proper charging and discharging of dc-dc using mosfet - Page 1
Forum section: electronics / projects

author: john23
author_rank: Frequent Contributor
timestamp: June 11, 2024, 08:14:05 am
content: Hello,tc1261lcs8 needs to be both turn on and off using esp8266 3.3V GPIO pulse and the output needs to charged and discharged with this GPIO pulse.When my GPIO =0V output off when GPIO=3.3V output is ON.Which is the opposote of what the SHDN does as shown in the data sheet below.I have made an inverter as shown below .I want to discharge the negative voltage to ground using NMOS.Vg=5V and Vs=-2V so the NMOS is open.However the output only goes from 0 to -1 instead of 0 to -2.4 expected by the formula below as it was withoud the SHDN switching.Why its not charging back to -2.4V?another problem is that i want the output to be discharge at logical 1 (5V) but here i got the opposite)Is there something that could be done?Ltspice model is attached.Thanks.https://eu.mouser.com/datasheet/2/609/1261lfa-3124642.pdf

author: xvr
author_rank: Frequent Contributor
timestamp: June 11, 2024, 07:48:38 pm
content: QuoteWhen you 'close' MOSFET and set 0V to its gate you still have -2V on source, so MOSFET still open. Only when Source drops below Vth MOSFET will be closed.This is a reason of -1V output.

author: john23
author_rank: Frequent Contributor
timestamp: June 12, 2024, 09:35:37 am
content: Hello XVR , When Vg=5V output discharges very good to 0V.Vth=1V.But when the Vg=0V the mosfet suppose to close in order to allow the output capcitor to charge to -2.4V.As you said the Vgs<Vt only works till -1.Vs from 0V it start to lower it self till the mosfet will open.Vg-Vs<Vt0-Vs<1So Vs<-1 is the boundary to which the mosfet could lower itself without opening the mosfet.So if it tries to further lower itself beyong -1V suppose -1.5VVs=-1.5V Vg=0V Vgs=0--1.5>1 and -1.5 meets the GND potential :-) so basickly we are right on the border of the MOSFET opening.This is backly my thoughts no question.I solved it only because I found a MOSFET with Vth=-2.75 .What if i would need to charge the output to -5V ,and there would not be a -6V=Vth what could be done to solve the problem?How can make the MOSFET stay closed and make the output to charge till -5V with my Vth=-2.75V?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 12, 2024, 10:26:25 am
content: QuoteThan you need to change discharge schema. The simplest one will be small SSR instead of MOSFET.

author: john23
author_rank: Frequent Contributor
timestamp: June 12, 2024, 05:04:38 pm
content: Hello,SSR is a very new concept :-) i only know how to charge and discharge using mosfets.Could you reccomend me a scematics i could try to implement?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 12, 2024, 05:13:47 pm
content: Quote from: john23 on June 12, 2024, 05:04:38 pmIt is basically MOSFET controlled by LED (all in one package)Take a look at:https://www.digikey.ie/en/products/filter/solid-state-relays-ssr/183?s=N4IgjCBcoCwdIDGUBmBDANgZwKYBoQB7KAbRAGYAGAdjBkpAKuvICYInzqY4QBdAgAcALlBABlYQCcAlgDsA5iAC%2BBMFSihkkdNnxFSIegFYAnADZTjI5XPma-IaMgTp8pavCmAHFYTbdXAJiSDIGPk8AWlZNJChpAFd9ELJjfii-LXipJODDCAjPGNCJcQAlRxBzWJkAEzFIsEoOEBExa2EAT0EcMTQsZGVlIAQuoteJust connect MOSFET part of SSR over load to discharge (may be with current limiter series resistor), LED part to control signal (through series resistor). That's all.

author: john23
author_rank: Frequent Contributor
timestamp: June 12, 2024, 05:36:31 pm
content: Hello , is there are some component i could use for simulation and see how it works?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 12, 2024, 05:42:24 pm
content: You can use CPC1017 for test

author: john23
author_rank: Frequent Contributor
timestamp: June 13, 2024, 04:17:41 pm
content: Hello , I was suggested another way which i am trying to undestand its logic.As you can see in the photo below and the attached LTspice file.The output charges exactly to the value of the Vth.Could you show on the example of AO6408 mosfet how i need to update the circuit so it would make the output discharge to -2.4?So I could understand the intuition of this method?

author: xvr
author_rank: Frequent Contributor
timestamp: June 13, 2024, 04:45:19 pm
content: This is not discharge scheme but power switch. For discharge you should plug M1 transistor directly to output voltage source (use original MOSFET from your first schematic instead M1). It should work

author: john23
author_rank: Frequent Contributor
timestamp: June 15, 2024, 09:49:20 am
content: Hello XVR, I have succseeded in using SSR for my purposes ans shown in the plots below .Two questions:1. how do i know the proper functionality of this SSR? how much current it needs or "gate" voltage?In the attached table I see the caracteristics but there are LED forward switch on and LED forward only .There 4 states and i dont know which apply to my case how do i open or close is like in a case of mosfet.2. the charging time is much mush higher then what i have with mosfet , is there a way to speed it up?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 15, 2024, 10:14:39 am
content: SSR controlled by LED, so MOSFET gate characteristics is not important. LED current should be about 10mA.Your slow discharge rate is a sequence of low LED current.

