Update : 13 of 50 MOSFET’s might be ok after testing!

I purchased 50 x IRF7862 MOSFET’s from Asia through AliExpress and while I usually am happy with what I buy here it happens that I get screwed by non-honest sellers. What some of them do is to pick rejected chips from the factory and sell them. These MOSFET’s arrived in a plastic bag, not in original packing so I should have rejected them due to that alone, but I was obviously not on my watch back in 2016.

These was for my motor driver and are supposed to have 3.3mOhm internal resistance as well as a very fast switch time. I was therefore a bit surprised that they generated so much heat as they did and that so many of them was destroyed so fast. After I destroyed my test channel yesterday I soldered on a new one and realized it was non-working as well. I tested the lot manually and had to reject 6 of them. Soldering on 2 new ones I still discovered that they did not work. Scoping the gate I even see they distort the gate signal, so 2 new ones and gate signals are ok, but output (no load) is crap.

This lot was bought back in November 2016 and costed 16.45 USD (0,329 USD each). Well, this is what you must expect from time to time buying samples from AliExpress. I believe this is the 2nd time I had bad luck like this. The alternative to buy all components from distributors would be far to expensive for a hobbyist scheme.

What I will do next is to test manually and see if I can find a few that hold ca spec. This is easy as you just use the PSU, connect on each end with 10V over the MOSFET, put the current limit to < 1A and bring the gate to 10V. This will bring the PSU down so I will not get 3.3mOhm as Voltage will be to low, but it will indicate if they work at all.

I must admit I thought these MOSFET’s snapped a bit fast, so I had actually planned to replace them With IRFH5300, but I need a New PCB layout for these. IRFH5300 is rated to 30A. I don’t expect to get 30A out of this design, but they have a much lover resistance and I should get 15A out with far less heat. Luckily the IRFH5300 was from a profiled seller and arrived in original package.

Returning to IRF7862 – what I will buy 20x from different sellers and pick a profiled seller this time. I also see that price range is huge + a good indication that something is wrong is if you don’t receive chips in original package – these came in a plastic bag! Prices range from 0.28 each to 0.90 each. I picked IRF7862 due to it’s low cost, but if I have to pay 0.90 each I can as well use IRFE5300.

One of the generic changes I have considered for some time is to replace my 2×5 pin SWD connector with a 1×5 pin Micro JST connector. These connectors use 1.27 pitch and you get plenty of ready made cables. Firstly I only use 1×5 pin anyway and usually find other solutions for debug UART. But, most important is that connecting the adaptor directly to the board is not always convenient in tight places. A JST connector with a cable will fit more smoothly and allow me more freedom.

The picture don’t really show how small these connectors are, but you find plenty of them searching for “micro jst”. I plan to modify the existing adaptor and use this cable between the adaptor and the PCB Board so that I don’t need to adapt it directly.

Pin layout is still the same except for the New 3.3V:

• 3.3V (new on 6-pin)
• NRST
• SWDIO
• GND
• SWCLK
• BOOT

I assume I have to make it 6-pin and add 3.3V as I want Reset and Boot jumpers/switches on the next adapter board. Basically I don’t really need to do anything as I can use this directly as a drop in replacement on the existing Connectors (1×5-pin).

Also, I often have lack of connector space on my designs, so I am tempted to use these several places where the connector size is an issue!

I am adding 2 CAN/USB Adaptors. The upper one is not galvanic isolated while the lower one is 2cm longer and uses ADM3053 which also makes the board ca 5-8.- USD more expensive. The MCU is STM32F105RB and the USB is full host or Device capable. The SN65HVD233 based one have a few Control functions ADM3053 does not have, but the Boards are basically identical.

I just realized that ADM3053 is NOT a 3.3V device. It says 5V or 3.3V on VIO, but VCC requires 5V. The schematics I have used is also wrong as it does not feed power to VIO at all. The example on the datasheet is very misleading in this case. Looking at ADM2583 that I uses for RS485 this is all sorted – it actually IS a 3.3V device. This is a bit annoying because it means I need to maintain both 3.3V and 5V since 3.3V is needed elsewhere.