“Different from the traditional voltage mode or current mode control architecture, the DCAPx control system has two direct output feedback paths: one through the feedback Resistor divider network, and the other through the DC resistance (DCR) injection circuit, as shown in Figure 2. Show. The DCAPx control system does not have a large direct current (DC) current gain error amplifier like the traditional Type II or Type III compensator. The PWM pulse is modulated at the FB pin. The FB pin is usually the negative input of the error amplifier of the traditional control architecture. For DCAP, DCAP2, DCAP3, it is an input terminal of the PWM comparator.
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Author: Melinda Xie
The control loop gain can be plotted in the Bode Plot, which is an indicator that can better evaluate the stability of the system. The control loop bandwidth can also directly affect the transient response performance.
DCAP™ or DCAP2™/DCAP3™ regulators (I will call them DCAPx in this discussion) are popular because of their simplicity. When it comes to the measurement of control loop gain, DCAPx poses a challenge to engineers. By cutting off the loop from the top of the feedback resistor divider (as shown in Figure 1), it is easy to measure the Bode plot. This is suitable for traditional control architectures, which have only one output feedback path, and the feedback passes through the compensator before pulse width modulation (PWM).
Figure 1: Traditional control loop gain settings
Different from the traditional voltage mode or current mode control architecture, the DCAPx control system has two direct output feedback paths: one through the feedback resistor divider network, and the other through the DC resistance (DCR) injection circuit, as shown in Figure 2. Show. The DCAPx control system does not have a large direct current (DC) current gain error amplifier like the traditional Type II or Type III compensator. The PWM pulse is modulated at the FB pin. The FB pin is usually the negative input of the error amplifier of the traditional control architecture. For DCAP, DCAP2, DCAP3, it is an input terminal of the PWM comparator.
Figure 2: Block diagram of DCAP regulator with DCR injection circuit
If the measured output value of a feedback path is discarded, the Bode plot measured with the setup shown in Figure 1 is not directly related to the transient response. Therefore, to correctly measure the loop gain Bode plot, the loop cut-off point should include two feedback paths, as shown in Figure 3.
Figure 3: Correct DCAP regulator control loop Bode plot measurement setup
For DCAPx regulators, what determines the PWM modulation gain is the falling slope of the triangular waveform formed at the FB pin by the DCR injection network and the equivalent series resistance (ESR) of the output capacitor. The parasitic inductance and resistance injected into the cable along the disturbance and the noise coupled to the wire will tamper with the triangular waveform at the FB pin, making the PWM modulation gain different from the gain of the regulator without a test setup.
To maintain accuracy, a bypass capacitor (Cpass) is added in parallel to the 20Ω resistor. This 20Ω resistor and Cpass form a high-pass filter. The corner frequency is set to be lower than half of the converter’s switching frequency so that the triangular waveform at the FB pin is basically similar during the test and during normal operation.
For a converter with a switching frequency of 500kHz, I use a 0.22μF capacitor. For most applications, the appropriate Cpass value range: 0.1μF to 0.47μF. In order to minimize the impact on the system, the DCR injection capacitor (Cp) should be smaller than one-tenth of Cpass, as shown in Figure 3.
Figure 4 shows the results of the Bode plot measurement using the test setup shown in Figure 3. Cpass = 0.22μF, CP = 22nF. By adjusting Rp and Cff, the crossover frequency is set to one-sixth of the switching frequency, and the phase margin is 66 degrees. The reference design used by the author for these experiments: a step-down converter (PMP8824) that supplies power to the rails in Altera’s Arria V FPGA.
Figure 4: Bode plot measured with the recommended test setup of TPS53319 (with DCR injection capacitor, Vout = 1.2V)
Figure 5 shows the corresponding transient response when the load is stepped up and stepped down. The author also used PMP8824 for these experiments.
Figure 5: Load transient response of TPS53319 (Vout=1.2V)
For DCAP2 and DCAP3 control systems, the DCR injection circuit is integrated inside the silicon chip. The same techniques are applicable. Figure 6 shows the loop Bode plot test setup of the DCAP2 and DCAP3 regulators.