author: john23
author_rank: Frequent Contributor
timestamp: June 17, 2024, 11:47:40 pm
content: Hello XVR, I need to do similar thing to provide 5V for PMA5-83-2W+I can do this turn on and off trick with l3505 DC-DC and i can do it with regulator like LM7805 or LT1761.I can supply both voltages to PMA5-83-2W+ using these .What is the key difference between DC-DC and a regulator with performance?https://www.sparkfun.com/datasheets/Components/LM7805.pdfhttps://www.minicircuits.com/pdfs/PMA5-83-2W+.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/1761sff.pdfhttps://www.qorvo.com/products/d/da007774https://www.analog.com/en/products/lt3505.html

author: xvr
author_rank: Frequent Contributor
timestamp: June 18, 2024, 05:41:44 am
content: For +5V you can use linear regulator with Enable input (take a look in your other topic - I'd give you part number there).Difference between DC/DC and linear here is simplicity and low noise level of Linear. Drawback of Linear (high power loss) is not relevant in your application because of low current - power loss proportional to current, so it will be neglectiable.

author: john23
author_rank: Frequent Contributor
timestamp: June 18, 2024, 07:45:27 pm
content: Hello, Suppose I have a general bias of +15V gnd -15V.For 5V Vg2 i will use LT1761 LDO.What device you reccomend to use for -2V ? the low noise option would be great :-)What device you recommend for 12V step?https://www.analog.com/en/products/lt1761.html

author: xvr
author_rank: Frequent Contributor
timestamp: June 18, 2024, 07:58:54 pm
content: Quotehttps://www.eevblog.com/forum/projects/dc-dc-recomendations-for-pma5-83-2w-sequencer/msg5543047/#msg5543047https://www.eevblog.com/forum/projects/dc-dc-recomendations-for-pma5-83-2w-sequencer/msg5542941/#msg5542941

author: john23
author_rank: Frequent Contributor
timestamp: June 20, 2024, 05:42:31 pm
content: Hello ,I have tried to upgradethe lt1761-5 a circuit so i could connect INPUT to SHDN when pulse is 3.3V and to discharge the output when pulse is 0V.I see the error in the circuit below where whr mosfet discharges when pulse is 3.3V so i tried to reverse the opamp inputs to make the mosfet be closed .But the bigger problem is that SHDN not equal Vin when i open the M2.there is 2V gap between them.How do you reccomend to solve this issue?Thanks.LTSPICE file attached

author: xvr
author_rank: Frequent Contributor
timestamp: June 21, 2024, 08:37:52 am
content: Top level of SHDN pin is 2V maximum. So there is no need to bring it up to 5V. 3V us enough

author: john23
author_rank: Frequent Contributor
timestamp: June 21, 2024, 10:16:15 am
content: Hello , for the -2V i used LTC1261L ,I dont see that an LDO the less noisy version in the datasheet below.For G1 I need -2V and 4mA according to the datasheet shown below.But as you can see in the plot i get 400mA peaks of current.How can i improve this -2V circuit so i will meet the required demands on the datasheet for Vg1?LTSPICE and relay simbols of the relay compnent are attached.DC_Dc_relay_m2p4.aschttps://eu.mouser.com/datasheet/2/609/1261lfa-3124642.pdf

author: xvr
author_rank: Frequent Contributor
timestamp: June 21, 2024, 06:13:57 pm
content: 400mA is your discharge current. You don't have load at all.

author: john23
author_rank: Frequent Contributor
timestamp: June 22, 2024, 08:10:34 am
content: Hello XVR,Vg1 is a gate so i assume its a highZ resistance.I put 0.5uF capacitative load as shown in the schematics above.What load capaciltance should i put?Thanks.https://www.minicircuits.com/pdfs/PMA5-83-2W+.pdf

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 08:17:26 am
content: Discharge current mostly determined by R4 and turn on resistance of U1. Load capacitance could affect mostly discharge time, not initial current. And discharge current taken from load capacitance, it not affect regulator.

author: john23
author_rank: Frequent Contributor
timestamp: June 22, 2024, 08:39:22 am
content: Hello XVR, Its really hard to reach the 4mA mark :-) and it ruind my sqare signal.I have tried to play with the capacitors and resistors.Is there any more productive way?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 08:58:08 am
content: 4mA is a load current (not capacitive but resistive). You don't have any resistive load in scheme, so 4mA not relevant for it. I don't know what 0.5uF in your circuit means. If it real capacitor, then it will be real discharge current but it has nothing in common with 4mA of load current. If it model of Gate capacitance of amp then it TOO high - MOSFET capacitances ordinary have a tens of nF maximum

author: john23
author_rank: Frequent Contributor
timestamp: June 22, 2024, 09:34:49 am
content: Hello XVR , Yes you are tottaly correct. I put 0.5UF as a load with no thought at all.4.5mA is the maximal rating so i prever 2mA so it wont burn :-)So given -2V and 2mA then load resistance is 2/2mA =1Kohmsuppose my gate capacitance is 100nf and i put series resitance as 1Kohmi get a very bad Vout signal (its jumping a lot)but the good news that the current on the load capacitor is around 1-2mA.How can i fix the voltage signal?

