Transformer tap-switching circuit for linear power supplies but without relays - Page 1
Forum section: electronics / projects

author: drksy
author_rank: Regular Contributor
timestamp: January 17, 2025, 06:13:20 am
content: Usually when switching transformer taps, one uses relays to switch different taps of the transformer with the goal that the pass transistor in a power supply will dissipate less wasted heat. A typical tap switcher will do something likeI want to explore how to do this type of switching without using any mechanical relays.A while ago I came across this circuit which is from the schematic of the HP E3630A. There they use this clever circuit using SCRs and optocouplers to basically extend the bridge rectifier and switch the top tap in depending on the output voltage.Can this idea be extended to more than one transformer tap?What are some other, maybe even better way to do this without relays?

author: jbb
author_rank: Super Contributor
timestamp: January 17, 2025, 06:20:36 am
content: It’s not quite the same, but controlling the phase angle of the SCRs (ie when in the mains cycle you provide the trigger pulse) allows for variable rectifier output.

author: drksy
author_rank: Regular Contributor
timestamp: January 17, 2025, 07:34:08 am
content: Quote from: jbb on January 17, 2025, 06:20:36 amI think you might be talking about an SCR phase-cut pre-regulator (https://www.eevblog.com/forum/projects/very-low-noise-preregulator-for-benchtop-power-supply/msg626382/#msg626382) which is a totally different thing than what I'm asking here. Here SCRs are either fully on or fully off.

author: moffy
author_rank: Super Contributor
timestamp: January 17, 2025, 07:40:15 am
content: It's simple and clever, the bridge rectifier with the highest AC voltage wins, I don't see any immediate reason why it couldn't be extended to more taps.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2025, 10:06:57 am
content: Why not a relay?A transistor can be used. Here's a crude design I came up with awhile ago.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2025, 01:39:31 pm
content: Quote from: drksy on January 17, 2025, 06:13:20 amSure, add more pairs of SCRs and more comparators.QuotePower MOSFETs could be used, likely on the negative instead of positive side so n-channel parts can be used without high side gate drive.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 17, 2025, 06:59:42 pm
content: Quote from: drksy on January 17, 2025, 06:13:20 amThis use case has such low duty cycles that relays are KISS, but maybe relays are too tall ?you can use MOSFETS as relays, using duals and photovoltaic isolator for gate drive is only 2 packages per switched arm. The price of that is falling over time.

author: drksy
author_rank: Regular Contributor
timestamp: January 18, 2025, 12:20:38 am
content: Quote from: Zero999 on January 17, 2025, 10:06:57 amWell it turns out relays arc, they wear down, they only have a few thousand switches of lifetime, and they don't really like to be in situations where instead of pure dc, the supply outputs a low frequency waveform. And who wants to hear the acoustics of a click-clack opera in that scenario?Quote from: Zero999 on January 17, 2025, 10:06:57 amThat looks like a linear pre-regulator where all the heat gets dumped in Tr1 instead of U1. Or maybe I'm not understanding it

author: drksy
author_rank: Regular Contributor
timestamp: January 18, 2025, 12:25:21 am
content: Quote from: David Hess on January 17, 2025, 01:39:31 pmSCRs aren't really more expensive than high current relays are they? Then I wonder how come big manufacturers aren't doing this. Cheaper, near infinite life, silent, no relay coil to drive hence less power, simpler. There's no gotcha here? Just add a pair of SCRs per tap?

author: drksy
author_rank: Regular Contributor
timestamp: January 18, 2025, 12:32:27 am
content: Quote from: PCB.Wiz on January 17, 2025, 06:59:42 pmA pair of back to back p-channel/n-channel mosfests and a photovoltaic isolator would essentially be a typical solid-state relay, with the disadvantage that it would only be be single throw switch. Not sure if this is cost-effective or simpler than relays because you would need a lot of them, and have to be extra careful not to short some transformer windings during switching.

author: bdunham7
author_rank: Super Contributor
timestamp: January 18, 2025, 12:34:20 am
content: Quote from: drksy on January 18, 2025, 12:20:38 amNo, the transistor will mostly be on or off. What you have is a CT full-wave half-bridge until the output reaches ~12.7V and then it converts to a full-bridge configuration.

