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I am designing PWM speed control for 110v DC motor. Since it is 110v, I am going to power it from bridge rectifier connected to mains, with MOSFET on low side for switching.

For driving MOSFET I plan to use dedicated driver (though it doesn't matter for the question I have, so let's assume I drive gate directly from the low-voltage schematics).

For low-voltage side (microcontroller/logic) I want to use typical wall charger, which is switched power supply, as I understand.

The tricky part here is that driver supply voltage (or gate voltage) will have to be applied relative to the "power ground" ("-" of bridge rectifier/MOSFET source). This means "-" of the 12v switching power supply will be connected to the "-" of bridge rectifier. If the 12v power supply was transformer-based power supply, it wouldn't be an issue since 12v part would have been isolated from the mains and connecting two grounds would be fine.

However, somehow I think that connecting these two grounds in case 12v is provided by switched power supply would result in a short, since, as I understand, there will be no galvanic isolation between 12v and 110v DC (bridge rectifier output).

So, the first question, if I am understanding the problem correctly?

Second question is, what options do I have for supplying power for gate driver/MOSFET gate? I can isolate logic part using optical isolator, but I would still need low voltage power supply for driver/gate.

It seems to me that simple option (without using transformers) would be to use resistive/capacitive power supply connected to the bridge rectifier? Although driver should be able to provide relatively high current to the gate (let's say, 5A), as I understand, what matters is how much charge is actually transferred. Assuming Qgs = 150nC and switching frequency of 20kHz it would be Qgs*F per second, which gives Isupply = Qgs * F ~= 150nC * 20kHz = 3mA. Which means, resistive power supply providing 50mA would be plently. 50mA with 100v drop would give 5W of burned energy, which is OK.

So, the third question, am I right in these calculations?

Here is the schematics, but there is not much to draw. I omitted all other components (like snubbers, etc). Box on the bottom is switched power supply. L1 is motor. Imagine I want to open MOSFET by applying 12v to gate-source. My understanding is that I cannot just connect two grounds and connect 12V to Vg -- this would result in short.

schematic

simulate this circuit – Schematic created using CircuitLab

P.S. I think, I am wrong about switched power supply. It seems like some of them (most of them?) are actually isolated. Which means I should be able to use it for MOSFET driving circuit.

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  • \$\begingroup\$ Please add a schematic. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 4 '14 at 0:00
  • \$\begingroup\$ You do realize that the rectified voltage will be higher than the RMS voltage, and that a non isolated system will be more dangerous to work on, right? \$\endgroup\$ – W5VO Jun 4 '14 at 0:55
  • \$\begingroup\$ Yes, I expect DC to be 110*sqrt(2). Yes, I do realize it would be dangerous. Though it is still 2 times less voltage than in the country I came from :) \$\endgroup\$ – Ivan Dubrov Jun 4 '14 at 3:55
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TI offers some isolation gate drivers that let your do exactly this, without destroying yourself or the other circuits.

Also, beware of the minimum clearances for this type of circuit.

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