Mosfet RDSON test - Page 1
Forum section: electronics / projects

author: dietert1
author_rank: Super Contributor
timestamp: January 10, 2025, 02:31:27 pm
content: Recently i got some IRFB3207 and IRF1405 mosfets. As i have some doubts about fakes, i wanted to test their Rdson and used a 0.5 A test current at Ugs= 12 V, measuring Uds=11.9 mV. That gives about Rdson = 23.8 mOhm, much more than the datasheet spec of 4.6 - 5.3 mOhm.Except the datasheet spec is at a much larger current = 101 A (that i can't test). The DUT does not get hot during this test. Are there any rules about the current dependence of Rdson?Regards, Dieter

author: eugene
author_rank: Frequent Contributor
timestamp: January 10, 2025, 03:32:51 pm
content: I have no specific experience with this, butit seems to meTMthat you should be able to infer RDSON from any of the graphs of IDvs VDSin the datasheet. The slope of any of the curves under any particular conditions has units of amps/volt and is thus resistance.

author: wraper
author_rank: Unknown
timestamp: January 10, 2025, 03:39:42 pm
content: Rdson will rise when you push too much current. But you won't get higher value than specified due to lower current. So the part you tested is fake.

author: mawyatt
author_rank: Super Contributor
timestamp: January 10, 2025, 03:54:30 pm
content: Rdson has a +TC, so at lower drain currents one would expect a lower Rdson due to a lower channel temperature.If your setup is per the data sheet (specifically Vgs), either you have a setup measurement issue or counterfeit devices.Best

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 10, 2025, 05:46:12 pm
content: You may be introducing an error in the way you are performing the test. See in the first image that the connection of the GND of the source to the lead of the Mosfet source is being made via protoboard. And as a result I am reading 17mv instead of 3.5 ~ 4mv (test current of 1A). Note in the second image that I am connecting the GND of the source directly to the lead of the Mosfet, without going through the protoboard. The result is now correct, 3.6mv.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 10, 2025, 06:09:45 pm
content: I designed a milliohmmeter, with the main objective of measuring RDS and identifying fake MOSFETs. See the project link where I tested several MOSFETs. It has been very useful to me ever since. See the image below of the device measuring the RDS of the IRF1404, at the correct value of 3.5mR, using only 50mA.https://www.eevblog.com/forum/projects/diy-milliohmmeter-how-to-solve-grounding-issue/msg4258852/#msg4258852

author: dietert1
author_rank: Super Contributor
timestamp: January 10, 2025, 07:03:38 pm
content: Quote from: eugene on January 10, 2025, 03:32:51 pmYes, if i look at the Id vs. Uds graphs in Fig. 1 or 2 in the IRF1405 datasheet i see a linear relationship, at least at reasonable currents, e.g. between 10 and 100 A. They don't show data below 1 A, but certainly the slope will be the same. When i read the slope, it is about 100 A at 1 V, that is 10 mOhm. So one may wonder how they specify Rdson = 4.6 to 5.3 mOhm in the same datasheet. Maybe there is an extra 5 mOhm package resistance involved.My own measurement was directly on the package between source pin and cooling tab for drain.Regards, DieterEdit: Meanwhile i understood that Fig. 1 is for room temperature and Fig. 2 for 175 °C. The factor two i was missing is due to difference in chip temperature. In Fig. 1 (at room temperature) i read 200 A at 1 V, that is 5 mOhm.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 10, 2025, 07:04:22 pm
content: If you have a reason to expect fakes (i.e., they are not from distributors but e.g. Ebay), they most likely are fakes.Rds(on) is constant (i.e., voltage drop is linear per current) as long as die temperature stays constant. As others have said, at high currents you can expect rising Rds(on) due to heating, but as long as current is low enough so that the device does not heat up significantly, you should be pretty close to datasheet room temperature value.

author: dietert1
author_rank: Super Contributor
timestamp: January 10, 2025, 11:14:54 pm
content: Quote from: Siwastaja on January 10, 2025, 07:04:22 pmYes, meanwhile i am pretty convinced that i got fakes. The transistors were bought at Mult Comercial and Mercadolivre.com.br as it can be expensive to import from digikey or mouser. One has to calculate about 150 % extra for taxes and postage. What is available locally seems to be chinaware.

