|Ripple & noise rejection||Poor – Poor|
|Output impedance||Poor – Fair|
The capacitive filter is simply a capacitor in parallel with the output of the bridge rectifier. The capacitor reduces the ripple caused by the pulsating power from the transformer / bridge rectifier combination.
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 740mV 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.
Output impedance of the entire power supply
(This includes the transformer and bridge rectifier)
At the lower end of the frequency spectrum the impedance is just shy of 120mΩ. This remains reasonably constant until around 8Hz after which it starts falling. At 1kHz the impedance is only 22mΩ and now slowly falling until well beyond 100kHz.
The low frequency impedance is dominated mostly by the internal resistance of the bridge rectifier (at the specific load) and transformer impedance. The high frequency impedance is dominated mostly by the capacitor ESR figure.
Output impedance of the filter only
(This excludes the transformer and bridge rectifier – power is supplied by a perfect voltage source)
There is no point in showing this graph for the capacitance filter because a perfect voltage source would bypass the capacitors such that the output impedance would be 0 throughout.
(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.
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 has a direct path to the output because there is nothing much to filter it. For this reason the simple capacitive power supply does not have great noise rejection figures.
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 and transformer while at higher frequencies the impedance is dominated by the capacitor ESR.
The standard capacitance filter is very easy to implement and can work well in many applications but does not perform well in most other regards. 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.
Best used where your circuit has very good PSRR (power supply rejection ratio) or where the ripple and voltage sag will not overly influence operation.