BOD100 loop analyzer application switching power supply gain stability analysis
With electronics, self-control, space, communication, medical equipment, etc., the continuous development of depth and breadth, it is necessary to require higher stability as a power supply, that is, not only there is a good linear adjustment rate, and load adjustment rate is also There is a fast dynamic load response. These factors are related to the control loop, and the control loop is generally operated in the load state, called the voltage negative feedback. If the feedback control loop is not used in the converter (H portion in Figure 1), the transfer function is generally c / R = g, where g is the product of the input filter, power conversion, rectifier filter portion, etc. (it is In the form of a joint, this article is expressed in total maternal factor g), it can be seen that the output changes with the input change, but due to the nonlinearity of the filtering network throughout the time domain, this change should be It is approximately linear, so when the input power is changed, it does not have a good gas to regulate. If the feedback loop design is not good, the transient change of the corresponding load cannot make timely adjustments, then the output power supply will be high or low, and even the oscillation of the power system, The first level is damaged. If you can measure the loop measurement, it is important. So how is the loop section affect the entire loop?
Refer to the Delta AC-DC series, the loop gain model in the feedback loop control section shown below, its transfer function is C / R = g / (1 + gH), where G: open gain. H: Feedback coefficient. GH: loop gain (can be derived by Figure 1)
First, the standard of ring gain stability
From the transfer function C / R = g / (1 + gH), the open loop gain G of the amplifier is a function of the frequency, and as it is reduced as the frequency increases, it is also related to the phase of the amplifier, when GH = -1, The value is, that is, the gain is infinite, it can be considered that any small input disturbance can cause an infinity of the output, and if this output infinite signal is feedback to the power conversion link, it will inevitably cause the last output oscillation, The entire system is not in voltage regulation. Therefore, it is possible to determine the stability of the system by analyzing the gain and phase of GH.
Also because when GH = -1 is oscillated, there is a phase shift ∠GH 180 ° (because the negative feedback itself has a 180 ° phase shift), the loop gain | GH | = 1 (0dB). So to make the transport Playing stability requires the following conditions:
1.
The phase condition is to be smaller than 180 °, that is, a margin of 45 ° or more, and the gain conditions generally requires the -12dB overhead.
2. Closer by 20 dB / DC through the frequency. Related explanations explained below.
Second, BODE map foundation
We can determine the stability of the system through the frequency characteristics of the loop gain GH, and the frequency characteristics of the loop gain | GH | and the circuit phase difference ∠GH can be represented by the BODE map (FIG. 2), and the stability of the system can pass through Bode The phase margin, a gain margin, and Crossover Frequency are measured. In which
Phase margin means that the phase corresponding to the gain is reduced to 0 dB, and the degree (DEG) is indicated (Fig. 2).
Gain margin refers to: The gain size corresponding to 0 is indicated by decibel (DB) (Fig. 2).
Crossover Frequency refers to the frequency point when the gain curve is route 0dB (Figure 2).
The function of the phase margin is to ensure that it is possible to stabilize under certain conditions (including the error of the components, the input voltage, the load change, temperature rise, etc.) can be stabilized. Using the nominal input rated load room temperature, there are 45 degrees. Amount. If the power supply is input, the load is very large, the phase margin should not be less than 30 degrees. In order not to close the unstable point, the gain margin is generally considered to be necessary.
The size of the frequency band width can reflect the slower response of the control loop. It is generally considered that the wider width is wide, the better the suppression ability of the load dynamic response, the smaller the overshoot, the less the restoration time is, the more fast, the system can be more stable. However, due to the impact of the right half flat zero, the bandwidth of the raw material, the bandwidth of the op amp cannot be infinite, and the bandwidth of the power supply cannot be improved without restriction, and generally take the switching frequency 1/20 ~ 1/6.
Third, loop test
For the gain and phase of the loop, we can usually measure using a loop analyzer (FRA) or gain-phase analyzer. These instruments are pre-processed by the sampled analog signals, and then by A / D conversion, the gain and phase are obtained by using the DFT (Discrete Fourier Transform) operation, and finally use the curve PLOT.
This article will analyze the Omicroni-Lab company's loop analyzer (Bode100), mainly in accordance with the connection map below, especially need to note the position of the injected resistor, and the size, in order to reduce the measurement error, the experiment is generally selected 10 ~ 100Ω resistors, the size of the disturbance signal can be read through the oscilloscope during the test, or the amplitude compression function of FRA can be used, but the amplitude of the disturbance cannot exceed 5% of the voltage, Otherwise, the result is not accurate.
Fourth, switching power supply stability test case:
After understanding factors and determination criteria affecting product stability and how to detect the product, the measured product results can be analyzed. Below, an experiment is made with a product of DELAT's wide pressure series, and the actual grounding part is seen (Figure 3), the test results are shown (Figure 4). Gain (can be derived by Figure 1).
In this case, it can be seen from the above figure that its gain margin is -9.499dB, the phase margin is 83deg, the bandwidth is 20.49 kHz, but the 83deg phase margin is that the dynamic is underneatical, in fact, can be converted to the error amplifier The compensation network is implemented in a slightly low frequency. Under the premise of ensuring the phase balance, the gain of 0 dB can be attenuated at -40dB / decade, which is more advantageous to suppress high frequency interference, which reaches the optimal response.
The frequency of 0 dB intersection is 20.49kHz. If a higher bandwidth is required, a smaller output filter capacitor can produce a smaller ripple voltage, so that the use of the use of the raw material and the volume of the module power supply can cause the module power source. Be more energy efficient. Considering the very conservative gain and phase margin, of course, some small changes can be made to the compensation link of the error amplifier, reasonable lift bandwidth, so that the system reaches a faster response and improves stability.
In addition, during the test, the loop gain and phase of the switching power supply are generally suggested to be tested at no-load, semi-load and full load. As for the power of the power of the excitation signal, in general, it is necessary to set the power of the injected AC excitation signal when the measurement frequency is low, for example, -20dbm or -30dbm. The capacitance ESR test is also very important during the loop test, and OMICRON-LAB's loop analyzer Bode100 is up to 40MHz frequency, which greatly facilitates the frequency characteristic measurement of passive device.
V. Summary:
From the perspective of the feedback loop gain, how to measure the stability of the switching power supply, give stability, the understanding of the Bode map, the use of FRA includes link diagrams, measurements of loop characteristics, and simple analysis of the experimental results, finally Propose improvement suggestions. Of course, the stability of the system can also be qualitatively analyzed by the time domain manner, but through the loop analyzer, it can be quantified in the frequency domain, thereby making more intuitive judgment on the performance of the system. Especially in the case of intense competition in the industry and the customer's delivery is very important, the power supply is extremely important. In the measurement switching power supply negative reflast loop frequency response, it is recommended to use a dedicated loop analyzer or vector network analyzer, such as Bode100, E5016B.
Our other product:
