|Ripple & noise rejection||Poor – Good|
|Stability||Can have problems|
The C-L-C filter power supply is similar to the C-R-C filter with the only difference being that the resistor in the C-R-C supply is replaced with an inductor. The supply consists of a bridge rectifier followed by a capacitor in parallel, followed by an inductor in series, followed by a capacitor in parallel. The first capacitor acts as a storage device and reduces the ripple caused by the pulsating power from the transformer / bridge rectifier combination. A second-order low-pass filter is created by the combination of the resistor and the following capacitor which greatly reduces ripple and noise at the expense of a reduced output voltage.
The inductor in the schematic below (L1) is shown as a 1mH unit. This should preferably be an air-core inductor made from heavy gauge wire in order to keep the DCR of the inductor low. For the purposes of this article it is assumed that L1 has a DCR of 0.1 ohm.
For the purposes of this article, we will assume the use of the following components:
Transformer: Nuvotem Talema toroidal; 2 Output Toroidal Transformer, 300VA, 2 x 25V ac. Spec sheet
Bridge Rectifier: Vishay GBPC3506W-E4/51, Bridge Rectifier, 35A 600V. Spec sheet
Capacitors: Epcos 4700μF 63 V dc Aluminium Electrolytic Capacitor, B41303 Series. Spec sheet
Input to the transformer is via an AC power source operating at 50Hz. Voltage is shown as 325.22V peak to peak which is about 230V RMS.
We will be using only 1 of the secondary taps on the transformer. Looking at the spec sheet you will see that the primary coil has a DCR of 4.7Ω. This is represented by R_Prim in the figure below. For a voltage rating of 25V on the secondary, we have a DCR of 0.1667Ω and output current of 6A per secondary. This is represented by R_Sec in the figure below.
The bridge rectifier is chosen because it is readily available and capable of high voltage and current.
The capacitors are chosen because they display excellent characteristics with very low ESR of only 39mΩ (shown as ESR and ESR1 in the schematic below).
The following graph shows the output voltage when faced with a constant DC load stepped from 0 Amp up to 4 Amp in 1 Amp steps.
Ripple voltage for a 1 Amp load is around 2.2V P-P which is worse by a large amount than either the capacitance filter supply or the C-R-C supply. As expected the ripple voltage gets worse with increasing load current. Also interesting to note is the significant output voltage sag with increasing current demand. This is the price we pay for simple unregulated power supplies. This could be made better by increasing capacitance but will ultimately always be there to some degree.
The nice thing about this kind of supply is that the shape of the ripple in the graph above is very nearly sine wave in form. This indicates significantly less higher order harmonics which is great! One note of caution: The output voltage under load for a C-L-C supply is lower than the capacitive supply under the same load conditions. This is because the inductor (L1 in figure 1 at the top of the page) is dropping some voltage across is because of its DCR value.
Output impedance of the entire power supply
(This includes the transformer and bridge rectifier at a current draw of 1 Amp)
WHOA! That looks different!
Immediately obvious is the sharp peak centered around 76Hz where the impedance shoots up dramatically. This is one of the dangers of working with inductors and capacitors – it is very easy to create a LCR resonant tank circuit which is exactly what we are seeing here.
The next graph shows the output impedance of the circuit but with the size of inductor L1 stepped from 0.5mH (blue) to 1.0mH (green) to 2mH (red).
By changing the size of the inductor, the magnitude of the impedance peak is changed dramatically. The change in inductor size also has an effect on the center-frequency of the peak but not enough to make it usable.
The only sure way of damping the peak is to increase the DCR of the inductor. I tried different values of DCR for the 1mH inductor, and values less than 0.5Ω all show some amount of peaking. From 0.5Ω upwards the resistance damps the LCR tank sufficiently.
The figure below shows the effect on output impedance when L1 is 1mH with a DCR of 0.5Ω
So now that we have damped the resonant peak, let’s look at what that’s done to the output voltage under different loads. The following graph shows the output voltage when faced with a constant DC load stepped from 0 Amp up to 4 Amp in 1 Amp steps.
