A power supply plays a critical role in making modern electronic systems work flawlessly
By Richa Chakravarty
Monday, December 16, 2013: A modern electronic system comprises a complex network of ICs and FPGAs working in sync with each other. With our never ending need for speed, these ICs operate faster than ever, leading to a highly dynamic demand for power during different test scenarios. These fast changing current waveforms can lead to anomalies in the system or even a voltage drop at ICs, which can even reset the microprocessor if the voltage drop is significant enough. This is one of the scenarios in which power supplies play a critical role in making modern electronic systems work flawlessly, shares Mukul Pareek, marketing programme manager, electronic measurement group, Agilent Technologies India Pvt Ltd.
Fluctuations and variations in the electricity supply can be destructive to electronic systems. Hence, power supplies are now designed in such a way so as to minimise such anamolies and prevent damage to systems. These supplies also help in converting AC to DC and vice versa, as well as AC to AC and DC to DC, explains Pranay Rana, senior executive, business development, Rishabh Instruments Pvt Ltd.
Power supplies come as bench-top variants for conducting experiments in laboratories and also as industrial models for heavy duty applications. Linear or switch mode power supplies are also popular.
Specifications to look for while selecting a power supply
According to Srinivasa Appalla, manager, test and measurement product support and application, Rohde & Schwarz India Pvt Ltd, a good power supply has a high output power, a compact-sized housing and high efficiency, combined with a small residual ripple. Due to its electrically insulated, earth free, overload and short circuit proof output, it can be opened in series or in parallel to deliver high voltage or high current output.
From a broader perspective, power supplies are used in two major ways:
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To act as a clean and regulated power source to supply power to systems, PCBs and components as per the needs of different applications.
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To do stress testing of electronic components and systems, subjecting them to various power situations (glitches, noise, fast transients, etc)
“It is important to note that all power supplies are not created equally. So there is nothing called as ideal spec for a power supply as these specs vary with different applications. While voltage and current would definitely be one of the important specs, they might not be the only ones to consider and it’s time to think beyond voltage and current (V&I),” explains Mukul Pareek.
Beyond V&I, the key aspects one should consider while selecting a power supply can be categorised under the following points:
Output and power requirements: Multiple outputs are vital when a power supply is used to test many devices at the same time. When choosing a multiple output power supply, check if you need the outputs to be isolated from one another, and also look into the power supply’s tracking capabilities. Auto-ranging power supplies, also called autorangers, are designed to provide a greater range of operation than a typical power supply. When buying these, look at the maximum rated volts, AMPS and output power.
Operational specifications—hold time: This is the amount of time the power supply takes to produce its output even if it loses its input. This value indicates the length of a blackout that the power supply may be able to tolerate before dropping the signal. Generally, the time is about 20 milliseconds. It is important for the user to check this and also to compare it against the switch time. The greater the hold time, the lower the chances of problems while testing.
Voltage/load regulation: Voltage variation is a common problem while carrying out a test. However, the ability of the power supply to control the output voltage level, as the load on the power supply increases or decreases, is what one should look for. The voltage of a DC power source tends to decrease as its load increases, and vice versa. Power supplies from reputed firms do a better job of smoothing out these variations. Hence, one should look for this specification before putting it to test. “Intelligent power management in modern power supplies enables the maximum current possible with minimum voltage and reduced currents at higher voltages by not exceeding maximum output power. This enables the power supply to utilise the rated maximum power,” informs Srinivasa Appalla.
Over voltage protection (OVP) and over current protection (OCP): In addition to specifying the normal maximum voltage level, a good power supply will always include protection against output voltage that exceeds a certain critical level. If, for some reason, the voltage surges beyond the critical level, the power supplies shut down the output. In most cases, on detecting an over voltage, a power supply will reset. The user should carefully check this specification—at what percentage over the maximum rating for each voltage output will this occur.
The PSB-2000 series offered by GW Instek can set the OVP or OCP to 10-110 per cent of the rated voltage or current. When the voltage and current outputs exceed the preset conditions, the PSB-2000 series shuts down the output function to prevent DUTs from damaging. Also, PSB-2000 allows users to preset the automatic shutdown time. The range of preset step time is from 10 minutes (at the minimum) to 99 hours and 50 minutes (the maximum). See details about the product on page 80.
