Troubleshooting
5. The procedure described thus far is to be used when there are no specific characteristics given. When characteristics (such as shown in Table 4-1)
are given, make the load-regulation test using the correct values. For ex
ample, for the 5-V output load-regulation test, adjust the input voltage Vs
to the correct +4.5 V, and adjust R2 for a 75-mW output load, or R2 = 333 Ω (52/0.075).
6. Now (using a Vs of +4.5 V and R2 of 333 Ω), assume that the no-load voltage is 5 V and that the full-load voltage is 4.985 V. The percentage of regulation is [(5 - 4.985)/4.985] x 100 = 0.3%. This is above the typical 0.2%, but within the maximum 0.5% shown in Table 4-1.
4.4 Basic Line-Regulation Tests
Line regulation (also known as source effect) is usually expressed as a per
centage of output voltage (possibly with a given load) and represents maximum al
lowable output variation for a given input-voltage variation. For example, using the 5-V output line-regulation characteristics of Table 4-1, the output is measured (1) with an input or Vs of +5.8 V; and (2) with an input of +15 V. If there is no change in output, line regulation is perfect (and probably impossible). If the output varies by 0.15 V from the desired 5 V, the output variation is 3% (0.15/5). Again, this is above the typical 2% but within the maximum 4% shown in Table 4-1.
4.5 Basic Efficiency Tests
Power-supply efficiency is usually expressed as a percentage and represents output-power divided by input-power (times 100 to find percentage). Although the calculation is simple, you must have some means of measuring input current as well as voltage. If you do not have an ammeter that will measure input current, use a re
sistor in series with the input supply. Then measure the voltage across the resistor, and calculate current (I = E/R). If you use a l-Ω resistance, the result will be in am
peres (A). A 1000-Ω series resistance will indicate milliamperes (mA). Although the steady-state input current of most switching-regulator ICs is low, the starting- current surge can be quite high.
Assume that the full-load output voltage is 4.985 V with a load of 75 mW and that the input is +4.5 V with an input current of 20 mA. The input power is 90 mW (4.5 x 0.02), and the output power is 75 mW, so the efficiency is 83.3% (75/90). This effi
ciency would be very good for most battery-powered regulator/converter circuits.
4.6 Basic Ripple Tests
Any switching supply, no matter how well regulated and filtered, has some ripple (also known as output noise). The ripple can be measured with a meter or
scope. Usually, the factor of most concern is the ratio between ripple and full output voltage. For example, if 0.03 V of ripple is measured, with a 5-V output, the ratio is 0.03/5, or 0.006, which can be converted to a percentage (0.006 x 100 = 0.6%).
4.6.1 Measuring Ripple wiffi a Meter
Use the following procedure to measure output ripple with a meter.
1. Connect the equipment as shown in Fig. 4-1.
2. If R2 is adjustable, set R2 to the correct value, using the equation in Fig.
4-1.
3. Apply power. Measure the DC output voltage at position 3 (full load).
4. Set the meter to measure AC. Any voltage measured under these condi
tions is ripple.
5. Find the percentage of ripple as a ratio between the two voltages (AC/DC).
One problem sometimes overlooked when measuring ripple is that the voltage is rarely a pure sine wave. Most meters provide accurate AC-voltage indications only for pure sine waves. A better method is to measure ripple with a scope, where the peak value can be measured directly.
4.6.2 Measuring Ripple with a Scope
A scope can be used to display the ripple waveform from which the ampli
tude, frequency, and nature of the ripple voltage can be determined. Usually, the scope is set to the AC mode when measuring ripple, because this blocks the DC out
put of the supply. Normally, ripple voltage is small in relation to the supply voltage.
If scope gain is set to display the ripple, the supply DC voltage drives the display off screen.
1. Connect the equipment as shown in Fig. 4-1.
2. Apply power. If the test is to be made under load conditions, close switch Sj. Open Sj for a no-load test. The value of Rj must be chosen to provide the supply with the desired load.
3. Adjust the scope controls to produce two or three stationary cycles of each wave on the screen. Figure 4-2 shows the theoretical ripple waveform for one of the CMOS voltage converters described in Chapter 5.
4. Measure the peak amplitude of the ripple on the scope's voltage-calibrated vertical scale.
5. Measure the frequency of the ripple on the scope's horizontal scale, using frequency = 1/period of complete cycle. For example, lx and t2 in Fig. 4-2 represent one complete cycle. If the total period of tj + t2 = 40 μ8, then fre
quency = 25 kHz.
J-(V)
Figure 4-2. Theoretical ripple waveform for CMOS voltage converter (Harris Semicon
ductor, Linear & Telecom ICs, 1991, p. 2-101)
4 . 7 Measuring Transformer Characteristics
If a flyback converter design does not prove satisfactory, the fault may be with the transformer. Although a supply component (such as a diode, capacitor, or IC regu
lator) is usually easy to replace during the experimental or design stage, a transformer can more easily be tested than replaced at this stage (unless another transformer is readily available for substitution).
The obvious test is to measure the transformer windings for opens, shorts, and the proper resistance value with an ohmmeter. In addition to basic resistance checks, it is possible to test a transformer's polarity markings, regulation, impedance ratio, and center-tap balance with a voltmeter.
4.7.1 Checking Transformer Polarity Markings
The transformers used in switching supplies are generally marked as to polar
ity or phase with dots, color-coded wires, or a similar system. Unfortunately, be
cause transformer polarity markings are not always standard, they can prove very confusing during the experimental stage.
In general, transformer polarities are indicated on schematics as dots next to the terminals. When standard markings are used, the dots mean that if electrons are flowing into the primary terminal with the dot, the electrons flow out of the sec
ondary terminal with the dot. Thus, the dots have the same polarity as far as external circuits are concerned. No matter what system is used, the dots or other markings show relative phase, because instantaneous polarities are changing across the trans
former windings.
From a simplified-design standpoint, there are only two problems of concern:
the relationship of the primary to the secondary, and the relationship of markings on one transformer to those on another.
The phase relationship of primary to secondary can be found using the test circuit of Fig. 4-3. First check the voltage across terminals 1 and 3, then across 1 and 4 (or 1 and 2). Assume that there is 3 V across the primary, with 7 V across the secondary. If the windings are as shown in Fig. 4-3(a), the 3 V is added to the 7 V and appears as 10 V across terminals 1 and 3. If the windings are as shown in Fig.
Input