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EEZ Bench Box 3 (BB3)

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prasimix:
EEZ BB3 crowdfunding campaign on Crowd Supply
Forum discussion about crowdfunding: https://www.eevblog.com/forum/crowd-funded-projects/eez-bench-box-3-sequel-to-eez-h24005/

User manuals on English and German are available online: https://www.envox.hr/eez/eez-bench-box-3/bb3-user-manual/1-introduction.html



I'm opening this new thread since it does not strictly belongs to existing two threads: one about what is called EEZ H24005 programmable power supply and another discussing DIB ("DIY instrumentation bus").

This one is about making EEZ H24005 a) even more modular and b) more “completed” design/project that does not include ready-made modules. That also does not imply that EEZ H24005 project is dead, and that people who built it or get it via crowdfunding campaign cannot count on further support.

Currently EEZ H24005 is modular in the sense that so-called Power modules are independent from digital control (Arduino Shield). Theoretically one can make another Power module with different capability and functionality as long as its dimension are within 165 x 74 mm and can use 26-pin connector for both power and control lines.
One possibility is to redesign that module in line with proposed DIB where PCB could be 145 mm tall and 170 mm wide (or even wider). That gives enough additional space to host a complete AC/DC power converter, what will be presented in future posts.

More “completed” design mean replacing of used ready-made modules with "home made" design. In case of the EEZ H24005 that means giving up from Arduino Due, AC/DC module (from Mean Well) used to deliver 48 V for Power Board and AC/DC module (from Vigortronix) for delivering 5/12 V for powering Arduino Shield and 12 V cooling fan.

Mentioned changes are not trivial at all (i.e. cannot be done in few days): replacing Arduino Due means migrating firmware to the new platform and replacing AC/DC parts include playing with mains voltage that require different discipline during development and testing to stay alive and also provide something that is robust enough to be safe and secure during operation under all (or at least imaginable but practically possible) conditions.
Just that two reasons are enough to keep topic separated from existing EEZ H24005 project since even if I succeed with it that don't necessarily means that many people will be interested to cope with different MCU and especially with circuit that works with mains voltage.

prasimix:
I was entertained with the idea to replace AC/DC power module or mains transformer with some “in-house” solution almost from the beginning of my programmable power supply adventure. But shortly after EEZ H24005 crowdfunding campaign fulfillment was completed I starts to think more seriously about it. The similar situation was with thinking about alternative for used Arduino Due. I'd like here first to address AC/DC power module since I spent much more time on it then to MCU alternative.

AC/DC power module that should replace existing Mean Well LRS-150-48 (48 V/155 W) is designed taking into account the following features:

* Wide input AC voltage (e.g. 85-265 Vac) without using a 115/230 Vac switch (also called universal AC input, full-range input, etc.)
* Widely adjustable output DC voltage 2-42 Vdc or more. Therefore it can replace not just Mean well's power AC/DC module but also pre-regulator stage of the Power board build around LTC3864.
* Over-current/short-circuit protection
* under-voltage and over-voltage protection
* Min. 200 W of continuous power
* Bias power supply with all needed fixed voltages for the existing Power board (replacing LTC3260 on the Power board for generating required negative voltages)
* Active/Synchronous rectification for better power efficiency (e.g. lower thermal losses)
* Compact design that both power and bias stage are within Mean Well’s module dimensions (i.e. 159 x 97 mm)I've decided to give a try to the AC/DC converter topology that is possible unusual for such purposes but was recommended by the person who is a sort of “living legend” when comes to power analog electronics who is actively, massively and selflessly helping many DIYers (including me) on couple of regional (Serbian) forums. His name is Dragoljub Aleksijevic, known as Macola. Therefore presented AC/DC converter (that acts as power pre-regulator and bias power supply) is more or less my attempt to make alive what Macola in one moment suggested.
A couple of disclaimers/clarifications is needed here: first, he also suggested more advanced topologies that can serve as efficient power pre-regulator and has better EMI since there are not hard-switching, but since this was my first attempt to make power AC/DC converter I selected one that will be presented shortly.  As you will see there include a lots of interesting details and challenges. Secondly, I was already spent many months on it and I'll try to squeeze a whole adventure in a few posts. Therefore it's quite possible that I'll forget to mention many important details so feel free to ask any kind of questions and I'll try to answer it by myself or by asking Macola for assistance.

