|Ripple & noise rejection||Very good – Good|
The 78xx family of IC linear voltage regulators has been around for decades. The reason for this is simple to understand – they are easy to implement, cost effective, provide reasonable performance and typically have internal current limiting, thermal shutdown and SOA (Safe Operating Area) compensation.
The xx in 78xx is replaced by 2 digits which indicate the regulated output voltage, so a 7805 would be a 5V regulator and a 7812 would be a 12V regulator. The commonly available devices have output voltages ranging in discrete steps between 3.3V all the way up to 24V and are normally housed in a 3 pin TO-220 package.
The 78xx regulators typically have a dropout voltage of around 2V. This means you need an input voltage of at least 2V higher than your required output voltage, so for a 15V output you would need a minimum of 17V input.
While this article deals mainly with the 78xx regulators which are for regulating positive voltages, you will also find 79xx series of regulators which handle negative voltages.
These regulators typically have a maximum of 1 Amp current rating, though there are some versions which will provide up to 2.2 Amp. Because of this they are not normally used where high power regulation is needed. Very importantly, these regulators have a maximum input voltage of around 35V.
Because of the low maximum input voltage and maximum current carrying capability of these regulators we will not use our normal 300VA transformer with 25V AC secondaries or the 35A bridge rectifier for this article – rather we scale these down to a 50VA transformer with 15V AC secondaries and a Vishay 6 Amp bridge rectifier. The filter capacitor has also been scaled accordingly:
For the purposes of this article, we will assume the use of the following components:
Transformer: Nuvotem Talema toroidal; 2 Output Toroidal Transformer, 50VA, 2 x 15V ac. Spec sheet
Bridge Rectifier: Vishay G5SBA60-E3/51, Bridge Rectifier, 6A 600V. Spec sheet
Capacitors: Panasonic 2200μF 35 V dc Aluminium Electrolytic, FR Series. Spec sheet
Regulator : Fairchild LM7815ACT 15V Positive Regulator. 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 49Ω. This is represented by R_Prim in the figure below. For a voltage rating of 15V on the secondary, we have a DCR of 0.6411Ω and output current of 1.6A per secondary. This is represented by R_Sec in the figure below.
The bridge rectifier is chosen because it is readily available and cost effective with 6 Amp current carrying capacity.
The capacitors are chosen because they display excellent characteristics with very low ESR of only 14mΩ (shown as ESR in the schematic below).
I1 in the schematic is a constant current load. This components allows us to apply different loads to the power supply in order to review its performance.
Because the 7815 voltage regulator has a maximum current of 1 Amp, we will not load it with the usual 0 – 4 Amp load, but rather look at loads between 0 Amp and 0.8 Amp in 0.2 Amp steps:
No load output voltage is around 15.5V. At 0.2 Amp load the output voltage is around 14.8V. As can be seen the output voltage displays a linear sag with increasing load, but the ripple voltage is very nicely taken care of giving us a clean smooth output voltage.
Voltage ripple at 0.2 Amp load comes in at a fantastic 770uV (0.77mV) P-P .At 0.8 Amp load the ripple voltage is only slightly higher coming in at 5.3mV P-P which is a commendable result.
The graph below shows the voltage ripple for the 0.8 Amp load:
In our power supply circuit above, the transformer has a secondary voltage rating of 15V RMS. Once rectified, this becomes 15V x 1.414 = 21.21V DC. In order for the 7815 regulator to regulate this input voltage down to 15V output the difference between input and output voltage is burned off inside the regulator, causing it to heat up.
The graph below shows the heat dissipation through the regulator for loads between 0 Amp and 0.8 Amp in 0.2 Amp steps with an input voltage of 21.21V DC:
Dissipation is around 1.4 W for a load current of 0.2 Amp and up to 4.5 Watt for 0.8 Amp loads. From the datasheet, we know that the regulator IC has a junction to air thermal resistance of 65 ºC/W.
This means that when the regulator is dissipating 1 Watt of power through it, the temperature will rise 65ºC above ambient. If your room temperature is 25ºC, this means the regulator will be at a temperature of 25ºC + 65ºC = 90ºC. This is not good so we need to employ a small heatsink.
For higher power dissipation (ie higher load, or higher input voltage) we will obviously need a larger heatsink.
Output impedance of the entire power supply
(This includes the transformer and bridge rectifier at a current draw of 0.5 Amp)
Higher low frequency output impedance is caused mostly by the bridge rectifier and transformer secondary combination.
While this is interesting to look at, the load currents are rather small (less than 1 Amp). Because of this the output impedance being on the high side at just above 10 ohm for low frequencies isn’t too important and shouldn’t cause you any concern in practice. At 16Hz the output impedance has already dropped to under 4 Ohm and by 64Hz it is down to around 1 ohm.
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 0.5 Amp)
This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.
Low frequency output impedance is good – down to around 105mΩ with the impedance increasing from around 65kHz onwards. This is because the ability of the feedback mechanism inside the 87xx regulator becomes less efficient at higher frequencies (loop gain drops with increasing frequency).
(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. It doesn’t sag very much and output voltage ripple is pretty good so this power supply has a “good” rating for load regulation.
(How well the PSU is able to maintain a steady output voltage with changes to the input voltage.)
The output of this power supply is regulated reasonably well, so as long as we remain within the specifications of the regulator we will not have any problems with changes to the input voltage. This power supply therefore receives a rating of “excellent”.
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 voltage regulator.
The datasheet gives an output noise of 90uV (0.000’090 V) for frequencies between 10Hz and 10kHz which is pretty good!
The regulator is very stable under most normal conditions as long as you have an output capacitor (C3 in the schematic) connected as shown.
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.
As discussed above the output impedance is interesting to look at, but remember that the load currents are rather small (less than 1 Amp). Because of this the output impedance being on the high side at just above 10 ohm for low frequencies isn’t too important and shouldn’t cause you any concern in practice.
The 78xx regulated power supply is very easy to implement and can work well in many applications where you don’t need voltages higher than 24V and loads of less than 1 Amp.
Performance is surprisingly good for the cost making this type of power supply easy to recommend.