IFR4905 P-Channel MOSFET - Page 1
Forum section: electronics / projects

author: raptor1956
author_rank: Frequent Contributor
timestamp: January 18, 2017, 10:11:32 pm
content: I have a cheap on/off temp control board I picked up for less than $10USD on Amazon that I'm using to control a battery heater but the board comes with a relay output and I'm thinking about replacing it with a transistor to eliminate the failure mechanism of the relay. At present I'm using a hysteresis of about 0.5C as that reduces the on/off switching frequency, but with a transistor I could lower that to 0.1C and let it switch away to its hearts content. So, looking at the circuit a NPN transistor is driving the relay coil (pulling low side low) so to keep things the same a P-channel MOSFET makes the most sense as it's almost a direct replacement with only a pull up for the gate and maybe a current inrush limiting resistor for the gate. So, given the loading of just a couple amps and an average duty cycle of less than 30% I could use just about any P-channel MOSFET but many P-channels have a pretty high Rds on resistance. The IFR4905 has an Rds on of 20mohm and that's about as low as I can see -- any other choices with better Rds On?Brian

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 19, 2017, 12:42:30 am
content: Does the battery heater element has one end already at 0V or you are free to connect both ends?If one end of it can be wired to high side, then you can switch its lower end to 0V and thus use a NMOS instead. They are much cheaper and have a much lower Rds. You would need to replace the relay by a PNP transistor and use its collector to drive the NMOS.

author: raptor1956
author_rank: Frequent Contributor
timestamp: January 19, 2017, 04:19:31 am
content: Quote from: eblc1388 on January 19, 2017, 12:42:30 amAs wired the negative of the heating element goes directly to the negative supply and the plus side is switched through the relay. So, if I removed the relay and used a PMOS like the IFR4905 then I could get by with a simple swap of the 4905 for the relay and adding a couple resistors. I can live with 20mohm though it would be nice to have even less Rds on. The 4905 is a bit pricey if purchased through Amazon but at Digikey it's only a couple bucks.Brian

author: Codebird
author_rank: Regular Contributor
timestamp: January 19, 2017, 06:58:57 am
content: Why a P channel? If you use a cheap little inverter circuit (I usually use a BS170 and 100k pull-up) as well, you can use an N-channel power MOSFET with only two additional components.

author: langwadt
author_rank: Super Contributor
timestamp: January 19, 2017, 09:19:40 am
content: something like an AUIR3313 will give you high side switching and protection against most faults

author: raptor1956
author_rank: Frequent Contributor
timestamp: January 20, 2017, 02:11:06 am
content: The NMOS options are going to require more components and with the PMOS approach just one active component and a couple resistors. For my application 20mohm is just fine and I would only need to dissipate about 75mW accross the 4905. So, one active components and a couple resistors and there's no need to invert anything and I can use the output of the NPN transistor to drive it using a 22Kohm pull up to limit the NPN and pull the gate of the NMOS to plus rail. I can optionally add a 2.2Kohm as input to the gate to limit inrush to the gate cap. Simple, neat and only a couple bucks. Yes, there are other elegant methods that have the appeal of there elegance, but usually simple beats elegant. Additionally, the net power consumption will be less as the NMOS will eat less power than the relay coil.Brian