Switch Mode Power Supply

Switch Mode Power Supply – An “Unconventional” Design from CET Technology

A typical Switch Mode Power Supply (SMPS) or switching supply has an input of ~120VAC to 240VAC and outputs a low DC voltage of say 5, 12 or 24V.  Recently, a major fire & security hardware manufacturer approached CET for a more “unconventional” supply that had a low input voltage of 24VAC and required high current, 2.5 amps at 5VDC output.  Unconventional, because most switching power supplies do not run in such low AC input voltage.

The need came from their unique application. The customer had to implement the functionality of media converters using 24VAC power found on existing analog cameras.  The location of the media converters with respect to the analog cameras can never be assumed near a standard AC outlet or DC source.

The media converters alter the analog picture data into encapsulated digital data for transmission over IP/Ethernet networks.  The analog signal is transmitted to the media converter using fiber optics. Once there, the media converter performed analog to digital conversion, along with the proper TCP/IP encapsulation, for packet data transmission.

The packet data can then be transmitted over any distance, reassembled, and converted back to analog with no loss in fidelity or quality.  The design and subsequent supply of this SMPS provided by CET Technology met the demands of the environmental and electrical specifications required for a stringent need within fire safety and security.

The part number of this power supply is CT-7186

How Switch Mode Power Supplies Work

A Switch Mode Power Supply (SMPS) or switching supply is an electronic power supply with an integrated switching regulator to convert AC power into DC. The SMPS takes power from the mains source and converts and transmits it to DC-powered electrical components. Switching supplies are extensively used in many industries across the globe due to their high efficiency and low power consumption.

Linear power supplies are good for low-power devices such as cordless phones and video game consoles. Although new video game consoles demand higher power so Switch Mode Power Supply is being embedded in the latest generation consoles. There are several components that work simultaneously in switching power supply:

• The input comes from the AC power which goes through EMI filtering (bridge rectifier).
• There are two more capacitors on board
• A switching transformer is present in SMPS that converts the current.
• Output filter coil
• Output filter capacitors
• PWM controller chip
• One or more switches (FETs). This is where the switch-mode power supply gets its name.
• There are several other input connectors which are connected to the power input and are not part of the main board.
• There are two large heatsinks, one for primary and one for secondary. A few other small heatsinks are also onboard in high-end PSUs which cool down MOSFETs.

Most switch-mode power supplies come with active Power Factor Correction (PFC) while simple PSUs exclude PFC to cut costs. SMPS takes the input voltage (110VAC to 240VAC) which goes through transient filtering and skips voltage doubler. The current then passes through a rectifier then a switching stage; the power then goes through a transformer that converts the current to DC voltage and passes through a rectifier again. Lastly, the filtered current goes out to the electronic component that requires power. The process is regulated through a PWM controller on board.

Where a linear power supply can only operate over a narrow input voltage range (such as 120V±10%), Switch Mode Power Supply can tolerate a wide range of input voltage (90-265VAC or higher) without damaging the electronics, allowing a single power supply to be plugged in anywhere in the world. Also, circuitry is present to protect the supply.  For instance, if there is a fault condition that overloads the power supply such as a short circuit, it shuts down automatically until the fault is removed.

 

Switch Mode Power Supply Applications

The most widely used applications of SMPS are in the computer hardware industry, medical equipment, mobile phone chargers, automobiles, consumer electronics, and lighting. In third world countries with power outages, a switch-mode power supply is used in Uninterruptable Power Supplies (also known as UPS and inverters). This helps take power from 12v or 24v batteries and convert it to AC current which is used to power household items such as lights, fans, home appliances, etc. Switch-mode power supplies can be external, such as with laptop PCs or integrated into the electronic device such as LED TVs, game consoles or home theater systems.

 

 

Switch Mode Power Supply vs. Linear Power Supply

The biggest advantage of SMPS is that it wastes less power in the form heat compared to linear power supplies. High-performance power supplies have >80% efficiency up to 95% which is extraordinary compared to linear power supplies in the same power range. Many computer PSU manufacturers have introduced fanless operations in their SMPS. When the PC is in idle state it does not take more than 20% of power from the PSU as a result, the fan inside the power supply is switched off. There are six types of 80+ certifications on PC PSUs:

1. 80 Plus: 80% power efficiency at 100% 115v load.
2. 80 Plus Bronze: 81% power efficiency at 100% 230v load.
3. 80 Plus Silver: 85% power efficiency at 100% 230v load.
4. 80 Plus Gold: 88% power efficiency at 100% 230v load.
5. 80 Plus Platinum: 91% power efficiency at 100% 230v load.
6. 80 Plus Titanium: 91% power efficiency at 100 230v load. The highest efficiency is 96% at 50% load.

SMPS are even used on the International Space Station (ISS) to convert the voltage produced by the solar array and battery system because different voltages are required by different equipment on-board.

SMPS holds plenty of advantages over linear power supplies. SMPS is much lighter in weight and smaller in size. Although some very high-end PC power supplies can weigh up to 10 kg but compared to linear supplies it is still significantly low.

Switch mode power supplies can tolerate more voltage variations of input before the voltage output. So users can expect steady voltage being delivered to their components. Secondly, SMPS have higher efficiency compared to linear supplies and better heat and power dissipation. In terms of complexity, switched-mode mains (AC-to-DC) supplies, several voltages can be generated by one transformer core, but that can introduce design and use problems: for example, it may place minimum output current restrictions on one output. So technically, SMPS has less complex structure which results in low repair costs.

For PC users, the biggest concern is noise and heat. High-end power supplies with silent operation ensure noise reduction even at high loads. Heat dissipation is already very good with switch mode power supply. Lastly, SMPS have many built-in protective circuits to address issues such as over-voltage protection, low-voltage protection and many more.

 

International Regulations

The growth in the use of switch mode power supplies has been driven by an ever-increasing number of laws relating to the no-load power draw of external supplies.  In 2004, the California Energy Commission enacted the first standards.  More recently, the European Union has adopted Level 5 standards and the US Department of Energy will enact Level VI starting February of 2016.  As such, original equipment manufacturers must ensure they specify power supplies in accordance with these new standards.  CET Technology can assist you in the design and manufacture of this new class of switching supplies.

 

About CET Technology

CET Technology specializes in the design and manufacture of standard and custom magnetics.  CET manufactures a full line of ROHS compliant components including electrical transformers, power supplies, inductors, coils, chokes, and DC/DC converters.  They use the latest manufacturing techniques to guarantee lower production costs with excellent repeatability and performance.