I think that I've found some solutions to the problems I offered below. Just wanted to come back and share. As I addressed before, Julian Ilett offered some interesting optoisolated drivers... > During a search for MOSFET drivers I came across a couple of interesting > tutorials by Julian Ilett using a dual complementary optoisolators to > switch the gate of a high side N-channel MOSFET between the source termin= al > and a bootstrap capacitor. If you're interested here are some of them: >=20 > https://www.youtube.com/watch?v=3DtebprQvrqk4&t=3D425s >=20 > https://www.youtube.com/watch?v=3Duff1nzFo140 >=20 > https://www.youtube.com/watch?v=3DiNYeww1Sjzk&t=3D61s >=20 > Each have an excellent explanation of both the use of a bootstrap capacit= or > for the gate and the use of optoisolators in a complementary configuratio= n. >=20 > Before moving on, personally I'm not interested in black box gate drivers > at the present time. I'm really interested in how those drivers work and > how they can be duplicated with junkbox discrete parts. >=20 I found a solution in a Arduino thread titled "Automotive PWM MOSFET drivers." One of the posters offers a really simple circuit, which uses a NPN, diode, and a pullup resistor to create an emitter follower that actively pulls up the emitter to the upper rail with the lower NPN turns off, then turns off once the output has reached that point. I believe that it solves most of my problems. The second circuit is a form of a Dickson charge pump. The charge pump is a simple device that ties the top end of a cap to steering diodes, then uses a clock signal on the lower leg of the cap to alternately charge the cap through one diode when grounded, then squeezing the charge into a second cap through a second diode when the clock is raised. A storage cap after the second diode maintains the higher voltage when the pump cap is dropped back to recharge. The adaptations I made was to the charge pump were: 1. Source the pump cap to 12V or so, which is more than enough to raise the voltage on the gate to turn the N-channel fully on. 2. To clock the charge pump cap between 0V and the top rail of the charger. At 0V, the pump cap charges to 10.8V. When the clock switches to the top rail voltage that 10.8V is added to the top rail, squeezing 10.2V + the top rail voltage into the storage cap. 3. Tie the bottom leg of the storage cap to the top rail voltage. So the storage cap is really only charged the 10V or so. But the top leg of the cap will be 10V above the top rail voltage. So essentially driving the charge pump clock circuit with a PWM signal will provide a permanent gate voltage for the high side N-channel MOSFET for the half bridge. So here's how these two discoveries fix all the original problems: > My real interest is how to adapt what is a trivially simple driver into > specifically some step down lithium battery charging projects I have near= the top > of my project list. However I'm running into some issues in adapting thes= e. > Here are the main problems: >=20 > 1. In Julian's examples the load (a lightbulb) is grounded. So when the > mosfet is off, the terminals of the bulb, along with the source terminal = of > the mosfet are at ground potential. A battery as a load will never be > grounded at the source. So for example charging a 40V pack, the source of > the high side mosfet is always at 30V or higher. The charge pump solves the problem by independently switching the lower leg of the pump cap between GND and the top rail using the steering diodes to isolate the pump action between 12V and ground from the switch action at the battery voltage at the top. >=20 > 2. In the same vein, unlike Julian's example where the top voltage in the > circuit is 12V, the main charging voltage will be about 50V. While 12V is > clearly good as Vgs, 50V clearly is not. I have some golf cart DC/DC > converters that will output 12V or so, but getting it switched is going t= o > be a bit of a challenge. The golf cart converter will supply the steady 12V to the charge pump. The totem pole switch will switch the charge pump between 0V and the top rail now. >=20 > 3. The optoisolators are the third issue. They are 6N139. Their CTR is > excellent, so they can drive the totem pole for the gate, However, they > have a maximum Vce of 18V, running into the same problem as #2. I technically only need the opto in one place: the totem pole switch between the gate voltage from the storage cap and source terminal of the high side mosfet. But there is also a low side mosfet that is driven with a complementary signal. So I could use a DCOI to drive both MOSFETS. But neither gate is going to be driven more than 10-12 volts away from the source. So the 18V Vce of the optos won't be exceeded. >=20 >=20 > So given these circumstances, I'm interested in the following: >=20 > 1. How to switch the bootstrap cap negative terminal between the battery > voltage and ground to charge it? Totem pole to switch the bottom leg of the charge cap between 0 and the top rail voltage. 2 NPNs, 2 base resistors, and a diode. >=20 > 2. How to protect the optos which may have up to 44V between the the > collector and the emitter? By not having 44V between the collector and the emitter. All gate voltages only swing 12V or so. >=20 > 3. How to limit the gate voltage to a standard Vgs of 15-20V? By having the charge pump cap only charge to 12V. >=20 > Any suggestions welcome. Still interested in seeing if anyone has put together such a driver... =20 BAJ =20 --=20 Byron A. Jeff Associate Professor: Department of Computer Science and Information Technol= ogy College of Information and Mathematical Sciences Clayton State University http://faculty.clayton.edu/bjeff --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .