Summary:
The 90Vac to 305Vac wide input range for high power PWM dimming LED street lighting is increasingly used, and the UCC28019A Controller is ideal for this application. However, the PFC inductor noise problem caused by output load PWM dimming may be the main problem. In this paper, we will analyze the root cause of this phenomenon based on the small-signal model and propose a solution. To verify the effectiveness of this proposed solution, we used the UCC28019A average model and used experiments to test. It has been proved that the experimental results are in good agreement with the analytical results and the simulation results.
Key words:
UCC28019A, LED lighting, APFC, average model, load dynamics, simulation, noise problem
1 Introduction
The average current control of CCM operation is the most typical control scheme, which is widely used in high-power APFC converters, such as UC3854-based converters. It has many advantages over peak current control, such as: no external compensation slope, higher inspection current signal noise rejection, and lower input current THD. However, the traditional CCM control scheme using a multiplier inside the chip complicates the external current design. Recently, a new CCM (an 8-pin solution) using a 1-D control model, such as the TI UCC28019A controller, has become the first choice for engineers.
The UCC28019A controller utilizes the pulse and non-linear characteristics of the switching converter to achieve instant control of the rectified voltage or current average. The purpose of this control scheme is to provide faster dynamic load response and better input disturbance rejection than other PFC controllers.
In actual engineering, most engineers are impressed with the high PF value of the wide input range UCC28019A controller and the excellent performance of no output overshoot boost. This advantage makes the UCC28019A more suitable for this application than the traditional BCM PFC controller. This is especially true in applications where the 90-300Vac wide input range of high-power PWM dimming LED street lighting is used more and more. However, when using dynamic response, its unique loop characteristics can cause visible noise problems.
In view of the above issues, the goal of this article is to introduce you to a suitable solution that can improve this dynamic performance. First, the second section details this issue. In order to study the reasons, Section 3 analyzes the small signal of the current loop; based on this, the root cause is explained in detail and a correct solution is proposed. In order to fully verify this solution, we also validated the experimental tests and results in Section 4 using the corresponding UCC28019A averaging model. Finally, combined with practical engineering applications, the article outlines some of the design techniques for this solution.
2, UCC28019A load dynamic visible noise description
Dynamic noise problems can occur in the following two situations:
• The load rises when the loop is not optimized (see Figure 1), at which point the PFC peak inductor current increases instantaneously, causing the ferrite inductor to saturate.
Figure 1 Load rise for saturating the PFC inductor
• Figure 2 shows the load drop operation when the loop is not controlled during OVP operation. During this process, the irregular frequency causes the PFC to have a high peak inductor current, resulting in visible noise.
Figure 2 shows the load drop of the PFC inductor saturation
About LEDs - This phenomenon is suppressed for outdoor applications due to late PWM load dimming requirements.
3. Analysis of the root cause based on the working principle of UCC28019A
Small signal modeling is the most practical way to study the stability of converter control loops. This section focuses on the main small-signal transfer function of the UCC28019A internal current loop, because the inductor current response under voltage loop voltage disturbance is our main research goal.
3.1 Analysis of PFC inductor current noise according to VCC change during UCC28019A load rise period
In the case of a conventional PFC converter, the key to achieving power correction is to let the input current track the input voltage. [1] The document details the implementation of the 1-D control circuit. In order to study its small-signal characteristics, in this section we only introduce the implementation of the small-signal transfer function. In fact, inside the UCC28019A, there are two loops: the current loop and the voltage loop.
After an in-depth study of the 1-D control scheme of the APFC converter, it was found that the 1-D functional equivalent circuit can be moved to the control module of the current loop. See the internal function module of [2]. Figure 3 shows the compensation current loop using the 1-D control scheme inside the UCC28019A:
Figure 3 Cc based current compensation loop
( 
A small signal that is perturbed to the PFC, Icomp, and 1-D current sense resistor voltages; m1 and m2 are nonlinear gains, k1 is the internal controller constant, Cc is the compensation capacitor, and Ts is the switching time).
Compensation transfer function of current loop 
The deduction is as follows: 
As for power factor correction, the main problem is the operation of the input current that tracks the input voltage. We know that in the low frequency range of 90 to 120 Hz, the input current always tracks the input voltage; therefore, the current loop is the only low frequency characteristic problem. Like the UC3854, the power factor principle of the UCC28019A combines the low frequency characteristics of the current loop. From equation (1), we can see that the low frequency gain under steady state operation is:
Also, at low frequencies:
Combine equations (2) and (3) at low frequencies and the result is:
See the boost converter principle:
Finally, the input current formula for tracking the input voltage is derived as follows:
It shows that PF is reached. From the previous formula, the standard formula based on equation (1) is:
The Iin perturbation of Vcomp gives:
among them 
. Use the MathCAD expression as follows:
Figure 4 m3 (Vcomp) curve under Vcomp
By (8), we know that 
depending on 
The steady value, which indicates that the Iin current is still disturbed by these parameters under certain stable operating conditions. This also shows that the PFC current responds quickly after Vcomp changes rapidly.
From the previous analysis, we can conclude that if there is a small change in the PFC inductor current when injecting a Vcomp disturbance, it must be reduced at the same time. 