Figure 6: Correct DCAP2 and DCAP3 regulator control loop Bode plot measurement settings
For DCAP or DCAP2/DCAP3 regulators, the Bode plot is measurable. The techniques provided in the previous discussion can be used to measure Bode plots to ensure system stability and as a guide for optimizing transient performance.
Author: Melinda Xie
The control loop gain can be plotted in the Bode Plot, which is an indicator that can better evaluate the stability of the system. The control loop bandwidth can also directly affect the transient response performance.
DCAP™ or DCAP2™/DCAP3™ regulators (I will call them DCAPx in this discussion) are popular because of their simplicity. When it comes to the measurement of control loop gain, DCAPx poses a challenge to engineers. By cutting off the loop from the top of the feedback resistor divider (as shown in Figure 1), it is easy to measure the Bode plot. This is suitable for traditional control architectures, which have only one output feedback path, and the feedback passes through the compensator before pulse width modulation (PWM).
Figure 1: Traditional control loop gain settings
Different from the traditional voltage mode or current mode control architecture, the DCAPx control system has two direct output feedback paths: one through the feedback resistor divider network, and the other through the DC resistance (DCR) injection circuit, as shown in Figure 2. Show. The DCAPx control system does not have a large direct current (DC) current gain error amplifier like the traditional Type II or Type III compensator. The PWM pulse is modulated at the FB pin. The FB pin is usually the negative input of the error amplifier of the traditional control architecture. For DCAP, DCAP2, DCAP3, it is an input terminal of the PWM comparator.
Figure 2: Block diagram of DCAP regulator with DCR injection circuit
If the measured output value of a feedback path is discarded, the Bode plot measured with the setup shown in Figure 1 is not directly related to the transient response. Therefore, to correctly measure the loop gain Bode plot, the loop cut-off point should include two feedback paths, as shown in Figure 3.
Figure 3: Correct DCAP regulator control loop Bode plot measurement setup
For DCAPx regulators, what determines the PWM modulation gain is the falling slope of the triangular waveform formed at the FB pin by the DCR injection network and the equivalent series resistance (ESR) of the output capacitor. The parasitic inductance and resistance injected into the cable along the disturbance and the noise coupled to the wire will tamper with the triangular waveform at the FB pin, making the PWM modulation gain different from the gain of the regulator without a test setup.
To maintain accuracy, a bypass capacitor (Cpass) is added in parallel to the 20Ω resistor. This 20Ω resistor and Cpass form a high-pass filter. The corner frequency is set to be lower than half of the converter’s switching frequency so that the triangular waveform at the FB pin is basically similar during the test and during normal operation.
For a converter with a switching frequency of 500kHz, I use a 0.22μF capacitor. For most applications, the appropriate Cpass value range: 0.1μF to 0.47μF. In order to minimize the impact on the system, the DCR injection capacitor (Cp) should be smaller than one-tenth of Cpass, as shown in Figure 3.
Figure 4 shows the results of the Bode plot measurement using the test setup shown in Figure 3. Cpass = 0.22μF, CP = 22nF. By adjusting Rp and Cff, the crossover frequency is set to one-sixth of the switching frequency, and the phase margin is 66 degrees. The reference design used by the author for these experiments: a step-down converter (PMP8824) that supplies power to the rails in Altera’s Arria V FPGA.
Figure 4: Bode plot measured with the recommended test setup of TPS53319 (with DCR injection capacitor, Vout = 1.2V)
Figure 5 shows the corresponding transient response when the load is stepped up and stepped down. The author also used PMP8824 for these experiments.
Figure 5: Load transient response of TPS53319 (Vout=1.2V)
For DCAP2 and DCAP3 control systems, the DCR injection circuit is integrated inside the silicon chip. The same techniques are applicable. Figure 6 shows the loop Bode plot test setup of the DCAP2 and DCAP3 regulators.
Figure 6: Correct DCAP2 and DCAP3 regulator control loop Bode plot measurement settings
For DCAP or DCAP2/DCAP3 regulators, the Bode plot is measurable. The Bode plot can be measured with the techniques provided in the previous discussion to ensure system stability and as a guide for optimizing transient performance.
The Links: PH150A280-24 BSM50GD60DN2