I've been working on making a flipper for a custom built pinball machine and have a few questions regarding driving the flipper coil.
Typically, pinball manufacturers have two coils in one: one with low resistance for the initial "kick" of the solenoid, and another that is mechanically switched in series with the first with much higher resistance. This ensures the flipper stays up, but that the solenoid doesn't burn out. (I think the current is lowered by an order of 10 or so when the switch is hit.)
My plan is to use an ATmega328 to send a PWM signal to a logic level MOSFET to regulate the current instead of using a mechanical switch. I would prefer to have the uC drive the FET directly, rather than use a dedicated driver, in order to simplify the circuit. Eventually, I'd like to have many solenoids on the playfield and want to keep the number of components down.
My solenoid has 3.5 Ohms of resistance, and I want to drive it at 24V (I'd rather not mess with the 50V that pinball machines sometimes use.), so I'm looking for a FET that can handle maybe 16A at 50V.
The uC will send 100% PWM for a fraction of a second, and then reduce to about 10% or so when the flipper is fully actuated. I want to use a 150 Ohm resistor between the out pin and the FET gate, which gives me a tested 30mA current out of the I/O pin. The PWM frequency will be 20-30 kHz to keep it out of the audible range.
This MOSFET I'm considering is an IRLU3105 (http://www.irf.com/product-info/datasheets/data/irlr3105pbf.pdf). My concern is about how much heat will dissipate with only 30mA to charge the gate capacitance. I estimate a couple of Watts using calculations and simulating with LTSpice, but I wanted to get some educated opinions before I fry my uC (again . My uC informed me it doesn't like providing shoot through, nor acting as protection against back emf). The formula I used for switching Power is P = 1/2(ton + toff)*Vds*I*f = 1.4µs*24V*7A*20kHz = 4.7Watts. Although, I assume 2 or 3Watts since 24V probably won't be dropping across the FET during the entire rise/fall time. But, I don't like to assume anymore.
I calculate that the worst case static Power (correct usage?) would be 43mA*1.75 (derated at 125C) * 24V ~= 2W, which might be close to the Power dissipated from switching.
I'm attaching my shematic as a reference. The area plotted shows how the Drain voltage drops from 24V to 0V in about 600-700ns. This seems to check out when I calculate the Gate charge from the datasheet divided by the current: 20nC / 30mA ~= 700 ns. And, also 5V * 710pF / 30mA * 2*pi ~= 700 ns.
Are my calculations accurate? And, am I interpreting the datasheet correctly?
On a side note, would placing a resistor from the uC ground pin to the 24V ground add any appreciable protection for the uC?
Thanks for your time in reading this.
Typically, pinball manufacturers have two coils in one: one with low resistance for the initial "kick" of the solenoid, and another that is mechanically switched in series with the first with much higher resistance. This ensures the flipper stays up, but that the solenoid doesn't burn out. (I think the current is lowered by an order of 10 or so when the switch is hit.)
My plan is to use an ATmega328 to send a PWM signal to a logic level MOSFET to regulate the current instead of using a mechanical switch. I would prefer to have the uC drive the FET directly, rather than use a dedicated driver, in order to simplify the circuit. Eventually, I'd like to have many solenoids on the playfield and want to keep the number of components down.
My solenoid has 3.5 Ohms of resistance, and I want to drive it at 24V (I'd rather not mess with the 50V that pinball machines sometimes use.), so I'm looking for a FET that can handle maybe 16A at 50V.
The uC will send 100% PWM for a fraction of a second, and then reduce to about 10% or so when the flipper is fully actuated. I want to use a 150 Ohm resistor between the out pin and the FET gate, which gives me a tested 30mA current out of the I/O pin. The PWM frequency will be 20-30 kHz to keep it out of the audible range.
This MOSFET I'm considering is an IRLU3105 (http://www.irf.com/product-info/datasheets/data/irlr3105pbf.pdf). My concern is about how much heat will dissipate with only 30mA to charge the gate capacitance. I estimate a couple of Watts using calculations and simulating with LTSpice, but I wanted to get some educated opinions before I fry my uC (again . My uC informed me it doesn't like providing shoot through, nor acting as protection against back emf). The formula I used for switching Power is P = 1/2(ton + toff)*Vds*I*f = 1.4µs*24V*7A*20kHz = 4.7Watts. Although, I assume 2 or 3Watts since 24V probably won't be dropping across the FET during the entire rise/fall time. But, I don't like to assume anymore.
I calculate that the worst case static Power (correct usage?) would be 43mA*1.75 (derated at 125C) * 24V ~= 2W, which might be close to the Power dissipated from switching.
I'm attaching my shematic as a reference. The area plotted shows how the Drain voltage drops from 24V to 0V in about 600-700ns. This seems to check out when I calculate the Gate charge from the datasheet divided by the current: 20nC / 30mA ~= 700 ns. And, also 5V * 710pF / 30mA * 2*pi ~= 700 ns.
Are my calculations accurate? And, am I interpreting the datasheet correctly?
On a side note, would placing a resistor from the uC ground pin to the 24V ground add any appreciable protection for the uC?
Thanks for your time in reading this.
Attachments
-
71.4 KB Views: 185