Isolated Flyback Converters: Performance Evaluation and Check Points
Performance evaluation using an evaluation board: Measurement method and results
- Evaluating power supplies
- High voltage safety
- Measuring high voltage
- Measuring power supplies
This section describes the measurement method for and results of an evaluation to determine whether design goals have been attained, using the circuit and circuit board that were verified in the preceding section. The circuit board is, available in the market, is one that is referred to as the BM2P014, designed to evaluate the power supply IC.
The table below, the same as the table that was shown in the preceding section, represents the design goals for this section.
|No load input power||-||-||50||mW||Input:100VAC/230VAC|
|Output ripple voltage||-||-||100||mV||Bandwidth: 20MHz|
The parameters listed below are to be measured. The table below shows the measurement method and conditions, and measurement equipment to be employed, as well as the measurement points to be included.
|Input voltage||Using a step up/down transformer, apply 90VAC, 100VAC, 230VAC, and 264VAC voltages.||Voltmeter (AC) and wattmeter|
|Input current||Measure with each input voltage and output load current.||Clamp ammeter and wattmeter|
|Input power||Measure the input power.||Voltmeter (AC), wattmeter, and clamp ammeter (AC)|
|Output voltage||Measure with each input voltage and output load current.||Voltmeter (DC)|
|Output current||Apply a 0A to 1.5A current using variable load equipment.||Ammeter (DC)|
|Output ripple voltage||Observe the waveform with an oscilloscope.||Oscilloscope|
|Efficiency||Calculate the efficiency from the above measurement results.||Output power ÷input voltage (%)|
As may be clear from the above diagram, the parameters involved are basically voltage and current, which can be measured easily with a multimeter and a wattmeter. For the measurement of alternate current, there is no question that a wattmeter comes in handy, although a clamp ammeter can also do the job.
For output ripple voltages, use an oscilloscope to observe the output waveforms. Observing with an oscilloscope is a mandatory requirement due to the fact that in order to determine an output ripple, peak voltages are required.
The following devices are needed to set up measurement conditions: a transformer capable of stepping up or down the voltage in order to generate a 90 VAC to 264 VAC power from 100 VAC; and a variable load equipment that sets an output load current.
An important precaution is that the process involves the handling of high voltages. For input voltage, a maximum of 264 VAC may need to be handled. The primary side rectified voltage will be 372 VDC, which obviously could cause life-threatening injuries. Use extreme caution in avoiding shorting or inadvertent contact. Before undertaking measurements, be sure to take the necessary and adequate safety measures.
The table below shows actual measurements.
The above figures represent measurements of minimum, nominal, and maximum input voltages, under six conditions by varying the load current from zero to10 mA, 100 mA, 500 mA, 1 A, and 1.5 A. The encircled figures indicate those that are equal to design goals.
The output ripple voltage produced the waveform shown below, based on an oscilloscope probe. Grounding the equipment with a standard clipped grounding wire can produce disturbance and spikes in the waveform that actually do not exist. The best technique would be to use a dedicated connector to insert a probe directly. However, it can also be effective to take measurement through a grounding wire that is cut as short as possible, as shown in the photograph.
The measurement results are summarized below:
|Input voltage||90||-||264||VAC||Normal operation in this range|
|No load input power||-||-||50||mW||32mW at 100VAC input
36mW at 230VAC input
|Output voltage||11.4||12||12.6||V||12.08V Min
|Output current||1.5||-||-||A||Normal operation at 1.5A|
|Output ripple voltage||-||-||100||mV||74.0mVp-p|
|Efficiency @1.5A||80||-||-||%||83.8% Min
The results indicate that for each parameter the maximum and minimum requirements are met, and the design goals are attained. It goes without saying that the circuits, parts, and board used are evaluation units, with proper adjustments and revisions completed so that design goals would be satisfied. In the actual design process, items that do not meet requirements may crop up. Since the purpose of this task is debugging, we need to uncover issues, identify the causes of them, and address the problems.
In addition, in performing measurements, beyond the set conditions, such as an input range of 90 to 264 VAC, the conditions have to be tightened and likely trends need to be provided for, in consideration of margins inherent in the components. In this case, because the 90 to 264 VAC range includes a ±10% margin, up to ±15% should be verified. There may be cases where “the device operated OK up to ±10% but at ±11% it suddenly failed to function.” This suggests a lack of adequate margin, in which case the design goal must be revamped.
If the design goals are to be guaranteed values for a power supply unit, separate standards may need to be set up as to what margin must be provided.
・ With basic instruments, power parameters can be measured with relative ease.
・The measurement process involves the handling of high voltages. Learn the steps necessary to ensure safety, and work in strict adherence to the safety precautions.