author: drksy
author_rank: Regular Contributor
timestamp: January 18, 2025, 12:40:06 am
content: Quote from: bdunham7 on January 18, 2025, 12:34:20 amyou're right, I didn't look at it carefully at first. That totally works too! But it relies on D1 being normally reverse-biased and not conductive. How might we do this with multiple taps?

author: bdunham7
author_rank: Super Contributor
timestamp: January 18, 2025, 12:53:53 am
content: Quote from: drksy on January 18, 2025, 12:40:06 amLets say you have X taps plus a common. Put a bridge rectifier on each tap and then a transistor after each to select the one you want to use.

author: garrettm
author_rank: Frequent Contributor
timestamp: January 18, 2025, 01:20:20 am
content: Maybe not the best approach, but a triac (two SCRs in anti-parallel) with zero voltage switching (full conduction cycle) would be as close to a relay as you can get with low EMI.Rather than use the SCR pre-reg idea of partial conduction angle, you could trigger full half-cycle pulses to keep the voltage at the storage cap above the dropout threshold of the linear regulator. This should have lower EMI at the expense of higher ripple current. However, this would cause the line current to be non-sinusoidal (since we are pulsing random half-cycles). Though the same is true of SCR pre-regs (particulary at turn-on). And I suppose all linear power supplies: The current waveform has high crest factor because current is primarily drawn during peak line voltage due to the storage capacitor being partially charged. Not much that can be done here aside from a line filter.

author: garrettm
author_rank: Frequent Contributor
timestamp: January 18, 2025, 01:23:38 am
content: Quote from: bdunham7 on January 18, 2025, 12:53:53 amThat topology would not be a good choice for low voltage taps. Two diode drops can be a significant fraction of total losses for low voltage output. But with linear regulators, and the required dropout voltage, maybe it would be okay.STmicro makes MOSFET based bridge rectifiers, so for the lower voltage taps, the losses would be much less than a standard silicon bridge rectifier.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 18, 2025, 09:58:59 am
content: SCRs still need quite some gate current. So the power saving compared to a realy is not that large.Today MOSFETs may be an option, though there can be a slight issue with the rather high current spike for charging the filter capacitor. So the idea with the BJT has some benefits in limiting the current.Quote from: Zero999 on January 17, 2025, 10:06:57 amAlternative to just tap switching there is the option to have the regulator with a power stage that can use 2 and possibly more DC voltage voltage levels. This is similar to a class H audio amplifier. It is a bit extra effort with more power transistors and filter capacitors and usually only 2 levels (though more are possible). On the up side the transistion is so fast that even the ripple part can be included and very little headroom needed. There is also no current spike to charge the filter capacitor to a higher level.

author: David Hess
author_rank: Super Contributor
timestamp: January 18, 2025, 02:35:39 pm
content: Quote from: Kleinstein on January 18, 2025, 09:58:59 amMOSFETs are only an option because economy of scale reduces their cost. SCRs are more area efficient.The larger problem with MOSFETs is that they conduct in reverse when turned-on, so conduction has to be controlled to prevent reverse current from the input filter capacitor into the transformer. Rectifiers could be added in series with the MOSFETs to solve this at the cost of their forward voltage drop. Properly done this also prevents surge current when charging the filter capacitor because the MOSFETs only turn on when the difference in voltage is small.

author: bdunham7
author_rank: Super Contributor
timestamp: January 18, 2025, 03:22:18 pm
content: Quote from: garrettm on January 18, 2025, 01:23:38 amI think the goal in most voltage-tap setups for linear power supplies is to simply limit the power dissipation in the output stages by keeping the delta between the filter cap and output voltages to below some maximum value. I don't think overall efficiency is a big consideration and with the required allowances for regulator drop and transformer sag, the lower voltages are going to be very inefficient anyway.