The topology is called current-fed dual inductor converter (CF-DIC), that evolved from single inductor converter (or SIC) presented for the first time in the article Filho, Barbi (1996), A comparison between two current-fed push-pull DC-DC converters-analysis, design and experimentation.

The following picture shows basic components for converter's topologies presented in above mentioned article:

SIC:



… and DIC:



A list of DIC benefits over SIC topology is also presented and I'll mention it here for getting a better picture:

* Voltage across the switch is 50% lower, hence switching components stress is lower
* Transformer require only one primary coil and peak volt-ampere is 50% lower
* Input inductor current is 50% lower (a cheaper/smaller inductor can be used)
* Smaller current ripple and rms current in the output capacitor
* Slightly smaller switches rms current
* Inductor switching frequency is 50% lower (hence smaller losses)What Macola proposed is a little bit different and with having in mind from the start what controller IC could be used to serve that purpose:



It has buck stage at the input that can be used to change duty cycle when PP (push-pull) stage is working with fixed duty cycle. In a way PP stage serve as a “DC transformer” (mind quotation marks here) that isolate primary from secondary side of high-voltage buck. Both buck (synchronous) and PP driver stages can be found in TI's LM5041B PWM cascaded controller. It include push-pull outputs that can be used to drive PP stage directly but with fixed duty cycle that is set to 50%. Depends of chosen topology, i.e. voltage-fed or current-fed PUSH and PULL outputs can be generated with programmed dead-time or overlap-time.
Driver signals overlapping is of paramount importance since neither of DIC inductors should be disconnected at anytime. That will induce a huge voltage that will shortly destroy one of the switching elements (MOSFETs in our case).
PP switching frequency is derived from buck stage frequency that is twice as much higher and set with external resistor.

How it works

Using Macola's words the following steps is an overview of important facts during operation:

1. Vin is switched on (first half cycle). HI-BUCK MOSFET and one of the PP MOSFETs (e.g. PUSH) are turned on. DIC1 inductor between them is connected to the full voltage (almost Vin) and current in it is rising following the dI = U / L * dt

2. When PWM time is expired, HI-BUCK is turned off and DIC1 inductor is trying to keep its current (Iend) to flow in the same direction. That current flow is preserved thanks to LO-BUCK MOSFET that is turned on (when expire dead-time after HI-BUCK is turned off). The PUSH MOSFET is still conducting. At the DIC1 inductor ends we have almost short-circuit condition (i.e. the major voltage drop is caused only by Rds, on of LO-BUCK and PUSH MOSFETs). Following the same law of dI = U / L * dt current change will be minor since voltage is small (about 1 V) hence we can consider that current is constant (e.g. Iend is still unchanged).

3. Just prior then PUSH MOSFET is switched off, PULL MOSFET is switched on (as defined with overlap time) and connect its end of primary coil to the ground. Otherwise when PUSH MOSFET is turned off both primary coil's ends will be left unconnected and the voltage on both drains will go into infinity (with disastrous outcome for one or more components). PULL MOSFET also connect its inductor (DIC2) to the ground and new Buck cycle is starting that is now charging DIC2 inductor.

4. Short overlap time is expired and PUSH MOSFET is turned off and voltage on its drain reach primary coil's Vclamp value (since its other end is grounded). Other end of the primary coil, that is connected to the switched off PUSH MOSFET is now behave as an accumulator that is charging and has Vclamp potential. Current thru DIC1 has Iend value and is now flowing thru the primary coil.

5. DIC1 inductor potential is now Vin – Vclamp, because Buck hi-side period is active. Since it was previously shorted and preserve Iend current, that current is now decrease slowly since a small voltage difference exists between its ends.

6. HI-BUCK is turned off again, and its voltage drops to LO-BUCK voltage drop (e.g. -Vd). DIC1 inductor has now Vclamp - (-Vd). The voltage difference is now much higher and current thru DIC1 inductor is falling much faster supplying the primary coil. DIC2 inductor has a constant current and its captured by short-circuit caused by PULL MOSFET conduction state and waiting that PULL MOSFET be turned off that it can start to flow into primary coil that will be reversely polarized in that moment.

7. Prior then PULL MOSFET is switched off (during the overlap period) now the PUSH MOSFET is turned on and “catch” its side of the primary coil, PULL MOSFET is turned off and current is continue to flow in other end of the primary coil that is reversely polarized.

… and the whole cycle is repeated over and again.