Now this looks much better! The amount of voltage ripple at 1 Amp load is significantly reduced from 2.2V P-P previously to around 710mV P-P. The improvement carries through to all loads.
Therefore, be very careful with the C-L-C circuit – it is very easy to make things worse by placing an inductor into your power supply.
The size and DCR of the inductor should be checked for resonances using spice simulation and confirmed in-circuit using an oscilloscope.
Output impedance of the filter only
(This excludes the transformer and bridge rectifier – power is supplied by a perfect voltage source with a current draw of 1 Amp)
This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.
Low frequency impedance is dominated here by the DCR of the inductor with higher frequencies dominated by the ESR of the capacitor after the inductor (C2). The rise in impedance starting around 150kHz is due to the self inductance of the capacitor C2.
The inductor specified is an air-core unit. Steel core inductors used with DC can very easily reach core saturation. When the core becomes saturated the magnetic field collapses and the inductor no longer functions as an inductor. In this case it just becomes a short circuit – as if the inductor is no longer there but has been replaced by a piece of wire. The best way to overcome this problem is to use an inductor without a core – ie there is nothing to saturate. If a steel core must be used, be sure to have it specified for a maximum DC current well above the peak current you expect to draw from your power supply.
(How well the PSU is able to maintain a steady output voltage with changes to the load applied to it.)
From the graphs above it can easily be seen that the output voltage sags with increasing load current therefore the load regulation of this type of supply isn’t good.
(How well the PSU is able to maintain a steady output voltage with changes to the input voltage.)
If the AC Mains voltage fluctuates this will directly impact on the output voltage because there is no mechanism to stop it doing so – this type of supply has no line regulation at all.
The C-L-C is a low-pass filter. With the values shown in the schematic above this C-L-C filter has a -3dB point of around 85 Hz. The -3dB point can be lowered by using a higher value of capacitance or by increasing the value of the inductor L1.
Any noise present or induced into the AC mains line will be damped by the transformer primary because it has resistance, inductance and a little capacitance. The dominant factor which determine noise transfer in the transformer will be the large inductance of the primary coil. This inductance causes rising input impedance with frequency, ie it is a low-pass filter. Any high frequency noise on the line should therefore be reduced rather well.
On the secondary winding things are slightly different. There are only 2 significant sources of noise: the noise the bridge rectifier introduces (switching noise) and the possibility of induced noise on the line. When a power supply is housed inside a grounded metal case there shouldn’t be any significant amount of external noise which will affect the secondary winding or subsequent rectified supply line.
The potential exists for an AC power carrying cable which passes near-by the power supply to induce some 50Hz noise into the line. With a bit of forethought and care in placement this shouldn’t be a problem. In difficult situations you could always use shielded cables to carry the AC mains inside the case which will significantly reduce any stray magnetic coupling into the supply.
If some noise manages to be transferred onto the output stage of the power supply it is further filtered by the L1; C2 low pass filter in the supply. For this reason the C-L-C power supply has a “good” rating for noise reduction. While the reduction in noise is not spectacular it does help matters if you are dealing with a noisy supply.
Even though there are no active components in this power supply and you would expect it to be stable there are issues related to causing LCR resonant tank circuits when using an inductor in conjunction with capacitors. Stability can be very good if your design ensures that no resonance exists but this is not necessarily an easy task.
Temperature has no significant impact on the circuit. As usual, capacitors suffer most with high temperature which drastically shortens their life. A rule of thumb is that for every 10ºC rise in temperature the capacitor’s life is halved.
The output impedance of this type of supply is heavily dependent on the components used. At lower frequencies the impedance is dominated by the bridge rectifier, transformer and L1 DCR combination while at higher frequencies the impedance is dominated by the ESR of the capacitors after the L1 inductor.
The C-L-C filter is moderately difficult to implement but will work well in many applications if you can manage to stop an LCR resonant tank from forming. Cost can vary from relatively inexpensive to very expensive depending on the size of the transformer, large air core inductor and the amount of capacitors needed to ensure sufficiently low ripple voltage on the output.
Best used where your circuit has good PSRR (power supply rejection ratio) or where the ripple and voltage sag will not overly influence operation.