fake MOSFETs on Ebay?

tom66

Joined May 9, 2009
2,595
Am I missing something? I can't see the datasheet. The page shows 59A. I (keep in mind I really don't know what I'm doing since I've never done it before) am looking for something with at least 110A (which I will limit to 70A) capacity so that @125C I can still get the 70A out of it that the legs are rated for. After that requirement, then I would like at least 180 or 200V (leaving room to grow; later if my design works I will be using higher voltage motor) and a low Rds(on). MOSFETs like that can be found, but not for cheap, so I'm in a battle as to if I want to spend the money for the good stuff and blow it up. If I have to skimp on something, I guess it would be voltage; for my lower voltage I might go down to 50V or something; can't see myself going above 50V on a starter motor. I am tracking down several things in parallel. For every mosfet I look at, I also need to figure out what driver to use and I still haven't figured out how to do that.
What I'm trying to say is $1.75 a MOSFET would be a steal, if you could get it. Saw a nice 30mOhm 400V MOSFET. How much each? $28!

You'll need a high voltage driver, like IR2110, which has a built-in 500V driver.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,527
What I'm trying to say is $1.75 a MOSFET would be a steal, if you could get it. Saw a nice 30mOhm 400V MOSFET. How much each? $28!
DOH! I get you now, yeah, 1.75 would be a steal, but remains to be see if He'll sell it to me. My offer was pretty low-ball.

You'll need a high voltage driver, like IR2110, which has a built-in 500V driver.
Ok, looking at this driver, the things I don't get:
1. In the literature I've read, I have seen more than once that the gate of the mosfet needs a voltage higher than the voltage on the drain in order to turn on. Yet, I continually see 200 & 300V mosfets that specify a gate voltage of 10 or 12 volts. for example, The mosfet in post#1, Vdss 300V, Vgs 20V. So, would I need a driver that goes above 300V or not? why?
2. Driver states it's a high & low side driver. I'm Not clear on what high & low side means. I assume That I am looking for a low side driver since the mosfets are N-type and I am switching ground with them. whatever the case, it seems this driver would work.
3. How do I determine how many mosfets I could parallel with this driver? I know it has to do with Ciss of my mosfet, which in this case is 6300pF, and it also has to do with how fast I want to switch them, which in this case would be 20khz, and it has to do with the amp output of the driver, which is 2A the math escapes me. I have seen the formula (think) but was confounded by it, and then distracted by something else and never got back to it.
4. how do I connect it to my μC? I assume the Lin would go to my digital pin, and I would send PWM here. Vss would connect to my uC ground, and simple as that. (plus 5-20V on Vdd)
 

SgtWookie

Joined Jul 17, 2007
22,221
DOH! I get you now, yeah, 1.75 would be a steal, but remains to be see if He'll sell it to me. My offer was pretty low-ball.
Unless you are buying from the factory or a factory-authorized distributor, you may well be buying counterfeit parts, which simply won't perform up to the specs in the datasheet. "You pays yo' money and takes yo' chances". If you want to be certain you're getting the genuine item, check the manufacturer's authorized distributor list and buy only from that list.

Ok, looking at this driver, the things I don't get:
1. In the literature I've read, I have seen more than once that the gate of the mosfet needs a voltage higher than the voltage on the drain in order to turn on. Yet, I continually see 200 & 300V mosfets that specify a gate voltage of 10 or 12 volts. for example, The mosfet in post#1, Vdss 300V, Vgs 20V. So, would I need a driver that goes above 300V or not? why?
You're getting Vdss and Vgs confused.
Vdss is the maximum voltage that can be applied across the drain and source terminal before the junction breaks down.

Vgs is the voltage on the gate, referenced to the source terminal (ie: using the source terminal as the "ground" or 0v point). Most typical MOSFETs have a maximum Vgs of ±20v, but there are a number that have a much lower limit. To avoid smoke, verify the limits in the datasheet for the particular MOSFET in question.

2. Driver states it's a high & low side driver. I'm Not clear on what high & low side means. I assume That I am looking for a low side driver since the mosfets are N-type and I am switching ground with them. whatever the case, it seems this driver would work.
An IR2110 is a half-H bridge driver.
If you are only switching the low side of a load (between the load and ground) then you only need a low-side driver.
3. How do I determine how many mosfets I could parallel with this driver? I know it has to do with Ciss of my mosfet, which in this case is 6300pF, and it also has to do with how fast I want to switch them, which in this case would be 20khz, and it has to do with the amp output of the driver, which is 2A the math escapes me. I have seen the formula (think) but was confounded by it, and then distracted by something else and never got back to it.
Read this application note:
http://www.ti.com/lit/ml/slup169/slup169.pdf
It covers a wide variety of techniques.
4. how do I connect it to my μC? I assume the Lin would go to my digital pin, and I would send PWM here. Vss would connect to my uC ground, and simple as that. (plus 5-20V on Vdd)
Well, it's more complicated with an IR2110; the Hin and Lin need to go nearly to the IR2110's Vdd, which has to be somewhere above 10v (I think 12v is minimum) or the driver will go into shutdown; so you'd need a driver for the driver....