author: garrettm
author_rank: Frequent Contributor
timestamp: January 18, 2025, 11:34:36 pm
content: A bit off topic, but worth pointing out, is that the transformer is literally the most expensive part of a classic linear power supply. And the largest, and heaviest component (aside from an oversized passive heat sink). Weight adds cost to shipping and creates issues when stacking equipment, etc. Ultimately, a 50 to 60 cycle, single phase transformer itself is the enemy of a modern / good design. (Three phase transformers with delta and wye output windings--to produce an effective six phase output--improves on the situation, but isn't something that can be used outside of special situations. )As an example, I have a fairly compact GW Instek 20V, 10A linear supply that uses an absolute monster of a transformer that takes up at least 60% of the internal space inside the PSU and is probably 70 to 80% of the total weight. It also uses relays to select taps as the output voltage is adjusted. The transformer is the noisiest part of this PSU--even louder than the variable speed fan.For a modern design, I think it would make sense to use an active PFC stage to the storage capacitor and then a low noise, isolated buck converter that feeds an LDO linear reg. Switching converters can be made surprisingly low noise by carefully controlling the rise/fall time of the switching elements or using resonant operation (making the parasitic L and C work for you rather than against). And additional filtering can be added before the linear regulator to meet noise requirements. In the end, the PSU would have great power quality at the AC mains side (high power factor / low harmonics) and be significantly more efficient in the AC to DC power conversion and likely have similar EMI / output noise characteristics as a standard linear power supply but much smaller and lighter for the same power output. It's likely cost would also be comparable, if not less for high power designs. All modern ATX PSUs use this basic design, less the LDO regulator on the output, and they approach 90% or so conversion efficiency. Making the switcher operate with lower noise would drop conversion efficiency, as would the LDO regulator, but it would still be worth it. Might need two or more outputs on the isolated buck converter to get better switcher efficiency over the adjustable output range. And with multiple rails, it might make sense to do a class H output like Kleinstein mentioned.

author: TheoB
author_rank: Regular Contributor
timestamp: January 19, 2025, 12:36:19 am
content: Quote from: drksy on January 17, 2025, 06:13:20 amA Variac.Most are autotransformers but isolated ones seem to exist. You want to control them automatically to provide constant headroom to the final LDO. Not very practical to implement but it would make a very efficient linear power supply.

author: drksy
author_rank: Regular Contributor
timestamp: January 19, 2025, 12:46:20 am
content: Quote from: TheoB on January 19, 2025, 12:36:19 amAre you suggesting to put a motor in there to turn it?

author: drksy
author_rank: Regular Contributor
timestamp: January 19, 2025, 12:49:54 am
content: Quote from: garrettm on January 18, 2025, 11:34:36 pmIf you are using a switch-mode AC/DC, might as well use differential feedback to output ~2 volts above what's needed for the regulator, then can skip the buck converter. I remember reading a thread about this a long time ago, where some french guy designed an LLC SMPS, then fed the output to a pair of LT3081s.

author: johansen
author_rank: Super Contributor
timestamp: January 19, 2025, 03:31:21 am
content: If you feed forward the output current, and store enough energy in the inductor, you can adjust the variac fast enough to preserve the output current requirement..

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2025, 05:55:21 am
content: Quote from: drksy on January 19, 2025, 12:46:20 amPower Designs supplies like this 3650-S used a ganged variac and potentiometer to adjust the voltage. The potentiometer controlled the output regulator circut while the variac changed the input voltage.https://www.ebay.com/itm/186892028190

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2025, 10:15:04 am
content: A PFC circuit can make some sense, but it adds some higher frequency EMI. The power factor of simple rectifier and filter cap is around 1.6. So one could safe maybe 1/3 on the transformer power rating when also using PFC. A good transformer (avoiding core saturation) does not have to be noisy. A really noisy transformer may be an issue with the rectifiers taking more current with one polarity than the other. This can magentize the core in one direction and cause saturation on one side that than adds noise. One can do some preregulation with PFC, but the range is usually limited.Sometimes one wants a linear supply with old style transformer because of low EMI and than PFC is not such a good option. This usually is with lower power.Controlling the transformer taps from the set voltage only can work for some power supplies, but not for a lab supply that could also be used in current controlled mode. One can still have the voltage set to a high value, but operate in constant current mode with relatively low voltage. With the high transformer tap one would still get the full power at the transistors. It can still make sense to have a manual setting, e.g. for a high or low range that than also effects the maximum current.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2025, 11:11:47 am
content: Quote from: drksy on January 18, 2025, 12:20:38 amIf you put the relay after the smoothing capacitors, then it will will last much longer because it doesn't have any large current surges.

