Power To Spark: 31 power supplies side by side （2）

Published:2011/8/9 22:22:00 Author:Li xiao na From:SeekIC

Outputs

As already mentioned, it can be convenient to have several outputs available. Besides units with several adjustable outputs, there are also power supplies available with one or more fixed-voltage outputs in addition to a ’normal’ adjustable output. The fixed voltage is usually 5 V or 12 V.

Most lab supplies nowadays are fitted with sockets that accept banana plugs. The familiar combined banana jack and terminal post is slowly vanishing from the scene. This has to do with legal regulations related to shock protection. With certain power supplies (particularly if they are connected in series), the output voltage can be quite high. Under such conditions, terminal posts are naturally taboo. Incidentally, it’s also convenient if the power supply has a separate switch for disabling the output, in order to prevent switch-on phenomena from reaching the connected circuitry.

Measured results

In order to judge the quality of the power supplies, we made measurements to check two important specifications for each unit: ripple and load regulation.

Most readers will probably know what ripple is: it’s simply the residual ac voltage found at the output following conversion of the mains voltage. With a conventional power supply (transformer, rectifier, regulator circuit and associated filter), the ripple is usually a low-amplitude signal with a frequency of 100 Hz.

With a switched-mode power supply, the frequency of the ripple voltage is determined by the switching frequency




Terminology

Conversion

Linear power supplies use a transformer to convert the mains voltage to a lower ac voltage, which is then rectified and filtered (smoothed). Switched-mode power sup plies first rectify the mains voltage, and this rectified voltage is then converted into an ac voltage at a relatively high frequency. This allows o much smaller transformer to be used for conversion to a lower voltage. These two techniques can also be used sequentially (mixed mode).

Output

In the output-range specification, the smallest possible increments for adjusting the voltage and current are shown in parentheses. Naturally, this does not apply to power supplies with analogue adjustment (using a potentiometer).

Ripple

The ripple, which is the residual ac voltage at the mains frequency or switching frequency, is given as an rms ac voltage measured with a bandwidth of 300 kHz. The inaccuracy of the meter used for the measurements (Fluke used in the supply. This is also the case with supplies based on a combination of these two methods. Naturally, it goes without saying that the lower the ripple voltage is, the better (see also the Terminology inset below).

Load regulation

What’s more interesting than the ripple is how a power supply handles a ’difficult’ load. Ideally, the
output voltage (or current) should remain constant under all conditions. None of the supplies can actually manage this in practice, but that’s a perfectly normal situation. Most manufacturers also specify how well the power supply can handle a difficult load. This is usually given in the form of ’load regulation’, which specifies the maximum change in the output level for a sudden change in the load. Elsewhere in this issue, you can read more about our testing methods and how you can test power supplies yourself. Refer also lo the Terminology inset in the survey section on the following pages.
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Terminology

Conversion
Linear power supplies use a transformer to convert the mains voltage to a lower ac voltage, which is then rectified and filtered (smoothed). Switched-mode power sup plies first rectify the mains voltage, and this rectified voltage is then converted into an ac voltage at a relatively high frequency. This allows o much smaller transformer to be used for conversion to a lower voltage. These two techniques can also be used sequentially (mixed mode).

Output
In the output-range specification, the smallest possible increments for adjusting the voltage and current are shown in parentheses. Naturally, this does not apply to power supplies with analogue adjustment (using a potentiometer).

Ripple
The ripple, which is the residual ac voltage at the mains frequency or switching frequency, is given as an rms ac voltage measured with a bandwidth of 300 kHz. The inaccuracy of the meter used for the measurements (Fluke 1 87) is included in the stated values. The load and dc voltage for the measurement are stated in parentheses.

Load regulation
The slated values are maximum values. The following values are given in the order listed: the peak voltage of the overshoot when the load is disconnected (U0), the duration of this overshoot (tp), and the (quasi)static deviation from the set value (U5). These values include the inaccuracy of the instrument (Tektronix TDS3020) and reading errors. The load consists of a fixed part and a variable having the same value, which is switched in parallel with the fixed part at a rate of 300 Hz with Uout = 0.33 Umax. The load regulation values cannot be directly compared with each other, since each of the supplies v/as set to a different voltage for this test. However, the individual values do provide an indication of the qualify of the design and regulation of the power supply (lower values are better).

Recommended retail price (RRP)
Unless otherwise stated, this is the recommended retail price including VAT, as specified by the supplier who provided the unit.

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