Value. However, this is difficult to control during the actual function optimization design process. Therefore, the standard solution relies on the extent to which the Vcomp variation is reduced as the entire voltage loop is closed.
3.2 UCC28019A During the load rise period UCC28019A Vcomp change root cause analysis
The UCC28019A product specification describes its voltage error amplifier as follows:
Figure 5 internal principle of the voltage loop
When more than 5% of the output voltage disturbance occurs at the VSENSE input, the amplifier goes out of linear operation. In the undervoltage condition, the UVD function calls EDR, which immediately increases the voltage error amplifier transconductance from 42μs to 440μs. This high gain causes the compensation capacitor to charge more quickly to a new operating level. This shows that the EDR generates a large amount of Vcomp charge, which greatly increases the Vcomp boost, especially when the output current increases dramatically. Therefore, if the Vcomp effect is reduced according to the EDR function requirements, the voltage loop response speed must be slightly increased to avoid the UVP point if possible. As shown in Figure 6, we must slightly lower the response speed of the voltage feedback circuit (usually Ccv2) to make it slightly faster than the loop response time.
Figure 6 voltage feedback compensation loop
3.3 Analysis of PFC Inductor Current Noise during Bucking of UCC28019A Controller
In most cases, PFC inductive noise may occur during PFC load drop. Experiments have shown that this inductive noise occurs when the output OVP is triggered. In addition, this noise may exist for a considerable period of time if the OVP remains triggered, especially when the load is switched to light load mode. Therefore, the noise is closely related to the output OVP protection mode.
According to the product manual, the UCC28019A has a very simple OVP protection mode—if the OVP protection is triggered, it directly shuts down the drive. However, in actual experiments, we found that the driver is abnormal in this state, and the inductor current also has some abnormally high peak electric shock.
Many experiments have shown that Vcomp drops very slowly with this process. If the buck time is shortened, the noise is reduced. Therefore, a good solution is to use some external method to quickly discharge the compensation capacitor when the OVP is triggered. Once the Vcomp voltage drops, the output also leaves the OVP level and there is no longer a noise problem.
3.4 UCC28019A PFC Inductor Current Noise Solution During Load Drop
As analyzed in Section 3.3, there is a way to quickly reduce the Vcomp voltage. In some cases, this does not present a serious problem because we chose a small value compensation capacitor and the noise is not so obvious. However, in most cases, when the PCB layout is not ideal and the higher PF value is not reached, there is no room for optimization in the voltage compensation loop, but the noise is still significant; in this case, an external circuit is required. To solve this problem.
The suggested solution is as follows:
For ease of understanding, we use the standard OP and TL431 or TL103, the circuit is shown in Figure 7.
Figure 7 Suggested solution compensation loop simple schematic
Figure 8 shows the complete solution using TL103. Under normal circumstances, half of the TL103 can be used for high temperature protection, which is required by the safety standards in actual engineering.
Figure 8 Complete solution to improve load dynamics with TL103
In the actual design, the focus of this solution must meet the following requirements for the high tolerances of R1, R2 and TL103:
4. UCC28019A average model and actual experiment verification suggestion solution
To verify the feasibility of the solution mentioned in the previous section, we built the UCC28019A average model and simulated it. At the same time, the prototype was manufactured and the solution was verified.
The simulation model and experimental prototype are based on the parameters listed in Table 1.
Table 1 List of prototype parameters 
Figure 9 Average model of the UCC28019A application
When the PFC changes from no load to full load transient, and the EDR still works in the PFC operating state, the peak current of the PFC inductor is unavoidable; however, there is no inductor saturation problem and no visible noise. However, when the PFC transitions from full load to no load transient, there is noise in the inductor. Figure 10 shows the simulation results for the initial application.
Figure 10 No TL103 output and inductor current simulation results
From the previous simulation results, we know that when the PFC load drops, the noise can be clearly observed. Now, Figure 9 depicts this external TL103; Figure 11 shows the simulation results for the output voltage, inductor current, and OP output.
Figure 11 Simulation results of output, inductor current and TL103 output
From the simulation results shown in Figure 11, we can see that the noise disappears, and TL103 discharges the capacitor current of the voltage loop. Therefore, the output voltage can quickly enter the adjustment range. However, a very important issue is that no-load power requirements must not affect no-load operation.
In order to verify the actual work situation, we did an experiment on the prototype. Figure 12 shows the measurement results based on the initial application; we can clearly see the presence of noise during the output load drop. However, with the improved solution, the noise disappears (see Figure 13), which is identical to the simulation results.
Figure 12 Output without TL103 and PFC inductor current measurement results 
Figure 13 Measurement results of output, inductor current and TL103 output
5 Conclusion
Based on the initial UCC28019A application, this paper details the root cause of the noise problem during the rise and fall of the output load. This is an urgent problem for PWM dimming LED street lighting applications. We have proposed the corresponding solution for you, and verified its effectiveness through theoretical analysis and simulation as well as experimental measurements. The results show that they are very satisfactory.
In summary, by recommending the TL103 external circuit, we can optimize the voltage loop parameters to avoid load rise noise, and also avoid load drop noise. Note that another part of the TL103 can be used for high temperature protection in practical engineering, which means that this proposed external circuit is of great value.
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