author: Njk
author_rank: Frequent Contributor
timestamp: January 19, 2025, 09:04:14 pm
content: Quote from: drksy on January 17, 2025, 06:13:20 amAs an option, you can take the Back UPS approach and implement the switching on the primary side. In that way, less opto-couplers will be required. And it's easy to switch AC off for over-current protection

author: Njk
author_rank: Frequent Contributor
timestamp: January 19, 2025, 10:25:13 pm
content: Quote from: drksy on January 18, 2025, 12:25:21 amYes but SCRs are not rad-hard

author: drksy
author_rank: Regular Contributor
timestamp: January 20, 2025, 11:14:57 pm
content: Quote from: bdunham7 on January 19, 2025, 05:55:21 amThis ancient technique works well only in constant-voltage mode. The dual gang mechanism is not smart enough to drop the pre-regulator voltage to track the output when the supply is in constant current. Then it has the dissipate the full set voltage times the output current in the event of a short circuit.Quote from: Kleinstein on January 19, 2025, 10:15:04 amIf the tap switching circuit follows the output then it will "know" when the supply is in constant current and will follow it by changing to the lower taps. This is how the HP circuit that I have initially shared works.Quote from: Njk on January 19, 2025, 09:04:14 pmThis can work too, but not as practical as secondary tap switching imo. First of all, you'll be very hard pressed to find a 60hz transformer that has multiple primary windings that can be switched. Oh the other hand it's common to find transformer with many different secondary windings. Heck you could even buy a toroidal transformer, unwind the secondaries, and add more taps as needed (which I've actually been seriously considering).

author: Circlotron
author_rank: Super Contributor
timestamp: January 20, 2025, 11:38:07 pm
content: Quote from: garrettm on January 18, 2025, 01:20:20 amA choke input filter is your friend here. Quite unfashionable for passive bench supplies, I know, but they do have their good points.

author: Circlotron
author_rank: Super Contributor
timestamp: January 20, 2025, 11:45:29 pm
content: Have a look at the voltage selection circuit for a SMPS where they use a single SPST switch to select 120V or 240V input by reconfiguring the rectifier as either a voltage doubler or a bridge rectifier respectively.Have a main transformer with a single untapped secondary and a second smaller transformer with the same current rating that is used to 1/buck, 2/do nothing, 3/boost the main transformer voltage. With that setup feeding the above mentioned doubler/bridge rectifier you could have six different rectified DC voltages available.

author: Circlotron
author_rank: Super Contributor
timestamp: January 21, 2025, 04:03:28 am
content: Quote from: Circlotron on January 20, 2025, 11:45:29 pmA spreadsheet to play with various transformer voltages using the above method.

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2025, 04:08:09 am
content: Quote from: Njk on January 19, 2025, 10:25:13 pmHave used that fact as a radiation exposure detector for space applications. An SCR with no gate drive and a DC bias across it, if it triggers it's because it was exposed to radiation.

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 21, 2025, 04:23:18 am
content: It is easy to turn on a MOSFET through a cap-coupled AC charge pump. Does not need to be so fast; 1 mSec might be good. I have done a soft start ramp up controller which drives a high side Nch MOSFET in this way.See attached.