author: Postal2
author_rank: Frequent Contributor
timestamp: January 10, 2025, 11:35:50 pm
content: Quote from: dietert1 on January 10, 2025, 02:31:27 pmDo search "YR1035+".https://www.bulcomp-eng.com/datasheet/Yaorea%20YR1035_%20-%20User%20Manual.pdf

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 11, 2025, 01:02:56 pm
content: Another method that people often use is to measure the zener voltage of the internal diode. From the datasheet, we can see that we only need to apply a current of around 250µA to perform the test. I use a very simple circuit, rectifying and filtering the 120V line to obtain up to 170V and thus test several MOSFET models.

author: wraper
author_rank: Unknown
timestamp: January 11, 2025, 02:55:27 pm
content: Quote from: Avelino Sampaio on January 11, 2025, 01:02:56 pmThat circuit is dangerous. Even without it being under mains voltage, it does not have a discharge resistor. So it remains dangerous after disconnecting form the mains. 47uF is way too much as well and is another source of danger.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 11, 2025, 03:59:57 pm
content: It was reckless of me not to inform and indicate in the schematic that I use a resistor to discharge the capacitor. Yes, the capacitor used has a very high value but it was the one I had here with a voltage of 250v.Here is an example of a zener voltage test with three original MOSFETs.Image 1: IRF1404 - zener voltage 40v indicated in the datasheetImage 2: IRFB3306 - zener voltage 60v indicated in the datasheetImage 3: IRFZ44 - zener voltage 55v indicated in the datasheet

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 11, 2025, 05:44:45 pm
content: I found a fake IRFZ44N MOSFET here. I am bringing the results of both the Rds reading and the Vds zener reading.Image 1: IRFZ44 OK - Correct Rds reading, at 17mR.Image 2: IRFZ44 Fake - reading of 39mR, where it should be around 17mRImage 3: IRFZ44 Fake - Zener voltage reading of 77v, very far from the 59v of the real MOSFET.

author: dietert1
author_rank: Super Contributor
timestamp: January 11, 2025, 06:12:09 pm
content: My application does not require very low Rdson nor a high Uds, and it is linear near DC (audio bandwidth). So i just bought the mosfets with the largest current rating i could get. I could use them even if they are fakes. Just wanted some kind of quality check.The measured Rdson values of about 20 mOhm are about 4x worse than specification yet good enough. I also checked gate currents to be "zero" and the gate threshold voltages. The Uds zener voltages are next..Regards, Dieter

author: mawyatt
author_rank: Super Contributor
timestamp: January 11, 2025, 11:31:03 pm
content: The 4X higher Rdson might "hint" that the die is much smaller than expected by the part # and data sheet. This could mean a significantly less capable device wrt to power dissipation.Best

author: inse
author_rank: Super Contributor
timestamp: January 12, 2025, 06:17:47 am
content: I would like to agree to mawyatts statement.Without proper datasheet you won’t know the SOA curve either.How much power do you want to dissipate, whats the application anyway?

