Rebuilding a vintage lab power supply (you can see how I gutted it here
https://youtu.be/P1JIkpT76wY) I found myself in need of a tracking pre-regulator. Staying with the vintage theme I didn’t wanted a switch-mode type and I wanted it to be as simple as possible.
During my search I stumbled on the EEVblog forum upon a design that fascinated me because of its use of a thyristor for main zero crossing detection:
https://www.eevblog.com/forum/projects/very-low-noise-preregulator-for-benchtop-power-supply/. So I took it and simplified it (maybe to a fault, but it’s my thing to use as few parts as possible) …
Advantages of the simplified design:
A) Of course much simpler (10 instead of 21 parts – not counting rectifier, capacitor and linear regulator)
B) Can pre-regulate down to lower voltages (using a BJT instead of MOSFETs)
C) Injects two orders of magnitude less current into the output of the linear regulator
Drawbacks of the simplified design:
A) Considerably less efficient (using a BJT instead of MOSFETs)
B) More expensive parts (I really maxed out on the parametric searches of the distributors)
C) Creates more noise (the pass transistor is switched off hard)
I haven’t ordered the parts for it yet, because some of them are quite expensive (e.g. $10 for a MJ11032G TO-3 BJT). Instead I wanted to put the design under your esteemed scrutiny first. So if you spot an error, please stick it to me. Or, even better, if you think it can be done with even fewer parts, please let me know. Here it goes …
DESIGN CONSTRAINTSNot really technical constraints, but anyway …
1) The switching power transistor has to come in a TO-3 package (the vintage power supply has two external TO-3 heat sinks – one will be used for the pre-regulator and the other for the linear regulator – so a purely “aesthetical” constraint)
2) Can’t do anything about the AC input voltage and its wide swing (in Europe AC mains is specified as 230V +/-10%, and I’m reusing the transformer)
3) The input voltage range of the linear regulator (7V - 37V) and the maximum current (5A) is a given (OK, these are actually technical constraints)
CIRCUIT DIAGRAMsee attachment
PRINCIPLE OF OPERATIONUnless otherwise noted “current voltage” references the output of the bridge rectifier D1 at it cathodes (rectified sine wave), its anodes being the ground (0V).
The footnotes contain information in relation to the actual choice of values and parts.
A) Charging OperationA constant current source (U1, R1) delivers a current to any point low enough below the current voltage (1).
Assuming D3 is “off” – see B) Voltage Feedback and C) Zero Crossing Detection – that current flows to the base of Q1, if the base is pulled low enough below the current voltage by the voltage drop across the collector and emitter of Q1 (2). This voltage drop includes the base-emitter forward voltage of Q1 (3).
As Q1 becomes conductive, current flows through D2 and charges C1, if the voltage drop across D2 (4) plus the voltage of C1 is below the current voltage minus the voltage drop across the collector and emitter of Q1 (3).
(1) 2.7V
(2) 4.2V - 6.2V
(3) 1.5V - 3.5V
(4) 0.2V - 0.8V
B) Voltage FeedbackA voltage divider (R2, R3, and R4) delivers a part of the voltage differential between C1 and the output of the linear regulator (1) to the base of Q2. If it is low enough below the voltage of C1 (2), a current flows through the base of Q2.
As Q2 becomes conductive, a current flows through its emitter and collector via R5 to the gate of D3, and switches it on.
The current from the constant current source (U1, R1) is then flowing through D3 to ground, instead to the base of Q1. The charging of C1 stops.
(1) nom. 7V
(2) 0.63V
C) Zero Crossing DetectionAs long the current from the constant current source (U1, R1) flows through D3 to ground, D3 stays conductive, even if its gate no longer receives any current from Q2 through R5.
The constant current source (U1, R1) will be able to maintain that current as long as the current voltage doesn’t fall too low (2). So the charging of C1 can only resume after the current voltage has fallen to that low point.
That low point (2) is below the required voltage drop across the collector and emitter of Q2 and D2 to resume charging C1 (3). In effect the charging of C1 is further delayed until the current voltage has risen again beyond that required voltage drop, that is after its zero point.
(1) 2mA
(2) 3.6V
(3) 4.4V
CHOICE OF PARTSU1: LM317HVT (
http://www.ti.com/lit/ds/symlink/lm117hv.pdf) – At first glance a no brainer for a simple constant current source working at a maximum of 60V. I choose one in a TO220 package to keep it cool (I don’t like to burn my fingers on my breadboard). However, there are constant current LED drivers available that would do the job with just one part (e.g. AL5809-20P1-7), not needing a resistor. Alas, they are only available in SMD packages (doesn’t fit into the vintage theme at all). I also had a classical Zener (ZPY1)/transistor/resistors 4-part solution designed. Its minimum operating voltage was much lower, shaving off 12W from Q1’s worst case power loss. But its two parts more and “Winter ins Coming” (12W will help to fight the cold).
Q1: MJ11032G (
http://www.onsemi.com/pub/Collateral/MJ11028-D.PDF) – Expecting high peak currents I choose the beefiest transistor in a TO-3 package I could find. It had to be a Darlington in order to keep the rest of the circuitry lean and simple.
D2: FERD40H100S (
http://www.st.com/content/ccc/resource/technical/document/datasheet/group3/4e/b5/b8/47/b6/af/41/d6/DM00281757/files/DM00281757.pdf/jcr:content/translations/en.DM00281757.pdf) – A strong enough Schottky diode with an acceptable voltage drop. For various reasons I restricted myself to types coming in a TO220 package (mainly because I have about two dozen TO220 heatsinks lying around).
Q2: BC560C (
https://www.fairchildsemi.com/datasheets/BC/BC556.pdf) – Nothing special here, but for the C models providing a high current amplification, thus enabling the voltage divider being high impedance. Its maximum collector-emitter voltage of 45V should be enough here, because it’s only seeing the already pre-regulated voltage.
D3: EC103D1 (
http://www.ween-semi.com/documents/EC103D1.pdf) – I choose this one for its low gate trigger current. Again this allows the voltage divider in the voltage feedback part to be high impedance, because it enables Q2 to be driven with low base currents.
Thank you so much for your time!
Again, if I made some mistakes – stick it to me.
And if you think it can be done with fewer parts – please let me know.