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    1. Requirements for an output filter

    Switching controllers exhibit a residual ripple in their output, which disturbs electronic device and may cause electromagnetic interference. Output filters are therefore often used for interference suppression, which may, under certain circumstances, exert an influence on the control loop. Compensation of the control loop may be required to prevent losses in output power. Irrespective of which switching controller topology is in use, the output current causes unwanted residual ripple through the parasitic series resistance ESR and the parasitic inductance ESL of the output capacitor. A relatively large residual ripple arises which depends on the capacitor type selected and has different waveforms. For example, a common electrolytic capacitor causes a ripple voltage of up to several hundred millivolts depending on the output voltage of the switching controller. Selecting a ceramic capacitor, the remaining ripple voltage may only be a few tenths of a millivolt. High residual ripple is undesirable and can disturb the electronic devices. Special analog and HF circuits require a stable, smoothed and clean supply voltage. However, the high frequency component of harmonic oscillations on the output voltage cannot be overlooked, as this may lead to increased electromagnetic interference emission. Here the output filter can reduce the residual ripple and filter out high frequency components.


    2. Reduction of residual ripple

    In practice, the decision is usually made for a LC low-pass filter in order to reduce the residual ripple to a few millivolts and to suppress high frequency components. Figure 1 shows such a low-pass filter, which can, for instance, be realized with an unshielded coil, a WE-PD2 and a conventional electrolytic capacitor.

    Figure 1: Simple low-pass filter


    If a particularly clean output voltage is required, the LC low-pass filter is extended with a second stage low-pass filter consisting of a ferrite and a capacitor. Figure 2 shows this type of two-stage output filter, which can be realized inexpensively, for example with a WE-PD2 coil and a WE-MPSB SMD ferrite.

    Figure 2: Two-stage output filter


    "Lfilter" and "Cfilter1" act as a low-pass filter, which filters out the clock frequency of the switching controller and attenuates its harmonic oscillations. Further high frequency components of the switching controller output voltage are converted into heat by the SMD ferrite and together with "Cfilter2" their amplitude is attenuated. An output filter of this type reduces the residual ripple to a few millivolts and even allows sensitive analog circuits to be supplied.


    3. Direct current losses at the output filter

    Above a certain switching controller output power, the output filter causes large DC losses of the output power and thus a reduction in the efficiency of the switching controller. The DC resistance RDC of the coils and ferrites now generates a significant voltage drop across the output filter and therefore causes a reduction in the resulting output voltage. Figure 3 shows this effect.

    Figure 3: Voltage drop at the filter coil


    According to the dependence of the size of the coil, the RDC may be between a few milliohms and several ohms and is therefore not negligible for high output currents. Even an SMD high current ferrite can have an RDC of up to 0.04 Ω. To determine the actual voltage, for switching controllers the output voltage is tapped from a voltage divider and is passed to the feedback connection of the switcher IC. To reduce output voltage losses due to an output filter, there is the option of implementing the output filter in the control loop by determining the actual value after the output filter. Figure 4 shows the schematic arrangement of this method.

    Figure 4: Implementation of the output filter in the control circuit


    4. Stability of the control circuit

    However, the filter coil, ferrite and the filter capacitors cause an unwanted phase shift, which disturbs the stability of the control loop. This unwanted phase shift gives rise to a reduction in the amplitude and phase margin. In extreme cases, it leads to instability and the output voltage tends to oscillate. In order to ensure stability, an amplitude margin of > 12 dB and a phase margin of > 45 is necessary in practice, such that the converter does not oscillate on excitation. The control circuit is considered dynamically stable if the loop gain drops to 0 dB before the associated phase shift has attained a value of -180 . The loop gain should cross the x-axis, i.e. at 0 dB with 20 dB / decade. Figure 5 shows a Bode plot of a stable-regulated step-down (buck) converter. This example shows an amplitude margin of 32 dB and a phase margin of 56°.


    Figure 5: Bode plot of a stable-regulated switching converter


    If the stability criteria of a switching converter are not met with an output filter, compensation of the control loop is required to ensure a stable output voltage. The stability of the control section thus exerts an influence on the stability of the output voltage.


    5. Transient response

    In the event of a voltage fluctuation at the input of the switching controller, the output voltage should remain stable. Analogously, in case of a sudden drop or rise of the output current, the output voltage should readjust quickly. This is what transient response means. Figure 6 shows the transient response of a stable-regulated switching converter (yellow curve) with an output voltage of 5 V and a sudden change in load from 0 A to 1 A (green curve).


    Figure 6: Transient response of a stable-regulated switching converter


    A sudden change in load should lead to a rapid step response of the control circuit, which promptly regulates the output voltage to its target value. The step response should not cause too great an amplitude in the output voltage, as otherwise electronic circuits could be destroyed by overvoltage. Ideally, the output voltage should be promptly regulated to the target value following the step response, without generating an overshoot or even ringing. Ringing during the compensation phase would therefore be attributable to instability of the switching controller. If there is a fast step response and a timely compensation phase, the switching controller may be considered to be stable-regulated.


    6. Summary

    If the output filter is implemented in the control section, the control section is 2nd order. The switching controller must therefore be operated with a higher integration component, which attenuates the control section and hence slows it down. Complicated compensation of the control section is now required. The method of implementing the output filter in the control section is therefore not recommended. The switching controller output voltage should be tapped directly at the switching controller output capacitor, i.e. prior to an output filter. It is recommended to select filter coils and ferrites with the lowest possible RDC in order to reduce DC losses in the output filter.


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