Ripple: Periodic and random deviation (PARD), or plain noise in other words, is another spec to be considered while buying a power supply. Ripple values are usually given in terms of millivolts peak-to-peak (mVp-p). Peak-to-peak refers to measuring the AC voltage from its negative maximum to its positive maximum. The Hameg brand HMO 4030/HMP 4040 offered by Rohde & Schwarz is a low residual ripple < 150 µV, with high set/readback resolution. This can be comfortably used both in parallel as well as in series mode. (See details about the product on page 80)
“A good power supply in general should have zero ripple, continuous regulation (both line and load), and should be flicker-free,” says Pranay Rana.
Transient response: The drastic change in load (either an increase or a drop) may suddenly cause the voltage level to shoot up drastically. This sudden change is called a transient. Transient response measures how quickly and effectively the power supply can adjust to these sudden changes. This specification should also be looked into when evaluating power supplies. “Normally, DC power supplies are constant current and constant voltage types. A user should look for a power supply that can be isolated from the mains, if required, for experimentation,” informs JK Baldua, director, technical, Scientech Technologies Pvt Ltd.
Safety and certifications: Generally, all power supplies have their safety and quality certified by one or more agencies. These certifications indicate that the power supply has been tested and has passed a certain standard. Safety and quality certification is probably the most important thing to look for. Including other safety standards, tests that assess how much electromagnetic interference (EMI) or radio frequency interference (RFI) the power supply generates should also be considered.
Apart from these operational functions, there are some additional features that not only aesthetically improve the device but make it more user friendly, while improving its functionality. These features include preset functions, low noise level, warranty, rotatable control panel, off timer function, LCD screen display, etc.
Cost of ownership: An important factor that should be considered while buying a power supply is the cost of ownership. One should take into account not only the price of the power supply but the whole product experience. This includes the cost of downtime for calibration and repair, protection for the DUT and test instruments, and reliability.
| POWER SUPPLIES Offered by agilent |
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Agilent’s advanced power systems—(APS) N6900 and N7900 series with Versa power architecture, and Agilent N8900 series auto-ranging system DC power supplies offer a range from 1 kW to 15k W, expandable up to 60 kW fall in the high wattage power supplies portfolio.  Key features of APS series
 Key features of auto-ranging DC power supplies
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| POWER SUPPLy Offered by Scientech Technologies |
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Scientech Technologies’ 4073P programmable power supply is a versatile instrument with the capability to interface with the PC. The front panel with keypad and cursor makes it user friendly. It can be set to be used as a constant voltage and constant current source. With low ripple and excellent line and load regulation, Scientech 4073P is fully protected against over load, over voltage, over current, over heat and short circuit. Its 4075i programmable dual DC power supply can also interface with the PC via Ethernet. (For its latest product, see page 80).  Key features for 4073P
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| Power Supply Offered by Rishabh Instruments |
| Rishabh Instruments offers Rish Flex series which comes in single-phase, two-phase and three phase systems. It has the best derating factor. |
Key features of single phase supplies
- Input: 115-230V AC
- Output: 24V DC, 50 degree C
- Efficiency: Up to 88 per cent
- Strong overload without switch off
- Flexible power continuity: From 36 to 72 W
- DIN rail mountable
- Extremely small size
- Three-year warranty
- Power boost feature upto 1.5 times the rated current for more than 3 minutes at 60˚
- Three operating modes for short circuit mode including continuous mode, hiccup Mode and manual reset
- Designed for various applications
| Merits and demerits of linear and switch mode power supply | |||
| DC power supplies are of two types: Linear and switch mode. Each offers significant advantages over the other based on the application | |||
| Linear power supply | Switch mode power supply | ||
| Advantages | Disadvantages | Advantages | Disadvantages |
| Low output noise | Larger in size | Smaller in size | Slower transient response |
| Fast transient response | Low efficiency | High efficiency | Higher output noise |
| High programming speed | More cooling required | Less cooling required | Slower programming speed |
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