There are several factors to consider when choosing a power supply. An important first step is to categorize the major attributes of the supply. These can be generally be summarized as Fit, Form and Function:
Fit is the ability to interconnect, interface or become an integral component of another item.
Form is the size, mass, weight, shape and any and all dimensions that uniquely identify a power supply.
Function defines the performance of a power supply under a quantified criteria (environment, electrical stress, physicality etc.).
Four Important Guidelines
Every power supply has a data sheet which identifies fit, form and function. The designer should consult the manufacturer data sheet to make sure the supply meets the requirements of their application. While all specifications should be considered, different applications may find some parameters more critical than others. In some designs, fit or the ability to conform to existing footprints and connections may be the most important. Others may have a need for a special form factor, while still others may require a specific electrical function and the ability to perform under a special set of circumstances. Consider the most important factor(s) when choosing a supply and assign the appropriate priority to them.
A key consideration is that one manufacturer's definition of fit, form and function may be different from other information of the same parameter. This can be caused by differences in terms that mean the same thing. Here's an example, one manufacturer publishes the rated current as "full load" while another uses "max load". You may call it operational current or static load or something else. Also, keep in mind that space does not allow for all data to be published, therefore some information must be extrapolated from curves or other given data. Sometimes the curves themselves are formatted differently. Inquire to the manufacturer's technical support if you have trouble understanding the information or need information not shown on the data sheet. Manufacturers will typically furnish additional data upon request.
Lastly, keep in mind that manufacturer is only responsible for the published data, which may be subject to change without notice. If full document control is necessary, it should be discussed with the manufacturer.
When looking for replacement supplies, manufacturer part numbers and data sheets are good tools for discussion In the event that an exact replacement is needed, it's advisable to present a comprehensive written specification to the manufacturer for a full technical review. This is often done in military applications where the designer will submit a procurement document. The procurement document assures that the manufacturer will meet all requirements. Changes to the part may be allowed when it still fulfills the requirements of the procurement document. The end customer can request full document control to scrutinize any manufacturer changes before accepting them in all revised documents.
New technologies and packaging have led to many changes in power supply forms, with multiple connection options, and electrical characteristics.
As your particular market evolves, it may be worthwhile to make a periodic review of your requirements. You may consider using a higher density supply with a smaller footprint to gain space for additional build out. Be vigilant about any changes that may have occurred to your operating environment. Advances in thermal management can help increase your operating spectrum, making for a more robust design.
Review your power budget, it may have decreased and you're paying for unnecessary power. Consider any special connectors or feature sets that might have changed or been eliminated. Cost savings may be realized through standardization where possible. Stay up to date on the necessary safety approvals for your market and any updates to those approvals. Re-certifications and/or amendments are often required for continued sale to certain markets and countries.
Once your review is finished, you're ready to choose a power supply. PSUI engineers and sales staff are ready to work with you throughout the process with over 10,000 available SKUs. Let us help you make the right decision for all your power supply needs.
By John Benatti
Power supplies are employed in various applications. Many have standard load requirements that fall within the rated output current of the supply. In these cases, Vout x Iout will yield a sufficient power rating for the power supply selection. However, some applications will require a high peak load or Ipeak to initialize their operation. Examples of these are electric motors, pumps, fans, disk drives and other devices that need a start-up or peak current. Peak currents can be several orders of magnitude higher than the static (steady state) load. Even though an Ipeak duration tends to be relatively short, typically a few seconds or less, these periods can be problematic for many supplies. Manufacturers rate an acceptable amount of over current, shown as an Ilimit threshold, but peak currents will usually far exceed these limits. When this happens, the peak current will be inhibited by any of several types of current limit protection ranging from hiccup mode to total latching off. Before the evolution of peak rated power supplies, one solution was to overrate the power supply to the high peak load, but this solution unnecessarily increases the size and cost of the power supply.
Fortunately, PSUI carries many high peak load power supplies, such as the Tri-Mag DG160 series (see link for datasheet below)*. The Tri- Mag DG160 can handle high peak loads to 360 watts, while still operating as a traditional power supply. The only provisions of this type of supply are that the high peak load applied not exceed the specified peak load for the duration and duty cycle per the data sheet. This is essential to ensure that the total power, including peak power, does not exceed the nominally rated average output power of the supply. If a high peak load event is within the manufacturer's peak load ratings, then we can calculate how much power is available during the non-peak power period to drive the steady state operation.
