Developing a modular switching tool for Research

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Vidura posted this 12 May 2019

Hello All ,
In this thread I will present tool which is designed especially for researching and developing many devices threated here on the forum. It consists of three different modules(for the moment), and is very flexible. Many different topologies of switching can be easily assembled by combining the modules in different ways and with different setups and configurations. The signal source is a two phase PWM module with combined analogue and digital features. It is based on an enhanced 8bit MC, the PIC16F785 with specific peripherals, like a dual phase PWM module and two hi speed comparators, which are used for analogue duty cycle adjustment with infinite resolution. To overcome the limited period resolution typical for digital PWM devices, an adjustable external oscillator is used to clock the MC, which in combination with the pre-scaler allow an infinite period resolution over a wide range of frequencies as well. The second module is the switchboard, which is designed as single floating switch, which allows to assemble simple Hi or Lo side pulsing, half bridge or full bridge topology, or floating coil shorting as well as multiple other configurations as required.
The third module is the floating driver power supply, which provides a galvanically isolated supply for various switching drivers. In the thread we will discuss different solutions for specific applications. Schematics , images and videos on the development will be released as well as firmware and software solutions for specific requirements. If you find this work useful you can support this project HERE:

Your support will be very much appreciated, and will help
rising founds to make this tool readymade available for order.

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Vidura posted this 12 May 2019

In a first stage I nave build the Two Phase PWM circuit on stripboard and test to find the appropriated values for the discrete components and work out the source code. For the moment the work and progress will be posted here, participation and suggestions are welcome as always. The tool resulting from this work it will be released as open source, for those who wish to build it, and readymade modules and tool kits will be available for order soon, part of the incomings will return for support the forum. This first schematic is a draft, to be improved while developing and testing is going on.

Edit:Some additional data about the basic operation of the device: the components in the rectangle at the right side RV4 R4 C5 are the external adjustable clock for the MC.RV1 D1 C4 makes a sawtooth generator for the timing of phase 1 duty cycle, same as RV2 D2 C3 for phase 2. When the output signal of the respective phase goes hi the capacitor charges up until the comparator reaches the reverence value and terminates the pulse.RV3 R3 are for a coarse adjustment of the reverence voltage for both comparators, this is necessary to set properly the value for different frequency ranges. The Dip switch is to configure the frequency range by system clock dividers and shift value between the phases. With the Sync Signal available on J5 another MC can be synchronized for example to use a synchronous rectifier with certain time delay. On J4 two additional GPIO ports are available for sensing purposes. 

The signals can be configured as free running two phase signal with independent adjustable duty cycles, or as complementary drive signals for half or H bridge topology with programmable deadtime or overlapping. A synchronization feature allows to connect various MC in parallel in order to add more phases if needed.

Vidura posted this 12 May 2019

I have simplified the circuit, reducing it to only the two phase signal source, the other features will be available on the definitive PCB for optional use. Anyway to make the circuit operational a PIC programmer is needed, so also the programmable configurations of the MC can be set up this way, simplifying the basic circuit considerably.
I have built this circuit for first testing:

A connector socket for the PIC programmer has been added, and all additional(optional) circuitry removed. Some component values might need to be modified to match the required frequency and adjustments ranges, I'm still waiting for some components for final adjustments.
here a video from the first test:

Vidura

Vidura posted this 12 May 2019

In this place I would like to share some experiences of my own regarding the signal source. In the beginning I worked with basic equipment only, a 555 PWM circuit with duty cycle adjustment and a other digital PWM built with a PIC micro. I had some good results with the analogue 555circuit, as it is possible with two potentiometers adjust frequency and duty cycle at the same time, and analogue devices have the advantage of infinite resolution, this can really make a difference when dealing with very narrow values for the sweet spot of a device. In all conventional digital PWM circuits there is a kind of rippling, due to the own clock frequency of the MC, this limits the outputted signals to be always harmonics to this processor clock. This can in some cases make devices fail to get properly tuned. Anyway the 555 circuit has its drawbacks and limits as well. Later I bought a SG, not the best but quite useable , a good tool for many things, but not nearly close as good for most pulsing experiments as the analogue device. Too much time turning the encoder wheel, pressing switches , changing ranges  , and modes. It might take you an hour to find a specific spot of tuning, wich can be don in a few minutes with the analogue source. But there is no doubt that MCs have many advantages, and are more stable as a 555timer circuit. MCs are programmable and can manage lot's of tasks, they are very flexible devices. So I was looking at a solution which combines the most important features of both technologies. The result can be viewed in this thread.

