1 Introduction
A stable feedback loop is very important for switching power supplies. If there is not enough phase margin and amplitude margin, the dynamic performance of the power supply will be poor or output oscillation will occur. The following describes the various zero and pole amplitude and phase frequency characteristics of the three control modes, and then analyzes the zero, pole and characteristics of the most commonly used feedback regulator TL431. Topswitch is a smart chip for flyback power supply widely used in the market. Its control method is more complicated voltage type control. It integrates some compensation functions internally. Finally, it analyzes the power supply of a Topswitch design and dissects its loop.
2 Loop compensation mode and TL431 characteristics
2.1 Single pole compensation
Suitable for current mode control and power supply operating in DCM mode with low ESR zero frequency of the filter capacitor. Its main function is to lower the control bandwidth and reduce its gain to 0 dB before the power portion or the portion with other compensation reaches 180 degrees.
figure 1
2.2 Bipolar point, single zero compensation
Applicable to the compensation of only one pole of the power part, such as all current type control and discontinuous mode voltage type control.
figure 2
2.3 Three-pole, double zero compensation
Suitable for output topology with LC resonance, such as all inductor current continuous mode topologies without current mode control.
image 3
2.4 Zero-Pole Characteristics of TL431 Output Power Supply
The TL431 is the most commonly used reference and error amplifier for secondary feedback of switching power supplies. The different power supply modes have a great influence on its transfer function.
Figure 4
among them:
As can be seen from the above equation, in the case where the output directly supplies power to 431, the position of the zero point is at 1/[2Ï€(R+R1)C] instead of 1/(2Ï€RC). Even if there is no R, only one C is connected, the zero point still exists. If R1 is much larger than R, the position of the zero point is mainly determined by the upper voltage dividing resistance of the feedback network. In order to suppress the switching ripple of the output, sometimes an LC filter is added, and the resonant frequency is generally about 1/10-1/20 of the switching frequency. This frequency is usually much larger than the bandwidth of the feedback loop, and its influence can be neglected.
3 case analysis
The following figure is a typical Topswitch power supply control loop, wide range input, 12V / 2.5A output, the schematic diagram is as follows;
Figure 5
The feedback loop diagram of the power supply is as follows:
Figure 6
Its open loop transfer function is:
KPwr-power partial transfer function; KLC-output LC filter partial transfer function; KFb-feedback partial pressure partial transfer function; Kea-feedback compensation part and optocoupler part transfer function; KMod-modulator part transfer function. Before making the compensation design, calculate the frequency characteristics of K1=KMod × KPwr × KLC × KFb, determine the required design target KKa according to the actual situation, and then complete the KEA requirements by designing the corresponding compensation of TL431.
3.1 Small signal transfer function K1 except the compensation part
In this design, Kfb=1 because the upper divider resistor is directly connected to the 431 reference.
Design the compensation part, first determine the target bandwidth, and then design the compensation part so that the phase margin at the target bandwidth is greater than 45°. The amplitude margin is automatically satisfied regardless of the above compensation method, so the design is generally not used. Special consideration. In the flyback power supply designed by Topswitch, the target bandwidth is limited by the general limit of the flyback power supply (the bandwidth is less than 1/2 of the switching frequency; 1/4 of the zero point of the right half plane; the op amp gain limit; the output capacitor The type of selection) is also limited by the internal 7KHz pole, generally not too high, about 1-2KHz.
3.2 TL431 partial small signal transfer function
Since the TL431 is powered by the output, its transfer function is:
The function has a pole at the origin and a zero:
The R6 and R9 sizes determine the gain. Since R9 is determined by the position of the zero point, the overall gain is determined by adjusting R6. CTR is the measured current transfer ratio of the optocoupler PC817C. The compensation section has only one pole and zero, which together with the 7kHz pole in the Topswitch form a π-type compensation network. 7KHz pole is used to offset the output filter
Zero tolerance, attenuation noise and switching ripple interference.
3.2 Total open loop response
The open loop gain of the entire loop is the product of K1 and KEA, which is the algebraic sum of the gain and phase of the two parts.
Finally, the crossover frequency is 1.16 kHz and the phase margin is 66.5°, which satisfies the loop stability requirement.
4 Summary
This paper introduces some concepts and basic design methods of loop compensation, analyzes the small signal characteristics of the TL431 in the output power supply, and then analyzes the control loop of a specific Topswitch flyback application. These methods can also be applied to other methods. In the analysis of the topology, when the secondary operational amplifier is used for feedback control, if the optical coupling is connected to the op amp output and the power output
The analytical method of TL431 is also applicable.
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