Anyway, read that application note for starters.
Then, if you're just going to be switching the low-side MOSFETs, there are quite a few other low-side drivers that would be easier to interface. Some have quite high current outputs (>6A). Then you also have things like trace inductances, etc. to worry about, but try to not take too much in at once or you'll get confused.
 

tom66

Joined May 9, 2009
2,595
Ah, I didn't know whether he'd be using a low-side MOSFET or a high-side MOSFET.

A problem with placing MOSFETs in parallel is that the gate charge increases. The total on-resistance of the array falls. This causes lower static power losses (static = on or off, not switching between states) but higher switching losses.

A good driver is http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en553187

Heed my warning about using a single MOSFET for drive. (Or even multiple parallel MOSFETs.) You do not want a shorted MOSFET causing the motor to run out of control.

How will you implement reverse, if necessary?
 

Thread Starter

strantor

Joined Oct 3, 2010
5,527
Unless you are buying from the factory or a factory-authorized distributor, you may well be buying counterfeit parts, which simply won't perform up to the specs in the datasheet. "You pays yo' money and takes yo' chances". If you want to be certain you're getting the genuine item, check the manufacturer's authorized distributor list and buy only from that list.
thanks, I think I will; digikey
You're getting Vdss and Vgs confused.
Vdss is the maximum voltage that can be applied across the drain and source terminal before the junction breaks down.
yes i was
Vgs is the voltage on the gate, referenced to the source terminal (ie: using the source terminal as the "ground" or 0v point). Most typical MOSFETs have a maximum Vgs of ±20v, but there are a number that have a much lower limit. To avoid smoke, verify the limits in the datasheet for the particular MOSFET in question.
mine is +/-20V
An IR2110 is a half-H bridge driver.
If you are only switching the low side of a load (between the load and ground) then you only need a low-side driver.

Read this application note:
http://www.ti.com/lit/ml/slup169/slup169.pdf
It covers a wide variety of techniques.


Anyway, read that application note for starters.
skimmed over all of it, read most of it in detail, understood some of it.
Here's what caught my attention, on page 9:
the most important characteristic of
the gate driver is its source-sink current
capability around the Miller plateau voltage level.
Peak current capability, which is measured at full
V DRV across the driver’s output impedance, has
very little relevance to the actual switching
performance of the MOSFET. What really
determines the switching times of the device is
the gate drive current capability when the gate-to-
source voltage, i.e. the output of the driver is at
~5V (~2.5V for logic level MOSFETs).
so after a bit of searching I found this:http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FAN3121TMXTR-ND

from the datasheet:
FAN312X drivers incorporate the MillerDrive architecture for the final output stage. this bipolar/MOSFET combination provides the highest peak current during the Miller plateau stage of the MOSFET turn-on/turn-off process.
11.4A peak sink at Vdd = 12V
9.7A sink/ 7.1A source at Vout = 6V
Does that look like a good driver? Do you think it could switch a few in parallel easily? or should I get 5 or 6 drivers?

It has 2 types available, one for CMOS, and one for TTL; not sure which one is better - I will probably be driving this with an arduino to start.

also, it is available in inverting and noninverting; I assume that inverting would turn the mosfet OFF if the input is high, and noninverting would turn the mosfet ON if input is high?

try to not take too much in at once or you'll get confused.
You got that right. The past few days have been very challenging. I'm a hands-on kind of guy, and all this reading bland writeups is not in my nature. I actually last night realized that I can't remember everything I'm reading and that my mind so "all over the place" that I haven't even really learned and understood anything since I started this quest, so I got a notebook and started writing down all my questions with notes and tying to answer them systematically one at a time. I'm a very scatterbrained person and until today, I've just been running around in circles.

Ah, I didn't know whether he'd be using a low-side MOSFET or a high-side MOSFET.

A problem with placing MOSFETs in parallel is that the gate charge increases. The total on-resistance of the array falls. This causes lower static power losses (static = on or off, not switching between states) but higher switching losses.

A good driver is http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en553187

Heed my warning about using a single MOSFET for drive. (Or even multiple parallel MOSFETs.) You do not want a shorted MOSFET causing the motor to run out of control.

How will you implement reverse, if necessary?
Yes I got a big 400A fuse to use; I would rather have a contactor as well... will see.
I just started learning about drivers so pardon my ignorance; Why did you recommend the MCP14E7? Is it better than the one I linked to? All I know to look for is the Amp capacity and then the miller thing

implementing reverse by swapping field wires probably
 
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