Figure 1 illustrates this concept. The basic equation for a high peak load supply is α = [(Wm x T) - (Wp x t)] / (T - t)
α = the available non-peak power (W)
Wm = Maximum Average Output Power (W)
Wp = Peak Pulse Power (W)
T = Total Period (S)
t peak = Ipeak duration (S)
Using the Tri-Mag DG160 data sheet we can examine the example below.
Example: An application needs 300 watts of peak power for 3 seconds every 25 seconds and has static load of 100 watts. Can I use the Tri-Mag DG160? If so, how much available power is there for my static load.
Following the equation:
α = [(160 x 25) - (300 x 3)] / (25-3)
Therefore, 140.9 watts is available during the non-peak period of 22 seconds. This is below the average output power of 160 watts per the data sheet.
This satisfies both the peak load requirement and static load obligation. We can manipulate the high peak load equation to produce various peak, non- peak and duty cycle solutions.
Over the past few years, power levels for DC/DC converters were only available from larger size modules. Now they've migrated downward into much smaller packages. Not only are relative footprints becoming smaller, but they're also providing more cost effective power solutions.
Advancements in converter technologies and packaging have resulted in a significant decrease in the cost per watt, while also optimizing available board space. Given the large selection of brick sizes, the system designer can now choose the right sized module at the right price.
PSUI is pleased to offer these exciting designs through their partners at Semiconductor Circuits Inc. an experienced leader in power design. Semiconductor Circuits Inc engineers robust designs for all market spaces and applications. One of their featured products is the new 1"x1" 40W DC-DC Converter.
Small Footprint, Cost Effective Power Solution
The CP40 Series is the only 40 Watt, 4:1 input module available on the market today in a 1"x1" form factor. This module is ideal for industrial or distributed power architecture applications and is compliant with the standard 1"×2" pinout and interface standards. It has an ultra wide input voltage range of 18 to 75VDC (9 - 36 Vin versions available) and provides 2250VDC input to output isolation in open frame configuration, 1600VDC in the encapsulated module.
The CP40 4:1 input series is ROHS II Directive 2011/65/EU Compliant, meets UL/EN 60950 requirements and is UL94 V-0 flammability rated. The module complies with all of the typical industry requirements including no load operation, 2X nominal input voltage transient and pre-biased load startup.
Other features include: fixed-frequency operation, auto-restart over-voltage, over-current & over-temperature protections, undervoltage lockout and +/-10% output voltage trim. Remote ON/OFF is standard with positive or negative logic options available. Modules can also be
ordered without enable pin, trim pin or with neither.
The modules feature a standard 1×2 pinout with dimensions of 0.94" L x 0.94" W x 0.35" Max. Height (23.9mm x 23.9mm x 8.9mm.) The encapsulated versions are standard 1" x 1" x 0.4" Tall (25.4mm x 25.4mm x 10.2mm.)
Custom output voltages in the 4:1 input series are available upon request. We also welcome quick-turn modifications and custom configurations to meet our customer's particular requirements.
Cause: When an electrical device like a power supply is turned on, an inrush current will momentarily flow in that device. In the case of an AC/DC switching power supply, the inrush current is primarily due to the charging current of the large electrolytic storage capacitor(s) employed in the rectification stage. See the simplified schematic Figure 1 . These currents can easily exceed 50 amps. The reason for this is that there is very little impedance present at turn-on, typically just a few ohms. Consider the Ohm's law equation, V/R = I . Ex. If turn on is at the peak of the AC cycle, 120VAC and we assume an input impedance of 3 ohms, then (120 x √2)/3µ = 56.7A. If the nominal input voltage is 240Vac in rush currents in this example can be greater than 100A (Fig 1).
