|Ripple & noise rejection||Fair – Good|
|Output impedance||Poor – Fair|
The C-R-C filter consists of a bridge rectifier followed by a capacitor in parallel, followed by a resistor 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 first-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.
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 520mV P-P and as expected gets worse with increasing load current. Also interesting to note is the 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 much more rounded than what we find in the standard capacitive filter supply. This indicates less higher order harmonics which is always a good thing! One note of caution: The output voltage under load for a C-R-C supply is lower than the capacitive supply under the same load conditions. This is because the resistor (R1 in figure 1 at the top of the page) is dropping some voltage across is – in effect we are trading that voltage drop for a more benevolent output ripple voltage.
While we are on the topic of the resistor, let’s take a look at the power dissipation through the resistor. In the schematic above the resistor is shown as a 0.47 ohm unit. The graph below shows the power dissipation through the resistor for load currents of between 0 and 4 Amp in 1 Amp steps.
Dissipation is around 0.5 W for a load current of 1 Amp; 2 W for 2 Amp; 4.2W for 3 Amp; 7.5 Watt for 4 Amp loads. You would expect this to be a linear relationship but it definitely is not. This is because the formula for calculating power is given by W = I2 x R.
If you are planning to use a CRC filter be sure to calculate the power dissipation of the resistor under worst case conditions for your intended use and then add a factor of at least 3x for the resistor power needed. While a 5 Watt resistor should handle 5 Watt worth of dissipation readily enough it will become excessively hot! By ensuring that you have a resistor with power rating at least 3x the calculated value the temperature will be far more manageable. For high power use I strongly recommend chassis or heatsink mount type resistors!
Output impedance of the entire power supply
(This includes the transformer and bridge rectifier at a current draw of 1 Amp)
Compared to the standard capacitive filter, the C-R-C filter has much higher low frequency output impedance. As with the capacitive filter the low frequency impedence is dominated mostly by the internal resistance of the bridge rectifier (at the specific load) and transformer impedance but now also with the addition of the R1 resistor between the two capacitors. The high frequency impedance is dominated by the capacitor ESR of the last capacitor before the load output. It’s important to note that the high frequency impedance in this case is not benefiting from the lowered ESR of the 2 capacitors in parallel because the R1 resistor is in effect isolating the high frequency power draw to only the last capacitor. This means that both the low and high frequency output impedance is higher than the simple capacitive filter.
Output impedance of the filter only
(This excludes the transformer and bridge rectifier – power is supplied by a perfect voltage source at 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 resistance of the resistor R1 with higher frequencies dominated by the ESR of the capacitor after the resistor (C2). The rise in impedance starting around 150kHz is due to the self inductance of the capacitor C2.
(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-R-C is a low-pass filter. With the values shown in the schematic above this C-R-C filter has a -3dB point of around 68 Hz. The -3dB point can be lowered by using a higher value of capacitance or by increasing the resistance R1. Note that increased resistance will cause more power dissipation in the resistor as well as more voltage drop across the resistor.
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 cary 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 R1; C2 low pass filter in the supply. For this reason the C-R-C power supply has a “fair” rating for noise reduction. While the reduction in noise is not spectacular it does help matters if you are dealing with a noisy supply.
There are no active components in this power supply therefore stability should be very good under most conditions.
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 R1 combination while at higher frequencies the impedance is dominated by the ESR of the capacitors after the R1 resistor.
The C-R-C filter is very easy to implement and can work well in many applications. Cost can vary from relatively inexpensive to very expensive depending on the size of the transformer and the amount of capacitors needed to ensure sufficiently low ripple voltage on the output. Be careful with the thermal dissipation of the resistors – these can become hot enough to cause fire in some cases if not designed with enough dissipation headroom.
Best used where your circuit has good PSRR (power supply rejection ratio) or where the ripple and voltage sag will not overly influence operation.