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| Saturating a MOSFET with a constant current sink driver |
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| Wimberleytech:
--- Quote from: Oleenick on January 06, 2020, 11:16:34 pm --- --- Quote from: Wimberleytech on January 06, 2020, 02:18:30 pm --- --- Quote from: Oleenick on January 06, 2020, 11:54:02 am ---I am using +5V as VCC, I should have stated that earlier. Also I think I will be going with the IRLS3036TRLPBF N-channel MOSFET now anyways because of its very low RDSon. That will fix my thermal issues as I'd like it to be capable of a continuous current of each layer just incase the program hangs during switching between the 8 levels. (Attachment Link) There's the schematic that I would follow with the new MOSFET. The resistor to ground would be 620ohms and the capacitor 100uF. In this case should I use a resistor on the gate? --- End quote --- The threshold voltage for that transistor is min of 1v and max of 2.5 volts. Therefore, your output voltage will be a maximum of about 4 volts and a minimum of about 2.5 volts. A gate resistor is not necessary. --- End quote --- That is potentially problematic. Do you mean that Vds (output voltage) can only be 2.5-4V? Where did you find that on the datasheet? Or by output voltage do you mean gate voltage? (Attachment Link) This figure from the datasheet shows that at a voltage of ~3V Id can be 10A, which would be enough for my application. Do you mean that the gate voltage needs to be between 2.5 and 4V to achieve the current I need? --- End quote --- Your latest circuit is a source follower. Therefore, the source will be VDD-VGS and VGS will be what it needs to be for a given current. So, from the graph you posted, at 10A, VGS will be ~3V. The Gate voltage maximum is 5V, so the source voltage will be 5-3=2V. Are you happy with the output voltage being 2V? If so, you are fine. |
| Oleenick:
--- Quote from: Wimberleytech on January 07, 2020, 12:01:29 am --- --- Quote from: Oleenick on January 06, 2020, 11:16:34 pm --- --- Quote from: Wimberleytech on January 06, 2020, 02:18:30 pm --- --- Quote from: Oleenick on January 06, 2020, 11:54:02 am ---I am using +5V as VCC, I should have stated that earlier. Also I think I will be going with the IRLS3036TRLPBF N-channel MOSFET now anyways because of its very low RDSon. That will fix my thermal issues as I'd like it to be capable of a continuous current of each layer just incase the program hangs during switching between the 8 levels. (Attachment Link) There's the schematic that I would follow with the new MOSFET. The resistor to ground would be 620ohms and the capacitor 100uF. In this case should I use a resistor on the gate? --- End quote --- The threshold voltage for that transistor is min of 1v and max of 2.5 volts. Therefore, your output voltage will be a maximum of about 4 volts and a minimum of about 2.5 volts. A gate resistor is not necessary. --- End quote --- That is potentially problematic. Do you mean that Vds (output voltage) can only be 2.5-4V? Where did you find that on the datasheet? Or by output voltage do you mean gate voltage? (Attachment Link) This figure from the datasheet shows that at a voltage of ~3V Id can be 10A, which would be enough for my application. Do you mean that the gate voltage needs to be between 2.5 and 4V to achieve the current I need? --- End quote --- Your latest circuit is a source follower. Therefore, the source will be VDD-VGS and VGS will be what it needs to be for a given current. So, from the graph you posted, at 10A, VGS will be ~3V. The Gate voltage maximum is 5V, so the source voltage will be 5-3=2V. Are you happy with the output voltage being 2V? If so, you are fine. --- End quote --- By the way thank you so much for your help. Is there a way that I can make the circuit with this IRLS3036 MOSFET that will output 5V (max 10A)? Is that just not possible because this MOSFET is designed for fast switching operation? |
| Wimberleytech:
--- Quote from: Oleenick on January 07, 2020, 12:08:28 am --- Is there a way that I can make the circuit with this IRLS3036 MOSFET that will output 5V (max 10A)? Is that just not possible because this MOSFET is designed for fast switching operation? --- End quote --- You need to provide more information. What are you driving that needs 10 amps at 5 volts? The TI part is driving all of the LEDs, right? |
| Oleenick:
--- Quote from: Wimberleytech on January 07, 2020, 12:20:00 am --- --- Quote from: Oleenick on January 07, 2020, 12:08:28 am --- Is there a way that I can make the circuit with this IRLS3036 MOSFET that will output 5V (max 10A)? Is that just not possible because this MOSFET is designed for fast switching operation? --- End quote --- You need to provide more information. What are you driving that needs 10 amps at 5 volts? The TI part is driving all of the LEDs, right? --- End quote --- I'll be using 8 of the MOSFETS to control each of the 8 anode layers of the LED Cube. So each MOSFET will have a duty cycle of 1/8. So you could call these the high side drivers for the LEDs. The TI TLC5927 are constant current sink drivers so they will be on the low side controlling 192 cathodes per layer. I'd be running the LEDs at an absolute max of 50mA (if 30mA is not bright enough): 192 * 0.05 = 9.6 or ~10A. If 30mA is bright enough, which I'm thinking it will be, the current required will be ~6A (I miss calculated and said 7A before). I'd like the MOSFET to be capable of a constant load of 10A at 5V so if the code breaks and the layer stays on for greater than 1/8th second, no overheating will occur. |
| Wimberleytech:
--- Quote from: Oleenick on January 07, 2020, 12:36:47 am --- I'd like the MOSFET to be capable of a constant load of 10A at 5V so if the code breaks and the layer stays on for greater than 1/8th second, no overheating will occur. --- End quote --- I think you need a PCH as in your first design. Here is one (Fuji 2SJ475-01) that can handle the power dissipation and the current. Looks like this is discontinued but seems to be still available. Here is another that may be just on the edge w/r to power dissipation: https://www.mouser.com/datasheet/2/427/siss05dn-1595673.pdf |
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