Can I just scale down converter to measure how it will behave in full size? - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 09, 2022, 01:40:49 pm
content: Hi folks.I have a practical question. Can I just take a big DC/DC converter and replace all inductors, capacitors, and current sense resistors by a factor of 10 (for example)?While keeping all the semiconductors in place.Will it behave the same as a full-scale one? I know there are some parasitics that will be the same. But in theory, all the waveforms shall be pretty similar just 10 times smaller.I want to design some bigger DC/DC with digital control and test its behavior under various conditions. Yet the full-scale power levels are beyond my current measuring equipment. Plus it shall avoid spectacular failures during tuning.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 09, 2022, 03:48:02 pm
content: Not really. A lot of the circuit design values and waveform are greatly dependent on the characteristics of the inductor used, the inductor or transformer of a switching converter is the heart of the whole circuit.You'll have to build the actual thing, full size. Start testing with current limited source, and small load on output, work your way up. But ultimately in order to prove the design functions correctly you'll need to be able to test it under full rated load, no way around it really. You can do all the simulation and small scale testing you want and still have things go wrong once you really load it up.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 09, 2022, 04:37:43 pm
content: With theoretical ideal components (say, inductor with only inductance, capacitors with only capacitance, MOSFETs with zero Rds_on and infinite switching speed, and no other limitations), infinitely small, this would work.In reality, power converter design is 1% calculating L, C values, and 99% understanding all the parasitic features of the components, choosing the right parts for the job, and designing the correct layout, and measuring the performance correctly.But you totally canlearnwith the scaled down prototype, and I strongly suggest that. Then you can apply that experience when designing the bigger one, which likely is more challenging.A lot can be simulated in absence of equipment. Protection circuitry can and should be designed, and these circuits tested and verified without applying actual power. This will minimize number of expensive blown parts in large converters. For example, to test that your pulse-by-pulse current limit circuit works, you don't even need to solder power transistors in; just charge an elcap to say 1V and connect it across the current shunt resistor. Measure the output of the shunt amplifier, and MOSFET gate drive reaction with an oscillosscope to determine if it did turn off, and how long it took.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2022, 06:31:20 pm
content: As mentioned, sort of. Depends on what you're scaling.If you want to observe switching dynamics, you'll need to scale down the transistors as well, and scale up the loop inductance (including source inductancewhich affects the gate switching loop as well!). And lower voltage transistors don't have the same C(V) curve of HV ones, so you're still kind of stuck on the exact particulars there. Next best, then: you can do reduced current (transistors at same voltage, same product line, are basically just scaled down in area), so also lower power though at the same voltage.Another way to observe is by keeping the same current -- so as long as dI/dt is the same, you get representative transients due to loop inductance. Catch: same transistors have exponentially higher Coss, so dI/dt won't be the same. Smaller transistors (high current, low voltage) could perhaps be used to get this a bit closer.Probably a good compromise is doing full current test at reduced, but not low, voltage. Above 50 or 100V, C(V)_off should be representative, and while the switching times won't be quite the same (due to relatively more time spent in the <20V high-Coss range), the voltage overshoot/ringing should be similar, while turn-on current overshoot / hard switching will be... well, worse in some respects (because of the "relative more time at low voltages" aspect), but better in others (as the loop inductance isn't charged from as much voltage, so current doesn't increase as sharply before the off-transistor is discharged / body diode recovers).Mind, I'm assuming this is using like 600V+ MOSFETs of recent production, so, SuperJunction types with extremely aggressive C(V) curves. If it's a lower voltage project to begin with, 100V or less say, they'll be regular VDMOS in any case, and scaling is less dramatic, take your pick of the above methods.Whereas if you're interested in cycle-to-cycle control behavior rather than switching dynamics, yeah, no problem. As long as it's doing the right waveforms overall, and the feedback values are scaled appropriately, the control won't even know anything's different.Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 09, 2022, 06:54:49 pm
content: Thanks, TimIt brings me an ideaAs I have a digital controller I can use it to generate test switching to make just a single cycle on full (or defined) power as it will be peak current mode controlledTo observe switching dynamicsAnd test loop and peak current behavior on scaled passives while keeping the same semiconductors

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2022, 12:09:28 am
content: Yes, pulsed tests are a good idea as well.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2022, 09:54:06 am
content: With digital controller, you can also capture the data and plot in excel/matlab/python/whatever.Like this:Code:typedef struct{uint16_t voltage;int16_t current;uint8_t some_internal_state_variable_you_could_not_even_measure_with_oscillosscope;} trace_item_t;volatile int trace_i = TRACE_LEN;trace_item_t trace;void control_loop_isr(){int16_t current;uint16_t pwm;// PID controller or whatever hereif(trace_i < TRACE_LEN){trace.current = current;. . .trace_i++;}}main_loop(){trace_i = 0; // trigger start of traceif(trace_i == TRACE_LEN) // it's finishedprint_it_to_uart_in_csv();}This can greatly augment what you can get with an oscillosscope.

author: EEVblog
author_rank: Unknown
timestamp: May 10, 2022, 10:22:18 am
content: Quote from: Siwastaja on May 09, 2022, 04:37:43 pmThis.It's not really going to work in practice.It might work ok-ish if you aren't anywhere close to margins at either end of the efficiency/performance curve, but even then it can be a crap shoot.Magnetic component (transformers and inductors) saturation are a major problem, but just the basics of frequency and pulse currents with different cap values can change everything as well.