Boost converter- Measuring current with current sense resistor - Page 1
Forum section: electronics / projects

author: FerdinS
author_rank: Contributor
timestamp: August 13, 2020, 07:10:10 am
content: Hello everyone;My project is to make boost converter. From 18V approx 0.5 A to approx 100V and max 0.085A.I am using those components:- 220 uH inductor- 3uF capacitor- SQJ454EP-T1_GE3 - mosfet.Because I have to measure income current and output current I am using 2 current resistors, as it is shown on the picure :because I have to make this "voltage drop" current sense resistors I am using rail to rail operational amplifiers so that voltage goes to max. 3.3 V, bcs my ADC on IC is measuring max that.It is shown on the picture:When I am measuring current now it is shown that the voltage drop on current sense resistors is too high. Also tried to put on output voltage around 100 V and it seems my resistors got burned, so it really has to be too high voltage drop on current sense resistor.I am asking now, is 0.5 and 0.05 ohm too high ?Also what resistor should I use on the out put ? I tried with 1k - and also 20k ohm and it seems that it gives me same result.Thank you a lot for your kind answers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2020, 11:58:49 am
content: What's up with the duplicated diff amps? U1A and U1B seem to be doing the same thing, except B with some looser resistors for some reason. And U2 similarly, but R15, R18, R22 and R24 are seemingly wrong values.The AA4, AA41 and AA4R connections aren't shown, I'm assuming they're across the output resistor somehow...The MCP6022 has a common mode range of -0.3 to 3.6V as shown. You're applying up to 16.3V to U1 (or more if the maximum input voltage is higher), and up to 93V (if it ever got there) to U2. Needless to say the ESD diodes will be activated and the amps will die a pitiful death.Have you considered a low-side current sense resistor? If the output doesn't need to be common-ground with the input, this is an excellent choice. It can also be used with the transistor, typically used with a peak current mode control since you miss the diode current in that configuration. Peak mode would be a fine choice at this low power level.A purpose-made current sense amp is a good idea. These have a wide input voltage range, low offset, and some offer a current output that can be used with external components to sense arbitrarily high voltages.The ordinary diffamp configuration can still be used, but the inputs need to be biased negative enough to meet the input range requirements. This will attenuate the signal being sensed (technical term: increased noise gain), so much lower offset op-amps would be desirable. You may not have enough bandwidth to do a proper average current mode control in that case, either.Tim

author: FerdinS
author_rank: Contributor
timestamp: August 13, 2020, 05:02:20 pm
content: Hi Tim;Sorry for postng wrong picture and low explanation.So I am making boost converter that is gonna be suitable with ISO26262 functional safety. So it is ment to measure input current and output current 2 times( every measurement has to be duabled in case that one of them get bad.So I am sorry I send you wrong picture, the resistors that you mantioned are changed to:For "Izhodni toki = output current" measurements I am using 0.5 ohm current sense resistor and 36 times amplifying. Since there will be maximum on the input:- 18V - 500 mA = 9WAnd on output max :approx : 100 V - 0.085A = 9WSo on first current sense resistor there will be voltage drop of 0.25 V and using 12 times amplifying comes out with 3V, and on output 0.0425 and using 36 times amplifying comes out with 1,53 V which is okej for 12bit ADC resolution. ( please correct my math if I am wrong ).Hope you guys understand what is my struggle.Ferdin