Figure 1 Effects:
- High inrush currents can unduly stress the bridge rectifier, causing catastrophic failures
- Fuses become overheated and degraded over time
- Line circuit breakers can trip, especially in applications with multiple supplies sharing the AC mains
- Switch contacts are subjected to arcs resulting in premature failures
- High currents cause voltage dip disturbances which may affect other circuits
These effects cause system lifetimes to decrease, which in turn adds significant repair/replace costs. Unplanned downtime from tripped circuit breakers or power supply failures is expensive and more importantly can be dangerous as in medical or mission critical situations. Solutions are necessary to mitigate the inrush current and a few are shown here. Solutions: Solutions generally fall into two categories, Passive and Active. 1.) Passive... dissipate energy at turn on A.) Inrush limiting resistor: A fixed resistor in series with the line will help limit the inrush current. This approach should only be used in applications less than a few watts because there will be losses in efficiency due to dissipated power in the resistor. B.) NTC resistor: An NTC is a Negative Temperature Coefficient resistor. This is a simple method of inrush current limiting. The way it works is that at initial turn on the resistor is cold and has a high resistance to the inrush current. Shortly thereafter, the NTC resistor itself heats up due to the power it's dissipating. When that happens the resistance decreases significantly and dissipates much less power for the rest of the on time. When the power supply is turned off, the NTC cools off and goes back to it's initially high resistive state. It's now ready for the next turn on. The disadvantage is as the name implies, in the temperature coefficient of this component. Adequate time is needed between turn-ons for the NTC resistor to cool down. If not the NTC offers little resistance to the inrush current. NTC resistors are also affected by their ambient temperatures and will act accordingly. Never warming up in cold environments and never cooling down in hot climates. Despite the disadvantages, this simple inexpensive method is widely used. C.) Bypassed resistor: When the charging current of the electrolytic capacitor is reached the resistor will be bypassed by triac, relay or IGBTs. Greatly reduces efficiency losses. This method adds some expense (Fig 1a). D.) Bypassed NTC: Just as in the case of the fixed resistors , the NTC resistor can be bypassed after charging current is reached by triacs, IGBTs or relays. This makes the system more efficient and ensures that the NTC will have a high impedance at turn on. As expected, it also adds expense. These last two methods are sometimes known as Active Passive.
1.) Active ....controlled start up
A.) Zero crossing technique: The Vac input is monitored by a control circuit and the supply turned on when the line is approximately 30Vac or below. Synchronous solid state relays can be used or triacs. This eliminates random turn on at peak voltage and keeps inrush current low. No NTC is needed and there is no temperature dependency. Not recommended for designs over 500 watts as the inrush current can still be large at these power levels regardless of low line voltage (Fig 2a).
B.) Time delayed switching: This behavior is accomplished by a pre-charging module that controls the pulses used to slowly charge the storage capacitor. Since the current used to charge a capacitor depends upon the rate of change of the voltage I = C dv/dt, pre charging the capacitor with "soft" pulsing reduces the inrush current. Delaying the turn on this way also allows other consumer circuits to meet their inrush requirements uninhibited by the power supply turn-on. Not temperature dependent (Fig 2b).
C.) Mosfet limiting: Mosfet limiting is another way to effect the voltage rate of change to the bulk capacitor and mitigate inrush current. A Mosfet power switch will soft start the voltage to the capacitor. The change in the capacitor current is equal to the slew rate of the mosfet output voltage as configured below (Fig 2c).
Inrush current can be lowered by additional impedance or by a controlled start up circuit, using discrete components or advanced semiconductor designs.
These are some of the accepted methods to reduce inrush current. They are others as well but the basic concepts are the same. Use the method that best suits your application and budget.
Note: These techniques are specific to AC/DC power supply inrush and may not be advisable for use with other electrical devices.
By John Benatti
Global users of Audio Visual (A/V) and Information Technology Equipment (ITE) products number in the billions. The safety of these users, whether lay person or professional, is a growing concern. Since these two markets share many technologies, the IEC (International Electrotechnical Committee) has established a hazard-based standard to combine both IEC 60065 (A/V) and IEC 60950-1 (ITE). The standard also includes Communication Technology Equipment (CTE).
The principal methodology used in the development of this standard is a new approach called Hazard-Based Safety Engineering.
What is Hazard-Based Safety Engineering?
