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I am trying to build a simple H bridge as part of a learning curve. At the moment it is on a breadboard. I am using 2 P-Channel MosFets (IRF9540) and 2 N-Channel MosFets (IRFZ440N). They are tied to a 3.6v screwdriver motor. I've tested the current draw on this at stall when powered by a 6v lantern battery. The max current I have recorded is 0.8A. When testing on the breadboard it is not under any stalling load. At the moment I am not using an MCU controller - i'm just jumpering the control wires (going into the gates of each side) straight into the power line. The P-Channels are getting very hot; which is somewhat to be expected as the P-Channel RDS is relatively high at c200mΩ. (but I have to say that I am surprised by how high the temp is getting. ambient is c.25C. However this question is more aimed at the fact that the motor is running very slowly. It looks like somewhere in the circuit I am getting serious voltage drop - it appears to be random but sometimes the motor will not turn at all, sometimes it turns slowly and gradually gets faster, but never achieves anything like the speed when directly connected even to a small LiPo 3.7v cell (900mAH).

sometimes I am reading no voltage across the motor pins, sometimes 1-2v. but nowhere near the 6V across the battery pins.

is this all down to poorly balanced fets? how can I really be dropping 4+V? when the gates on the right hand side of the H bridge are brought high I am getting the following voltage drops across drain and source on each of the fets

Q1 3v Q2 1v Q3 4v Q4 4v

(note annoyingly my normal multimeter is on the blink so I am using one that is granular only as to 1v).

the circuit schematic is here: https://www.dropbox.com/s/ih7bx6hlj40vxtf/Screenshot%202015-05-18%2018.24.49.png?dl=0

a photo of the breadboard assembly can be found here (fwiw): https://dl.dropboxusercontent.com/u/194358/IMG_20150518_183350.jpg

(please forgive the absence of colour coding in the wiring).

I am using jumper cables which for sure can't take multi-amps but they should be ok with smaller currents - none are getting hot in any event.

many thanks in advance for any pointers you can give me.

Justin

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2 Answers 2

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Without looking at your schematic I can tell you the problem- the MOSFETs are specified at 10V Vgs and you are giving them a few volts only, so they are not fully turning on. The 6V is probably dropping from the motor too.

You need to find higher voltage drive for the gates to get them to turn on, or (easier) find MOSFETs that are specified to have low Rds(on) with a few volts Vgs. This kind of circuit should also have a UVLO (undervoltage lockout) so if the voltage is too low for them to fully turn on the MOSFETs don't get dangerously hot.

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  • \$\begingroup\$ Hi My data sheet suggests that Vgs is -4v(Max). Otherwise I'd completely agree with you. This has all the hallmarks of undervoltage. \$\endgroup\$
    – Justin Adie
    Commented May 18, 2015 at 21:05
  • \$\begingroup\$ Vgs(th) is the voltage at which you get 250uA minimum. Doesn't sound like enough to run a motor. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 18, 2015 at 21:14
  • \$\begingroup\$ thanks again. you're probably right. Looking at the curves for Vgs, at -5v it should be delivering 4A at 25C ambient. My datasheet is here to make sure we are looking at the same thing (i believe that this particular fet is now manufactured by Vishay, but the examples I have are still IR): dropbox.com/s/6o9se44ijcgbcd3/irf9540n%20datasheet.pdf?dl=0 I guess the question remains as to why there is such a large voltage drop? \$\endgroup\$
    – Justin Adie
    Commented May 19, 2015 at 8:12
  • \$\begingroup\$ You're looking at typical curves. Shift that typical curve so that it reads 250uA at 4V and you'll get a better idea of what the worst case may be. But did you actually measure Vgs accurately? \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 19, 2015 at 12:07
  • \$\begingroup\$ dropping the curve down by 50uA still shows a healthy current @5v. the issue seems to be that despite feeding the circuit with 6V the voltage at the source is too low. which is bizarre as it is directly connected to the battery terminals. The gate is connected to ground when the P-Fet is turned on - so logically Vgs should be somewhere close to 6v. but clearly it isn't. I have not, however, measured Vgs, just Vds for each fet. i will rebuild the circuit this evening and measure Vgs. and post back. \$\endgroup\$
    – Justin Adie
    Commented May 19, 2015 at 16:04
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As vgs of MOSFET is not able to drive MOSFET to saturation this problem occurs..since here MOSFET is in ohmic region possess more resistance than RDS(on) .u can check by probing multimeter across drain to source of each MOSFET...u will find missing voltage..

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