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 09:41:16 am
content: LTC1261 required capacitor on its output to operate. This is NOT load capacitance (see recomended value in datasheet). Discharge current for THIS capacitor will be significant (much more than 2mA). Unfortunately nobody can help there😕 Just add resistor into discharge net to limit current by maximum allowed for SST

author: john23
author_rank: Frequent Contributor
timestamp: June 22, 2024, 05:33:09 pm
content: Hello , I think that it i open g1 then at any mosfet its highZ its pure capacitative load.The current that they write i think is I=CdV/dt.so i think there is no resistance in the load.Where is the error in my logic?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 05:36:43 pm
content: No, it's has resistance load. 4mA is a pure resistance load. Capacitance load is not taken in account at all.

author: john23
author_rank: Frequent Contributor
timestamp: June 22, 2024, 06:46:20 pm
content: Hello, I am confused.you say that the current will be much higher then 2mA.So how it wont burn the component?Where is the reccomended values in the datasheet?I cant see the section.Thanks.https://www.minicircuits.com/pdfs/PMA5-83-2W+.pdf

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 07:57:12 pm
content: This current don't have any connection with component at all. This current will be ONLY in circle comprised output capacitor and discharge SSR. It will not affect amplifier.

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 08:29:57 pm
content: Once again - I suggest treat Vg1 not as power rail but as control voltage (2mA of load allow to do so). Remember - you need some facility to adjust this voltage in range -2V till -0.5V. With DC/DC it will be another round of hacks to do so.So, you will need stable negative voltage, without any adjustment or enable/discharge circuits and one opamp.Here:DAC output 0 to +3.3V mapped to Vg1 output 0 to -2VC1 is a blocking capacitor (take a look in amplifier datasheet which one is needed)+V is a positive power rail for opamp. It can be 0 (ground) if opamp is RRIO-V should be not less than 2V or more, it depends of output voltage swing range of opamp.R5 used to isolate opamp output from capacitor load to suppress possible oscilation.opamp should be able to provide at least 2mA of output current

author: john23
author_rank: Frequent Contributor
timestamp: June 24, 2024, 03:55:22 pm
content: Hello XVR, An important issue is the load.From the datasheet we have two hints shown in the photos below.The basic caracteristic says its -0.8V with 15 micro amp which meeans that the resistance at Vg1 is almost highZ.So we start with -2V and increase till we reach 15uA.What load capacitor you reccomend to use?https://www.minicircuits.com/pdfs/PMA5-83-2W+.pdf

author: xvr
author_rank: Frequent Contributor
timestamp: June 24, 2024, 04:05:53 pm
content: DS keep silence about load capacitance of VG1 & VG2. In any case recomended circuit from DS includes 3 capacitance on each VG? pin with total capacitance about 4.7uF

author: xvr
author_rank: Frequent Contributor
timestamp: June 24, 2024, 04:07:46 pm
content: QuoteUntil we reach 400mA. We change VG1 and track Idd (not Ig1)

author: john23
author_rank: Frequent Contributor
timestamp: June 24, 2024, 07:49:15 pm
content: Hello XVR, I am still confused regarding the 250mA over the capacitor.Suppose the gate capacitance is 100pF i put in parralel 10nF capacitor .when i charge this gate to get -2V .I saw 250mA going threw that capacitor, why you are saying its not going to touch the gate capacitance?my 4mA rating is very fragile. could you say what exact load i should use?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 24, 2024, 08:59:47 pm
content: You will get 250mA discharge flowing through your 10nF capacitor, and much less through gate capacitor. Put both of them in LTSpice and draw current through gate capacitor only. Current of external blicking capacitor (10nF) will not flow neither through gate capacitor nor through VG1 in general

author: john23
author_rank: Frequent Contributor
timestamp: June 29, 2024, 09:11:14 am
content: Hello, I have tried to impement your idia in lt spice using AD8034 as shown below.I know that OPAMP is a comparator.But it has feedback also.What is the formula for the output of the opamp system you showed so I could see where dod i go wrong?Thanks.

author: xvr
author_rank: Frequent Contributor
timestamp: June 29, 2024, 09:45:17 am
content: QuoteNo, it's a opamp. So it's ok to use it here.As for schematic - V2 polarity is wrong. Swap its pins.QuoteOpamp with negative feedback will keep potentials of its input at the same value. Positive input has zero potential, so negative also will be zero.By Kirchhoff's law currents through R1 and R2 are equal by module and has different signs.So V1/R1 = Vout/R2You can scale both R1 and R2 by the same value, it will change current from DAC and Opamp output to feedback circuit. Now it's 1 mA