Electronics - Simulation and Judgement

Created; 14/04/2016, Changed; 12/03/2024, 13/03/2024

Previous page; Electronics - Minimum design for Electromagnetic Compatibility and Electrostatic Discharge

I use spreadsheets and other tools provided by wound component and switch mode power supply IC makers, and they are very good.  This and linked pages show my evaluation of spice simulation tools for general circuit modelling.  I have been recommended simulation tools in places that I have worked, but I observe that they will mislead an engineer if he has not thought through the circuit, PCB layout parasitics and what is required.  In another case, a spice tool was recommended to me for power handing and therefore thermal modelling.  I agree that spice tools can be good but taking care with the result.  I have also been advised that simulation would do things that I have not seen so far, instead of using a calculator, spreadsheet or mental arithmetic and figuring out before I apply myself to putting any part in to a draft circuit.  Then I would most likely go straight to circuit and PCB design to figure layout parasitics and component package shape implications, these things being very important. 

The simulation tool I use is SiMetrix and also;

LTspice has a good support group; 

• • https://groups.io/g/LTspice

  • http://ltwiki.org

I have only come to using simulation at home, but not at work.  For work, I keep a design straightforward and so robust.  But at home I can test things without using a soldering iron, and that is adequate for this website.

Blog with design examples; http://blog.andrew-lohmann.me.uk/p/electronics.html

Peak detector worked through from my blog discussion; Electronics - Robust design methods - Theoretical example outdoor

Positive and negative Peak detector;

• Electronics - Positive and negative Peak detector circuits and models

  • Electronics - Peak detector circuits and models

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More features of simulation tools. (also see; Electronics - Control Loops)

The power transistors are designed to be soldered to a PCB on top of a close matrix of small vias to conduct heat to the under side of the PCB, where the heat sink would be fitted.  The current probe is placed near the capacitor where the transitions are slower, so less noise is injected to the PCB, though functionally it is sensing the transistors current to the first inductor.

LTspice

• Parameters can be changed automatically and tested using the .step function, which could run simulation's with different output loads.  

• .tran Transient simulation runs the circuit in the way it will be used normally, but the alternative .ac is used to test the frequency response.   You can use the GUI menus to change the parameters.

• The LTC4444 driver was substituted with IR2010, the low voltage power supply was increased to 12V for this part.  This works better because there is a delay built in, ensuring that there is a real-time between one transistor turning off and the other being turned on.  A 2.7V drop to below 10V was added for the 10V rated comparator.

• The output return could be to a pair of capacitors instead of a centre tapped power supply if this circuit were used as a loudspeaker driver.  Other options;

  • For 8R, 600W variant, 2x 25mF 225V working.  This is an expensive option.

  • For doubly isolated gate, driver such as; UCC21222, UCC21222-Q1, UCC21220 which Texas Instruments make.

  • A gate drive transformer would not be efficient because the long pulse drive would need to be low voltage in order to maintain a symmetrical power waveform. 

  • Input moved to symmetrically placed +-5V supplies with 10uF input decoupling capacitor, with output being +-100V supplies Direct coupled.  This configuration will minimise the offset voltage in the output.

AL-0052-01A Block diagram circuit revisions required to implement this.  The input and driver circuit needs to be changed for +-2.5V operation and the gate driver changed to a fully isolated drivers type. 

Features of the circuit;

• The output low pass filtering is not part of the feedback loop and therefore keeps the feedback simple.  Consequently, the circuit should be optimal as a low distortion with reasonable voltage accuracy audio amplifier.  The circuit can be described is a relaxation oscillator with an output filter.  Coilcraft also stock a range of chokes for Class-D amplifiers with a flat magnetic curve, but I have not selected one of those.   There is a small output series resistance equal to the output inductors resistance.

• The circuit is stable with the resistive loads shown, it is designed to work with 4R load or higher.  A lower value load will cause the inductive reactance to increase and the output bandwidth to be reduced consequently.  If the power supply is increased to 200V and an 8R load, therefore, the output filter could be changed for optimum performance.

• The circuit is simple, but there is a lot of power circuiting in the output inductor, so the losses will be high and continuous even under light load.  Even so, this compromise is worthwhile in an audio amplifier in which minimal distortion would be the most important parameter.  The circuit in any case consumes a lot less power than a linear amplifier would.

• The circuit operates at a fixed 100KHz.

• Four or six amplifiers could be configured stereo with active cross-over bands from Woofer, Mid and Tweeter loudspeakers.  With the double pole filters on the teeter amplifiers only and single pole filters on the other amplifiers.

Out standing design work;

• The MOSFETs are not necessary the most suitable, although they work for simulation well enough.  Selecting parts from the LT library is not easy, particularly when looking for a part that also has the thermal model.  There are two parts for the one transistor on the circuit show (it is not one part) and that is why two references and two part numbers are show.

  • With LTspice an N-channel MOSFET with thermal modelling outputs can be selected from the component library. 

• • It is important to not over or undersized these transistors in order to achieve optimal efficiency.  Therefore, do not constrain a design to use parts that are included in the LTspice library, unlike what I have done here.

• The circuit could be implemented using a small microcontroller instead of the operational amplifier and comparator.  In that case, the only parameter that would need checking is that the microcontrollers A/D input's filter capacitor is adequate.

• The output drive could be replaced by a full bridge power driver IC and http://www.st.com such as L6208 may be a suitable IC designed for stepper motor drive.  But this may not be a cheaper solution, but the IC has various overload protection features built in.  Using one side of the half bridge as a polarity switch, the L and C network could be more efficient under light load.

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Conclusion

Modelling tools allow an engineer to try more variations without damaging a PCB that traditional prototyping method would tolerate easily.  I have shown some cases where modelling looks useful, but in most cases a design would either be trivial or an engineer should not pursue such a marginal design anyway.  For example, there are refinements in the conventional peak detector that I have played with, but I would not be pursued though they were useful for play to evaluate simulation tools.

I probably would not use the novel peak detector circuits with either BJT or feedback steering, that I have not included but work.  In playing with those ideas, some useful features of some components are demonstrated.  Simulation tools are beneficial for calculating power dissipation in switch mode power output circuits, snubber networks for switch mode inductive part and gate series resistors to minimisation shoot through current in CMOS type drive.  The simulation gives an ideal case without circuit board parasitic capacitance and inductance.  But a circuit performance depends on good PCB design and tolerant circuit design.  Do break and simplify the circuit into small sections and simulate each sections that need simulation, or else the simulation time will be very long or impossible.

The simulation is not as good as doing the worse case design and if necessary prototyping.  Simulation looks useful in the Class-D amplifier's at Electronics control loops, but I made the original successfully some decades ago without simulation but using pen and paper modelling easily enough.  It is best to make the real circuit board and test that in practice, but not make a veroboard prototype unless you are unsure.

Theoretical design example with the circuits above; http://blog.andrew-lohmann.me.uk/p/electronics.html

Next page (blog); http://blog.andrew-lohmann.me.uk/2016/02/electronics-dicussion.html

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