Difficulty level Moderate
Load regulation Good
Line regulation Good
Ripple & noise rejection Very good – Very good
Stability Excellent
Thermal drift Good
Output impedance Fair

The Zener series regulator is based on a similar topology as the capacitance multiplier filter.  In the capacitance multiplier where we tap off the power supply rail and heavily filter this to give us a clean “signal” which we then feed into a pass transistor (BJT or MOSFET) which then amplifies the current based on this filtered “signal” voltage.

The only difference here is that we employ a Zener diode to provide a constant voltage which is then filtered and fed to a pass transistor.

The name I have given to this regulator (Type 1) is not it’s real name.  It is only a designation to show that I will be covering other types which all work on the basis of a series pass transistor with a Zener diode generated reference 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
Transistor: ON Semi TIP140G NPN Darlington Transistor, 10 A 60 V.  Spec sheet
Zener diode: Fairchild BZX85C27 Zener Diode, 27V 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 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).
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.

zRefSr_Type1_Fig1_Schematic

 

How does it work?

Resistor R1 allows around 10mA worth of current to flow into the zener diode D1.  D1 is a 27V Zener diode, therefore at the junction between R1 and D1 we will have a potential of around 27V.  This junction then feeds the BASE pin of the series pass transistor Q1.

The voltage of Zener diode D1 – the Vbe of our pass transistor sets the output voltage of the regulator.  By using a different voltage Zener diode you can set the output voltage to suit your needs.  Note that by either changing the Zener diode to a different voltage type, or by using a different power supply rail voltage you will need to recalculate the resistor R1 to ensure you have enough bias current flowing into the Zener diode.

In our example we have a darlington-pair transistor (2 transistors in a single package) so we have 2x Vbe drops we need to take into consideration.  Output voltage should therefore be:  27V – 0.65V – 0.65V = 25.7V.

There is an inherent danger with these types of regulator:  Because a BJT transistor needs base current it in effect “steals” some of the 10mA available which is passing through to the Zener diode D1.  In order to maintain a reasonably stable voltage across D1 we need at least 1.5mA to flow through it.  We have around 10mA worth of current in total passing through D1.  If we need a minimum of 1.5mA, then it leaves us with 8.5mA headroom which the transistor can use.

Now if the pass transistor has a low DC current gain (Hfe), then it may draw a significant amount of current through its BASE when delivering higher output currents to a load.  If we draw too much current through the BASE, then our Zener diode D1 will no longer be at the expected 27V and the regulation will collapse.  Therefore it is very important to ensure we have a transistor with a high enough DC Current gain that this will not happen.

Our example here uses a TIP140G darlington transistor.  Per the spec sheet, it has a DC Current gain (Hfe) of 1’000 for collector currents of 5 Amp.  This means that if the transistor was asked to deliver 5 Amps from the COLLECTOR to the EMITTER pins, it would require a BASE current of 5 Amp / (Hfe) = 5 Amp / 1’000 = 0.005 = 5mA.  This ties in well with our setup because it still leaves us a bit of headroom even at 5 Amp load.

Load step response

The graph below shows the regulator output voltage when faced with a constant DC load stepped from 0 Amp up to 4 Amp in 1 Amp steps:

zRefSr_Type1_Fig2_LoadStep

As can be seen above, the voltage regulation is very good indeed.  For load currents between 1 Amp and 4 Amp there is less than 200mV worth of total rail sag!  Impressed yet?  It gets better 🙂

At a load current of 1 Amp the output voltage ripple is around 3.1mV P-P.  At the full 4 Amp load the ripple is still only 22mV.  Wow!

…but…

we can do better still by adding 2 additional components into the circuit:

zRefSr_Type1_Fig3_Schematic

We have added resistor R2 and capacitor C3 to the circuit.  In effect we now have an RC low-pass filter on the output of the zener diode which will greatly reduce ripple and noise.  This filtered signal is now fed into the BASE  of the pass transistor Q1.  Let’s see how this has affected things by looking once more at the load step response between 0 Amp and 4 Amp:

zRefSr_Type1_Fig4_Loadstep

It’s not too easy to see from the graph, but we now have a very slightly lower overall output voltage across the different loads.  The difference the additional components have made come into play when we look at the output voltage ripple.

