Power Electronics / Power Management


The switching power supply: a primer

31 May 2006 Power Electronics / Power Management

The switching power supply has replaced the linear power supply gradually. It is widely used in all kinds of electronic equipment, including module type sources, driver type sources, UPS and battery chargers. Applications of switching power supplies can be found in many industrial and commercial areas.

Switching power supplies are a large improvement on the high volume, heavy weight and low efficiency of linear power supplies. The principle of operation of the switching power supply is shown in Figure 1. It connects directly from the AC power (off-line) without going through the low-frequency transformer. Because of the high input AC voltage, the buck capacitor can be smaller. Switching power supplies use high-frequency transistors to chop the high DC voltage (rectified from AC source) into a high AC voltage and then convert it into the rated voltage through the high frequency transformer. It is then rectified into the rated DC voltage. Due to the high frequency switching, there are ripple noises generated from the switching transients, which need to be taken care of.

Figure 1. Principle of operation of a typical SMPS
Figure 1. Principle of operation of a typical SMPS

Switching power supply circuit theory

Among all the DC-to-DC converters, SMPS (switch mode power supplies) can be divided into three basic circuit structures based on input voltage, output voltage and polarity:

* Step-down or buck converter: used for output voltage lower than input voltage.

* Step-up or boost converter: used for output voltage higher than input voltage.

* Inverter or buck-boost converter: the third one is used when the output polarity is inverted from the input. This kind of circuit can also be used in both step-up and step-down conditions.

If we need to isolate the input and output, the above three basic circuits cannot be used any more. Instead we must convert these three types to Forward type, Flyback type, Half-bridge type, Push-pull type, or Full-bridge type circuit structures. There are two ways to generate the switching signal: one is the self-oscillation circuit where its frequency is decided by output load and input voltage; the other is the pulse-width modulator (PWM) IC where its frequency is decided by the control IC.

Non-isolated types

* Buck regulator: when the switch is ON, power is transferred to the load through L1 and also stored in L1 at the same time. When the switch is OFF, power will be supplied by L1 and transferred to the load through D1 and L1.

Figure 2. Buck regulator
Figure 2. Buck regulator

* Boost regulator: when the switch is ON, power is stored in L1. When the switch is OFF, power will be transferred to the output load through L1 and D1. Output voltage can be higher than the input because of the pre-stored voltage at L1.

Figure 3. Boost regulator
Figure 3. Boost regulator

* Buck-boost regulator: when the switch is ON, the power is stored in L1. When the switch is OFF, the power is transferred to the output load through L1 and D1.

Figure 4. Buck-boost regulator
Figure 4. Buck-boost regulator

Isolated types

* Flyback converter: when the mosfet (switch) is ON, the power is stored in the transformer, and when the mosfet is OFF, the power is transferred to output load from the transformer.

Figure 5. Flyback converter
Figure 5. Flyback converter

* Forward converter: when the mosfet (switch) is ON, the power is transferred to the output and stored in L1 through D1 and the transformer. When the switch is OFF, the power stored in L1 will be transferred to the load through D2.

Figure 6. Forward converter
Figure 6. Forward converter

* Push-pull converter: when switch 1 is ON (switch 2 is OFF), power is transferred to the output load through the transformer and D1. When switch 2 is ON (switch 1 is OFF), power is transferred to the output load through the transformer and D1.

Figure 7. Push-pull converter
Figure 7. Push-pull converter

* Half-bridge converter: when switch 1 is ON (switch 2 is OFF), power is transferred to the output load through the transformer, C2, and D1. When switch 2 is ON (switch 1 is OFF), power is transferred to the load through the transformer, C1, and D2.

Figure 8. Half-bridge converter
Figure 8. Half-bridge converter

* Full-bridge converter: when switches 1 and 4 are ON, switch 2,3 are OFF), power is transferred to the output load through the transformer and D2. When switches 2 and 3 are ON (switches 1 and 4 are OFF), power is transferred to the load through the transformer and D1.

Figure 9. Full-bridge converter
Figure 9. Full-bridge converter

The characteristics of each type are shown in Table 1. For assistance and advice about your power supply requirements, contact your local MeanWell representative.

Table 1. Characteristics of each circuit type
Table 1. Characteristics of each circuit type



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Power efficiency and robustness in electronics design
Power Electronics / Power Management
Mouser Electronics recently announced a new eBook in collaboration with Analog Devices highlighting essential strategies for optimising power systems.

Read more...
USB Type-C-powered controllers
Future Electronics Power Electronics / Power Management
Diodes Incorporated has released two USB Type-C PD 3.1 extended power range sink controllers that can be embedded into battery-powered devices.

Read more...
Multicell battery monitoring
Altron Arrow Power Electronics / Power Management
The LTC6811 from Analog Devices is a multicell battery stack monitor that measures up to 12 series connected battery cells with a total measurement error of less than 1,2 mV.

Read more...
Full telemetry in tiny DC-DC converters
RS South Africa Power Electronics / Power Management
The FS160* series of µPOL DC-DC converters from TDK all offer full telemetry, provide increased performance, and are remarkable for extraordinary power density in the smallest sizes.

Read more...
Power IC supplies 1650 W
EBV Electrolink Power Electronics / Power Management
Power Integrations has announced a two-fold increase in power output from the HiperLCS-2 chipset with the new device now being able to deliver up to 1650 W of continuous output power.

Read more...
High-voltage step-down DC-DC converter
Altron Arrow Power Electronics / Power Management
The MAX17793 is a high-efficiency, high-voltage, synchronous step-down DC-DC converter with integrated MOSFETs operating over an input voltage range of 3 to 80 V.

Read more...
High-voltage contactors
RS South Africa Power Electronics / Power Management
TDK Corporation has announced two new additions to its high-voltage contactor portfolio for harsh environments: the HVC43MC with integrated mirror contact and the HVC45 with enhanced short-circuit current handling capability.

Read more...
Chokes rated at 36 A
RS South Africa Power Electronics / Power Management
TDK Corporation has launched the EPCOS SurfIND series, a new range of current-compensated ring core double chokes for high currents and surface mounting.

Read more...
RF arrestor provides robust protection
RFiber Solutions Power Electronics / Power Management
NexTek’s range of coaxial RF surge and lightning arrestors are designed and built to provide robust protection for any radio or coaxial RF transmission application.

Read more...
Wide-Bandgap Developer Forum 2025
Power Electronics / Power Management
To give designers the ultimate in design flexibility, the entire range of WBG power semiconductors will be provided including discretes, modules, and highly integrated solutions ranging from 40 V to 700 V for GaN and 400 V to 3,3 kV for SiC.

Read more...