author: KerimF
author_rank: Frequent Contributor
timestamp: January 12, 2025, 07:53:15 am
content: Quote from: mawyatt on January 11, 2025, 11:31:03 pmMe too, I agree with 'mawyatt'.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 12, 2025, 09:18:56 am
content: Quote from: dietert1 on January 10, 2025, 02:31:27 pmMost probably they can't either. When you see weird values like 101A, read the fine print. Might be yet another marketing lie, like the 169A continuous current supported, when the maximum current supported by that type of packaging is only 75A. The 169A is thecalculatedmax possible current by a die of the same size and with ideal cooling, so bollocks, because I buy transistors, not ideal dies in ideal infinite radiators.For example see remark 6 in the datasheet:https://www.infineon.com/dgdl/Infineon-IRF1405-DataSheet-v01_01-EN.pdf?fileId=5546d462533600a4015355db084a18bbor the fine print at note 1 here:https://www.infineon.com/dgdl/Infineon-IRFS3207-DataSheet-v01_01-EN.pdf?fileId=5546d462533600a401535636708e215eThey should let the engineer write the datasheets, and the marketing should write only the bloatware flyers filled with empty emphasis words and pics. I'm very upset about infesting engineering specs with marketing crap.I didn't study in full the datasheets, but I suspect the 5m\$\Omega\$ specs do not apply to the kind of test you are doing. Probably the 3-5m\$\Omega\$ is yet another one of those values that are true only when all the planets align just right.My bet is the transistors are probably not fake, but to be sure, I would ask Infineon what are the expectedmeasuredRds values for your particular test setup.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2025, 09:41:37 am
content: The maximum current cited on the first page is usually so that 1/2 R_on * I²_max = P_tot. This gives rather unrealistic values. This starts with using a too high a P_tot and R_on at roomtemperature.The IRFB3207, even the real one is anyway no suitable for linear operation, e.g. in a power supply or MOSFET amplifier. The forward SOA is rather poor, as expected for a modern relatively low voltage FET that is made for switching. The IRF1405 may be a little better, but still not good - just no know to be bad. Attached is the SAO curve from the Infineon / IRF datasheet for the IRFB3207.For linear operation one should more looks at higher voltage MOSFETs and preferrably a good size case as a TO220 is also limited in power handling. Also old types like BUZ10 have a good chance to work. A old time favorite is the IRFP250.

author: inse
author_rank: Super Contributor
timestamp: January 12, 2025, 09:49:29 am
content: @RoGeorge:I have seen datasheet where the maximum current or power dissipation were specified at t_junction of 25 Celsius, how to achieve this practically - “not our business“

author: Postal2
author_rank: Frequent Contributor
timestamp: January 12, 2025, 09:50:14 am
content: Quote from: RoGeorge on January 12, 2025, 09:18:56 amTrue. Three welding points, 25A each.

author: dietert1
author_rank: Super Contributor
timestamp: January 12, 2025, 09:18:37 pm
content: The application is a TEC driver. The circuit is a full bridge where the lower mosfets get switched while the upper ones run in linear mode in order to realize a current source for the TEC. Currents will be up to 3 A at a supply voltage of about 7 V.Under these conditions even a mosfet with Rdson = 20 mOhm should survive.Will report later, how the idea of getting a "strong enough" fake worked out. They may in fact be other, older mosfet chips similar to those BUZ... types.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2025, 11:06:45 pm
content: Eben with only 7 V the SOA limit for the IRFB3207 would be just marginal for 3 A. So really a speciallized switching type. One may still not have the full 3 A and 7 V Drain-source at the same time.

author: dietert1
author_rank: Super Contributor
timestamp: January 13, 2025, 11:26:43 am
content: That IRF3207 SOA diagram is a bit difficult to understand, as they specify about 0.5 K/W as thermal resistance from junction to case elsewhere. That seems to be the same as for other TO-220 power mosfets. As far as i understand the 175 °C - 25 °C = 150 K temperature difference between junction and case assumed for the SOA diagram means a continuous load of about 300 W. Those SOA curves seem to be about avalanche capability under heavy load conditions.Anyway i have enough mosfets to use IRF1405 for linear control and IRF3207 for switching.Regards, Dieter

author: DrijSkij
author_rank: Contributor
timestamp: January 13, 2025, 11:30:58 am
content: I love and hate protoboards. Very easy to wire something up. Then, all the intermittent performance issues when testing it.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 12:24:28 pm
content: The DC curve is the lowest of the curves shown in the SOA diagram. It first follows the R_on limt, than up to some 5 V the power limit (e.g. thermal resistance and max junction temperature) and than it drops quite fast because of possible thermal instabilty (analog to 2nd breakdown). This rather early dop makes the type not well suited for analog operation. It is usually only lower voltage fets that have problems already at such low votlage.The avalance part is a different curve at higher voltage, above the normal maximum.The IRF1405 datasheet, that I found has not DC curve an even already stops at 10 ms. As the chips is super low R_on and only 55 V the SOA could be similar bad. So it may not be so bad to fake FETs, as old more standard types like IRF540 would likely have a better SOA.

