|Load regulation||Very Good|
|Ripple & noise rejection||Very good – Poor|
In order to achieve higher current regulation using the standard zener diode shunt regulator we need to add couple of additional components into the mix – most importantly a transistor.
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
Shunt Transistor : On-Semi BD139G NPN Bipolar Transistor, 1.5 A, 80 V. Spec sheet
Zener diode : Fairchild BZX85C15 Zener Diode, 15V 5% 1 W. 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.
How does it work?
A shunt regulator draws a constant current from the power supply regardless whether a load is placed on the regulator output or not. When no current is being draw from the regulator, transistor Q1 will assume the burden of drawing nearly the full rated load current. As the load on the output of the regulator is increased, transistor Q1 eases off so as to maintain a steady total current draw through resistor R1. This will continue until such point that the regulated voltage falls below a the “threshold” at which point the output will no longer be regulated and the output voltage will start falling with increasing current draw.
The “threshold” voltage is the voltage at which no current flows through the BASE pin of transistor Q1. This is set by the voltage of the zener diode (D1) which is 15V in this case + the Vbe(on) voltage of transistor Q1 which should be around the 0.65V mark, so our threshold voltage then is 15V + 0.65V = 15.65V. Incidentally, this is also the output regulation voltage. By changing the zener diode D1 to a different voltage component, you can set the output votlage of the regulator.
Resistor R2 sets a bias current up through the zener diode D1 ensuring it is operating inside its stable region, thereby increasing regulation accuracy of the circuit.
Resistor R1 limits the total amount of current which can be drawn by the circuit. It is very important to have this resistor in place when (or some other method of current limiting) because the power dissipation through shunt transistor Q1 is directly impacted by it.
If we did not have R1 in the circuit above, then the shunt transistor Q1 will try to load the entire power supply to it’s limits until such point as the output drops to the “threshold” value which will cause unnecessary strain on components and most likely cause large amounts of heat in transistor Q1.
Let us assume we need a maximum current supply of 100mA from the regulator at 15.5V output. There are some “losses” in a shunt regulator which need to be taken into account, so I like to provide around 10% headroom on the current rating. Therefore our maximum current becomes 110mA.
If our input voltage (transformer, rectified and smoothed by filter capacitor C1) is 15V AC x 1.414 = 21.21V DC then it means we need a voltage drop of around 21.21V – 15.5V = 5.7V across resistor R1. Using Ohm’s law, we can now calculate the size of the resistor we need:
V = I x R
5.7V = 0.11A x R
R = 51.8 Ω
That’s pretty close to a 51 ohm resistor, so let’s use that.
Resistor R2 has also been adjusted to ensure we have at least 1.5mA flowing through it whilst the output voltage is at the threshold value. The circuit now looks like this:
Let’s take a look at the load step response of the regulator. The following graph shows the output voltage when faced with a constant DC load stepped from 0 Amp up to 125 mA in 25mA steps:
As can be seen above, the voltage regulation is very good indeed. For load currents up to 100mA there is nearly no difference in output voltage. At a load current of 125mA which is beyond our 110mA target you can see the output voltage is now outside of regulation and some ripple is breaking through.
Ripple voltage for a 25mA load is around 4.3mV P-P. At 100mA load the ripple is still only 10.2mV making this a very good regulator indeed!
The load regulation is so good in fact that it deserves a closer look. Note the Y axis scale on the graph below!
Resistor R1 which is the current limiting device in this circuit will have a constant power dissipation because the load through it will be constant. In our example this resistor is dropping 5.7V across it and with a current of 110mA passing through it, the power dissipation will be:
P (Watt) = V x I
P (Watt) = 5.7V x 0.11A
P = 0.627 Watt.
Importantly, this is continuous power dissipation so to be safe and maintain reasonable temperatures you would need to apply a factor of at least 300% to size the resistor. 0.627 Watt x 3 = 1.88 Watt. I would use a minimum of 2 Watt for this application – 3 Watt would be better.
The dissipation of the shunt transistor Q1 will vary with different loads as shown below:
With no load at all, the dissipation in transistor Q1 is near the 2 Watt mark. As the load on the regulator is increased the dissipation drops. At a load of 125mA the dissipation is essentially 0 because at that load the output voltage is below the threshold figure and no longer being regulated.
If you design this regulator with close tolerances for the maximum load current you could potentially get away with not using a heatsink on the transistor. I would however always recommend at least a small heatsink be used – they are inexpensive and can save you from catastrophe.
Output impedance of the entire power supply
(This includes the transformer and bridge rectifier at a current draw of 0.1 Amp)
Low frequency output impedance doesn’t look too good at all. This is mostly as a result of the resistor R1 (51 ohm) which is in series between the power supply and regulator. Not that you need to worry about it – the output impedance is part of how this circuit works, and was part of our initial calculations. In other words – for a 100mA load we don’t need a lower output impedance than this.
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 0.1 Amp)
This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.
Output impedance is pretty stable and flat at just shy of 2 ohm. This isn’t great for a high current design, but is plenty good enough for the 100mA this regulator is designed for. The reason why this figure is on the high side is because of the low gain of the regulation circuit.
(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 is very stable with increasing load current up to the point where we draw more current than the regulator was designed for. Therefore the load regulation of this type of supply is very good.
(How well the PSU is able to maintain a steady output voltage with changes to the input voltage.)
AC Mains voltage fluctuation will impact the maximum current that the regulator is designed for because our current limiter is a simple resistor.
This could be solved by employing a constant current source rather than a resistor, but as is this circuit has a “fair” rating for line regulation.
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 will be filtered to some extent by the regulator circuitry, but this won’t be much.
Stability should be very good under most conditions.
Temperature has no significant impact on the circuit. Changes in temperature will have a small effect on the absolute voltage across the Zener diode D1, so there will be a little drift as things warm up to operating conditions, however in most circuits this will be small enough to not cause any real concern.
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 dominated by the current limiting scheme at low frequencies, but that is by design, so nothing much to worry about! It received a “Poor” rating only because the impedance is on the high side.
Scaling for different voltages and / or currents
In theory this type of regulator can be made to handle just about any output voltage and / or load current.
Always ensure that the transistors you use are capable of handling the voltages and currents to start with. Next take the time to calculate the power dissipation through the current limiting resistor R1 and shunt transistor Q1 in order to work out heatsinking requirements, etc.
Be warned however that at significant currents the power dissipation through R1 and Q1 can become enormous and extremely difficult to keep cool.
Having said that, if you can manage to keep everything cool enough then there is no reason why this scheme cannot work for any voltage and current you can think of.
The higher power zener shunt voltage regulator performs exceedingly well for such a simple circuit.
This type of regulator is rather wasteful because it draws the full output current regardless of the load being applied, so efficiency isn’t one of it’s strong points.
Care needs to be taken with input voltage fluctuations and power dissipation through both the current limiting resistor (R1) and the shunt transistor (Q1), but other than that it is highly recommended.