Power supply ripple rejection ratio (PSRR) is the ratio of the change in input power supply voltage (in volts) to the change in converter output voltage (in volts), usually expressed in decibels (dB). For high-quality D/A converters, changes in the power supply voltage used by the switching circuits and operational amplifiers should have minimal impact on the output voltage. The power supply ripple rejection ratio is typically defined as the ratio of the percentage change in full-scale voltage to the percentage change in the power supply voltage.
Power supply rejection ratio (PSR) can be divided into AC PSR and DC PSR, and their specific meanings are as follows: AC PSR is calculated by first taking a measurement value at the nominal power supply voltage (5V), then superimposing a 100Hz signal with an effective value of 200mV onto the power supply voltage, and taking a second measurement value under the same input signal level. The percentage error calculated using the measurement error formula "percentage error = (second measurement value - first measurement value) / first measurement value" is the AC PSR. DC PSR is calculated by first taking a measurement value at the nominal power supply voltage (5V), then changing the power supply voltage by 5%, and taking a second measurement value under the same input signal level. The percentage error calculated using the measurement error formula (same as above) is the DC PSR.
Like other loss measures, the power supply rejection ratio (PSRR) in the parameter specifications refers to the input of the operational amplifier. Noise on the operational amplifier's power supply lines can also affect the output signal, so it is necessary to appropriately "suppress" noise. The PSRR measures the degree to which the operational amplifier suppresses this deviation. It is generally defined as: the gain from input to output divided by the gain from power supply to output. As operational amplifiers are increasingly trending towards low voltage and low power consumption, the requirements for the power supply are becoming more stringent. This parameter reflects the magnitude of the change in input imbalance voltage when the power supply voltage changes by a certain amount. It is calculated by dividing a 1V change in power supply voltage by the input imbalance voltage measured in microvolts. The calculation method for output voltage error is the same as that for voltage imbalance and drift. The regulation of the external power supply is directly converted into the output error of the operational amplifier network in the form of the PSRR.
Our ultimate goal is to reduce the output ripple to an acceptable level. The most fundamental solution to achieve this goal is to avoid the generation of ripple as much as possible. First, we need to understand the types of ripple in switching power supplies and their causes.
As the switch is turned on and off, the current in inductor L fluctuates around the effective value of the output current. Therefore, a ripple with the same frequency as the switch frequency will appear at the output; this is what is generally referred to as ripple. It is related to the capacitance and ESR of the output capacitor. The frequency of this ripple is the same as that of the switching power supply, ranging from tens to hundreds of kHz.
Furthermore, switches typically use bipolar transistors or MOSFETs. Regardless of the type, there is a rise time and a fall time during their conduction and cutoff. This results in noise in the circuit that has the same frequency as or an odd multiple of the switch's rise and fall times, typically in the tens of MHz range. Similarly, during the reverse recovery of diode D, its equivalent circuit is a series connection of a resistor, capacitor, and inductor, which can cause resonance, generating noise at a frequency also in the tens of MHz range. These two types of noise are generally called high-frequency noise, and their amplitude is usually much larger than the ripple.
For AC/DC converters, in addition to the two types of ripple (noise) mentioned above, there is also AC noise, with a frequency of around 50-60Hz, which is the frequency of the input AC power supply. There is also common-mode noise, caused by the equivalent capacitance generated when the power devices in many switching power supplies use their casings as heat sinks. Since I work in automotive electronics R&D, I have less experience with the latter two types of noise, so I will not consider them for now.
Measurement of switching power supply ripple
Basic requirements: Use an oscilloscope with AC coupling, 20MHz bandwidth limit, and disconnect the probe's ground wire.
1. AC coupling removes the superimposed DC voltage to obtain an accurate waveform.
2. Enabling the 20MHz bandwidth limit is to prevent interference from high-frequency noise and avoid obtaining incorrect results. Because the amplitude of high-frequency components is relatively large, they should be removed during measurement.
3. Disconnecting the oscilloscope probe's grounding clip and using a grounding ring for measurement is to reduce interference. Many departments do not have grounding rings, and if the error is permissible, they may directly use the probe's grounding clip for measurement. However, this factor should be considered when determining whether the measurement is合格 (qualified/acceptable).
Another point is to use a 50Ω terminal. Yokogawa oscilloscope documentation states that the 50Ω module removes the DC component and accurately measures the AC component. However, few oscilloscopes are equipped with this dedicated probe; most measurements use standard 100KΩ to 10MΩ probes, so the impact of this is currently unclear.
The above are basic precautions for measuring switch ripple. If the oscilloscope probe is not in direct contact with the output point, a twisted pair cable or a 50Ω coaxial cable should be used for measurement.
When measuring high-frequency noise, the full passband of an oscilloscope is used, typically in the range of several hundred megahertz to GHz. Other aspects are the same as described above. Different companies may have different testing methods. Ultimately, the first requirement is to understand your own test results, and the second is to gain customer approval.
Generally speaking, ripples are entirely detrimental and offer no benefits. The main harms of ripples are as follows:
a. Ripple in the power supply can generate harmonics in electrical appliances, reducing the efficiency of the power supply;
b. Higher ripple may generate surge voltages or currents, which can cause electrical equipment to malfunction or accelerate.
Equipment aging;
c. In digital circuits, ripple can interfere with circuit logic.
d. Ripple can also cause noise interference to communication, measurement and metering instruments and meters, disrupting the normal measurement and measurement of signals, and even damaging equipment.
Therefore, when manufacturing power supplies, we must consider reducing ripple to a percentage or less, and for equipment with high ripple requirements, we must consider reducing ripple even further.
Methods for measuring power supply ripple generally fall into two main categories: one is the identification of a single power supply, and the other is the adjustment of the product.
Try measuring.
When power supply companies and users evaluate power supplies, it is required to conduct the evaluation indoors (around 20°C), with humidity less than 80%, and minimal mechanical vibration and electromagnetic interference that may affect the measurement. The standard instruments and the power supply under test should be placed in the above testing environment for more than 24 hours.
For pure power supplies, when measuring power supply ripple, it is required to measure under load, and the applied load should make the output current greater than 80% of the rated output current.
For low-noise purely resistive or electronic loads, it is also necessary to select an appropriate measurement standard. Different standards will produce different measurement results.
Ripple voltage can be expressed as an absolute or relative quantity. The filtering performance of a DC power supply is generally evaluated by the ratio of ripple voltage to DC output voltage, i.e., the ripple factor. As an important indicator for evaluating a DC power supply, the ripple factor is calculated as the percentage of the effective value of the ripple voltage to the DC output voltage.
