By Steve Bush Posted on 24th March 2021 | Modified on 24th March 2021
Alpha and Omega Semiconductor has announced a family of active diodes which can reduce the 1.2V forward drop of 600V bridge rectifiers. microcar chinese auto rectifier
Branded AlphaZBL, the first two parts are:
“Both products are self-powered from the AC line and do not require external circuitry,” according to the company (which has forgotten the need for 1µF 25V Vcc capacitors. plus some optional components for the 7200). “The proprietary self-biasing scheme sips minimal power from the AC line making it very efficient in light load or stand-by mode of operation.”
Because these are aimed at mains voltage use, efficiency gains are not as high as can be achieved by low-voltage active bridges – improvements are more likely to be used to squash residual heat dissipation in very efficient power supplies.
“Today’s data centres strive for improved efficiency and thermals as cooling of the data centre takes as much, if not more, power than the servers themselves,” said marketing manger said Colin Huang. “AlphaZBL products make the design of Titanium grade efficiency power supplies easier to achieve.”
In its example typical 100W application, A&O sees the integrated mosfet AOZ7270DI improving efficiency by 0.89% at 115Vac and 0.44% at 230Vac over a diode bridge.
AOZ7270 block diagram – AOZ7200 externalises the mosfet
As is usual in active rectifiers, the mosfets are used ‘backwards’ – the n-channel conducting when the source is more positive that the drain (effectively shorting the conduction of the internal parasitic diode).
Inside, the chips sense their own cathode to anode voltage drop, switching on the mosfet if it is <-105mV and off at >1mV according to the data sheets. Assisting with understanding polarity here, the company describes this 1mV as the reverse threshold. A careful look at an accompanying diagram suggests the mosfet is switched on is when the anode (source) increases to 105mV above the cathode (drain) and switched off as current reverses when that voltage changes polarity and becomes more negative than -1mV.
A high voltage depletion-mode mosfet controls the charging of the external Vcc capacitor.
Surge resilience appears to be covered: “Both products exceed lightning surge requirements that are a critical requirement in ac-dc applications”, said the company – the AOZ7270 data sheet speaks of X-caps failing during testing: “Even [with the] X-cap broken by the 3kV surge, AlphaZBL still in work without fail.”
As well as in servers, A&O sees use in PSUs for desktop, game console and telecoms, and in adaptors for high-end laptops and televisions.
The AOZ7200 data sheet is here
The AOZ7270 data sheet is here
On the subject of data sheets (at Rev 1.0), I feel both need more work to improve understanding about these potentially fascinating ICs.
As well as the oddly described thresholds, the (more extensive) AOZ7270 data sheet has many tantalising but unexplained application circuits combining two active diodes with two standard rectifier diode – possibly to add reverse-blocking? (I can’t find a mention of reverse-blocking).
Another thing I can’t find is an explanation of the optional external RC networks in the AOZ7200 application diagram, nor and explanation of why not all chips get a Vcc capacitor in the AOZ7270 example layout – the provided publicity image (right) does suggest it might be possible to share Vcc capacitors.
Tagged with: diode rectifier integrated circuits
Hi Steve, I’d have to check on this because it is a very long time since I had to look at the Suzuki electrical system, but if I remember correctly… As used on a motor-cycle the alternator rarely sees an open-circuit in normal use because the alternator/rectifier is connected to both the battery and the electrical system in parallel. I’ve had broken battery connectors and a broken positive lead into the rest of the electrical system, but never both at the same time. The exception is when the ignition key is switched to ‘off’ whereupon everything is disconnected from everything else. In that circumstance any open-circuit voltages are only there for a short time until the engine stops. These rectifier units are sealed black blocks glued to a mounting-plate, which makes them very difficult to x-ray. They might well contain additional components to suppress high voltages. (Time passes…) I’ve just been looking at on-line circuit diagrams and guess what? The rectifier/regulator is just shown as a rectangle with some wires coming out. I’m getting old now, you know. Soon I won’t be able to remember my own name…
Luke Hear who? …. 😉 I have similar recollection issues Morning You just reminded me of fixing (amazingly) the ignition system on a Yamaha IT456. The circuit was potted with what looked like quartz chippings. It took three days of picking at those and the potting compound to clear enough space to get at the components – giving me some insight in to the patience required to clean a fossil from rock. BTW, what a motorbike – far too powerful for me (fixed it for a friend). I had one go and almost lost control on a series of long undulating off-road bumps as I increasingly got it out-of-shape (on a track between barbed wire fences…).
Hi Steve, Luke here. Re: the active diodes / bridge rectifiers, you seemed a bit mystified by the two active diodes / two ordinary diodes configuration. I have a possible explanation that could be completely wrong – I’ll leave you to do the maths! I have seen similar devices in use to control the battery-charging voltage on various motor-cycles with relatively low current electrical requirements, i.e. no radio/CD players, heated seats, etc. The source is a permanent-magnet alternator, the bridge rectifier consists of two diodes and two thyristors. The voltage from the alternator determines the thyristor gate voltage, the two extremes are: 1. A flat battery will present a low cathode voltage so the thyristor starts conducting when the alternator voltage is still very low, i.e. before the gate voltage gets very far up the half-wave coming through from the ordinary diode, and 2. A full battery, so a high cathode voltage, and the thyristor doesn’t start to conduct until the incoming voltage is nearly at the peak of the half-wave. In both cases the thyristors stop conducting when the incoming voltage is zero, i.e. at the bottom of the half-waves. When I first worked out what was happening, many years ago, I was extremely impressed! A superbly simple system with no hot-running Zener diodes, no extra poles needed on the headlamp switch to switch alternator coils in and out, and no tin boxes full of mind-boggling unreliable relays with moving parts. So perhaps the two active diodes / two ordinary diodes configuration is designed for use in battery chargers where safe charging and high efficiencies are required, e.g. heat pump systems. The advantage of a permanent-magnet alternator is that one can kick-start the engine when the battery is completely flat.
Hi Luke Hear (here) I am a bit mystified by many things 🙂 I have not seen thyristors used in series with permanent magnet alternators – intriguing (I am even trying to draw the circuit out as I type). I am guessing they could easily hold off the high open-circuit voltages involved? UPDATE: now I have (https://www.littelfuse.com/~/media/electronics/application_notes/switching_thyristors/littelfuse_thyristor_applications_in_2_wheeler_and_all_terrain_vehicles_atvs_application_note.pdf.pdf) Continued…. That said, I have seen something similar used in a simple mains-powered regulated 12V Pb-acid battery charger (in the great old Motorola book that Wise Mr Kurt found me an e-copy of. Just found a later copy of it here (caveat emptor) https://ae6pm.com/Semidata_books/Motorola/DL137-D.pdf and a 1988 copy here (again caveat emptor) http://bitsavers.trailing-edge.com/components/motorola/_dataBooks/1988_Motorola_Thyristor_Device_Data.pdf And have seen them used in threes to short-circuit a three-phase motorcycle generator when it is delivering more power than its battery could stand – used at least by Kawasaki. My confession is: I used to be a bit in love with thyristors – except for their forward voltage drop – until my head was turned by power mosfets….. I am not sure these active diodes can be being used like that as they cannot be turned off – they seem to be completely autonomous.
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