author: KerimF
author_rank: Frequent Contributor
timestamp: January 13, 2025, 02:52:35 pm
content: The following could be seen as off-topic.Since most of my products, I design and produce since around 15 years ago, use power MOSFETs which are locally available, I had to find out, once a while, if the new received patch is fake or not, before assembling.I don’t have sensitive accurate voltmeters, just an old digital scope. So, I made a small current transformer to get relatively high currents (up to 40 A, 50 Hz) which can be driven by a rotary autotransformer. Yes, the testing current is AC. Fortunately, Rds_on is bilateral (works for both current directions, positive and negative).

author: David Hess
author_rank: Super Contributor
timestamp: January 13, 2025, 04:56:35 pm
content: When fully on, the on resistance has very little dependence on current except for its positive temperature coefficient, so your test is good. A low current test is actually advantageous to remove most of the effects of temperature rise.At such low resistance, it is important to use 4-wire or Kelvin connections to remove effects from the test setup.It seems that the parts you bought are counterfeit.With a simple test circuit, the reverse transfer capacitance can be easily measured.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 05:40:25 pm
content: Especially modern MOSFETs have a rather nonlinear capacitance. They found a way to at least reduce the capacitance when the DS voltage is high. So the voltages when measuring the capacitance matters. In some cases even the frequency may matter.

author: dietert1
author_rank: Super Contributor
timestamp: January 13, 2025, 10:15:53 pm
content: Today i tested one of the IRF1405 fakes at a DC power level of 54 W (4 A at 13.58 V with Ugs = 3.3 V) at a case temperature of 50 °C and it ran stable for more than an hour. Whatever it is, it seems to be a well made mosfet that will serve my TEC drive application.I suspect that we can use the junction to case thermal resistance to determine safe DC operating conditions. If it is 0.5 K/W (typical spec for TO-220 mosfet) and the case runs at 50 °C then the 125 °C temperature margin up to 175 °C translates into a power limit of about 250 W.With my test setup rising heatsink temperature limits the test to about 100 W.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 11:04:28 pm
content: It is possible that for the low voltages like 10 or 15 V the fake chips could still be limited by the junction temperature and not jet by the termal instability / current bunching. With the origials it would be possible that they fail already at 4 A and 12 V, even though this is way below the power rating. Ideally there is also some safety margin in the shown SOA. Not all will fail at the same level.

author: dietert1
author_rank: Super Contributor
timestamp: April 02, 2025, 07:46:31 pm
content: Last week i received IRLB3034 mosfets from Digikey and today i checked one of them, with Rdson about 2 mOhm (1 A test current, 12 V Ugs). Datasheet spec is 1.4 to 1,7 mOhm. A minor deviation easy to get with non-ideal 4 wire setup. Maybe they get their specs with on-chip probes.I selected that type after Kleinsteins remarks about SOA specs. Its SOA diagram does show a DC curve and at 10 V drain voltage the spec is roughly 90 W. At 8 V drain voltage it allows about 50 A, that is 400 W. The parts were about 1 € a piece.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2025, 08:11:18 pm
content: The one resistance measurement could be tricky, as the pins can make up quite some of the resistance. The on resistance may actually be from a related version in a different case.The SOA looks a little better, but is still not great of a chip with rather high capacitance. It still seems to be cheap.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 03, 2025, 03:12:02 am
content: Quote from: dietert1 on January 12, 2025, 09:18:37 pmWhat is the source supply voltage and impedance here?A good reference is herehttps://www.edn.com/low-cost-driver-for-thermoelectric-coolers-tecs/If you have a near enough ideally matched main supply, you could avoid the buck regulator.You could also use a modulated drive, to avoid the maximum power operating point, if that is likely to be needed for any significant time.See Figure 9.2.10, Figure 9.2.11 graphs herehttps://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Semiconductor_Devices_-_Theory_and_Application_%28Fiore%29/09%3A_BJT_Class_B_Power_Amplifiers/9.2%3A_The_Class_B_Configuration