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Concept to Design to Manufacture of OEM power supplies

For every electrical application there must be a reliable power source. Each application has many variables, which must be taken into consideration during the concept phase. Many designers put a low emphasis on the power supply for new applications. To the contrary, the power supply is one of the most critical components of any electrical system. Should the power supply function incorrectly or worse yet stop functioning, the whole system stops.

We like to be involved from the beginning so that we can offer input and suggestions to insure that the right power supply is used in the right application. The all-important cost consideration plays directly into this. One of the critical portions of the design is the end market of the product. It is much less costly (in time and money) to consider this in the beginning and design around the issues. Some design considerations are size, cooling for the power supply, input voltages, output voltages, output regulation, regulatory issues, reliability, and always cost. For high quantity OEM applications off the shelf units are rarely attractive. In mass production the un-needed component or feature that costs $0.10 in quantities of 100,000 pieces adds up to $10,000. In today's marketplace with intense competition, this can make the difference between a go and no go situation.

Reliability is critical. The best solution is to always design around the worst case situations, whether it is ultra high or low input voltage or heat build up. If we can approach these issues from the beginning, the end product can function properly and economically.

Input Voltage

The input voltage is always a good place to start. The North American standard consumer requirement is typically 120 VAC +/- 10%. This gives you a good voltage range of 108 VAC to 132 VAC. Most well designed switching power supplies run from 100 VAC to 135 VAC and at 50 or 60 hertz. European applications typically run at 220 VAC although 240 VAC is also common. Areas with unusual voltage characteristics always present new and interesting challenges. Japan operates at around 100 VAC, requiring an operating input voltage as low as 89 VAC. Australia on the other hand has extreme surge and spike issues that must be addressed. It is important to evaluate and test your requirements in worst case scenarios, for example at the maximum operating temperature and highest input voltage. Good planning, from the beginning, keeps your power supply cost requirements low throughout you product life span.
Some additional issues of consideration are: Filtering of the input voltage can also be critical. In some situations excessive surge or transients can travel through the power supply and to the output. If the powered device is sensitive to these, they must be addressed from the beginning of the design phase. Surge current during start up can also become an issue if there are limitations to the voltages available. Rf and EMI issues must be addressed if the usage proximity of the power supply is near frequency sensitive equipment. Brown out or temporary loss of input voltage can also cause system to shut down, unless preventative measures are designed in.

Automotive applications require additional thought. Although most automotive situations are 12 volt DC, the constant voltage can range from 8 to 16 VDC. Special attention must be applied for those international applications that deal with positive ground or 24 VDC systems. Typically telecommunications equipment is very sensitive to Rf and EMI issues, the preventive filtering for these must be designed in during the concept phase.

Outputs

The power supply output is typically designed as the current and the voltage requirement. Most systems start with the voltage requirement, many of our systems require 12 volts DC. The next step is determining the current or amperage requirement. All power supply systems end up with a wattage rating. The wattage is established as output voltage X output current (amperage). For example a 12 VDC power supply rated at 5 amps is a 60-watt power supply. Cost is typically based on the output wattage.
The output voltage can float depending on the AC input voltage, in other words at a higher input VAC the output VDC is higher. Some systems require a constant output voltage, usually determined as a percentage independent of the input VAC. The most basic output range is +/- 10%, for example the 12 VDC output with a 10% regulation has a range of 10.8 to 13.2 VDC. In some situations the output may call for 1% regulation, for example 12 VDC output with 1% regulation has a range of 11.88 to 12.12 VDC. Of course the tighter the regulation the more the cost.
The output voltage will also move depending on the load or current requirements. This is especially true in "no load situations" or surge situations. In a no load situation, the power supply must realize there is no load and continue to monitor the output voltage to be sure it stays within the specified range. The opposite is also true, if the load changes from no current to maximum current, the output must continue to stay within regulation.