Regards Vidura

Vidura posted this 12 May 2019

The floating  Driver power supply:

In order to maintain all connected switches floating and galvanically isolated I designed this power supply, capable to feed four drivers on different switch modules:

EDIT: here the revised version

Note that some of the posted schematics are still drafts, so if you decide to build some of the devices please be patient, when they are updated with correct values I will replace them and put versioning f. ex.   Driver power supply v1.0  when approved, v0.x means a draft!

solarlab posted this 12 May 2019

Hi Vidura,

Excellent work and a very insightful approach (imho)!
Good luck; have a great week; and Happy Mothers Day to your "Commander and Chief!"

SL

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Vidura posted this 12 May 2019

The power switch module:
For greatest flexibility the power switch module is designed as simple switch unit, with an optical insulated MOSFET-IGBT driver, and integrated voltage regulator and filter for the driver supply.
And here a Switch modul with handcrafted prototype PCB version1:

version2, after some layout corrections:

 

 

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Vidura posted this 13 May 2019

a first switching test of some interconnected modules:

Really very happy with the optical drivers, 100ns fall time is not bad at all!

Here a Block diagram for a H bridge example:

The switch modules with the driver power supply can be fed safely by other signal sources, like SG or Arduino board, thanks to the opto isolated drivers.

 

Chris posted this 13 May 2019

Hey Vidura,

This is excellent work! Good to see such detail and progress!

You know, the last problems anyone needs is to find bad switching controller when doing intensive experiments after many ours of work!

If I may add a little think I found sometime back that I have not seen, or missed, if so please ignore my post, but a two way Diode on the Gate of the Mosfet. Like so:

 

D1 and D2 are in such a way as to stop noise turning on the Mosfet coming in on the negative rail. I have had a little of the trouble on H-Bridges especially.

Again I am sorry if I missed this and you have covered this already.

   Chris

Vidura posted this 14 May 2019

Hey Chris, you are correct with this , there are two bucking 18v zener diodes for gate protection, Specially important in noisy environment,and presence of longitudinal waves. Here i post the revised schematic of the powerswitch:

Also the driver power supply schematic is already updated, so anyone interested can start building this two modules.

Vidura

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Chris posted this 14 May 2019

Hey Vidura,

I found in noisy environments that two Zener Diodes were not enough. I have to replace D5 with a fast Diode.

Switching in environments where any sort of signals that can affect Gate Voltage from the Mosfet Source can be tricky to avoid problems, but I see you have that covered already. I only wanted to bring that up from my experiences to try to help others.

   Chris

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Vidura posted this 14 May 2019

Thanks for the feedback Chris

Later will make a test driving a teslacoil feeding a lower harmonic, a good noisy environment.

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Chris posted this 14 May 2019

Hey Vidura,

Forgive my ignorance, but may I ask why you have added Bidirectional Diodes?

I am no expert, and I am only going from my experience, but I would remove: D11 and D5 and reroute R10. D11 and D5 add potential for noise to trigger Mosfet. One of the safest Circuits I have used is based on the following:

 

 

The Gate is safe guarded, it has the Pull Down Resistor R10 if the Mosfet Driver does not do Pull Down, and the uf Diode D1 safeguards the Gate from reverse transients, and the Zener Diode D2, just in case the Gate hits Rated Voltage. Just some thoughts that may help?

Jagau, what are your thoughts?

   Chris

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Vidura posted this 14 May 2019

Chris, the second pair of diodes protect the gate from negative transients, which could not drain tru D1 in the proposed configuration, respectively could damage the low side of the driver ic . Anyway this considerations will only apply in extremely noisy applications. In my experience the board layout is as important as the best protection, I have many times used the driver directly connected to the gate, without additional components in quite noisy applications, and had seldom problems.

Regards

Vidura

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Jagau posted this 14 May 2019

Hi my firends

Here in the circuit of Vidura he uses an IGBT with gate driver A3120

The one I prefer is this one with the surge protection diodes in this direction.

 

If we use a mosfet instead of an IGBT R10 is very important, we must never wear out the gate of a floating mosfet.