At a load current of 1 Amp the output voltage ripple is around 83uV (0.000083V) P-P.  At the full 4 Amp load the ripple is still only 786uV (0.786mV).  WHAT?

We have not quite eliminated output voltage ripple and noise, but we have sure come close!  In reality you will probably have a bit higher ripple than this because the spice simulation software cannot include things like PCB layout, etc however even if we come within 10% of this figure it is still very good!

Power dissipation

Transistor Q1 dissipates heat because it handles the voltage drop between the power supply output and the regulator output.  In this case our transformer via bridge rectifier and filter caps provide a voltage of 25V AC x 1.414 = 35.35 V.  Our regulator has an output of 25.7V, thus the voltage drop across transistor Q1 is 35.35V – 25.7V = 9.65V.

If we have 1 Amp worth of current flowing through Q1, then the dissipation will be:

P (Watt) = V x I
P (Watt) = 9.65V x 1A
P = 9.65 Watt.

For a load of 4 Amp, this will increase to 38.6 Watt.  This kind of dissipation requires a more substantial heatsink to keep the transistor cool – the standard little PCB mount type will not work here.

If we use a heatsink with a thermal resistance of around 5°C/W this will result in a temperature rise of 48.25°C above ambient for a 1 Amp load.  This is getting on the high side, but still manageable – bigger would be better to reduce the temperature a bit more.

The heatsink below is made by Fischer Elektronik – model number SK08, and has a thermal resistance of 2.3°C/W (°C/W is the same as K/W) for a length of 50mm.

F1898555-01[1]

Using the heatsink above, we can safely dissipate 15 Watt with a temperature rise of only 35°C above ambient.

There is a way to reduce the total dissipation through transistor Q1 for a given load:  If the voltage differential between the output of the power supply and the output of the transistor Q1 is made smaller then the dissipation will be less.  This does however mean that we need to ensure the maximum ripple in the power supply output is small enough that it doesn’t break through the regulation threshold.  Normally this can be done by using more or larger capacitors.

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 1 Amp)

This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.

zRefSr_Type1_Fig5_OutputImp

Output impedance isn’t too bad at all – slightly higher than that of the standard capacitive filter, but less than the C-R-C, C-L-C and the capacitance multiplier.  As usual, the low frequency impedance is dominated mostly by the transformer secondary and the 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 1 Amp)

This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.

zRefSr_Type1_Fig6_FilterImp

 

Output impedance is good but not great.  Obviously it is much less than the standard Zener shunt configuration because we don’t have the current limiting in this circuit.

Load regulation
(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 with only minor voltage sag for increasing loads.  Therefore the load regulation of this type of supply is rated as good.

Line regulation
(How well the PSU is able to maintain a steady output voltage with changes to the input voltage.)

AC Mains voltage fluctuation will not impact the regulation as long as we are able to maintain enough headroom so that the power supply voltage does not drop below the threshold voltage.  This can be catered for during the design of the power supply so if taken care of it will not have any significant impact.  Therefore the line regulation of this type of supply is rated as “good”.

Noise

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, this will be heavily filtered by the RC low-pass filter created by components R2 and C3.  Because of this filtering effect, this supply is rated as “very good” for noise when used in conjunction with R2 and C3.

Stability

Stability should be very good under most conditions.

Thermal drift

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.

Output impedance

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.

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 pass transistor Q1 in order to work out heatsinking requirements, etc.

Be warned however that at significant currents the power dissipation through 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.

Conclusion

The higher power zener series voltage regulator (type 1) performs exceedingly well with regards to load regulation, ripple reduction and noise.  In addition it is very easy to scale this power supply to meet nearly any output voltage and current requirements.

This type of power supply is highly recommended.