I ‘m designing a high side switch using p-channel mosfet (NDP6030pl) to switch a high current load (around 20A max) via arduino Since I’m new to mosfets I wanted to clarify the term Vgs (gate to source voltage ). I’ve connected the mosfet source to +24v dc, the drain to load’s positive side and the gate to voltage divider leaving around -10v( Vgs) when shorted to ground via npn transistor (2n2222). Everything seems perfectly working but my question is:
When mosfet is switched off (gate connected to 5k 24v rail) that leaves 24v on the gate pin of the mosfet. Is that safe ? Does vgs mean the voltage between gate and source or just referring to the voltage connected to the base lead? The data sheet states max Vgs (+_ 16v) . Does that value has to to with the 24v on the gate pin (not referring the voltage difference between gate and source)?
It's all relative. When the NPN is off, both the gate and the source are at 24V, so the Vgs is zero. When the NPN is on, the gate drops to 12V, so Vgs is -12V, which is within the spec. The mosfet also sees a -24V Vds when the mosfet off, which is also within the 30V spec for that. Voltage is always a relative thing - the difference in potential between two points.
vaj4088:
When only one point is mentioned, the other point is assumed to be ground. However, Vgs mentions two points: gate and source.
I guess the point is that if you omitted the lower 5k resistor, and the transistor was fully on, the voltage Vgs would be (nearly) 24volts. For some mosfets, that would be out of spec.
Thanks for your replies however I still have issues. There seems to be a 0.9v leak when mosfet is off and also the 24v supply falls to 20.8v when mosfet is switched on. Why is this happening ?
For testing I’m switching an led but I notice that when led is switched off (mosfet off), voltmeter read 0.909 V between led anode and chathode and same reading appears between drain and ground. Also I notice that when mosfet is switched off (24v supply) voltmeter reads 22.3v between source and drain pins which drops to 20.8v when mosfet is switched on. Where are the other 3.2v being dissipated ?
Your circuit will be fine for Vgs. The spec is +/- 16 volts. You are a little close but should not be an issue. If this were a production design then I would agree a zener would be needed.
However you also need to worry about power dissipation.
Per Figure 1 in the ON Semi Datasheet at 20Amps the expected Vds voltage is ~ 0.5 Volts.
Power = Volts X amps = 0.5 x 20 = 10 Watts.
Without a heatsink the temperature would be: (spec = 62.5 degree C / watt)
Temp = 62.5 X 10 = much too hot ( a practical limit is about 125 °C)
So you will have to get a heatsink else you FET will last less than a minute at 20 Amps
Regarding the heatsink, for my project isnt’t required because I’m using the mosfet to pulse the 20 for a max of 1sec. The only issue I can’t figure out is that between drain and ground (when mosfet is switched off) my voltmeter reads -0.89V (and sometimes even -1v) (positive probe on drain and negative probe on ground). When drain is grounded via 10k resistor, these -0.89v disappear. Since my project is critical on having a 0v during the off cycle, is this reading (negative voltage leakage) due to something bad in my curcuit? How can I get rid of this leakage to have 0v when load is switched off (apart from grounding the drain pin)? I’ve tested different mosfets but the leakage is the same for all.
I’m using 2 separate power supplies. One for arduino and 1 for the load. Since I’m not using pure dc supplies, and arduino ground and the load’s ground are common maybe that’s the issue ?
What is this power supply you are using if it is not pure DC ? If you put a diode in series with it just for a test, do things improve ?
If you do put a high value resistor between drain and ground, I guess your load would not see it anyway.
The voltage drop of 0.9 volts could, if the mosfet were wired incorrectly, be attributable to the body diode. But I don't doubt you have checked this.
I’ve connected a full bridge rectifier to the supply but it seems that the leakage increased to 1.01v. This voltage leakage disappears on connecting drain to ground via 10k resistor.
If a 10k resistor gets rid of it, then it a very low current effect.
I'm no expert here but I guess these are properties which are not even mentioned in the data sheet. It could be body diode leakage or something similar. I'd be curious to see if putting a high value resistor (say 1M) between the mosfet gate and ground makes this effect disappear.
There is, contrary to what I said before, a specific statement in the data sheet which refers to gate-body leakage. It has a value of 100nA. That you saw around 0.9 volts instead of around 24 volts is probably attributable to losses in the measuring instrument you were using.
