|Load regulation||Very good|
|Ripple & noise rejection||Good – Good|
The 317 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.
There are 3 things that make the 317 linear voltage regulators different to the 78xx type regulators:
1. The 317 regulator has an adjustable output voltage
2. The 317 regulator uses feedback to actively maintain output voltage
3. The 317 uses a darlington output transistor, so the minimum voltage differential between IN and OUT is 3V.
These regulators typically have a maximum of 1.5 Amp current rating for TO-220 packages. Because of this they are not normally used where high power regulation is needed. Very importantly, these regulators have a maximum input / output differential voltage of 40V.
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 : Texas Instruments LM317T, Single Linear Voltage Regulator, 1.5A Adjustable 1.2 → 37 V. 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.
Now – before we continue let’s demystify what that ominous term “feedback” means exactly. It can sound complicated and difficult to understand, but it really isn’t:
Feedback just means we are comparing 2 things and taking corrective action if those 2 things are not the same. In this case the 317 regulator is comparing an internal reference voltage (1.25V) to an output voltage. If the output voltage we are feeding back does not equal 1.25V, then some form of corrective action (error correction) is taken to try and make it 1.25V. Nothing mysterious about it!
It may sound difficult to understand at first, but it is the fact that the 317 regulator has feedback that enables us to adjust the output voltage. In the datasheet it states:
“The device OUTPUT pin will source current necessary to make OUTPUT pin 1.25 V greater than ADJUST terminal to provide output regulation.”
So what does that mean exactly?
In simple terms it simply means this: The 317 internal circuitry will do whatever it can to try and make the OUTPUT pin 1.25V higher than the ADJUST pin.
So how do we “adjust” the 317 to give us the output voltage we want? It is done using a voltage divider, which is simply resistors R1 and R2 in the schematic above.
The output voltage of the 317 can be set by using this formula: Vout = 1.25 x (1 + (R2 / R1))
In the case of the schematic above, our output voltage will be:
Vout = 1.25 x (1 + (1.5k / 130Ω))
Vout = 1.25 x (1 + 11.54)
Vout = 1.25 x (12.54)
Vout = 15.675 V
Because the 317 voltage regulator has a maximum current of 1.5 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 15.7V – very close to our calculated output volatage!
Immediately noticeable is the fact that the output voltage doesn’t sag by much at all with increasing load current. This is as a direct result of the feedback utilized inside the 317 regulator.
Voltage ripple at 0.2 Amp load comes in at 33.5mV P-P.
At 0.8 Amp load something interesting has happened: the ripple voltage is significantly higher. What has happened here is that the bulk storage / filter capacitor C1 was no longer able to maintain a steady enough voltage between transformer pulses and the ripple before regulation is more than the input / output dropout voltage of the regulator and the ripple was therefore transferred through to the output. In order to correct this we can do 2 things: Either provide the 317 with a higher input voltage (so it has more headroom), or we can increase the size of the bulk filter capacitor C1.
To prove the case, I raised the regulator input voltage by 1.4V (by using a transformer with a 1V RMS higher output voltage), and this is the result:
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 317 regulator to regulate this input voltage down to 15.7V as used in this article 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 1.3 W for a load current of 0.2 Amp and up to 4 Watt for 0.8 Amp loads. From the datasheet, we know that the regulator IC has a junction to air thermal resistance of 50 ºC/W.
This means that when the regulator is dissipating 1 Watt of power through it, the temperature will rise 50ºC above ambient. If your room temperature is 25ºC, this means the regulator will be at a temperature of 25ºC + 50ºC = 75ºC. 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)
While this is interesting to look at, the load currents are rather small (less than 1 Amp) and in reality this won’t have much of an impact on your circuit. The graph doesn’t look too nice until you realize that the low frequency impedance is only around 0.475Ω. This is significantly better than the 78xx series of regulators which have a low frequency impedance in the order of around 10Ω for the same load current. What is the reason for this 20x improvement over the 78xx series? One word: Feedback
Most of this high impedance at the low frequencies is as a result of the transformer and bridge rectifier combination.
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 very good – down to around 41mΩ with the impedance increasing to around 420mΩ at around 70kHz. This is because the ability of the feedback mechanism inside the 317 regulator becomes less efficient at higher frequencies (loop gain drops with increasing frequency).
After the peak around 70kHz the impedance starts falling again. This is caused by the 1uF bypass capacitor C3 taking over at higher frequencies.
(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 very little with increasing load hence this supply has a “very 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 0.003% of Vout. If we have 15.7V output then the noise voltage is in the order of (0.000’473 V). Pretty good, but not as good as the 78xx series (if the specifications are to be believed).
The regulator is very stable under most normal conditions as long as you have an input capacitor (C1) near the regulator IC and a small output capacitor (C3) near the output of the regulator.
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). Regardless, the output impedance being at a maximum of around 475mΩ for low frequencies is more than good enough for low load applications.
The 371 regulated power supply is very easy to implement and can work well in many applications where you need voltages between 1.25 and 37V and loads of less than 1.5 Amp.
Performance is surprisingly good for the cost making this type of power supply easy to recommend.