Very cheap current limiter circuit

Question

I need to design the cheapest possible current limiter.

My system consists of:

half-bridge driver - NCP5901BDR2G - with the enable pin (high state of the pin is enable, low state disable outputs),
pairs of mosfets connected to the 8V line - half bridge configuration,
a 1:10 transformer whose primary winding is connected to the half-bridge output and half of the supply voltage - 4V,
the transformer output is connected to a variable load (human skin),
output is +/-40V square wave @ 1MHz.

The maximum output power I want is 9W. I need to design the cheapest possible current limiting system. Assumptions:

the system can use the driver's enable pin as a power limit
the price of the elements taken into account for valuation is the one for 1000 pieces
the cost of soldering a single SMD element is $0.015
the accuracy of the maximum current should be within the range of approximately +/- 10%
Maximum heat dissipation of current limiter (ex. shunt resister) should be below 0.4W, optimally <100mW

Any ideas how to make it cheaper than with using operational amplifier?

Since the load is described as being human skin shouldn't the design aim be the safest, rather than the cheapest? What happens if the cheap design causes harm to the human it is connected to? — Chester Gillon, Dec 28 '23 at 10:16
What about a capacitor because frequency ,voltage and hence reactance are known — Autistic, Dec 28 '23 at 10:34
Not speed but costs are the most important @ChesterGillon. Power limitation comes from device capability, not from human max absorbed power. — piotr, Dec 28 '23 at 10:38

score 2 · Answer 1 · edited Dec 28 '23 at 12:09

If you want minimal components count, I would look into a H bridge driver with integrated current limitation. You can also look into using one with integrated mosfets. This way you save on PCB space and on the number of components to solder. But I don't know if you can find a matching H bridge for a total cost bellow the current one (nb : you can also buy a full bridge and use it as half bridge if it turns out cheaper).

However, I think you are overthinking this problem :

@1000 pieces, you can find Op-amps at <0.05$, so with a few passives, and soldering, you will be at about 0.10-0.15$ of components, so 150$ for 1000 units.
I don't know your exact wages, and the extra costs your employer has to pay for you, but I expect that the time you spend developing this (if you are working in Poland as your profile suggests) cost probably more than that).
The certifications you will need to perform to sell this device (as it seems to be injecting current into the human body, I think it might even require medical level certifications) will cost you probably thousands of $

So as long as your quantity is 1000 pieces, aim for cheap, but don't waste your time for cheapest (the extra time is going to cost more that 0.05$ difference per PCB). If you start going into the 10 000 range, then it might start to be of interest to scrap a few cents per unit. If you reach 100 000 units, then every cent is very important.

Very cheap current limiter circuit

1 Answers1