Switching Power Supplies

Switch-mode power supplies (SMPS), also called switching power supplies, have become more popular than linear power supplies in the past ten years because they provide a regulated voltage with more efficiency and they don't require the larger transformers and filtering devices that the linear power supplies require. For example, the linear power supplies generally have average conversion efficiencies of 30%, while the SNIPS have efficiencies up to 80%. Since the SMPS don't need the larger components, they are more usable in modern circuits where cabinet space and board space is at a premium. Designers are continually trying to reduce the size and weight of electronic controls, and one easy way has been to change to SMPS.

Fig. 1 shows an electrical block diagram of a switch-mode power supply. This diagram will aid in understanding how the SMPS converts a dc input voltage to a new value of dc voltage that is filtered and regulated. The first block of the power supply is called the rectifier and filter section and it's shown in the diagram as a diode and capacitor, indicating the ac voltage is rectified to pulsing dc and then filtered to reduce the amount of ripple. The second block in the diagram shows the symbols of a MOSFET and bipolar transistor. This section is called the high-frequency switching section and it uses either MOSFETs or bipolar transistors to convert the dc voltage to a high-frequency ac square wave. The high-frequency ac square wave can be 20-100 kHz. The incoming ac voltage is rectified to dc and then the high-frequency switching section changes it back to ac for several reasons. First, the incoming voltage is always fluctuating and it's full of transient voltages that can be damaging to solid-state components if they are allowed to reach them. The two-step conversion helps to isolate these fluctuations and transients. The second reason: higher frequencies allow for higher conversion efficiencies.

Above: Fig. 1: Block diagram of a switch-mode power supply (SMPS).

The next section of the SMPS is the power transformer section. The power transformer isolates the circuits and steps up or steps down the voltage to the level required by the dc voltage. The output of the transformer is sent to a second rectifier section. Since the first rectifier section was for the input voltage, it's called the input rectifier, and since the second rectifier is used for supplying output voltage, it's called the output rectifier section. The output rectification section is different from the input rectifier in that the frequency of the voltage in the second section will be very high (20-100 kHz). This means that the output ripple of the high-frequency voltage will be nearly filtered naturally because of the number of overlaps between each individual output pulse. Since the ripple is very small, the actual capacitors in the filter section will be rather small.

The final section of the SMPS is the control and feedback block, which contains circuitry that provides pulse-width modulated output signal. The pulse-width modulation provides a duty cycle that can vary pulse by pulse to provide an accurate dc output voltage. This block of the power supply uses an operational amplifier to compare the output voltage to a reference voltage and continually make adjustments to the output voltage. The oscillator in this circuit provides the frequency for the duty cycle.

The final three blocks (power transformer, output rectification, and control/feedback) can have different circuitry called topology that is simpler or more complex than this example. Each of these topologies have advantages and disadvantages. The more common topologies will be presented in the next section.