Hazard-Based Safety Engineering or HSBE is a safety science that's been emerging in the world of safety compliance over the last 15- 20 years. It's a safety discipline that has two main objectives. The first is to identify sources of energy and classify the level of hazard (injury or damage) that could occur when that energy is transferred to body parts or combustible materials. See Table 1. The second objective is to identify and then qualify effective safeguards that will eliminate the transfer of any hazardous energy to body parts or combustible materials. The hazard-based standard, IEC 62368-1, specifies the compliance criteria needed to qualify the effectiveness of the safeguards against the transfer of hazardous energy. Safeguards are formally defined as "a device or scheme or system that is interposed between an energy source capable of causing pain or injury and a body part and reduces the likelihood of transfer of energy capable of causing pain or injury to a body part" There is a hierarchy of safeguards that should be considered. See Table 2.
Effects on Body
Effects on Combustible Materials
Not painful, but could be detectable
Ignition not likely
Painful, but not causing injury
Ignition likely with limited spread and increase of fire
Ignition likely with rapidly spreading and increasing fire
How does a hazard-based standard differ from previous standards?
In previous standards, designs had to closely prescribe to rules specific for that product. These are called prescriptive standards. Adherence to the rules and protocol of the standard was necessary for product certification. Hazard-based standards use a different philosophy. It's one that offers an additional performance based option to meet compliance criteria. Existing relevant standards, documents and engineering practices can all be used to identify and classify a hazard and then prove through performance that the hazard has been eliminated. The performance of the design speaks for itself and is largely technology independent. A significant benefit is that new constructions and technologies can be used for compliance without having to amend the relevant existing standard. This offers greater design flexibility.
However, a major consideration of the performance based option is that "all possible fault conditions" must be tested. Not only it is time consuming but testing for "all possible fault conditions" can lead to a subjective application of the performance based option. Certification body engineers may disagree with testing which will add additional costs and delays in product certification. Performance based compliance options should be given careful scrutiny before implementation, especially in complex designs. It's important to note that the performance method is an option. There is still an allowance for traditionally prescribed constructions if they've been proven safe in relevant standards (i.e. IEC60950-1 and IEC 60065).
Do I need to perform risk analysis?
No, risk analysis, assessment or management is not required for compliance.
Is the standard meant to be generic?
No, the standard has it's own specific requirements and compliance criteria, performance based and prescriptive. To that end, it's a stand-alone certification. It's also not a merger of IEC 60065 and IEC 60950-1.
Where will the standard apply?
The standard is to be applied internationally. It's compatible with the IECEE CB scheme and international safety mark certification.
What about country variances?
While the goal is to have as much international standardization as possible, variances will be allowed for local power sources and their specific characteristics.
What if my products are currently certified to IEC 60065 or IEC 60950-1?
The second edition of IEC 62368-1 : 2014 recognizes that requiring the transfer of all existing certified products to a new standard would cause an undue hardship on manufacturers. Clause 4.1.1 states that "Components and subassemblies that comply with IEC60950-1 or IEC 60065 are acceptable as part of equipment covered by this standard without further evaluation other than to give consideration to the appropriate use of the component or sub-assembly in the end product."
When will I need to be compliant?
Currently it appears that mid 2019 will be the timeframe when all previous standards of 60065 and 60950 (IEC, CSA,UL and EN) are phased out and compliancy must begin. Some power supply manufacturers are seeking compliance well ahead of schedule.
Integrated Power Designs, a valued partner of PSUI, already has a schedule of early adoption of IEC 62368-1 and will be fully compliant on all designs well before the mandated deadline. See the latest Integrated Power Designs innovations in 200 and 400 Watt designs that carry the IEC 62368-1 certifications.
This is a just general outline of the IEC 62368-1. If this standard will impact your products, you should be thinking about compliancy now. Currently, there are no provisions for allowing the shipment of non-compliant products after the deadline. Getting ahead of the compliancy mandate will ensure that your products will be allowed into the many international markets adopting the standard.
PSUI is dedicated to working with qualified power supply manufacturers like Integrated Power Designs to ensure that all your design and safety needs will be met. Please call PSUI for more information.