An example with L1 = 6.8 uH, C1 = 100 uF, L2 = 3 uH, and C2 = 22 uF is shown below.Īdding more L and C elements can modify the transfer function by introducing new poles. While the original filter stage may not have had problems with instability in its design, adding the new filter stage can produce new poles, which would be apparent in the combined filter’s transfer function. Modified buck converter with an output LC filter. This would look like the following circuit: When an LC filter circuit is added to an output filter in a switching converter, you have just converted a 2nd-order LC filter to a 4th-order LC filter. When adding an output filter, the goal is to make sure you do not add a new pole to the combined transfer function for the output stage. A more elaborate method, and certainly more effective from the noise perspective, is to use an additional LC filter on the output stage. If you want to beef up the capabilities of your output LC stage, the simple options are to use a larger inductor and/or a higher frequency PWM signal. Typically, in a buck converter, there is just enough resistance in the output and control loop to prevent these oscillations. In this case, we would expect the transient response to be underdamped with about 20 kHz oscillations. But the point stands: the output filter will have some transient response that can be severely underdamped. Note that the real transfer function will be limited by DC conduction losses in the switching elements, the inductor winding resistance, and the capacitor’s ESR value. Original undamped transfer function of the LC output stage in our simple buck converter. If we look at an example buck converter with output capacitor and inductor values of L1 = 6.8 uH and C1 = 10 uF, the circuit and transfer function for this simple buck converter would look like the following: For a buck converter, we can see this in the following equation: However, if you increase the size of that inductor or the switching frequency, you will reduce ripple by increasing the impedance seen by the rippling signal on the output. The LC stage is actually a low-pass filter, we just never talk about it as a low-pass filter in practice because everyone focuses on ripple. Together with a rectifying element (diode or transistor), these components are responsible for directing switched-polarity current to a DC with an average voltage set by the duty cycle of the switching PWM signal. There is a capacitor and inductor on the output stage that produces the required DC output plus some ripple overlaid on a nominal DC level. Think for a moment about the typical buck converter. The input portion of this topic is more extensive and is related to ensuring EMC compliance, so we will leave that for a future article. In this article, we’ll examine output filter design for DC/DC converters from the LC filter perspective as this is the most common way to produce a low-pass filter with reasonable rolloff in the stopband. What they don’t tell you in those reference designs is that the input/output/control loop transfer functions must be monitored during designs so that oscillations in the transient response can be prevented. The 2nd method is very common and will often be seen in reference designs for power product, both on the input and output. The 1st of these is more about efficiency than noise, although it is cited by experts as an important element of conducted EMI control. FCC and CISPR 22 both place limits on conducted emissions throughout power systems from 150 kHz to 30 MHz, which drives the need for innovative filtering and power stability solutions.īroadly, there are two ways designers can approach this challenge with noise: Similarly, if downstream from another converter or a rectification stage in a high power system, noise at the converter’s input stage and the noise returned to the power source should be limited. To deal with noise, the standard approach is to add filtering to the output stage of the converter. Switching DC/DC converters generate a lot of noise at high frequencies, so much that excessive noise can cause an EMC failure that requires redesigns.
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