R13 in the case of the IGBT is very important and must be calculated with the parameters of the driver a3120 and the parameters

OFF time of the IGBT

Using an IGBT driver is an excellent idea Vidura it avoids many problems.

It seem you will have valuable circuit 

Jagau

Vidura posted this 14 May 2019

The Gate drive Optocoupler:
First I had planned to use the FOD3182 from ON semiconductors, with slightly higher drive current, But as this device was not avaliable on the local market, the A3120 was a good choice as well .This opto-isolated gate driver is rated for driving voltage up to 30v, with a rise-fall time of 100ns. In the Power switch module it is set to 17v, this can be changed if needed by replacing the Zener diodes D4,D5,D6 by different values up to the rated maximum of the driver and switch. Many power IGBTs can be driven with up to 30V, although for most usages 17v will be fine, and can drive safely a MOSFET as well. One more option I wanted to refer to , that is the possibility to setup a negative offset voltage for the driver. It can be achieved by adding a voltage dividing resistor between the switch source(emitter) and the ground of the driving circuitry , with the result that the gate will be driven with a negative voltage at Lo state. The resistor value have to been chosen accordingly to the switching frequency and gate charge of the switch, typically a few ohms. An improved transient rejection and shorter switching time can be achieved with this technique. We will come back to this again later with a more detailed explanation and showing an application in  a experiment.

Jagau posted this 14 May 2019

One more option I wanted to refer to , that is the possibility to setup a negative offset voltage for the drive

 

 

You touch a very good point Vidura
I have a circuit that must use this concept in my research
  that I am testing.

I'll let you know my results if you're interested

Excellent
Jagau

Vidura posted this 14 May 2019

Thanks' for the feedback Jagau,

I have also mostly used the proposed configuration of two bucking zener diodes  and you are correct this is the better polarity , and to remark a important function of R13, is to protect the driver IC from excessive current and overheating.

Vidura

Vidura posted this 15 May 2019

Some minor updates made on the latest Powerswitch schematic, returned to the opposing zener diodes, in the polarity suggested by Jagau, I removed the additional diodes from the design, as I have already 4 boards assembled in this configuration for testing.  Pins 1 and 4 of the driver are connected to signal ground as recommended in the datasheet, for better noise immunity.

Another detail we have to be aware is the correct current for the emitter led of the driver, R9 has to be properly selected, according to the signal voltage. As the MC outputs 5V signal the current should be around 10mA with the selected 470R resistor, but due to the voltage drop of the led I measured 7mA, which is on the lower limit of recommended values(7 to 16mA for this driver). So I replaced with a 330R value for R9. If someone use a 3.3v MC, like the Arduino 220R would be better.

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Vidura posted this 16 May 2019

Hello all here the latest works on the Power switch board. I made some mirrored modules in order to keep power connectors as short as possible:

Vidura posted this 16 May 2019

After many tests of the PWM module at different frequency ranges some more adjustments and changes in the design where necessary. We will now have another look at the different modes of configuration of this PWM device. It can be set up for complementary output signal at 50% duty cycle with programmable deadtime(or overlapping). This mode is typically used for driving Half bridge or H-bridge topologies. The maximum duty cycle is set predeterminate at 50% in this mode(digitally by PWM clock counter) , but the pulse can also be terminated before with the analogue shutdown(combined mode). The third mode is analogically only with the complementary digital mode disabled, this might be the most flexible mode with widest adjustment possibilities. In the analogue modes, the duty cycle is set by absolute timing, not by relation to the frequency(%), as some already might know only a few more cost SG's have this feature, which can be very helpful in some applications. As we know ,when we feed a signal into a inductor and change the frequency with a duty cycle setting by % by increasing the frequency the impedance rises, and decreasing it diminishes, to a point at low frequencies the current will rise very much. Anyone who used a standard PWM will have observed this effect. In contrast the analogical PWM presented in this project will maintain the puls duration unchanged thru the entire frequency range, and the impedance of a given inductor will be maintained quite stable over a wide range(disregarding resonance effects). This feature can be very helpful in some tasks of tuning. The drawback of tis technique is the greater implementation of hardware to cover wide frequency ranges. To resolve some issues related to this is what I am actually working on. Soon I will post another updated version of the schematic. PS. I tried to get the frequency down to around 150hz with a maximum pulse duration of 2ms , for usage in magnetic resonance testing in this ranges and above. Will be fine with some switches for change the range.
Vidura

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