author: mawyatt
author_rank: Super Contributor
timestamp: January 21, 2025, 04:28:14 am
content: Quote from: moffy on January 21, 2025, 04:08:09 amAlso used for Crow-Bar protection of Power Supplies for electronic devices that must survive a Nuclear blast. The ionization radiation from the detonation causes all PN junctions to become forward biased and if the power supply can supply enough current during this event it can destroy sensitive semiconductor devices. The Crow-Bar discharges the supply very quickly before (hopefully) any sensitive semiconductors are damaged by the capacitive stored energy.Recall there were guide-lines for survivability for the amount of energy each supply line could host, which dictated the total amount of supply line capacitance allowed.Testing was done with a Flash-Xray setup in a shielded chamber in an isolated separate building, you didn't want to be around when they fired the X-RayBest

author: drksy
author_rank: Regular Contributor
timestamp: January 22, 2025, 01:43:34 am
content: Quote from: mawyatt on January 21, 2025, 04:28:14 amGuys, let me just say, I'm not building a power supply to take with me to the center a nuclear bomb detonation zone or a radioactive underground facility!

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2025, 06:24:08 am
content: Quote from: drksy on January 22, 2025, 01:43:34 amCrowbar protection is still a feature outside of nuclear and radiation applications. It protects against pass element failure, which is usually a short. Programmable power supplies benefit from it also.

author: mawyatt
author_rank: Super Contributor
timestamp: January 22, 2025, 03:28:55 pm
content: Quote from: drksy on January 22, 2025, 01:43:34 amLet's hope not!!!!As David mentioned the "Crow-Bar" has many uses for protecting against unexpected events. We've all seen a power supply fail where the output voltage goes to a rail, very common for a Buck SMPS or Linear Regulator, with the usual consequence of all the semiconductors downstream seeing a large over-voltage condition and failing. Many custom power supplies for Military Equipment have an over-voltage Crow Bar requirement, and suspect many commercial applications have such as well.Best

author: najrao
author_rank: Regular Contributor
timestamp: January 26, 2025, 03:11:11 pm
content: Related, may be, but not quite the subject of this thread.Tappings on a transformer impose penalties in design, extra copper and insulation, not to mention labour. In service, the unused coil sections do nothing useful, but take up precious window space. Reconnecting of coils in series or parallel is more efficient.In some situations, a 1:2 voltage step may be sufficient. I have used a simple one pole single throw switch to do this. See sketch.The extra diodes do add some drop when the switch isoff, but give the parallel connection. There is no penalty of this diode drop when series is chosen. Obviously, this method is more suitable at higher voltages, as diode drop becomes negligible.Diodes (and bridges) are very cheap, and work well at high temperature.I first used this on a railway traction alternator, which was required to provide 900V 200A for cruisingand200V1200A for starting. It was more complicated because of three phase, etc., but essentially used the principle.The basic idea can be stretched for three or more windings, but it can get cumbersome.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 26, 2025, 07:34:36 pm
content: Quote from: najrao on January 26, 2025, 03:11:11 pmYou might want to flip that bottom diode.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 26, 2025, 08:56:32 pm
content: Quote from: najrao on January 26, 2025, 03:11:11 pmNifty.The switch can be a single NMOS : it has to carry only one windings current and withstand a single winding voltage.The source does elevate when on, but only to half the Vout, so it does not have to be photovoltaic driven.A cheaper optocoupler, or even a current mirror using a HV transistor pair, (eg 150V DMMT5401) would work on bench supply voltages.

author: najrao
author_rank: Regular Contributor
timestamp: January 27, 2025, 01:00:18 am
content: Yes, in fact Ihaveused such a mosfet instead of a mechanical switch. Did not mention it, just to keep the presentation brief and simple. Don't forget that the method allows change-over up or downon the fly, without transients other than caused by the load itself. There is no possibility of short circuit due to relay malfunction, too. Much cheaper than a relay overall.Thank you, PCB.Wiz.Edit: I should have cautioned that the two transformer secondariesmustbe separate and isolated from each other. Many 'audio' power transformers have center tapped secondaries, and donothave this required separation. Even if the center tap is formed by joining the leads outside of the coil, there may not be enough separation to split them out.Edit: Sketch has a serious error, bottom diode needs to be the opposite way.

author: najrao
author_rank: Regular Contributor
timestamp: January 27, 2025, 07:06:30 am
content: You are absolutely right, PCB.Wiz. My bad. What was I thinking.najrao