration is always less than the input voltage.
In the buck circuit, a semiconductor switch is placed in series with the de input from the input rectifier/filter circuit. The switch interrupts the de input voltage providing a variable-width pulse to a simple averaging LC filter. When the switch is closed, the de input voltage is applied across the filter and current flows through the inductor to the load. When the switch is open, the energy stored in the field of the inductor maintains the current through the load.
In the buck circuit, peak switching current is proportional to the load current. The output voltage is equal to the input voltage times the duty cycle.
VO = VI x Duty Cycle
The Step-Up Regulator
Another basic switching regulator configuration is the step-up or “boost” regulator (Figure 6-3). In this type of circuit, the output voltage is always greater than the input voltage.
The boost circuit first stores energy in the inductor and then delivers this stored energy along with the energy from the dc input voltage to the load. When the switch is closed, current flows through the inductor and the switch, charging the inductor but delivering no current to the load. When the switch is open, the voltage across the load equals the dc input voltage plus the charge stored in the inductor. The inductor discharges, delivering current to the load.
The peak switching current in the boost circuit is not related to the load current. The power output of a boost regulator can be determined by the following equation:
The Inverting or Flyback Regulator
The third switching regulator configuration is the inverting or “flyback” regulator. This circuit is a variation of the step-up or “boost” circuit discussed previously. The flyback circuit is illustrated in Figure 6-4.
Flyback regulators, which evolved from “boost” regulators, deliver only the energy stored by the inductor to the load. This type of circuit can step the input voltage up or down. When the switch is closed the inductor is charged, but no current is delivered to the load because the diode is reverse biased. When the switch is open the blocking diode is forward biased and the energy stored in the inductor is transferred through it to the load.
The flyback circuit delivers a fixed amount of power to the load regardless of load impedance. It is widely used in photo flash, capacitor-discharge ignition circuits, and battery chargers.
To determine the output voltage of an electronic equipment supply, the load (RL) must be known. If the load is known, the output voltage may be calculated using the following equation:
The inductor current is proportional to the “on time” (duty cycle) of the switch and regulation is achieved by varying the duty cycle. However, the output also depends on the load resistance (which was not true with the step-down circuit).
Transient response to abrupt changes in the load is difficult to analyze. Practical solutions include limiting the minimum load and using the proper amount of filter capacitance to give the regulator time to respond to this change. Flyback type circuits are used at power levels of up to 100 W.