Hi,
I need to filter the input of a dc dc converter because of the conductive voltage emissions. With a Pi filter (as shown below).
I am really confused about selecting components because some sources say that C1 and L are the main filter components and other sources say that it's C2 and L that do the job (and C1 should have a really low ESR). C3 and R are used to damp the filter.
Is there a guide, a course or something that explains the pi filter in a complete way and without ambiguity ?
I followed this guide https://fscdn.rohm.com/en/products/...ower/inputfilterfordcdcconverter_ane.pdf but I'm not sure that what I calculated is correct. So,
Vin = 24V
The switching frequency (fs) of the converter is 330kHz
(The input current of the converter is 160mA at full load)
(The maximum ripple voltage of the converter is 60mVpp)
The capacitor should have a voltage that is at least twice the input voltage, so it should be 50V.
First, C1 is chosen for its low ESR (to not heat). I found a 33uF / 50V ceramic capacitor that has an ESR of 0.02 ohm at the frequency of 300kHz (near the resonance).
Next for C2, with L = 10uH (starting point, more or less arbitrary), we have the equation
\[ fc = \frac{1}{2\pi\cdot\sqrt{L\cdot C_{2}}} \]
And to reduce the noise, the filter frequency should be a tenth of the converter switching frequency.
So we have :
\[ C_{2}=\frac{1}{(2\pi\cdot0.1\cdot fs)^2\cdot L} \]
\[ C_{2}=\frac{1}{(2\pi\cdot0.1\cdot 330\cdot10^3)^2\cdot 10\cdot10^{6}}\cong2.32\mu F \to 2.7\mu F \]
For the damping resistor/capa, we have :
\[ R=\sqrt\frac{L}{C_{1}} \]
\[ R=\sqrt\frac{10\cdot10^{6}}{2.7\cdot10\cdot10^{6}}\cong3.7\Omega \to 3.3\Omega \]
And \[ 5\cdot C_{1} < C_{3} < 10\cdot C{1} \]
The minimum value of C3 would be 180uF but it seems impossible with a low ESR.
I would have C1 = 33uF/50V, C2 = 2.7uF/50V, C3 = 180uF/50V (electrolytic ?), L = 10uH / 192mA (at least + 20% of input converter current), R = 3.3 ohm
What do you think of this ? Is a 0.02 ohm ESR (C1) good ? How could I have a more precise idea of its impact ? Is it \[ I_{C2}=\frac{U_{ripple_{rms}}}{ESR} \] which would be 1A ?
Thank you in advance
I need to filter the input of a dc dc converter because of the conductive voltage emissions. With a Pi filter (as shown below).
I am really confused about selecting components because some sources say that C1 and L are the main filter components and other sources say that it's C2 and L that do the job (and C1 should have a really low ESR). C3 and R are used to damp the filter.
Is there a guide, a course or something that explains the pi filter in a complete way and without ambiguity ?
I followed this guide https://fscdn.rohm.com/en/products/...ower/inputfilterfordcdcconverter_ane.pdf but I'm not sure that what I calculated is correct. So,
Vin = 24V
The switching frequency (fs) of the converter is 330kHz
(The input current of the converter is 160mA at full load)
(The maximum ripple voltage of the converter is 60mVpp)
The capacitor should have a voltage that is at least twice the input voltage, so it should be 50V.
First, C1 is chosen for its low ESR (to not heat). I found a 33uF / 50V ceramic capacitor that has an ESR of 0.02 ohm at the frequency of 300kHz (near the resonance).
Next for C2, with L = 10uH (starting point, more or less arbitrary), we have the equation
\[ fc = \frac{1}{2\pi\cdot\sqrt{L\cdot C_{2}}} \]
And to reduce the noise, the filter frequency should be a tenth of the converter switching frequency.
So we have :
\[ C_{2}=\frac{1}{(2\pi\cdot0.1\cdot fs)^2\cdot L} \]
\[ C_{2}=\frac{1}{(2\pi\cdot0.1\cdot 330\cdot10^3)^2\cdot 10\cdot10^{6}}\cong2.32\mu F \to 2.7\mu F \]
For the damping resistor/capa, we have :
\[ R=\sqrt\frac{L}{C_{1}} \]
\[ R=\sqrt\frac{10\cdot10^{6}}{2.7\cdot10\cdot10^{6}}\cong3.7\Omega \to 3.3\Omega \]
And \[ 5\cdot C_{1} < C_{3} < 10\cdot C{1} \]
The minimum value of C3 would be 180uF but it seems impossible with a low ESR.
I would have C1 = 33uF/50V, C2 = 2.7uF/50V, C3 = 180uF/50V (electrolytic ?), L = 10uH / 192mA (at least + 20% of input converter current), R = 3.3 ohm
What do you think of this ? Is a 0.02 ohm ESR (C1) good ? How could I have a more precise idea of its impact ? Is it \[ I_{C2}=\frac{U_{ripple_{rms}}}{ESR} \] which would be 1A ?
Thank you in advance
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