Consumer applications occasionally have short circuit situations. The most basic process for preventing damage to the power supply is to use a fuse. The problem with a fuse is it's only good once, after being shorted the power supply will not function without the replacement of the fuse. The electronic fold back design allows the power supply to cease output when short circuit and resume operation after the short is removed.

Thermal regulation is also important. Should the power supply overheat (due to internal or external issues) the use of thermistors in the circuitry will prevent a costly meltdown.
Multiple outputs can either be dependent or independent of each other, depending on the design requirements. Also, variable outputs can be used to control the powered device (for example speed or temperature regulation).

Output Ripple

Ripple describes the frequency variations of the input voltage. The ripple activates the transformer, it can be 50 Hz or 60 Hz. Switching power supplies can produce a much higher frequency due to the design of the circuit. Lower ripple usually means higher output inductance and higher capacitance, which slows the transient response. Depending on the system this should be specified as peak to peak.

Regulatory Approvals

North America usually requires UL and ULC or CSA approvals, or if it is an integrated component of a larger item it can be a UR or UL registered product. Each country/market has their own requirements. The lead time for regulatory approval can be as short as 3 weeks or as long at 6 months depending on the agency.

Mechanical Considerations

Weight is one of the primary considerations with power supplies. Switching power supplies are the lightest and typically the smallest. Linear power supplies cost less (up to a certain wattage) but are heavier and slightly larger. Stand alone power supplies usually have few size and weight considerations. Integrated power supplies must mount within a specified area and mount securely. In addition cooling issues must be addressed. Airflow is required to dissipate the heat build up within the power supply. Higher wattage power supplies may require forced air-cooling. In addition regulatory issues such as drop test, moisture / environmental, cord pull tests and accessibility must be taken into consideration.
Environmental

There are two main temperature ranges to be evaluated. The first is the operating temperature range. This is the temperature range the power supply is expected to operate within. The second is the storage temperature range. This is the temperatures the power supply may experience when not in use. All components and materials must be rated for both of these temperature ranges.
Humid conditions can be hazardous for power supplies. This specification is typically handled by the housing although there are other measures such as coatings that can handle non condensing moisture.
Altitude is called out primarily to insure that the components can handle pressure and cooling issues. Although rarely an issue, this can become important if the product is portable and may be air shipped in a non pressurized cabin.

Shock and vibration are extremely important specifications. The most basic shock test is the drop test. The drop test is as simple as it sounds. The unit is dropped from a predetermined height (varies according to weight) onto a hard wooded surface (or as predetermined). The drop can be one side or multiple sides and a corner or all. The device should continue to operate normally after the drop test. Vibration can be a bit more complex and requires additional equipment. Utilizing frequency at the various axis the device is tested to insure that it can meet shipping or expected hazards in the field.

Specification of MTBF is extremely difficult. Pure calculation can be done to project what the expected MTBF is. However this is a pure calculation and can not truly account for issues from the field. Actual MTBF can be determined by physically operating the power supply at maximum operating temperature and maximum voltage at maximum load, however this process can take several years. Combining the two above procedures usually creates fairly accurate results.
By design the power supply should be more than adequate for it's application. If this is the case, the power supply should meet or exceed its life expectation. The best way to insure this is through good manufacturing procedures. A burn in at the end of the manufacturing cycle should weed out any nonconforming components. Typically a burn in is done by cycling all production units at maximum operating temperature, maximum load and maximum input voltage for 24 hours.

Quality Process

This is the final and critical portion of the power supply manufacturing process. OTE International pioneered the 100% Quality Assurance Manufacturing exit test. Each power supply we manufacture is individually serial numbered. Prior to being boxed and shipped to the customer we run each power supply through a series of 8 to 14 individual tests (output voltage load / no load, ripple, current, short circuit, efficiency, etc..) and store the information on our international database. Our customers can download this data prior to each shipment allowing them to shorten their incoming QC process. In addition we keep batch lot files for all components linked to their serial number. This information can be very beneficial especially when statistically analyzing single shipments or annual production runs or reviewing returned units from the field.