CF-DIC with short overlap time can be interpret as two boost converters that works in counter-phase with duty cycle a slightly over 50% and which load behind rectifiers is Vclamp. That means that the almost same rule is applicable for DIC inductor's calculation, where their supply is the buck stage, and average voltage of it pulses can be used as a DC source.

Output Vclamp on the Cout is reflected/mirrored on the primary coil proportionally to the transformer transfer ratio (Np/Ns). For example if Vclamp (i.e. converter's output voltage) is set to 10 V with transformer ratio N = 2 we'll have 20 V on the primary coil ends despite the fact that DC bus voltage is 325 Vdc (rectified 230 Vac).

Vclamp that is reflected/mirrored on the primary side can be seen as a voltage source with certain internal resistance. Therefore primary coil behaves as voltage source what is result of such heavy capacitance load. That load is necessary since it directly define its Vclamp. That is quite opposite from voltage-fed converters.

prasimix:
Now, I'll present what is assembled and tested so far, but first a short list of features:

* Wide AC input 85-265 Vac
* Wide DC output: 3-52 V controlled by tracker circuit
* Max. current 5 A continuously (i.e. max. power is 260 W)
* Synchronous rectifier
* Over-current protection (OCP) set to ~7.3 A
* Output over-voltage protection (OVP) set to ~54 V
* fsw, buck= ~68 kHz, fsw, pp= ~34 kHz
* QR flyback as bias power supply
A complete circuit (without QR flyback) can be split into two main section. Lets start with Buck/PP stage with DIC power inductors and transformer:



As already stated LM5041B is used as main controller for this CF-DIC. It's PUSH-PULL outputs can deliver respectable 1.5 A peak and it's used to drive directly PP stage MOSFETs. Situation with buck portion is different. Its outputs HD, LD are TTL and require driver IC. Buck is working with high voltage (e.g. 325 Vdc) therefore a HV driver is also required. But, additionally to just HV driver, an isolated driver Si8233 from Silabs is used that improve separation of low-voltage/small signal and hi-voltage/high power grounds. As two power inductors Murata 60B684C are selected and finally one very interesting part (also suggested by Macola): a transformer that is winded using VAC core (T60006-L2025-W380) with Vitroperm 500 F core material. It allows us to work with lower switching frequency (~34 kHz) while flux density could go easily up to 0.5 T instead of 0.3 T suggested as upper max. (Bmax for most of the other core materials.
Thanks for it's high permeability give us primary coil with high inductance using fewer turns. Making such transformer wasn't a big deal. I made first one in less then a ten minutes in the following way: first I make a braid with multiple wires for primary coil and insert it into heat shrinking tube with very small diameter. That tube alone is certified for 600 V that in combination with existing wire insulation is more then enough in our case, since voltage on primary coils in worst case (the highest Vclamp of 52 V) shouldn't go over ~104 V. Such prepared primary "braid" is combined with braid for secondaries of the same length (that is valid for 2 : 1 : 1 ratio), secondaries braid is bend on half and I simply start with windings. The end result is looks like this:



Doesn't look professional, but so far didn't make any trouble nor audible noise under any circumstances (if control loop compensation is properly set :)).

FB pin is grounded as usual in situation when secondary side is isolated and COMP pin is used instead for setting output voltage by changing duty cycle of HV buck stage.
LM5041B is powered from +12 V provided by QR flyback that will be presented later. But A-side of Si8233 needs +5 V and instead of providing additional bias voltage or stepped-down +12 V, a REF output from LM5041B is used for that purpose!
An diode clamping circuit is added in the PP stage to return back to the DCbus elcos (C34, C35) a part of reactive energy that exists due to L(sub)lk, pri(/sub) (primary coil leakage inductance)  reducing the stress of the RC snubber over primary coil. Anyway, that energy cannot be transferred to the secondary side.
As one can see all MOSFETs are SiC (Silicon-Carbide) that is especially beneficial for HV buck side. Also diodes D13, D15, D18 and D19 are SiC, too. SiC diodes are suggested since they don't have undesirable recovery time nor recovery current. Therefore they put less stress on related circuits: D13 to AUX bias power supply, D18 and D19 to PUSH-PULL MOSFETs (Q5, Q6). D15 is added as a “support” for low-side buck MOSFET (Q3) or its body-diode. D16 which is ordinary Schottky is added for more balanced supply between hi-side and lo-side buck supply (VOA and VOB).

jbb:
Hi Prasimix

I too have been wondering about a very similar topology (using a single buck inductor and H bridge for primary winding) for a similar application: wide range output.

