Vds drop in MosFET FS8205A - Page 1
Forum section: electronics / beginners

author: dangrie
author_rank: Contributor
timestamp: July 05, 2020, 06:21:25 pm
content: Hi everyone,I recently designed a small circuit consisting of a LiPo battery, battery protection, solar charger and a Micro to control 8 Channels of LEDs.The micro and LED's power is controlled via a mosfet which is driven my a LM393 comparator to only switch on when it is dark. The mosfets I used for power switching and driving the LEDs are the same as in the battery protection circuit (FS8205A,datasheet) which seemed perfect: Dual N-Channel Enhancement Mode with a low Vgs and low Rds(on) (35mOhm at 2.5Vgs). Seemed nice to be able to consolidate that BOM item.The circuit works "fine" when VBat=4.2 V, so really up there for a freshly charged LiPO. However I get a significant voltage drop across the drain-source junction of each mosfet. After the power mosfet (Q2), I have 3.5V at the source with a gate voltage of close to 4.2V from the comparator output (10k pulled up, open collector). Each LED channel then drops the voltage again by ~0.6-0.7V (gate voltage close to VCC of the micro, so ~3.5V) so I only have slightly under 3V left which is barely enough to reach the forward voltage of the LEDs. When I drop the battery voltage to even 4V, the voltage for the LEDs sinks too much and they don't light up at all.I probably have a really stupid beginner error, however any help would be appreciated. The Vgs seems high enough for me for the gate to be fully switched on and should have in the order of tens of milli ohms, which would not explain the big voltage drop. Is there something particular about these mosfets I overlooked in the data sheet? Can't they be used for high-side switching? I don't get any voltage drop from the battery protection circuit.I attached the schematic below.Thanks everyone who takes the time to help me.

author: dangrie
author_rank: Contributor
timestamp: July 05, 2020, 06:46:51 pm
content: Oh man, never mind. I just realised my mistake. When high side switching, I don't actually have a Vgs of 4V, I have a gate to ground differential of 4V, but as soon as the mosfet switches on, I have no voltage differential across gate and source. So I need to move the MosFETs to the low side.Could anyone confirm this please? Im going to rework some boards....

author: garethw
author_rank: Regular Contributor
timestamp: July 05, 2020, 06:58:14 pm
content: You are correct. You need to move the MOSFET to the low side so the source is connected to GND.

author: dangrie
author_rank: Contributor
timestamp: July 05, 2020, 07:12:40 pm
content: Quote from: garethw on July 05, 2020, 06:58:14 pmThanks, as I expected, a really stupid mistake. But well I guess that's how you learn.As I already have 5 boards assembled, I just ordered some DMP2240UDM. Those are P-Channel. Am I correct in the assumption that I can just replace the 9 MosFETs per board (8 for the LEDs and 1 for the power switching) and then invert the switching logic in there micro (easy) and in the comparator circuit (still easier than rerouting all LED and power traces), or have I overlooked something else?

author: garethw
author_rank: Regular Contributor
timestamp: July 05, 2020, 07:48:00 pm
content: P-Channel mosfets would work in that position. How are you going to replace the Mosfets? From what I can tell on the datasheets they are different packages (SOT-23 vs TTSOP-8)

author: dangrie
author_rank: Contributor
timestamp: July 05, 2020, 07:52:58 pm
content: Quote from: garethw on July 05, 2020, 07:48:00 pmGood catch!. However I actually used theFS8205(Not "A" variant)Sorry for the confusion, I copied the title from the data sheet.As far as I can tell, the FS8205A pinout should be compatible as a drop-in replacement for p-channel operation in that case