There are three things I really like about this style: it only uses a single primary windin, is easy to protect against output short circuits, and should be able to provide a wide output voltage variation to efficiently supply the linear post regulator.  There are a couple of things which make me nervous: where the CT goes, and the transformer isolation.

Maybe I'm just chicken, but I'm worried about people making their own transformers.  This is because it's a safety part which has to stand off big surge and spike voltages of 2500 V or more.  I'm concerned that someone might nick the insulating sleeve, or not allow enough creepage distance from the ends, or have some trapped metal particle that gradually rubs through, or...   I suggest you plan to source a commercially built and Hi Pot tested transformer for this project.  It will provide a margin of safety for the less experienced builders, and probably reduce the risk of you getting in legal trouble ("Your Honour, I made sure that the safety critical part was tested.")  Also, the leakage inductance will be more consistent in a commercial product.

As it comes to the switching devices, I think your primary switches could be superjunction Si MOSFETs because they don't switch in pairs.  Also, maybe the buck stage could use one Si MOSFET and an SiC freewheel diode (with a little efficiency loss...)

prasimix:

--- Quote from: jbb on June 21, 2018, 08:08:28 pm ---Hi Prasimix

I too have been wondering about a very similar topology (using a single buck inductor and H bridge for primary winding) for a similar application: wide range output.

There are three things I really like about this style: it only uses a single primary windin, is easy to protect against output short circuits, and should be able to provide a wide output voltage variation to efficiently supply the linear post regulator. 

--- End quote ---

Yes, in fact this topology is inherently resilient against output short circuit.


--- Quote from: jbb on June 21, 2018, 08:08:28 pm ---There are a couple of things which make me nervous: where the CT goes,

--- End quote ---

That is not visible on the first sheet of schematics. I'll explain that in the next coming post.


--- Quote from: jbb on June 21, 2018, 08:08:28 pm ---
... and the transformer isolation.

Maybe I'm just chicken, but I'm worried about people making their own transformers.  This is because it's a safety part which has to stand off big surge and spike voltages of 2500 V or more.  I'm concerned that someone might nick the insulating sleeve, or not allow enough creepage distance from the ends, or have some trapped metal particle that gradually rubs through, or...   I suggest you plan to source a commercially built and Hi Pot tested transformer for this project.  It will provide a margin of safety for the less experienced builders, and probably reduce the risk of you getting in legal trouble ("Your Honour, I made sure that the safety critical part was tested.")  Also, the leakage inductance will be more consistent in a commercial product.

--- End quote ---

I was instructed when both primary and secondary wires are insulated with heat-shrink tube that gives margin of 3 kV, or 1.5 kV when only secondary wire is insulated. My hand-made transformer shown above has insulation of primary side only. I presumed that can provide an equal strength to the former example.
As already described we have here a much favorable situation then in case of e.g. flyback where on primary coil we have Vin, dc + Vclamp * N that could easily goes over 450 Vdc.
But, please note that if anything is happen with this project in the future I'm not going to wind dozens (or hundreds) of transformer by myself :). Therefore I already contracted Polish company Feryster that send me yesterday picture of a prototype built in accordance with specification:




--- Quote from: jbb on June 21, 2018, 08:08:28 pm ---As it comes to the switching devices, I think your primary switches could be superjunction Si MOSFETs because they don't switch in pairs.  Also, maybe the buck stage could use one Si MOSFET and an SiC freewheel diode (with a little efficiency loss...)

--- End quote ---

I've used MOSFETs in my first prototype, IPA60R380P6XKSA1 for hi-side buck and push-pull MOSFETs and IPA60R120P7 for lo-side buck. Results was disappointing, but I suspect mainly due to badly routed PCB. That is one reason why isolated driver is used in current prototype. Maybe on the next PCB prototype I can try Si MOSFETs again but I think that one for hi-side buck  should remain SiC. Lo-side could be removed and leave just SiC diode.

As you can already see I'm not so concern about BOM cost for this converter. I'm more interested to get robust and flexible design that can be easily scaled in the future if required. Selected topology promise easy upsize to couple of kW of output powers, but also a even wider output voltage. In that sense I'm particularly interested to build in the next step a converter which output voltage can go up to 400 Vdc and e.g. 500 W that can be used while building another SMPS :). Additionally (if that happen at all), with different transformer I believe that is possible to make a converter with output of few kV that can be used as source for some ESD testing and HV experimentation.

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